80s vintage Yanmar 18 only hits 1800rpm

My girlfriends 31 foot sailing boat has a Yanmar 18hp and at full throttle the boat hits 5.5K. At this speed the engine is only doing 1800rpm and pumping a lot of diesel smoke. So we reduce throttle so the smoke stops. RPM only goes down a bit and speed is about 5K.

Do these symptoms sound like an over propped engine? What sort of RPM should this be reaching?

 

That looks like a classical symptom of overpropping. Are there any identical or similar boats you can check the prop size?

 

Thanks Gonzo. There are no identical or similar boats around that I can check. I will eventually dive down to scrub the bottom and measure the prop. I will also get exact engine specs.

Here is a pic of the boat. Not that it helps much but everyone likes pics Its unusual in that it has 7" draft on a 31" boat and huge free board for a 31". The boat looks huge next to other boats of similar length. Its strip plank. I think she said it is 5T?

 

Could be a cruise prop, could be a simple lack of fuel.

IF the smoke is mostly black , that's the sign of too much fuel , Cruise prop.

A 31 ft boat is probably 25 on the LWL , so 5K is the proper long range cruise speed , and 1800 is nice and quiet.

If you wish to add 1K at the cost of probably doubbling the fuel burn, some pitch can be removed from the prop.

Dive and be SURE its clean and not damaged.

FF

 

Simple enough. Just look at the specs on the engine. See where it develops max hp and tweak the prop till you achieve max hp rpm. You may want to use some Sea Foam to get rid of some of the black soot like stuff in the engine.

Easy Rider