The cavitation of the RMS Lusitania's propellers was so bad, she had to be reinforced with several tons of steel just so she wouldn't rattle herself into a pile of scrap metal! (She used direct-drive Parsons steam turbines and this happened a lot to early turbine powered ships)
Since air lubrication reduces friction on a ship's hull, wouldn't using air to lubricate the propeller using super-cavitation not only reduce drag but eliminate wear on the surface of the propeller?
Perhaps but complexity of having a hollow shaft and air ports on the propeller surface to allow bleed air to come out would probably increase costs enough to barely break even. And on a submarine you wouldn't have a great supply if air unless you used reactor power to perform water hydrolysis and blow oxygen from the blades.
Why not INJECT AIR on the back face of the propeller near the leading edge? Unlike water, air is not going to collapse back into liquid. This curtain of real air bubbles as opposed to temporary steam bubbles solves the problem, no?
Something I"m wondering about - cavitation is caused by the reduced pressure resulting in a decreased boiling temperature of the water, until the boiling temperature is below sea temperature. This makes it sound temperature dependent - will cavitation happen more in warmer seas (say 85°F water of the tropics) than in colder seas (such as 25°F seawater of arctic regions?) If so, how much difference does water temperature make?
Yes, cavitation can still occur with the tip of the propeller slightly out of the water. But in this case, ventilation is the larger problem. When the propeller breaks the surface, or is near the surface, the rotating blades can pull air into the water. When the air interacts with the propeller, it greatly reduces thrust, because the air is much lower density than the propeller.
True. Water vapor. Steam. Gaseous water. I target these videos for a non-technical audience. Sometimes I use less precise words in favor of easier to understand concepts. But you are quite correct. It is not technically air. Side note: we do get actual air in the propeller. That is called ventilation and occurs when the ship is light loaded and the propeller is near the water surface. On slow moving commercial vessels, this is a bad thing. For super fast power boats, we actually design the surface piercing propellers to take advantage of that and make ventilation a good thing.
Yes. It would be more accurate to say they are two different qualities. A fully submerged propeller can also be super cavitating. This is very common on surface piercing propellers because of the high speeds we use them in. The intent with a surface piercing prop is to have less of the propeller in the water, which reduces the prop drag. But this means the force from the propeller no longer lines up with the propeller shaft. So we need to reinforce the blades, hub, and shaft to handle that unbalanced load. Plus redesign the blade shapes to handle all the extra air they suck in.
No. Propeller get vortices too, but cavitation is different. Vortices result from flow patterns around the wing or propeller. Cavitation happens specifically because you have liquid water. The propeller creates a change in pressure. And that pressure change alters the state of the water from liquid to gas (this is only small layer right next to the propeller surface).
@@DatawaveMarineSolutions Thanks for your reply. So that I have this right, water comes of the propeller at a low pressure and once exposed to sea water (high pressure) bubbles form at a higher temperature (than the sea water) and then collapse in a micro-second!
@@MrBradley34 Almost. The temperature doesn't change. The temperature required to boil water changes, depending on the water pressure. This is why water boils faster at high altitudes in the mountains. It literally boils at a lower temperature. In the case of cavitation, the water pressure on the surface of the blade drops so low that water can boil at normal sea temperature. The water never changes temperature, it just changes the starting point for boiling water. And as you said, that only lasts an instant. The water is now vaporized, and as it leaves the blade surface, the pressure goes back to normal ocean pressure, which collapses that bubble of steam.
I'm pushing a 30' sailboat with a 15 hp outboard. It makes loads of foaming bubbles. What kind of prop can be put on it to get thrust and at least half the bubbles or is it just spinning too fast for such a heavy load?
That can be either ventilation or cavitation. If it is cavitation, you should see little pits and pockmarks on the propeller blade. Ventilation on the other hand happens when the propeller is too close to the surface. The prop sucks in air, which gets mixed with the water and generates lots of bubbles in the wash. If you have ventilation, the two fixes that I would try are: 1.) Try to mount the propeller deeper in the water. 2.) Find something to create a physical barrier between the propeller and water surface. I think I have seen some weed guards that are horizontal fins mounted on outboards. That may help.
They are not “air” bubbles they are vapour bubbles.That is the water changing from a liquid to a gas due to the water reaching its vapour pressure.There is no “air” underwater. You must get the basics right if you are going to profess to know what you’re talking about.
Fair point. I do favor the drama a little to make the subject interesting for a non-technical audience. Not sure about electrolysis, but I do know that ballast water treatment technologies are a large interest now.
On the subject of ballast water you could also take a look at the problem of introduced pest organisms being introduced via ballast water dumping by major commercial shipping. Australia faces numerous threats to its marine ecology due to ballast water dumping. I am of the opinion that ships should be made to cycle their ballast water several times while far out at sea as they transit between ports.
Thank you Nick for the useful video, Keep on the excellent work.
Exploding air bubbles is how ultrasonic cleaners clean parts.
and how my humidifier makes steam
I'd like to request a video focusing on super-cavitation
That's a good one! I'll add it to the list.
Nick talks about steam bubbles not air...
love your series, yould you discuss rake in propeller design
I'll add it to the list. You can also find the short answer in an article I wrote:
dmsonline.us/propellers-by-the-numbers/
The cavitation of the RMS Lusitania's propellers was so bad, she had to be reinforced with several tons of steel just so she wouldn't rattle herself into a pile of scrap metal!
(She used direct-drive Parsons steam turbines and this happened a lot to early turbine powered ships)
These are not "air" bubbles they are WATER VAPOR BUBBLES!!!
Yeah he knows that
Did you sat at 1:14 that proppellors create lift, surely you meant thrust?
No, theres lift and drag - watch Nick's video on propeller design for the finer details. Think aeroplane wing.
Since air lubrication reduces friction on a ship's hull, wouldn't using air to lubricate the propeller using super-cavitation not only reduce drag but eliminate wear on the surface of the propeller?
Perhaps but complexity of having a hollow shaft and air ports on the propeller surface to allow bleed air to come out would probably increase costs enough to barely break even. And on a submarine you wouldn't have a great supply if air unless you used reactor power to perform water hydrolysis and blow oxygen from the blades.
I told my roommate that bubbles are bad, they said that bubbles are not bad. I don't know what to believe.
Why not INJECT AIR on the back face of the propeller near the leading edge? Unlike water, air is not going to collapse back into liquid. This curtain of real air bubbles as opposed to temporary steam bubbles solves the problem, no?
Something I"m wondering about - cavitation is caused by the reduced pressure resulting in a decreased boiling temperature of the water, until the boiling temperature is below sea temperature. This makes it sound temperature dependent - will cavitation happen more in warmer seas (say 85°F water of the tropics) than in colder seas (such as 25°F seawater of arctic regions?) If so, how much difference does water temperature make?
Is the cavitation also occurs if the tip of the propeller is 3%-5% out in the water, Pls advise
Yes, cavitation can still occur with the tip of the propeller slightly out of the water. But in this case, ventilation is the larger problem. When the propeller breaks the surface, or is near the surface, the rotating blades can pull air into the water. When the air interacts with the propeller, it greatly reduces thrust, because the air is much lower density than the propeller.
Air bubbles? Aren't they water vapor?
True. Water vapor. Steam. Gaseous water. I target these videos for a non-technical audience. Sometimes I use less precise words in favor of easier to understand concepts. But you are quite correct. It is not technically air.
Side note: we do get actual air in the propeller. That is called ventilation and occurs when the ship is light loaded and the propeller is near the water surface. On slow moving commercial vessels, this is a bad thing. For super fast power boats, we actually design the surface piercing propellers to take advantage of that and make ventilation a good thing.
I've seen many old WW2 videos showing Liberty ships with their propeller halfway above the water while steaming around the old shipyards.
Is a super cavitation prop similar to a surface piercing setup like an Arneson or Levi drive? Racing props maybe?
Yes. It would be more accurate to say they are two different qualities. A fully submerged propeller can also be super cavitating. This is very common on surface piercing propellers because of the high speeds we use them in. The intent with a surface piercing prop is to have less of the propeller in the water, which reduces the prop drag. But this means the force from the propeller no longer lines up with the propeller shaft. So we need to reinforce the blades, hub, and shaft to handle that unbalanced load. Plus redesign the blade shapes to handle all the extra air they suck in.
Is cavitation like aircraft that produce vortices?
No. Propeller get vortices too, but cavitation is different. Vortices result from flow patterns around the wing or propeller. Cavitation happens specifically because you have liquid water. The propeller creates a change in pressure. And that pressure change alters the state of the water from liquid to gas (this is only small layer right next to the propeller surface).
@@DatawaveMarineSolutions Thanks for your reply. So that I have this right, water comes of the propeller at a low pressure and once exposed to sea water (high pressure) bubbles form at a higher temperature (than the sea water) and then collapse in a micro-second!
@@MrBradley34 Almost. The temperature doesn't change. The temperature required to boil water changes, depending on the water pressure. This is why water boils faster at high altitudes in the mountains. It literally boils at a lower temperature. In the case of cavitation, the water pressure on the surface of the blade drops so low that water can boil at normal sea temperature. The water never changes temperature, it just changes the starting point for boiling water. And as you said, that only lasts an instant. The water is now vaporized, and as it leaves the blade surface, the pressure goes back to normal ocean pressure, which collapses that bubble of steam.
I'm pushing a 30' sailboat with a 15 hp outboard. It makes loads of foaming bubbles. What kind of prop can be put on it to get thrust and at least half the bubbles or is it just spinning too fast for such a heavy load?
That can be either ventilation or cavitation. If it is cavitation, you should see little pits and pockmarks on the propeller blade. Ventilation on the other hand happens when the propeller is too close to the surface. The prop sucks in air, which gets mixed with the water and generates lots of bubbles in the wash. If you have ventilation, the two fixes that I would try are: 1.) Try to mount the propeller deeper in the water. 2.) Find something to create a physical barrier between the propeller and water surface. I think I have seen some weed guards that are horizontal fins mounted on outboards. That may help.
@@DatawaveMarineSolutions I hope there's a place to mount one of those barrier fins you suggested. Thank you.
They are not “air” bubbles they are vapour bubbles.That is the water changing from a liquid to a gas due to the water reaching its vapour pressure.There is no “air” underwater. You must get the basics right if you are going to profess to know what you’re talking about.
(too much drama) but a good subject.. next maybe "electrolysis" and sacrifice ??
Fair point. I do favor the drama a little to make the subject interesting for a non-technical audience. Not sure about electrolysis, but I do know that ballast water treatment technologies are a large interest now.
As a kid (10-12) I was fascinated when replacing the zinc plates
On the subject of ballast water you could also take a look at the problem of introduced pest organisms being introduced via ballast water dumping by major commercial shipping. Australia faces numerous threats to its marine ecology due to ballast water dumping. I am of the opinion that ships should be made to cycle their ballast water several times while far out at sea as they transit between ports.
read up on the "zebra" mussel in the US Great Lakes