Techie Prop Question

[Ribald]

The prop that came with my engine (Mariner 60 ELPTO) is ali with a dia of 10.13" and a pitch of 15". The previous owner added a Solas s/s prop with a 11" dia and a 15" pitch.

My question is - By using the s/s prop am I overloading the engine? The s/s prop is almost twice the weight of the ali one but does seem to bite the water better with far less cavitation/ventilation. The nearest s/s size that I can find in the workshop manual is 10.25 x 14

[Pete7]

The Mariner 60 2 stroke should rev to 5500 at WOT (wide open Throttle) with a normal load of passengers and fuel. Don't get to hung up if its close to this, however if it will not reach say 4800 to 5000 you need to change to a smaller pitch and if its going over 5500 revs then you must change to a larger prop. Swopping different props from the same manufacturer is reasonably simple but different manufacturers have different blade cupping and shapes, then you in for best guesses. If you need to change then see if a dealer will lend a prop for an hour to check before you buy.

[dmoore]

I think that the major forces encountered in the drive train would be the force required to push the water backwards. The force required to spin a heavier prop would not be in the same league as this. The larger diameter would increase the strain on the drive train as it would be shifting more water but the science of prop calculations is at best some tried and tested formulea and try a few out and see what works best. my own experience comes from calculating prop sizes for sailing boats with aux engines and I realise that there are major differences but with these craft I would not be bothered with the weight of the prop, only its diameter and pitch matched to engine power. If you want my spread sheet then email me and I will forward it.