Smokey diesel is it prop related?

[Homer J]

Jeanneau 37 with a yanmar GM30 (27hp engine)

It has a feathering 3 blade (Maxprop?) propeller

Questions
I'm only getting a maximum 6 knots - is this correct?
If the engine revs above 2,200 rpm is seems to be overfuelling as there is a black discharge in the water and gives about 5.5kts at 2,200
Max revs are about 2,500
Going quickly from ahead to astern will also trigger this discharge, which I'm thinking is due to overfuelling
It seems worse this year than last, which may be due to fouling on the prop but it was there a little last year
I had all the injectors out and serviced last year so I have to assume these are not at fault

I can only think that the prop is not set up correctly and the engine is over-fuelling because it can't turn the prop correctly and the additional fouling on the hull and prop is making it worse

What do others think? Could the prop be at fault and is it possible to adjust the prop correctly?

Any help gratefully received.

 

[KAL]

I think you’re right on the overfuelling idea.

Any engine will struggle to reach max revs if the load on it is too great. Bad fouling on the hull, or towing a heavy load, will increase the resistance that the engine has to overcome, so you will get black smoke if you give too much throttle in these conditions, as there will be unburned or incompletely combusted fuel exhausted.

I would imagine that, once you’re all scrubbed clean, the issue will be resolved. Good luck.

 

[AllanG]

The symptoms you describe could be caused by a blocked exhaust bend, so I'd be checking this first, which was the problem on my previous Yanmar 2GM20F engine.

 

[pcatterall]

I think you’re right on the overfuelling idea.

Any engine will struggle to reach max revs if the load on it is too great. Bad fouling on the hull, or towing a heavy load, will increase the resistance that the engine has to overcome, so you will get black smoke if you give too much throttle in these conditions, as there will be unburned or incompletely combusted fuel exhausted.

I would imagine that, once you’re all scrubbed clean, the issue will be resolved. Good luck.

Due to bad fouling in Caliari marina we experienced severe smoke and very slow hull speed.
Once cleaned up we were back to normal so being g sure that the prop and hull were clean would be my first check. that hull speed sounds a bit low too.

 

[Tranona]

Your engine should achieve at least 3400 and about 7.5 knots with a clear bottom in flat water. First check that it will achieve 3600 with no load. You can't do it any harm as it is governed to that speed.

So, assuming your exhaust is clear the pitch of the prop is approx 3" too much (an inch of pitch changes revs by 250-300).

If it is a Maxprop then the pitch can be adjusted, and rather than do it by trial and error suggest you contact the UK distributor who will calculate what it should be and tell you how to set it - or even worth having them check it over and reset it.

However I am surprised that the prop is set so far out and a blocked exhaust will give similar symptoms but usually noticeable by getting progressively worse over time. Your speed is what you would expect at the revs so checking the exhaust would be the first thing to do - relatively simple and no need to lift the boat.

 

[Homer J]

Thanks all. I will try the engine free revving on Friday as suggested. The reason I suspect the prop is that it was set up wrong at relaunch last year and had to be hauled out and redone as there was no drive.