Propeller Overhauling Process/Ship's Propeller Renewal procedure

Propeller is the most significant machinery system of the ship. No matter what type of propeller it is, ship owners never make compromises on such important equipment as it directly affects the efficiency and safety of the vessel. Here we discuss about fixed pitch keyless propeller renewal.
Why is propeller renewed?
There are various reasons for which a ship’s propeller could be renewed. Some of the significant reasons are:
1) Damage to the propeller
2) Dynamically unbalanced
3) Renewing the existing propeller with a new propeller that is of better material, larger blade area etc.
Before decoupling the intermediate shaft and the propeller shaft (tail shaft), make thorough inspection of the propulsion system. Generally, the shaft jack up test is done before removing the intermediate shaft.
Propeller Mounting
On completion and satisfaction of propeller bedding, tail shaft and other accessories that are removed are installed back. Then the propeller is mounted. This is a significant stage and careful attention is paid during the same. Here we will discuss about the oil injection method for a keyless propeller mounting.
1) Clean the propeller boss and the propeller shaft
2) Slide the propeller to the propeller shaft and align to the match marks. (Prior to this step, remember to record the temperature of propeller boss and propeller shaft)
3) Screw in the Pilgrim Nut
4) Dry fit: Actuate the high-pressure pump connecting the Pilgrim nut, allow the propeller to slide in to certain distance and set the dial indicator to zero. The maker always recommends initial load
5)Wet fit: Then actuate the high pressure pump connecting the propeller boss expansion oil port and simultaneously actuate the nut pressure pump. Raise the pressure gradually until the predetermined push up length is achieved (determination of push up length is explained below)
6) Once the required push up length is achieved gradually release the pressure of boss expansion port and then release the nut pressure
7) Remove all the connections and plug both propeller boss port and pilgrim nut port
8) Remember to take shaft jack up test post the installation
9) Verify the coupling alignment using sag and gap method,
Alternatively, there are graphs provided by manufacturer, which can be used in lieu of the above-mentioned formulae recommended by the class.
Permissible push up length can be captured for the corresponding temperature.
Propeller renewal is not about removing the existing propeller and just installing the new one, there are many other significant factors to overcome. Following are important points to be taken care of:
1) Good interference fit to transmit the torque: Damaging factor here is the varying temperature and differential thermal expansion of the propeller hub (bronze) and propeller shaft (steel).
Thermal expansion coefficient of bronze is about 10 and steel is about 7 (in 10-6 in/Fahrenheit). This will affect the fitting, hence keeping all this in mind required push up length is calculated such that the sufficient friction is achieved to transmit the torque.
2) Reduced stress on propeller hub and propeller shaft: Von misses stress should be less than 0.7 times yield stress of propeller material. Von misses criterion is one of the most commonly used failure theory.
To assess the von misses stress:
1) Derive the stress elements
2) Find the principal stresses (Mohr’s circle)
3) From the principal stresses von misses stress can be derived (for 2d). If it is greater than the yield strength, then the material fails. It is just a simple explanation about von misses stress. Taking all these into account, maker represent the minimum push up length graphically.