Yanmar 4LHA STP boat speed down

[Sam Jenkins]

Evening im working on a boat that has had a yanmar 4LHA STP fitted. The boat previously did just shy of 30knots with an older Mercruiser diesel engine.
The engine is fitted into an offshore 25 sports fishing boat with a bravo 3 duo prop sterndrive. Would anyone be able to advise the correct sterndrive ratio For this set up?
The engine makes 3300rpm but is only reaching a boat speed of 16knots.
Many thanks

 

[alldodge]

I think the drive ratio is correct, might need prop tuned up

Finding yanmar 4LHA-STP max rpm is 3100 and continuous max rpm should be 2900-3000

What is the current ratio?

 

[Sam Jenkins]

Hes currently running 24p props but has tried 22 and26 with not alot of difference. It’s supposedly a 2:1 ratio.
Im wondering if the engine is actually the lower hp engine as theres no tally plates on the engine. If this was the case do you think it would still make the revs but not the power
Cheers

 

[alldodge]

If 26P props turn the same rpm as the 24P props, then something else is happening. The props should reduce max rpm as pitch increase.

If boat is using the same outdrive so we are just looking at amount of power being produced. Now if rpm is reducing then need to increase pitch more or use a higher ratio drive

 

[QBhoy]

Hi...My first thoughts are that the numbers maybe don't make too much sense to me. If they are actually accurate, then the prop slip is enormously beyond anything reasonable and then some.
The power of the engine doesn't really come into the equation to me, if she is able to make 3300 rpm through a 2-1 ratio, that is turning 24' props and only seeing about 20 mph...then something in the drivetrain is compromised and slipping.
Are we thinking that the engine is definitely making those rpms for sure ? Is there a governor intervention perhaps, telling you that she is definitely right up there at those rpms for sure ?
Or is there a chance that the engine isn't actually making those rpms at all, perhaps maybe only indicating as much from a tachometer that is calibrated to an engine with a different number of cylinders previously ?
Is there any compromise in the ability of the props to stay driven as they should be...sacraficial hub failure (spun), spline failure or improper assembly of the props perhaps ?