Prop walk: the blade at the top of its turn, ie 12 o'clock, gets a bit of end plate effect so gives a bit more side thrust as it passes through 12 o'clock (tip vortex is reduced at 12 o'clock). That's why you get the imbalance of side thrust. At least that's what I was taught getting my bachelors in nav. arch. Prop walk can vary depending on the flow into the prop also, which is affected by speed and hull shape. Prop walk is really noticeable on tugboats that have old style fixed props on a shaft. Number of blades... usually you want to keep the blade number not an even fraction of the engine cylinders to keep vibration down (can be exceptions). I.e. if your engine has 4 cylinders, having 3 blades is better than 2 or 4 blades. This is due to the additional side thrust of the 12 o'clock blade. If you have an even multiple of blades to cylinders, you tend to get a sync of a cylinder firing at the same time you get that extra side force on the shaft, which can cause a vibration. Good vid, brings back memories of my segment in school on prop design.
@Mission Gameliel Not when crossing oceans or night sailing or if you don't have enough room for the amount of panels needed for the amount of power required. Solar is good and very good, but it's not a solve all resource. If solar alone could do as you say then why solar is not fitting for all solar powered boats? More so the small day boats! (SOLAR HAS IT'S PLACE BUT IT'S NOT A SOLVE ALL)
Interesting your opinion on Kiwi Props. My experience, and the advice of the creator /manufacturer is lube on annual haul out! I've found that to be effective. I've not seen any erosion of the plastic blades, probably due to application of Prop Speed, so other than lube and prop speed I've had no maintenance issues in 6 years.
I just want to point out that a kiwi prop costs us$1500 for a 3 blades and OR $150 each blade when you change them. So I think the doctor just looked at the prices for extra blades ☺️
The episode conveying the most 'wisdom' of all of your videos, imo. Very nice job, Herbie. Thank you for making that whole complicated issue/topic much clearer for me, and I'm guessing many others as well. Carry on, sailor.
Great presentation! I don't know what date it was recorded but it may require updated especially regarding Kiwiprop pricing and blade composite which are (now) made using Dupont's Zylan HTN composite.
Herbie, HERBIE, hERBIE, I love ya man but u gotta get over this molecular plastic prop phobia. The first time i heard you say that (on a different video) i stopped in my tracts and backed up the video to be sure I heard you correctly. Later I found out you are a dentist which makes your comment seem so strange. As an engineer i can assure you your dental practice and day to day living not to mention the sunglasses on your head - contributes far more environmental pollution than plastic prop's microscopic ware in the frigging ocean. Sorry just couldn,t take it a 2nd time LOL. What makes me truly sad is we keep dumping trash in the ocean and it shows up on the shorelines of even the most remote islands. Love you and your sweet wife( and the birds) and have learned soo much about sailing from u 2. Keep up the good work!! PS Please listen to your wife about boat design especially on the artistic aspects of modifying a lady boat's rear end!!.😍
Haha! It’s all good. Every bit counts, and there is a lot going out there!! Thankfully they are starting to clean up the massive amounts of fishing equipment that is floating out there to help the oceans. I do love these props and think they are the best design of feathering/folding props!
Except water is considered to be an incompressible fluid, so talking about the water being less dense up top, is nonsense. It may be more easily displaced toward a shallow surface, but it isn't any less dense.
My boat came with an Atomic-4 engine with a folding prop (forget the brand). I liked the prop. I had no problems with it, and, though people said the prop walk would be worse, I had very little prop walk. Maintenance was about the same as a fixed prop. I converted to electric, and stayed with the folding prop for the first year. It was fine. After that, I got a used fixed prop at a very good price and wanted to test regen, so I changed the prop out. I noticed no appreciable change in sailing, and I can get a little (very little) regen when I sail >5 knots.
Great video Herbie. Again, your depth of knowledge and ability to convey it, is impressive. Prop walk is a pain... I owned a diesel powered Pearson with an offset propeller shaft to counter prop walk. Backing out of a slip with that offset shaft can be a major PITA depending on which way you needed to travel out of the slip and how much room you have backing out. If you back out with low speed, she walks. Some spots were tight enough a hi speed burst may not be advisable. Then of course, add the effect of currents! Like you, I paid real close attention to where and how I tied up. ;-) Well done video. Great explanation!
Super information and delivery. A natural teacher. It's very clear that you researched the subject in great depth and left no stone upturned. Everything was covered in a very considered and systematic manner. Each question that came to my mind was subsequently answered. Your technical knowledge is second to none and your channel is an excellent resource for all things sailing related. Thanks for taking the time to make the video and share the knowledge.
For a professional skipper who specialized in 12-day trips France to Ireland and back (64 trips total), a folding propeller him gave a good half-day gain on the whole trip. That meant one more stopover in a beautiful anchorage. Speed prediction formulas used in handicap systems assume a mix and upwind and downwind. A folding/feathering prop can make a 1.5-knot difference going upwind, easily. If the conditions are rough (against the current), that could even mean making it to destination vs. not.
Guys, Kiwi props are not that cheap. They're about half the price of the others. There's just no way you can purchase a kiwi prop for $400. that just buys replacement blades!! I'm looking at them now and the 3 blade model is $1800 where a Max prop is close to $3k
You're suggesting that the 2-blade feathering Max-Prop would automatically set itself vertically when sailing. But that actually depends on the position of the shaft when turning the engine off. That will determine the angle of the propeller. If you absolutely want the blades to sit vertically, you'll have to check your shaft angle from the inside and potentially need to turn it, which is a pain..
The most efficient prop is one with the fewest blades (though one is not that practical), as big as possible, so it will have a smaller pitch to suit. Sailboat props are all about static thrust, think trying to make headway against a very strong wind.
.......and then there is also the "variable pitch" propeller, usually controlled via hydraulics. It allows for very fine, infinite control, however it's a fairly complex system that is very expensive. Not usually found on the average yacht, but on the larger mega-yachts owned by billionaires (both sail and power). By the way Herby: You did an excellent job with your propeller lecture. You pretty much covered all the bases and offered a few compelling arguments for and against various types of props and their design characteristics. It appears you've done your homework. In fact you were so convincing, I'm thinking about getting rid of the prop and switching to oars (totally eliminates prop walk😁).
Regarding the kiwi prop, there are a couple of comments that I find difficult to take seriously. Life span, what is short or long? I drove a kiwi on a Harmony 34 between 2007 and 2018. No problems to report from that period. It is probably more important to make sure that these propellers are well lubricated and that the seals that hold the grease in place are changed at appropriate intervals. Wear of material in the blades? I haven't seen any of that at all, the last few years a layer of antifouling was put on the blades, it disappears with time but not so fast either. I see no reason why it wouldn't have worked for another ten years. And a big advantage, the blades angles can be adjusted to have the engine run on desired revs.
Good to know! I have friends in the Chesapeake Bay who have had to replace blades every 5 years or so. Makes me wonder what is in the Chesapeake! One friend barely moved his boat and the sun always hit one blade in particular. One blade disintegrated and it is assumed to have been the one that always had the sun hitting it while in the slip. Another friend (who first introduced me to kiwi props) told me about the polishing blades. He works at a boat yard in Baltimore and frequently would have to replace worn down blades on kiwi props. I really love them! Their simplicity, reliability, and adjustability make them the best non-fixed propeller in my opinion. I’m glad to know you didn’t have the blade polishing issue :) it might just be an issue in the bay.
I’ve looked at the kiwi props before but it just occurred to me while I was watching your video. By the way you did an excellent job of explaining prop walk and why you have it and how to deal with it.
Hi Doc. ........... enjoyed your video. Picked up a few bits of interesting info. Cost !!! NOT CHEAP. Another thing in my research on the net was the composite the blades are made from Zytel, a plastic composite made by DuPont is now used for the blades. High strength, abrasion, and impact resistant thermoplastic poly-amide formulations. Is this newer info or not from what you know? Cheers
My info is from other cruisers who had the blades polish down over the years. It was all word of mouth but from first hand experience on their part. I do not know what material or age their blades were, or if they still have the problem. I contacted Kiwi Prop to see if they would send me a prop to test out on our return voyage. I offered to weigh the blades before and after the crossing to find out conclusively what happens over thousands of miles of sailing but they were not interested in my proposal. Of all non-fixed propellers, Kiwi is my favorite design; it has simplicity which I find elegant. I just wish that I knew definitively if the blades polishing off is an issue or not.
Yes especially the high end ones with synchronised gear meshing blades like "Gori" and "Wisco" -- Gori can go over centre which reduces the diameter under load thus having a gearing like effect for better utilization of power under load.
Prop walk occurs because the shaft of the propeller is usually at a downward angle toward the propeller. If the shaft was horizontal making the propeller perfectly vertical. You would not have prop walk.
In a single prop boat, yes you would still have some prop walk (think of it as a paddle wheel) but it wouldn’t be as noticeable. Most high powered twin engine boats have counter rotating props, even if the shafts are horizontal (saildrive & outboards)to cancel it out. I once owned a trawler with a massive four blade prop which was horizontally aligned with the keel, in an aperture, and it would only go one way in reverse.
we have a 2 blade maxprop. Working on getting it rebuilt after using it problem free for 16 years. After taking it apart we found some damage inside, but it still worked without issue. Highly recommended, there is a trick to get it to feather, but you can tell when it is not feathered because the shaft spins and you can hear that on our boat. A quick pop into reverse and then neutral when the engine is off feathers the prop and the shaft stops spinning.
I have a two blade J Prop on our C34 MkII. It has a slight prop walk to port but feathers beautifully. I love your splicing videos. Really having problems finishing the knots on soft shackles though! I used to be a merchant mariner, and was taught that our prop walk was a result of the pressure differential between the blades. We were talking 18 foot diameter props !
🫨 18 foot!! Sailboat props must look like a handheld personal fan! Are you doing a button knot or the lovers knot? I like the lovers knot for a few reasons, but you can splice the tails of the knot into each other to make a single tail that is really handy for guiding the knot through the loop.
I really dig those Rigging Doctor educational (about propellers, keels, rigging etc) videos. Thanks for making them! After watching this video about propellers it makes me wonder, why don't sailboats have Controllable Pitch Propellers (CPP)? It would give you both port and starboard (by changing the pitch of the propeller) prop walk that you can abuse to get in or out of marinas...
Some sailboats do have them and it’s very nice. The problem is they are complicated and expensive. Also, if the consumer doesn’t know how to work it, the boat builder will have to deal with all the complaints about how the boat doesn’t motor well. I’ve been on a few boats that had different styles and I loved them all! One was a high or low pitch, one for marina maneuvering and the other for cruising and range. Another had fully adjustable pitch instead of a gearbox. The prop was always spinning, but the blades could be rotated to be straight for neutral or pointing forward or back for the direction. By adjusting the pitch, you could make the boat have more torque or more speed without touching the revs. That was by far my favorite! The captain set the revs for docking and did everything by throwing the pitch one way or another. His only complaint was that the prop can’t free wheel while sailing and the blades don’t feather all the way. Also, if he ran over a line, he had to shut the motor down as neutral still had the prop spinning. Everything always has to be a compromise!
Do a informative video about tether safety harness.A person WILL DROWN IF FALL OVERBOARD.think it thru really well.ASK? Can a person pull him or her self back onboard while being dragged at 4,5,6,7 10,15 knots???in flat calm seas.? rough seas?cold seas?in the night when the othres are sleeping?SO always wear a locator beacon flashing light and a portabl ,gps position transmitter to your onboard screen.with a man overboard siren alarm to wake up the crew.BECAUSE by the time you drop the sails ,tun on the motor and turn around it will be a mile.CAN THE PERDSON WHO IS BEING DRAGGED WITH SAFETY TETHER SURVIVE A MILE???
Have a Kiwi for 10 years. Sent back to factory at 5 years for service. No blade wear evident Paid around €1600 euros. Let me know where I acan buy for $400 and I'll keep a spare.
I have s two bladed folding prop on a standard propshaft. I get prop walk to starboard in reverse. I use this to advantage when coming into my slip because I can come almost abreast of my slip give full rudder to port and a stab of reverse and she'll pivot about her keel and I can then motor straight in. Simples!
So, Saildrives with two blades and fixed propellers are the best for electric sailboats because they're reliable and cheap, and two blades have less drag than three or four blades, right?
Do you happen to know if kiwi props can help boat speed. I have a boat that doesn’t motor much, but is underpowered. Meaning on days with good 17-20 knot winds it’s a bit iffy on even taking her out. Due to the risks of her being overpowered by the wind when the sails aren’t up. Looking for not very expensive options for keeping this old YSM8 going but getting better performance out of her.
It’s going to be dependent on the pitch of the prop. If you decrease the pitch, you will increase your torque but cut your top speed. I’m not sure if the pitch is adjustable on Kiwi props. If it is, you are golden!
Nice interesting piece which covers it all. One point I'd like to add is I would serious miss my prop if it were not nice and close to the rudder blade. Manoeuvring does require experience as to which way the boat will turn when you put it in astern from standing still. BUT with the rudder close to the prop the walk is really small in ahead as you can blast water direct on to the rudder and make the boat turn any way you want almost immediately.
I know what you mean, it’s practically a stern thruster! I tried to dock a boat that the prop was far from the rudder and it was a very ungrateful event.
Prop regen only generates meaningful energy at minimum 6kn speeds maintained over many hours. At loer speeds the drag from the prop negates the regen. So prop regen makes sense on a 45ft cruising yacht out on passage 24/7
Agreed. When crossing an ocean, we enjoy it for the power it gives us. If we are coastal and making a short journey, we shut off the regen to avoid the speed penalty.
I have had my yacht for 11 years, it came with a kiwi prop and it still has the same prop. I Antifoul the prop so maybe that is why my prop hasn’t desolved as you suggest. Works for me
I'm sorry, but this is simply not the correct explanation for prop walk. Prop walk is simply a matter of asymmetric flow of prop wash over the hull. With astern power, the prop wash is directed forward (towards the bow) and the prop wash, as it leaves the propeller, isn't going straight forward. The total flow of the propeller has an average that is more or less perfectly straight and aligned with the propeller axis, but the wash coming of each indvidual blade will leave at an angle. The prop wash that is closest the hull is that coming off the closest blade, so it moves slightly sideways. As this flow impacts the hull, it does so mostly, or solely, on one side of the hull, which turns the flow into a side force. This can be easily observed as a difference in upwash on one side of the boat compared to the other, with stronger upwash ("bubbling water") on the opposite side of the direction the stern moves when reversing under power. Boats with a straight shaft angled down, and with a skeg or a long keel, will see a much more pronounced effect, as the flow will have more surface area to act on close to the propeller, and the flow will be diverted to one side more effectively. A prop mounted on a saildrive, and a hull that's almost flat and wide, close to and just forward of the prop, will see relatively little prop walk as the flow mostly continues forward, with very little vertical hull area to push against. The incorrect explanation with a depthwise pressure difference being the cause, can't explain the wildly different prop walk magnitudes that different hull, shaft and propeller combinations give. The brave boater could add strakes (long fins, like bilge keels but on a much smaller scale) on both sides of the propeller, to force the reverse prop wash into a straight and forward direction, blocking asymmetric upwash.
In the video I described many of the theories about the cause of prop walk. The reasoning behind the depth/pressure theory is pretty simple: prop walk is more pronounced on larger propellers. A 6 inch prop on an outboard will have less prop walk than a 48 inch prop on a shrimp boat. The pressure difference between the top and bottom blade tips on the 6 inch is nothing compared to the tips set 4 feet apart! As I mentioned, they are all theories which together try to explain the same phenomenon which has no definitive explanation. All the theories share a few points though: shaft angle, propeller size, underwater hull form, and RPM all play into the effects known as prop walk.
Thinking that boats/propellers travel straight ahead when sailing is wrong. No propeller hides perfectly behind any keel when sailing. Leeway causes the blades to see water coming at then from the side at all times when sailing. That is why folding propellers have less drag. Feathering propellers do not completely face water passing by perfectly. A little more drag than folding.
It’s not great. They will always put wear on the driveline in some way. If you have an electric motor though, the spinning actually charges your batteries while you sail!
Thanks! Great job. Third time I watched it. I’m going to emphasize P-factor. Airplane propellors get that in a high pitch. Boats with downward angled shafts get it even more. Propwalk will be most pronounced moving forward at low speed and high power and less so in reverse at low speed and low power. It just seems that it’s worse in reverse because of the pucker factor going into a slip and confusion over which way to turn the wheel. You guys are fantastic. Congratulations on your baby. And looking forward to more videos soon when you’re ready.
In my thinking you forgot to mention with regards to the folding propellor is the complex reversing mechanism that they do have and which ups their costs as well. Otherwise, you made a good show of a fairly complicated subject, congratulations.
A lot of the boats there were involved in smuggling blood diamonds from Africa. The owners were in jail and the boats left to rot! Oh, and it was super cheap!
Kiwiprops are not that cheap. $1,400 for 3 blade. 14" 2 blade feathering $2,000. Used fixed 3 blade $400. Fixed is cheap and always works even when barnicle start growing on it.
Great episode! It's nice to hear electric propulsion included in a talk about props. Regen is an other animal altogether. You mentioned propeller companies that are catering to EP, who are they?
The fatal flaw of a sail drive is it hangs out there to get caught on anything. If it gets ripped off your boat your going to have a very big hole at deal with. Pretty much headed for the bottom.
While I wouldn’t want one on my boat for reasons you have mentioned, they do a great job of minimizing prop walk. Some things are more important that others and for some people, backing up straight is more important than safety 🤪
What an interesting discussion, about the opposite prop walk when drifting forward compared to when the boat is still. I can't say I have practically seen that, maybe because I am usually drifting very slowly when entering a slip. Reading the comments, it seems to be another illustration of the Dunning Kruger effect, overconfidence by those with less experience. Many of us sail on just a few boats regularly over a lifetime. In the end, learn how your boat responds and use that to advantage.
You have to remember that a 3-blade fixed propeller is concave in the forward direction, and convex in the reverse, and therefore the thrust is a lot less at the same RPM in the backwards direction. This means that when the boat moves forward under sail, there is less drag, because the water is coming from behind the propeller. Furthermore, if the engine is put into neutral, then the propeller can spin under sail creating even less drag. However, my understanding is that spinning the propeller under sail is not good for the bearings (or something).
Some say it is bad for the transmission, but apparently it depends on the type of transmission that the boat has. We have an electric motor with direct drive so the spinning prop in neutral acts as a hydrogenerator to charge our batteries.
Thank you for this great presentation, we are seriously considering changing our 2 blade fixed for either a flexofold or featherstream . Your explanation of prop walk is clear and easy to understand.
Propwalk will be in the opposite direction when the gearbox is in forward (with the prop turning) compared to when it is in reverse. However, the direction of travel does not effect the direction of the propwalk. This was wrong in the video and should be corrected. The direction of travel (relative to the water) will effect the direction of flow over the rudder and therefore the action of the rudder. I suspect you are confusing this effect for prop walk. They are two different forces.
If your boat “walks to port in reverse” then you would expect the stern to walk to port when you go in reverse. This is true as long as you are still and starting to go in reverse. When you are moving forward and put it in reverse, you now expect the stern to walk to port but since you are still drifting forward, it will scoot your stern to starboard. This little hiccup can get you in a lot of trouble if you really needed to go to port and instead slammed into something on your starboard side that you were trying to avoid. That is what I was referring to and getting at. Walk is walk, but when you are maneuvering, this is a little asterisk to the statement “boat walks to ...”
Rigging Doctor No, this is incorrect. Or to be more accurate, if you are seeing this effect on your boat, it is not due to propwalk. Propwalk will always be determined by the direction of rotation of the propellor. The direction of travel does not effect the direction of the propwalk. When you shift the gearbox out of forward, the prop wash over the rudder will be lost which will effect the stern in either direction, but this is not propwalk. For most, boats shifting into reverse will act to swing the stern the same way irrespective of the direction of travel.
If you tell someone your explanation about prop walk and give them the helm and tell them to dock; they will give it a try and become utterly perplexed when it doesn’t work right! At a stop they walk one way but if they are drifting forward and expect to walk the same way, they will be shocked and confused as to what just happened when it “walks the other way”. The science of just the propeller is one thing but when you wrap a boat around it, the propeller becomes part of a system and the whole system reacts differently under particular circumstances. I want someone to understand how to use prop walk when they are maneuvering in a tight space. If I don’t mention that little caveat that the boat will walk opposite if you are drifting forward, how many scraped topsides do you think would come from that little omission?
Have a saildrive w/ 3-blade. As stated, virtually no walk. Yanmar makes my drive and engine and over the years (I've had 3 models), they seem to go back and forth about keeping the boat in gear under way. The current guidelines (maybe more since they state you warranty is invalid if ignored) is to let it freewheel which I guess reduces some of the drag versus it being fixed? I suppose they are more concerned about the stress placed on the gears being in gear versus the engine parts moving without the oil pump and such operating as God intended?
To free wheel or not to free wheel, that is the question. Weather 'tis nobler to suffer the thrums and degredation or, by selecting, end them? Personally, I would arrive slower but without the noise.
Thanks for a great discussion of different prop types, why and when you'd want them, prop walk, etc. Really good stuff! The theory about water density causing prop walk seems very wrong. The difference in density from a few inches of depth in the water is insignificant. r m's discussion from naval architecture school sounds right. The hull is acting as an end plate to constrain the water movement and cause a side thrust.
While the various methods for explaining prop walk exist, I still do not know for certain which reason it is, or if it is wholly one reason and not a combination of reasons acting in synchrony. Our outboard motor has no cavitation plate and is a 13 inch propeller. When I have the motor trimmed perfectly, I get very little prop walk (which makes docking hard for me). I tilt the motor up a notch or two and suddenly I have plenty of prop walk to make docking easier for me (as it behaves more like our sailboat which has a level shaft in an aperture).
@@RiggingDoctor Another explanation is that the hull impedes the water at the top of the prop's path, which allows the water at the bottom of its path to move more freely. Agree it's a bit confusing.
very good video. I have a 2 part question of drag difference of a fixed two blade vs. fixed three blade? I have heard it's a full knot per hour difference in speed in terms of drag of a three vs. two blade on a sailboat 1)Wondering if that is accurate. 2) as an example of making a 50 mile run across to Bahamas from Miami and motoring on a Baba 30 sailboat - is the speed increased greatly on the three blade vs. two blade? Have always heard less time on the water when making a crossing the safer it would be....so am trying to weigh the two against one another. (gain in forward speed and the control when backing up in a marina vs. the additional drag when under sail, added cost or savings on fuel etc.... Not so concerned about a racing speed as the point is to be on the water sailing in the first place. More of overall benefit is what I am pondering, and wondering your thoughts as a more experienced sailor......
I have only had a 3 blade on Wisdom but Windpuff has a 2 blade that I would rotate (I marked the shaft so that I could tell where the blades were) so that they would be blind behind the keel. Windpuff sailed better than Wisdom in light air! This is however a very unfair comparison as Windpuff weighs 9,000 pounds and Wisdom 34,000 pounds. I don’t know if it is a full knot of lost speed, and you can get thinner blade props. Wisdom has massive blades that are a lot of drag but I’m certain a thinner blade would not be as horrendous. The issue with maneuvering a 2 blade over a 3 blade is the constant fluctuation in the number of functioning blades. On a 2 blade, the oscillation will be 2 - 0 - 2 - 0 and so on. This is because you will have 2 blades out to the sides of the keel then both blades behind the keel and blind to it, then the two blades come out again, then they hide, then they come out again. This results in a fluctuation in thrust and lost efficiency. When you are moving, you want a constant thrust but a 2 blade will give you thrust “most of the time”. A 3 blade however will always have 2 blades working as the top or bottom blade will go behind the keel but the other two will be out to the sides. The fluctuations of a 3 blade will be 2-3-2-3-2-3 and so on. This will give you a constant thrust when you are motoring which also makes them more efficient. Less time on a passage makes it safer, and electric let’s you set the prop to barely spin so that it isn’t giving thrust but it also isn’t causing drag, or you can turn off regen and let it free spin which is less drag than having it stopped. On Windpuff, the prop is too small of a diameter to produce power from regen, but I am still going to put a 3 blade on. I will be looking for a very thin fixed blade prop though as I don’t want the complexity of a feathering prop or the drag of a wide blade. I would suggest looking into the Sharrow props, they are a torus blade and offer very little drag with incredible performance. If they have finally come to market when I’m at that stage, that is the exact propeller I plan on using.
@@RiggingDoctor that makes sense about being behind the keel and the 2,0,2,0,2,0 etc.... Is there still benefit of using this 3 blade Sharrow on a diesel engine as I don't have plans to go electric as far as motor. Sounds like they have much less drag?
They have the potential to be. It’s in the programming of the motor controller. Some setups don’t have regen, others do. Apparently, the Sevcon Gen4 has regen, so any motor hooked up to that controller will have regen (if I understand it correctly)
The 2-blade Max-Prop model that you showed is supposedly capable of re-gen if you reverse just before turning off the engine. I don't have the capability to re-gen, but I know how to make the shaft turn if I want (provided I sail fast enough). Answering to one of your points about fixed blade propellers being always more efficient, I have to say the feathering mechanics ensure that the leading edge of the blade remains a leading edge when reversing. The featuring Max-Prop has amazing reverse performance.
You might consider Aircraft propeller tendencies. You might get a better picture of what causes Prop-walk... Many of the same principals apply to both.
The Max Prop Easy Prop is, by far, the best propeller solution for a sailboat. A folding prop requires waking up to make it efficient, while a feathering prop is at the ready for rotation and work. Prop walk is almost non-existent with this propeller. It is at least 40% more powerful than a folding prop in reverse. Folding props absolutely suck in reverse. Go for the Max Prop.
A Kiwi made in bronze wouldn't be a Kiwi, it'd just be some other brand of feathering prop, which I believe already exists. Also, I'm not convinced sail drives are such a great idea. They suit boat manufacturers because they can fit the engine anywhere and don't need to worry about prop alignment. For boat owners, they are much more susceptible to corrosion and, in the case of Yanmars anyway, you can't change the sail drive oli with the boat in the water which is a real drag (pun intended).
Great job Herbie! I had a 2 blade ELICHE RADICE prop while sailing in the great lakes as part of preparing to head on down the erie canal then the icw and crossing oceans I decided to replace it with a 3 blade folding flex-o-fold. I ended up with the same speed but more torq. prop walk just a little less, under power. under sail I saw no difference. The big advantage I gained was coming into a dock was stopping distance, I would guess 1/4 boat length quicker. A fixed 14" prop under sail is like dragging a 5 gallon bucket behind the boat! But where you want the prop shaft to spin makes total sense. Bob
I came across your video. Very informative, thanks for making it! One thing i do miss, or better I thought of seeing your video is the relation of drag/ loss of speed to the boat LWL and hullshape, cat or tri etc. For a small monohull a fixed large prop can make a difference of 0.5 to 1 knot. meaning 0.5 to 1 Nm per hour less distance. which is significant enough to change a setup. For a bigger vessel this number is reduced 0.1 to 0.2 knots or Nm/h. As for Propwalk...Single engine aircrafts experience this too, specially dragging tail aircrafts at low speed. (Prop is on an angle) Propwash is cause that the effect going forward is different than goin in reverse.
The six seconds per mile is almost certainly calculated at racing speeds. The drag effect of the prop is much less at the cruising speeds that you guys travel at. All the more reason to stick with your original prop instead of buying new.
This is my first time watching one of your videos. As I was watching this I thought you sounded familiar somehow. You have a similar cadence of speech to someone I 'know'. It came to me at the end- and it is someone that YOU know! Ryan, from Jessica and Ryan Adventures.
I have a feathering 3 blade PYI style, love it. did not want the noise of the spinning fixed prop, did not want to deal with a shaft brake. Nothing wrong with prop walk, just have to know how and when to use it
Great talk and real life examples! Couple of thoughts , the extra drag from the extra blades requires more hp because of the extra drag from the same dia blade, gets complicated obviously but Tugs with unlimited power have 5-6-7 blades. Prop walk in forward is compensated on a properly designed single engine yacht by having the prop shaft at a slight angle to center line 1-2 deg is typical. Love you point about port propwash in FWD and stb in reverse ! Some interesting looking yachts in that yard too! Cheers Warren
I have a 2 blade fixed prop. I broke a blade and need to replace it. If I went to a 3 blade fixed. Can I use the same pitch, or need to go up or down. It is a 13" x 12 pitch right hand.
I have a U-8D that has twin props. It is actually an airplane, so they have prop walk of sorts as well. Some planes have counter rotating props and some don't. Mine does not. In fact if you lose a particular engine the torque effect may become more than the plane can handle and you get crash city. Though both props can be feathered with a control lever. With a boat I was always taught to put your rudder in the opposite direction of prop walk. That way if you can exactly control the boats speed and the prop speed you can get steerage in both directions at a relatively low relative boat speed.
They could now make Kiwi props from Basalt fiber reinforced resins.. thats like a greener version of carbon fiber and non toxic lower energy cost than carbon or Glass fibers.
@@RiggingDoctor 😎👍🏼😎 I heartily recommend that article ...i will try mentioning the source but if i put up the link U tube AI will probably remove it. Yes 🤦🏻♂️ it did.
@@RiggingDoctor Its also a very common igneous rock.. it's got a relatively low energy cost to produce and inherently low pollution impact. I just read a follow on link in that article relating to a 6Oft racer built of basalt fiber .. Designed for bumping growler ice bergs out the way.. in collisions its better than Carbon fiber ..and naturally hydrophobic.
I believe (hypothesize) that the reason prop walk works "backwards" when you have forward way on is that the wash is not acting on the keel but on the rudder until the boat is stopped, then it starts working on the keel. Just a thought from a guy with a full keel boat that (in reverse) walks to port like crazy!
@@RiggingDoctor It would ... Oooh you know just what to say !! The Darglow Featherstream has been in her mind.. Is there a way of writing to you other than here ? I only came upon your channel today and am very happy to tell you , an awful lot of sense is being spoken !!
Thanks for the great video! We were on the fence about buying a folding propellor (we have a fixed prop). Got to the end of the video and your information about time saved was invaluable! We had estimated more lost time from dragging. We are going to stick with our fixed prop for now. Thanks again!
There are so many factors to decide on what the correct propeller should be. Engine HP and max RPM, gearbox ratios, size of boat, type of through hull prop shaft. I think a prop with an adjustable pitch is critically important for efficient and reliable engine function. Don't burn up your engine with too big a prop or an over pitched prop, it's important to be able to fine tune!
I’ve met a few cruisers with variable pitch props and it’s amazing! They set the RPM and then play with the pitch to set their speed. It gives them incredible range and efficiency while taking it easy on the motor.
We are struggling about what to do with the two blade Max Prop on our , (new to us), 1989 Beneteau First 35s5. The prop is very pitted and suffering from lack of maintenance. PYI, Inc., the manufacturer, can refurbish the prop for about $1,000. When they install a Max Prop, they cut off about two inches and four or so threads off the end of the prop shaft. Will a fixed prop attach to our shortened shaft? Don't know until the prop shop sees the shaft. Then there is the other issue of the taper length of the shaft and the bushing issue of going from 25mm shaft to standard 1 1/8" bore of fixed props. The French didn't make it easy. We go back and forth on what to do.
I jus installed an electric motor. I have a two plane fixed prop. We live on the west coast of Florida where crab traps abound in the billions. I’ve only been on two long sails on this boat and on both of them I picked up a crab trap. I guess I was used to my old full keel Morgan that didn’t suffer this problem. Regardless I would rather have a folding prop to slip past these crab trap lines. I read that a folding prop will give regen if you excite the prop. I’m shopping now to find out. The idea is to use regen by day when you can avoid the traps and feather at night when you can’t see them. I read that the best best prop would be about 15 x 15 three blade but you mentioned a lower pitch for regen? What do you have?
We have a 16 x 14 three blade. The keel does a good job of pushing crab pots out of the way but we have snagged a few in the Chesapeake Bay over the years (thankfully they always slip free).
Nice simple explanation of props, good job. I've got a 3 blade fixed, fairly close to the rudder, hooked straight to a Yanmar 15 on my 28ft mid-eighties plastic tank. Pros? Simple as heck. Change the packing and anodes once in a while. Cons? Tons of walk and drag. A buddy of mine has a sort of similar boat with a sail drive and folding prop. Way better, really no comparison. That said, I'm not going to spend 6 to 10 grand to pick up half a knot. How about doing a video or three on boom rigging set-ups? I've noticed you went really minimalist and would like to know more about your choices.
Great presentation videos. I think your worry about leaving plastic in the ocean is slightly over dramatic. The blades are a compasite as listed below, I think any product that is sanded, scraped off during operation is minimal possibly less worrying that the antifoul products on the hull. The ability to replace a prop blade after a grounding with out damage to the stainless hub is a plus. There is a possibility you could use the compasite blade as a pattern and get some bronze blades cast. . Zytel HTN contains ~ 50 % glass by weight and is thus both very strong and stiff. The black colouring is not carbon based and thus does not have a corrosion potential. DuPont have extensive technical information on their web page regarding the physical characteristics of the many different grades of Zytel they have available. By choosing a 3 bladed design over a 2 bladed unit the forces involved are immediately reduced to 33 % per blade vs 50 % - a 33 % reduction. Another design issue is that composites and the economics they enjoy allow a blade to be sacrificed in a catastrophic situation. With a substantial impact on the blade tips ( always the first part to hit ) they invariably tear from the mounting hole outwards leaving the castings undamaged. We believe it is better to loose an easily replaceable blade costing $ 110 than a whole propeller or drive train when hitting the ground or a floating log or mooring chain. This can be very expensive in a Saildrive installation where the whole leg is at risk.
I actually think Kiwi Props are the best design for non-fixed propellers. My concern about plastic is rooted in a very disturbing time while becalmed out in the North Atlantic. We were not in the area of the garbage patch, we were much further NE than it is, but the surface of the ocean as far as the eye could see was covered in a white snow. I took a water sample and the top two inches of the ocean were full of floating micro plastics. I try really hard to not contribute further to that problem, so I’m always very mindful of these things when I look at something that will go on my own boat. That said, I really like Kiwi Props!
Prop walk: the blade at the top of its turn, ie 12 o'clock, gets a bit of end plate effect so gives a bit more side thrust as it passes through 12 o'clock (tip vortex is reduced at 12 o'clock). That's why you get the imbalance of side thrust. At least that's what I was taught getting my bachelors in nav. arch. Prop walk can vary depending on the flow into the prop also, which is affected by speed and hull shape. Prop walk is really noticeable on tugboats that have old style fixed props on a shaft.
Number of blades... usually you want to keep the blade number not an even fraction of the engine cylinders to keep vibration down (can be exceptions). I.e. if your engine has 4 cylinders, having 3 blades is better than 2 or 4 blades. This is due to the additional side thrust of the 12 o'clock blade. If you have an even multiple of blades to cylinders, you tend to get a sync of a cylinder firing at the same time you get that extra side force on the shaft, which can cause a vibration.
Good vid, brings back memories of my segment in school on prop design.
interesting addition on the blade count - thanks!
generic comment.... I never heard of your explanations.
Did you smoke a lot of funny tobacco when you were in college?
Comment is correct. Sail drives CAN have prop walk. Depending on the layout, they can have a lot of prop walk.
@Mission Gameliel Not when crossing oceans or night sailing or if you don't have enough room for the amount of panels needed for the amount of power required.
Solar is good and very good, but it's not a solve all resource.
If solar alone could do as you say then why solar is not fitting for all solar powered boats? More so the small day boats! (SOLAR HAS IT'S PLACE BUT IT'S NOT A SOLVE ALL)
Interesting your opinion on Kiwi Props.
My experience, and the advice of the creator /manufacturer is lube on annual haul out!
I've found that to be effective.
I've not seen any erosion of the plastic blades, probably due to application of Prop Speed, so other than lube and prop speed I've had no maintenance issues in 6 years.
I just want to point out that a kiwi prop costs us$1500 for a 3 blades and OR $150 each blade when you change them.
So I think the doctor just looked at the prices for extra blades ☺️
Now we know...
The episode conveying the most 'wisdom' of all of your videos, imo. Very nice job, Herbie. Thank you for making that whole complicated issue/topic much clearer for me, and I'm guessing many others as well. Carry on, sailor.
Great presentation! I don't know what date it was recorded but it may require updated especially regarding Kiwiprop pricing and blade composite which are (now) made using Dupont's Zylan HTN composite.
Herbie, HERBIE, hERBIE, I love ya man but u gotta get over this molecular plastic prop phobia. The first time i heard you say that (on a different video) i stopped in my tracts and backed up the video to be sure I heard you correctly. Later I found out you are a dentist which makes your comment seem so strange. As an engineer i can assure you your dental practice and day to day living not to mention the sunglasses on your head - contributes far more environmental pollution than plastic prop's microscopic ware in the frigging ocean. Sorry just couldn,t take it a 2nd time LOL. What makes me truly sad is we keep dumping trash in the ocean and it shows up on the shorelines of even the most remote islands. Love you and your sweet wife( and the birds) and have learned soo much about sailing from u 2. Keep up the good work!! PS Please listen to your wife about boat design especially on the artistic aspects of modifying a lady boat's rear end!!.😍
Haha! It’s all good. Every bit counts, and there is a lot going out there!! Thankfully they are starting to clean up the massive amounts of fishing equipment that is floating out there to help the oceans.
I do love these props and think they are the best design of feathering/folding props!
Except water is considered to be an incompressible fluid, so talking about the water being less dense up top, is nonsense. It may be more easily displaced toward a shallow surface, but it isn't any less dense.
Yeah. Just nonsense
You've outdone yourself once again Herbie. EXCELLENT JOB and well wort the watch. PEACE OUT! Ü
My boat came with an Atomic-4 engine with a folding prop (forget the brand). I liked the prop. I had no problems with it, and, though people said the prop walk would be worse, I had very little prop walk. Maintenance was about the same as a fixed prop. I converted to electric, and stayed with the folding prop for the first year. It was fine.
After that, I got a used fixed prop at a very good price and wanted to test regen, so I changed the prop out. I noticed no appreciable change in sailing, and I can get a little (very little) regen when I sail >5 knots.
Great video Herbie. Again, your depth of knowledge and ability to convey it, is impressive. Prop walk is a pain... I owned a diesel powered Pearson with an offset propeller shaft to counter prop walk. Backing out of a slip with that offset shaft can be a major PITA depending on which way you needed to travel out of the slip and how much room you have backing out. If you back out with low speed, she walks. Some spots were tight enough a hi speed burst may not be advisable. Then of course, add the effect of currents! Like you, I paid real close attention to where and how I tied up. ;-) Well done video. Great explanation!
Super information and delivery. A natural teacher. It's very clear that you researched the subject in great depth and left no stone upturned. Everything was covered in a very considered and systematic manner. Each question that came to my mind was subsequently answered. Your technical knowledge is second to none and your channel is an excellent resource for all things sailing related. Thanks for taking the time to make the video and share the knowledge.
For a professional skipper who specialized in 12-day trips France to Ireland and back (64 trips total), a folding propeller him gave a good half-day gain on the whole trip. That meant one more stopover in a beautiful anchorage. Speed prediction formulas used in handicap systems assume a mix and upwind and downwind. A folding/feathering prop can make a 1.5-knot difference going upwind, easily. If the conditions are rough (against the current), that could even mean making it to destination vs. not.
Guys, Kiwi props are not that cheap. They're about half the price of the others. There's just no way you can purchase a kiwi prop for $400. that just buys replacement blades!! I'm looking at them now and the 3 blade model is $1800 where a Max prop is close to $3k
Great video! You've helped me decide.
You're suggesting that the 2-blade feathering Max-Prop would automatically set itself vertically when sailing. But that actually depends on the position of the shaft when turning the engine off. That will determine the angle of the propeller. If you absolutely want the blades to sit vertically, you'll have to check your shaft angle from the inside and potentially need to turn it, which is a pain..
The most efficient prop is one with the fewest blades (though one is not that practical), as big as possible, so it will have a smaller pitch to suit. Sailboat props are all about static thrust, think trying to make headway against a very strong wind.
.......and then there is also the "variable pitch" propeller, usually controlled via hydraulics. It allows for very fine, infinite control, however it's a fairly complex system that is very expensive. Not usually found on the average yacht, but on the larger mega-yachts owned by billionaires (both sail and power). By the way Herby: You did an excellent job with your propeller lecture. You pretty much covered all the bases and offered a few compelling arguments for and against various types of props and their design characteristics. It appears you've done your homework. In fact you were so convincing, I'm thinking about getting rid of the prop and switching to oars (totally eliminates prop walk😁).
Hunsted makes a really good vari-pitch prop.
The dominant vs weak arm comes into play once you go with the human propulsion, though. ;)
@@SkySailor75 🤣🤣
When I was in highschool in Puerto Rico, we referred to it as the “sinning arm”
@@RiggingDoctor LOL
What i can't understand is, how do folding and feathering props not fold when they're in reverse?
Regarding the kiwi prop, there are a couple of comments that I find difficult to take seriously. Life span, what is short or long? I drove a kiwi on a Harmony 34 between 2007 and 2018. No problems to report from that period. It is probably more important to make sure that these propellers are well lubricated and that the seals that hold the grease in place are changed at appropriate intervals. Wear of material in the blades? I haven't seen any of that at all, the last few years a layer of antifouling was put on the blades, it disappears with time but not so fast either. I see no reason why it wouldn't have worked for another ten years. And a big advantage, the blades angles can be adjusted to have the engine run on desired revs.
Good to know! I have friends in the Chesapeake Bay who have had to replace blades every 5 years or so. Makes me wonder what is in the Chesapeake!
One friend barely moved his boat and the sun always hit one blade in particular. One blade disintegrated and it is assumed to have been the one that always had the sun hitting it while in the slip.
Another friend (who first introduced me to kiwi props) told me about the polishing blades. He works at a boat yard in Baltimore and frequently would have to replace worn down blades on kiwi props.
I really love them! Their simplicity, reliability, and adjustability make them the best non-fixed propeller in my opinion. I’m glad to know you didn’t have the blade polishing issue :) it might just be an issue in the bay.
Why not just add some epoxy to the kiwi prop blades and get a longer life out of them?
That is a really good idea! I love kiwi props but I couldn’t get behind the “polishing plastic” blades. Excellent solution!
I’ve looked at the kiwi props before but it just occurred to me while I was watching your video. By the way you did an excellent job of explaining prop walk and why you have it and how to deal with it.
Thanks 😊
Hi Doc. ........... enjoyed your video. Picked up a few bits of interesting info. Cost !!! NOT CHEAP. Another thing in my research on the net was the composite the blades are made from Zytel, a plastic composite made by DuPont is now used for the blades. High strength, abrasion, and impact resistant thermoplastic poly-amide formulations.
Is this newer info or not from what you know?
Cheers
My info is from other cruisers who had the blades polish down over the years. It was all word of mouth but from first hand experience on their part. I do not know what material or age their blades were, or if they still have the problem.
I contacted Kiwi Prop to see if they would send me a prop to test out on our return voyage. I offered to weigh the blades before and after the crossing to find out conclusively what happens over thousands of miles of sailing but they were not interested in my proposal.
Of all non-fixed propellers, Kiwi is my favorite design; it has simplicity which I find elegant. I just wish that I knew definitively if the blades polishing off is an issue or not.
Amazing channel. Best on You Tube.
Great explanation... I get it now... great video
Can feather propellers go in reverse? Very informative video by the way.
They can, and you can lock them in reverse for regen mode with electric motors
Yes especially the high end ones with synchronised gear meshing blades like "Gori" and "Wisco" -- Gori can go over centre which reduces the diameter under load thus having a gearing like effect for better utilization of power under load.
.
@@RiggingDoctor How is that done? Is it the same for all kinds of folding/feathering propellers?
Prop walk occurs because the shaft of the propeller is usually at a downward angle toward the propeller. If the shaft was horizontal making the propeller perfectly vertical. You would not have prop walk.
In a single prop boat, yes you would still have some prop walk (think of it as a paddle wheel) but it wouldn’t be as noticeable. Most high powered twin engine boats have counter rotating props, even if the shafts are horizontal (saildrive & outboards)to cancel it out.
I once owned a trawler with a massive four blade prop which was horizontally aligned with the keel, in an aperture, and it would only go one way in reverse.
we have a 2 blade maxprop. Working on getting it rebuilt after using it problem free for 16 years. After taking it apart we found some damage inside, but it still worked without issue. Highly recommended, there is a trick to get it to feather, but you can tell when it is not feathered because the shaft spins and you can hear that on our boat. A quick pop into reverse and then neutral when the engine is off feathers the prop and the shaft stops spinning.
This was so much of what I was searching for. Thorough and well organised explanation. I learnt a lot watching this.
You are very welcome
I have a two blade J Prop on our C34 MkII. It has a slight prop walk to port but feathers beautifully. I love your splicing videos. Really having problems finishing the knots on soft shackles though!
I used to be a merchant mariner, and was taught that our prop walk was a result of the pressure differential between the blades. We were talking 18 foot diameter props !
🫨 18 foot!! Sailboat props must look like a handheld personal fan!
Are you doing a button knot or the lovers knot? I like the lovers knot for a few reasons, but you can splice the tails of the knot into each other to make a single tail that is really handy for guiding the knot through the loop.
I really dig those Rigging Doctor educational (about propellers, keels, rigging etc) videos. Thanks for making them! After watching this video about propellers it makes me wonder, why don't sailboats have Controllable Pitch Propellers (CPP)? It would give you both port and starboard (by changing the pitch of the propeller) prop walk that you can abuse to get in or out of marinas...
Some sailboats do have them and it’s very nice. The problem is they are complicated and expensive. Also, if the consumer doesn’t know how to work it, the boat builder will have to deal with all the complaints about how the boat doesn’t motor well.
I’ve been on a few boats that had different styles and I loved them all!
One was a high or low pitch, one for marina maneuvering and the other for cruising and range. Another had fully adjustable pitch instead of a gearbox. The prop was always spinning, but the blades could be rotated to be straight for neutral or pointing forward or back for the direction. By adjusting the pitch, you could make the boat have more torque or more speed without touching the revs.
That was by far my favorite! The captain set the revs for docking and did everything by throwing the pitch one way or another.
His only complaint was that the prop can’t free wheel while sailing and the blades don’t feather all the way. Also, if he ran over a line, he had to shut the motor down as neutral still had the prop spinning. Everything always has to be a compromise!
Do a informative video about tether safety harness.A person WILL DROWN IF FALL OVERBOARD.think it thru really well.ASK? Can a person pull him or her self back onboard while being dragged at 4,5,6,7 10,15 knots???in flat calm seas.? rough seas?cold seas?in the night when the othres are sleeping?SO always wear a locator beacon flashing light and a portabl ,gps position transmitter to your onboard screen.with a man overboard siren alarm to wake up the crew.BECAUSE by the time you drop the sails ,tun on the motor and turn around it will be a mile.CAN THE PERDSON WHO IS BEING DRAGGED WITH SAFETY TETHER SURVIVE A MILE???
Very good topic idea! Thanks :)
Have a Kiwi for 10 years. Sent back to factory at 5 years for service. No blade wear evident
Paid around €1600 euros. Let me know where I acan buy for $400 and I'll keep a spare.
Three blade prop is now $1400 in the US and spare blades are $110 each as well.
I have s two bladed folding prop on a standard propshaft. I get prop walk to starboard in reverse. I use this to advantage when coming into my slip because I can come almost abreast of my slip give full rudder to port and a stab of reverse and she'll pivot about her keel and I can then motor straight in. Simples!
So, Saildrives with two blades and fixed propellers are the best for electric sailboats because they're reliable and cheap, and two blades have less drag than three or four blades, right?
Pretty much, but you won’t get Regen with a 2 blade, so you need to weigh out your options.
Do you happen to know if kiwi props can help boat speed. I have a boat that doesn’t motor much, but is underpowered. Meaning on days with good 17-20 knot winds it’s a bit iffy on even taking her out. Due to the risks of her being overpowered by the wind when the sails aren’t up.
Looking for not very expensive options for keeping this old YSM8 going but getting better performance out of her.
It’s going to be dependent on the pitch of the prop. If you decrease the pitch, you will increase your torque but cut your top speed. I’m not sure if the pitch is adjustable on Kiwi props. If it is, you are golden!
Nice interesting piece which covers it all. One point I'd like to add is I would serious miss my prop if it were not nice and close to the rudder blade. Manoeuvring does require experience as to which way the boat will turn when you put it in astern from standing still. BUT with the rudder close to the prop the walk is really small in ahead as you can blast water direct on to the rudder and make the boat turn any way you want almost immediately.
I know what you mean, it’s practically a stern thruster! I tried to dock a boat that the prop was far from the rudder and it was a very ungrateful event.
Prop regen only generates meaningful energy at minimum 6kn speeds maintained over many hours. At loer speeds the drag from the prop negates the regen. So prop regen makes sense on a 45ft cruising yacht out on passage 24/7
Agreed. When crossing an ocean, we enjoy it for the power it gives us. If we are coastal and making a short journey, we shut off the regen to avoid the speed penalty.
I have had my yacht for 11 years, it came with a kiwi prop and it still has the same prop. I Antifoul the prop so maybe that is why my prop hasn’t desolved as you suggest. Works for me
That’s great to hear!
Kiwi props are actually my favorite “non fixed” prop. They are the simplest design and have the fewest points of failure.
Very informative! Thank you!
I'm sorry, but this is simply not the correct explanation for prop walk. Prop walk is simply a matter of asymmetric flow of prop wash over the hull. With astern power, the prop wash is directed forward (towards the bow) and the prop wash, as it leaves the propeller, isn't going straight forward. The total flow of the propeller has an average that is more or less perfectly straight and aligned with the propeller axis, but the wash coming of each indvidual blade will leave at an angle. The prop wash that is closest the hull is that coming off the closest blade, so it moves slightly sideways. As this flow impacts the hull, it does so mostly, or solely, on one side of the hull, which turns the flow into a side force. This can be easily observed as a difference in upwash on one side of the boat compared to the other, with stronger upwash ("bubbling water") on the opposite side of the direction the stern moves when reversing under power.
Boats with a straight shaft angled down, and with a skeg or a long keel, will see a much more pronounced effect, as the flow will have more surface area to act on close to the propeller, and the flow will be diverted to one side more effectively. A prop mounted on a saildrive, and a hull that's almost flat and wide, close to and just forward of the prop, will see relatively little prop walk as the flow mostly continues forward, with very little vertical hull area to push against.
The incorrect explanation with a depthwise pressure difference being the cause, can't explain the wildly different prop walk magnitudes that different hull, shaft and propeller combinations give.
The brave boater could add strakes (long fins, like bilge keels but on a much smaller scale) on both sides of the propeller, to force the reverse prop wash into a straight and forward direction, blocking asymmetric upwash.
In the video I described many of the theories about the cause of prop walk.
The reasoning behind the depth/pressure theory is pretty simple: prop walk is more pronounced on larger propellers. A 6 inch prop on an outboard will have less prop walk than a 48 inch prop on a shrimp boat. The pressure difference between the top and bottom blade tips on the 6 inch is nothing compared to the tips set 4 feet apart!
As I mentioned, they are all theories which together try to explain the same phenomenon which has no definitive explanation. All the theories share a few points though: shaft angle, propeller size, underwater hull form, and RPM all play into the effects known as prop walk.
I assume that folding props are not s likely to catch on floating lines (when folded), whilst feathering props still have this issue.
I assume that as well, but if you are unlucky, it could be both snagged or pinched in the blades
Thinking that boats/propellers travel straight ahead when sailing is wrong. No propeller hides perfectly behind any keel when sailing. Leeway causes the blades to see water coming at then from the side at all times when sailing. That is why folding propellers have less drag. Feathering propellers do not completely face water passing by perfectly. A little more drag than folding.
Also shaft angle will make a feathering prop be more in the way than a folding prop.
What about gearbox wear when fixed props are spinning. Of Even when they are in gear and gearbox is fully loaded in reverse?
It’s not great. They will always put wear on the driveline in some way. If you have an electric motor though, the spinning actually charges your batteries while you sail!
Oooo wat a sales pitch.. " I can't stand all that plastic going to the ocean from the prop wearing out".. then buys an a$$ load of lithium... wtf?
Lithium isn’t plastic going into the ocean. I fail to see the point you were trying to make.
Hello nice video looking for the next I learned many more things about propellers. Saludos
Thanks! Great job. Third time I watched it. I’m going to emphasize P-factor. Airplane propellors get that in a high pitch. Boats with downward angled shafts get it even more. Propwalk will be most pronounced moving forward at low speed and high power and less so in reverse at low speed and low power. It just seems that it’s worse in reverse because of the pucker factor going into a slip and confusion over which way to turn the wheel. You guys are fantastic. Congratulations on your baby. And looking forward to more videos soon when you’re ready.
Thank you so much!
I was wondering about the P-factor but you explained it perfectly later in the comment.
In my thinking you forgot to mention with regards to the folding propellor is the complex reversing mechanism that they do have and which ups their costs as well. Otherwise, you made a good show of a fairly complicated subject, congratulations.
Thanks 😊
What is with that marina. Half the boats are junk. Is it super cheap?
A lot of the boats there were involved in smuggling blood diamonds from Africa. The owners were in jail and the boats left to rot!
Oh, and it was super cheap!
My paddle boat has no walk.
Kiwiprops are not that cheap. $1,400 for 3 blade. 14" 2 blade feathering $2,000. Used fixed 3 blade $400. Fixed is cheap and always works even when barnicle start growing on it.
kiwi prop $1350 3 blade $2700 4 blade..thats not $400
Very true, but still not $4000 like the nice bronze ones 🤑
Great episode! It's nice to hear electric propulsion included in a talk about props. Regen is an other animal altogether. You mentioned propeller companies that are catering to EP, who are they?
The fatal flaw of a sail drive is it hangs out there to get caught on anything. If it gets ripped off your boat your going to have a very big hole at deal with. Pretty much headed for the bottom.
While I wouldn’t want one on my boat for reasons you have mentioned, they do a great job of minimizing prop walk.
Some things are more important that others and for some people, backing up straight is more important than safety 🤪
Brilliant video, good timing also as was just considering changing my fixed 3 blade, have to say might not bother now
How does kiwi do reverse.
Bronze Kiwi propeller is a really great idea!
What an interesting discussion, about the opposite prop walk when drifting forward compared to when the boat is still. I can't say I have practically seen that, maybe because I am usually drifting very slowly when entering a slip. Reading the comments, it seems to be another illustration of the Dunning Kruger effect, overconfidence by those with less experience. Many of us sail on just a few boats regularly over a lifetime. In the end, learn how your boat responds and use that to advantage.
So informative, thanks Herby. Give Maddie a hug for us, please.
😊
You have to remember that a 3-blade fixed propeller is concave in the forward direction, and convex in the reverse, and therefore the thrust is a lot less at the same RPM in the backwards direction. This means that when the boat moves forward under sail, there is less drag, because the water is coming from behind the propeller. Furthermore, if the engine is put into neutral, then the propeller can spin under sail creating even less drag. However, my understanding is that spinning the propeller under sail is not good for the bearings (or something).
Some say it is bad for the transmission, but apparently it depends on the type of transmission that the boat has.
We have an electric motor with direct drive so the spinning prop in neutral acts as a hydrogenerator to charge our batteries.
@@RiggingDoctor Right, but I meant for diesel engines.
Thank you for this great presentation, we are seriously considering changing our 2 blade fixed for either a flexofold or featherstream . Your explanation of prop walk is clear and easy to understand.
So glad we could help!
Good info here. I will send people here instead of trying to explain it and have people looking at me like I’m speaking another language lol. 👍
Thanks! I appreciate it :)
Propwalk will be in the opposite direction when the gearbox is in forward (with the prop turning) compared to when it is in reverse.
However, the direction of travel does not effect the direction of the propwalk. This was wrong in the video and should be corrected.
The direction of travel (relative to the water) will effect the direction of flow over the rudder and therefore the action of the rudder. I suspect you are confusing this effect for prop walk. They are two different forces.
If your boat “walks to port in reverse” then you would expect the stern to walk to port when you go in reverse.
This is true as long as you are still and starting to go in reverse. When you are moving forward and put it in reverse, you now expect the stern to walk to port but since you are still drifting forward, it will scoot your stern to starboard.
This little hiccup can get you in a lot of trouble if you really needed to go to port and instead slammed into something on your starboard side that you were trying to avoid.
That is what I was referring to and getting at. Walk is walk, but when you are maneuvering, this is a little asterisk to the statement “boat walks to ...”
Rigging Doctor
No, this is incorrect.
Or to be more accurate, if you are seeing this effect on your boat, it is not due to propwalk. Propwalk will always be determined by the direction of rotation of the propellor. The direction of travel does not effect the direction of the propwalk.
When you shift the gearbox out of forward, the prop wash over the rudder will be lost which will effect the stern in either direction, but this is not propwalk.
For most, boats shifting into reverse will act to swing the stern the same way irrespective of the direction of travel.
If you tell someone your explanation about prop walk and give them the helm and tell them to dock; they will give it a try and become utterly perplexed when it doesn’t work right!
At a stop they walk one way but if they are drifting forward and expect to walk the same way, they will be shocked and confused as to what just happened when it “walks the other way”.
The science of just the propeller is one thing but when you wrap a boat around it, the propeller becomes part of a system and the whole system reacts differently under particular circumstances.
I want someone to understand how to use prop walk when they are maneuvering in a tight space. If I don’t mention that little caveat that the boat will walk opposite if you are drifting forward, how many scraped topsides do you think would come from that little omission?
Have a saildrive w/ 3-blade. As stated, virtually no walk. Yanmar makes my drive and engine and over the years (I've had 3 models), they seem to go back and forth about keeping the boat in gear under way. The current guidelines (maybe more since they state you warranty is invalid if ignored) is to let it freewheel which I guess reduces some of the drag versus it being fixed? I suppose they are more concerned about the stress placed on the gears being in gear versus the engine parts moving without the oil pump and such operating as God intended?
To free wheel or not to free wheel, that is the question. Weather 'tis nobler to suffer the thrums and degredation or, by selecting, end them? Personally, I would arrive slower but without the noise.
Thanks for a great discussion of different prop types, why and when you'd want them, prop walk, etc. Really good stuff!
The theory about water density causing prop walk seems very wrong. The difference in density from a few inches of depth in the water is insignificant.
r m's discussion from naval architecture school sounds right. The hull is acting as an end plate to constrain the water movement and cause a side thrust.
While the various methods for explaining prop walk exist, I still do not know for certain which reason it is, or if it is wholly one reason and not a combination of reasons acting in synchrony.
Our outboard motor has no cavitation plate and is a 13 inch propeller. When I have the motor trimmed perfectly, I get very little prop walk (which makes docking hard for me). I tilt the motor up a notch or two and suddenly I have plenty of prop walk to make docking easier for me (as it behaves more like our sailboat which has a level shaft in an aperture).
@@RiggingDoctor Another explanation is that the hull impedes the water at the top of the prop's path, which allows the water at the bottom of its path to move more freely.
Agree it's a bit confusing.
very good video. I have a 2 part question of drag difference of a fixed two blade vs. fixed three blade? I have heard it's a full knot per hour difference in speed in terms of drag of a three vs. two blade on a sailboat 1)Wondering if that is accurate. 2) as an example of making a 50 mile run across to Bahamas from Miami and motoring on a Baba 30 sailboat - is the speed increased greatly on the three blade vs. two blade? Have always heard less time on the water when making a crossing the safer it would be....so am trying to weigh the two against one another. (gain in forward speed and the control when backing up in a marina vs. the additional drag when under sail, added cost or savings on fuel etc.... Not so concerned about a racing speed as the point is to be on the water sailing in the first place. More of overall benefit is what I am pondering, and wondering your thoughts as a more experienced sailor......
I have only had a 3 blade on Wisdom but Windpuff has a 2 blade that I would rotate (I marked the shaft so that I could tell where the blades were) so that they would be blind behind the keel. Windpuff sailed better than Wisdom in light air!
This is however a very unfair comparison as Windpuff weighs 9,000 pounds and Wisdom 34,000 pounds.
I don’t know if it is a full knot of lost speed, and you can get thinner blade props. Wisdom has massive blades that are a lot of drag but I’m certain a thinner blade would not be as horrendous.
The issue with maneuvering a 2 blade over a 3 blade is the constant fluctuation in the number of functioning blades.
On a 2 blade, the oscillation will be 2 - 0 - 2 - 0 and so on. This is because you will have 2 blades out to the sides of the keel then both blades behind the keel and blind to it, then the two blades come out again, then they hide, then they come out again. This results in a fluctuation in thrust and lost efficiency. When you are moving, you want a constant thrust but a 2 blade will give you thrust “most of the time”.
A 3 blade however will always have 2 blades working as the top or bottom blade will go behind the keel but the other two will be out to the sides. The fluctuations of a 3 blade will be 2-3-2-3-2-3 and so on. This will give you a constant thrust when you are motoring which also makes them more efficient.
Less time on a passage makes it safer, and electric let’s you set the prop to barely spin so that it isn’t giving thrust but it also isn’t causing drag, or you can turn off regen and let it free spin which is less drag than having it stopped.
On Windpuff, the prop is too small of a diameter to produce power from regen, but I am still going to put a 3 blade on. I will be looking for a very thin fixed blade prop though as I don’t want the complexity of a feathering prop or the drag of a wide blade.
I would suggest looking into the Sharrow props, they are a torus blade and offer very little drag with incredible performance. If they have finally come to market when I’m at that stage, that is the exact propeller I plan on using.
@@RiggingDoctor that makes sense about being behind the keel and the 2,0,2,0,2,0 etc.... Is there still benefit of using this 3 blade Sharrow on a diesel engine as I don't have plans to go electric as far as motor. Sounds like they have much less drag?
So any electric drive is a motor/generator?
They have the potential to be. It’s in the programming of the motor controller. Some setups don’t have regen, others do.
Apparently, the Sevcon Gen4 has regen, so any motor hooked up to that controller will have regen (if I understand it correctly)
The 2-blade Max-Prop model that you showed is supposedly capable of re-gen if you reverse just before turning off the engine. I don't have the capability to re-gen, but I know how to make the shaft turn if I want (provided I sail fast enough). Answering to one of your points about fixed blade propellers being always more efficient, I have to say the feathering mechanics ensure that the leading edge of the blade remains a leading edge when reversing. The featuring Max-Prop has amazing reverse performance.
On the sailing ship I was on we had twin Eight cylinders Gardners engines with variable pitch and folding propellers.
Nice
Richard Mason , yes it was very nice
Very interesting thanks .
Very welcome
You might consider Aircraft propeller tendencies. You might get a better picture of what causes Prop-walk... Many of the same principals apply to both.
The Max Prop Easy Prop is, by far, the best propeller solution for a sailboat. A folding prop requires waking up to make it efficient, while a feathering prop is at the ready for rotation and work. Prop walk is almost non-existent with this propeller. It is at least 40% more powerful than a folding prop in reverse. Folding props absolutely suck in reverse. Go for the Max Prop.
A Kiwi made in bronze wouldn't be a Kiwi, it'd just be some other brand of feathering prop, which I believe already exists. Also, I'm not convinced sail drives are such a great idea. They suit boat manufacturers because they can fit the engine anywhere and don't need to worry about prop alignment. For boat owners, they are much more susceptible to corrosion and, in the case of Yanmars anyway, you can't change the sail drive oli with the boat in the water which is a real drag (pun intended).
Great job Herbie!
I had a 2 blade ELICHE RADICE prop while sailing in the great lakes as part of preparing to head on down the erie canal then the icw and crossing oceans I decided to replace it with a 3 blade folding flex-o-fold. I ended up with the same speed but more torq. prop walk just a little less, under power. under sail I saw no difference. The big advantage I gained was coming into a dock was stopping distance, I would guess 1/4 boat length quicker. A fixed 14" prop under sail is like dragging a 5 gallon bucket behind the boat! But where you want the prop shaft to spin makes total sense. Bob
I came across your video. Very informative, thanks for making it! One thing i do miss, or better I thought of seeing your video is the relation of drag/ loss of speed to the boat LWL and hullshape, cat or tri etc. For a small monohull a fixed large prop can make a difference of 0.5 to 1 knot. meaning 0.5 to 1 Nm per hour less distance. which is significant enough to change a setup. For a bigger vessel this number is reduced 0.1 to 0.2 knots or Nm/h. As for Propwalk...Single engine aircrafts experience this too, specially dragging tail aircrafts at low speed. (Prop is on an angle) Propwash is cause that the effect going forward is different than goin in reverse.
The six seconds per mile is almost certainly calculated at racing speeds. The drag effect of the prop is much less at the cruising speeds that you guys travel at. All the more reason to stick with your original prop instead of buying new.
This is my first time watching one of your videos. As I was watching this I thought you sounded familiar somehow. You have a similar cadence of speech to someone I 'know'. It came to me at the end- and it is someone that YOU know! Ryan, from Jessica and Ryan Adventures.
I have a feathering 3 blade PYI style, love it. did not want the noise of the spinning fixed prop, did not want to deal with a shaft brake.
Nothing wrong with prop walk, just have to know how and when to use it
Great talk and real life examples! Couple of thoughts , the extra drag from the extra blades requires more hp because of the extra drag from the same dia blade, gets complicated obviously but Tugs with unlimited power have 5-6-7 blades.
Prop walk in forward is compensated on a properly designed single engine yacht by having the prop shaft at a slight angle to center line 1-2 deg is typical. Love you point about port propwash in FWD and stb in reverse !
Some interesting looking yachts in that yard too!
Cheers Warren
I have a 2 blade fixed prop. I broke a blade and need to replace it.
If I went to a 3 blade fixed. Can I use the same pitch, or need to go up or down. It is a 13" x 12 pitch right hand.
I have a U-8D that has twin props. It is actually an airplane, so they have prop walk of sorts as well. Some planes have counter rotating props and some don't. Mine does not. In fact if you lose a particular engine the torque effect may become more than the plane can handle and you get crash city. Though both props can be feathered with a control lever. With a boat I was always taught to put your rudder in the opposite direction of prop walk. That way if you can exactly control the boats speed and the prop speed you can get steerage in both directions at a relatively low relative boat speed.
They could now make Kiwi props from Basalt fiber reinforced resins.. thats like a greener version of carbon fiber and non toxic lower energy cost than carbon or Glass fibers.
I just met a guy who is going to build his boat out of basalt fibers. It is pretty much bulletproof! Amazing material :)
@@RiggingDoctor
😎👍🏼😎
I heartily recommend that article ...i will try mentioning the source but if i put up the link U tube AI will probably remove it.
Yes 🤦🏻♂️ it did.
yachting world . Yachts and gear. The future of boatbuilding a greener approach. 👀👍🏼Read.
@@RiggingDoctor
Its also a very common igneous rock.. it's got a relatively low energy cost to produce and inherently low pollution impact.
I just read a follow on link in that article relating to a 6Oft racer built of basalt fiber ..
Designed for bumping growler ice bergs out the way.. in collisions its better than Carbon fiber ..and naturally hydrophobic.
Great to see a few bilge keels in the background🤙
They were a pretty common sight in Portuguese boat yards. Very cool designs!
@@RiggingDoctor love my bilgey
I believe (hypothesize) that the reason prop walk works "backwards" when you have forward way on is that the wash is not acting on the keel but on the rudder until the boat is stopped, then it starts working on the keel. Just a thought from a guy with a full keel boat that (in reverse) walks to port like crazy!
Perhaps a bit of overspeak, but certainly interesting stuff.
Very Informative video. Thanks. I learned something today.
Thank you SO much on the i sights regarding props... much appreciated! Will have to look into your system re electric motor.
thanks, that is a great explanation...of something the +average sailor probably thinks they have a handle on...like me
Will a 2 bladed prop position itself vertically behind the rudder or do you have to rotate it into position?
:)
This informative, instructive video, is the reason why I subscribed!
Out of curiosity, do cats have opposite turning props?
Excellent and very interesting .. Just saved myself 1800£ +, Thank you !!
You are very welcome. Would the possible prop be for that gorgeous gaff rig in your picture?
@@RiggingDoctor It would ...
Oooh you know just what to say !!
The Darglow Featherstream has been in her mind..
Is there a way of writing to you other than here ?
I only came upon your channel today and am very happy to tell you , an awful lot of sense is being spoken !!
@@Trevessa24what did you decide on? We are currently considering the 3 blade Featherstream .
Message me on my channel Bev.
@@Trevessa24 seem to be unable to comment in your channel?
looks like glades boat yard..?
It’s in northern Portugal. A lot of the boats that are rotting in the background were impounded from blood diamond smuggling
looks like one of those twin keels boat are like KARL on WHITESPOTPIRATES
Great video. Thanks for posting
Thanks for the great video! We were on the fence about buying a folding propellor (we have a fixed prop). Got to the end of the video and your information about time saved was invaluable! We had estimated more lost time from dragging. We are going to stick with our fixed prop for now. Thanks again!
Glad it helped! Keep your life simple so you can focus on other parts of the boat 😎
There are so many factors to decide on what the correct propeller should be. Engine HP and max RPM, gearbox ratios, size of boat, type of through hull prop shaft. I think a prop with an adjustable pitch is critically important for efficient and reliable engine function. Don't burn up your engine with too big a prop or an over pitched prop, it's important to be able to fine tune!
I’ve met a few cruisers with variable pitch props and it’s amazing! They set the RPM and then play with the pitch to set their speed. It gives them incredible range and efficiency while taking it easy on the motor.
We are struggling about what to do with the two blade Max Prop on our , (new to us), 1989 Beneteau First 35s5. The prop is very pitted and suffering from lack of maintenance. PYI, Inc., the manufacturer, can refurbish the prop for about $1,000. When they install a Max Prop, they cut off about two inches and four or so threads off the end of the prop shaft. Will a fixed prop attach to our shortened shaft? Don't know until the prop shop sees the shaft. Then there is the other issue of the taper length of the shaft and the bushing issue of going from 25mm shaft to standard 1 1/8" bore of fixed props. The French didn't make it easy. We go back and forth on what to do.
sounds like you need a new prop shop! plenty of metric bore props on the market. adapter is bad advice.
I jus installed an electric motor. I have a two plane fixed prop. We live on the west coast of Florida where crab traps abound in the billions. I’ve only been on two long sails on this boat and on both of them I picked up a crab trap. I guess I was used to my old full keel Morgan that didn’t suffer this problem. Regardless I would rather have a folding prop to slip past these crab trap lines. I read that a folding prop will give regen if you excite the prop. I’m shopping now to find out. The idea is to use regen by day when you can avoid the traps and feather at night when you can’t see them. I read that the best best prop would be about 15 x 15 three blade but you mentioned a lower pitch for regen? What do you have?
We have a 16 x 14 three blade. The keel does a good job of pushing crab pots out of the way but we have snagged a few in the Chesapeake Bay over the years (thankfully they always slip free).
Nice simple explanation of props, good job. I've got a 3 blade fixed, fairly close to the rudder, hooked straight to a Yanmar 15 on my 28ft mid-eighties plastic tank. Pros? Simple as heck. Change the packing and anodes once in a while. Cons? Tons of walk and drag.
A buddy of mine has a sort of similar boat with a sail drive and folding prop. Way better, really no comparison. That said, I'm not going to spend 6 to 10 grand to pick up half a knot.
How about doing a video or three on boom rigging set-ups? I've noticed you went really minimalist and would like to know more about your choices.
Good idea! They will be coming out soon ;)
Great presentation videos.
I think your worry about leaving plastic in the ocean is slightly over dramatic. The blades are a compasite as listed below, I think any product that is sanded, scraped off during operation is minimal possibly less worrying that the antifoul products on the hull. The ability to replace a prop blade after a grounding with out damage to the stainless hub is a plus. There is a possibility you could use the compasite blade as a pattern and get some bronze blades cast.
.
Zytel HTN contains ~ 50 % glass by weight and is thus both very strong and stiff. The black colouring is not carbon based and thus does not have a corrosion potential. DuPont have extensive technical information on their web page regarding the physical characteristics of the many different grades of Zytel they have available.
By choosing a 3 bladed design over a 2 bladed unit the forces involved are immediately reduced to 33 % per blade vs 50 % - a 33 % reduction.
Another design issue is that composites and the economics they enjoy allow a blade to be sacrificed in a catastrophic situation. With a substantial impact on the blade tips ( always the first part to hit ) they invariably tear from the mounting hole outwards leaving the castings undamaged.
We believe it is better to loose an easily replaceable blade costing $ 110 than a whole propeller or drive train when hitting the ground or a floating log or mooring chain. This can be very expensive in a Saildrive installation where the whole leg is at risk.
I actually think Kiwi Props are the best design for non-fixed propellers. My concern about plastic is rooted in a very disturbing time while becalmed out in the North Atlantic. We were not in the area of the garbage patch, we were much further NE than it is, but the surface of the ocean as far as the eye could see was covered in a white snow. I took a water sample and the top two inches of the ocean were full of floating micro plastics. I try really hard to not contribute further to that problem, so I’m always very mindful of these things when I look at something that will go on my own boat.
That said, I really like Kiwi Props!
Love this episode. It wasn’t the info I was searching for but learned/ crystallized prior learning so well. Thank you for doing what you do!
Thanks :)