A 15×13 RH rotation prop by Eliche Radice, supplied by Sillette Marine. Fitted to a 3GMD (2.36:1 gearbox) for efficient slow running rather than power: 5kn at 1450 rpm.
There are several reasons why you might be about to fit a replacement propeller. You might be trying to optimise the output efficiency of a new or reconditioned engine; at any rate, an engine that operates to its original specification, capable of running at its rated maximum rpm. If you are in that fortunate position, you could refer to the article at https://www.finesse-owners-association.co.uk/propeller-sizing for suggestions for the specification. On the other hand, you might be aiming at efficiency at the expense of speed, by increasing the diameter and/ or the pitch. Slower rpm means a quieter ride, too, as long as you are at peace with the knowledge that trying for higher revs may result in a smoky exhaust and quite possibly carbon deposits.
This hub depth was 2 1/4″, which revealed just enough thread for the nut and and a single washer.
The Finesse fleet was built in Essex and there are numerous fittings that are dimensioned in imperial units. The prop shaft is one example of this. On our Finesse 24, as shown in these images, the shaft has a diameter of 1″ and has a standard 1:12 taper. Your chosen propeller needs to match this specification, of course, but it also has to have a hub depth that is sufficiently short so that enough thread protrudes for its securing nut and washer to be fitted. In our case, this limited the hub depth to 2 1/4″. The keyway was 1/4″ wide. And finally, for our Yanmar 3GMD engine, the rotation sense was right-handed.
Prussian Blue applied to the shaft taper
The shaft taper and the propeller’s internal taper need to be matched so that the amount if contact surface between them is as great as possible, to ensure that the prop is centred (and therefore balanced) and to spread the torque load evenly. Start by applying a thin coat of Prussian Blue to the shaft taper, then push the prop on and give it a few turns. (At this stage the shaft can be prevented from turning by engaging the clutch. Later, for torquing up the locknut, the shaft will need to be locked solid.) Remove the prop and check where the Blue has been scuffed- that’s where contact has been made.
After rotating the prop, look for the areas where the Blue has been removed by contact. This picture suggests most contact was being made at the widest part of the taper.
It is likely that there will be significant areas where no contact has been made. To improve the degree of contact, clean off the Blue and apply some valve grinding compound. The prop is then pushed on and rotated clockwise and anticlockwise. A small amount of grinding will probably be sufficient; the prop is made from softer material than the shaft, and will lose most of the material.
After using valve grinding compound, there has been much more contact between surfaces. Stainless steel is harder than silicon bronze so more material will have been removed from the prop.
Note:
- Avoid skin contact with Prussian Blue
- Triple check that you have fitted the key in its slot before the final mounting of the prop!
The securing nut on our propshaft is 5/8″ BSF, and made from A2 stainless. The shaft and nut had been drilled to take the security split pin when the original propeller was fitted. Aligning these holes when the nut is torqued up to around 100lbft requires either a very accurate washer thickness or some judicious filing of the face of the nut. When I fitted a new prop in 2024, I found an M16 zinc-plated washer 2.8mm thick was just the ticket. However, when I checked it one year later, the washer had galvanically corroded beyond use. SoI took the other option, installed a 3mm A4 stainless M16 washer and faced the nut (carefully! I had no spare) to make the snug fit.
