Propeller question

[amaranth43]

I recently bought new props for my twin Yamaha 50's. (2016 &2022)
They had 11 5/8 x 11 props on so I bought the same again, but went for an aftermarket (PolaStorm) rather than genuine Yamaha. The boat is a commercial fishing catamaran (Cheetah) , 7.5m x 2.3m.

Before if I put the handles right down I would expect to get to about 5000rpm and be doing 18-20knts. Normal cruising speed about 13knts at 4200. Yamahas WOT is 5500-6000 for these engines I believe.

After I installed the new props the throttles won't go above 4300 but I get to my cruising speed at lower revs, about 3800. If I go to 4000 I am doing 15-16knots

All the above would suggest that the new props have a higher pitch, correct? But they are stamped the same as the old ones.

It seems too good to be true, lower revs for higher speed. They don't sound like they are working harder and my fuel consumption is, if anything, slightly lower. But I am reading a lot about 'lugging' and am concerned that I may be doing damage to the engines.

Any thoughts/advice appreciated.

[boscoe99]

I suspect that you have F50's. F and in four strokes.

When changing from one model of brand of propeller to a different model or brand of propeller, all bets are off as to what the results will be.

There is much much more than just diameter and pitch.

4300 RPM at WOT is not good for the health of the motors. They need to be propped to be at or near 6000 RPM at WOT with the boat loaded.

[amaranth43]

Just an update on this, I sent one of the propellers away to be measured for pitch. They found that the pitch was 11" at the root and 15" at the trailing edge, giving an average pitch of 13"; 2" above what is stamped on the propellers.

I've gone back to my original Yamaha props and am having my spares repaired. There'll be two nearly new Polastorm props for sale soon in the UK if anyone is interested ;-)

[boscoe99]

I recently bought new props for my twin Yamaha 50's. (2016 &2022)
They had 11 5/8 x 11 props on so I bought the same again, but went for an aftermarket (PolaStorm) rather than genuine Yamaha. The boat is a commercial fishing catamaran (Cheetah) , 7.5m x 2.3m.

Before if I put the handles right down I would expect to get to about 5000rpm and be doing 18-20knts. Normal cruising speed about 13knts at 4200. Yamahas WOT is 5500-6000 for these engines I believe.

After I installed the new props the throttles won't go above 4300 but I get to my cruising speed at lower revs, about 3800. If I go to 4000 I am doing 15-16knots

All the above would suggest that the new props have a higher pitch, correct? But they are stamped the same as the old ones.

It seems too good to be true, lower revs for higher speed. They don't sound like they are working harder and my fuel consumption is, if anything, slightly lower. But I am reading a lot about 'lugging' and am concerned that I may be doing damage to the engines.

Any thoughts/advice appreciated.

That is too low. WOT should be at or near 6000 RPM when the boat is loaded.