What revs?

[KREW2]

I have two 125hp Yanmar diesels which should give me a cruising speed of 15 knots. The max revs for the engine is 3800, I cruise at 2800 but only achieve about 12/13 knots. Google has told me the cruising revs should be between 80% and 85% of the maximum revs obtainable. This equates 3100rpm. What do forum members run at for a 5/6 hour passage.

 

[colhel]

On a trip like that where you'd probably just want to get on with it, I stay around 3200 but have also done leisurely cruises at around 2000 rpm.

 

[ari]

I have two 125hp Yanmar diesels which should give me a cruising speed of 15 knots. The max revs for the engine is 3800, I cruise at 2800 but only achieve about 12/13 knots. Google has told me the cruising revs should be between 80% and 85% of the maximum revs obtainable. This equates 3100rpm. What do forum members run at for a 5/6 hour passage.

The question is rather more complex than that. For example, if it's a planing boat and half on the plane at 2.800rpm but fully planing at 3,100rpm, you'll be putting far less stress on the engines and burning less fuel per distance at 3,100rpm.

Optimum cruising speed is about more than just what RPM.

 

[KREW2]

Its a semi displacement. not too worried about using a bit more fuel. Running at 3100rpm will take a chunk out of a channel crossing on a calm day. If it is not going to do any harm to run for 4/6 hours at those revs I'm happy to do it.