Props

[alistairh]

I have an 60hp yamaha 2 stroke on an Orkney 19+, and its prop need replacing badly. It used to have a small prop (pitch of only 15 and was a k model), i wanted to replace it with a stainless stell prop but its smallest pitch available is a 18". Can i still replace it with this or will it affect preformance, or revs.

 

[srm]

Its many years since I looked at prop size / power graphs but the short answer is that it will reduce revs and deliver less power.

If the engine performed correctly with a 15inch pitch prop going to 18 inch pitch (with same diameter and blade area) will probably overload it resulting in lower revs and less power output. Its called "overpropping" and is sometimes done deliberately to prevent an engine reaching max revs.

Its a bit like trying to keep a low powered car in top gear going up a steep hill, the engine can't reach the revs required to deliver the power needed and just chugs along.

No doubt others will come up with a far more techy answer, and I am happy to stand corrected.