Smoking Diesel

[Dave_Rolfe]

We've just bought a new (to us) yacht fitted with a Yanmar 2GM20 fitted to a saildrive leg.

Five weeks ago we had the boat pressure washed to get all the muck off the bottom for the survey and the boat has not moved from her marina berth since. Yesterday was the first time that we have been able to take her out.

In neutral the engine revved to 3400 rpm quite happily with no smoke. Once under way we got to about 2900 rpm under load and then the enginbe would rev no higher and increasing the throttle just resulted in black smoke and muck coming out the exhaust. I think that the bottom of the boat must have a load of slime and weed on causing the black smoke and lack of revs which has built up in the five stationary weeks. No antifouling has been applied since last winter so there may not be much left by now.

Could anyone please give an opinion as to the lilkely cause as I am due to deliver the boat back to her new home this coming weekend and will only worry if I am not confident of the cause. Engine hours are only just over 200 hrs and as far as I know the engine has been regularly (annually) maintained.

 

[Danny Jo]

Can't answer the question, but suspect that this is a feature of the engine-boat-prop configuration rather than a specific fault. My 38 footer has an 18-year old Perkins Perama M30 , which is supposed to generate 29 horsepower at 3500 revs. I get 5 knots in calm water at 2100 revs and 5.5 knots at 2300 revs. The extra speed at higher revs is not worth the extra fuel consumption, not to mention the sooty exhaust, the extra wear and tear, higher engine temperature etc.

 

[savageseadog]

Got the same setup. It will go black above a ceratain RPM due to the engine being unable to rotate any faster. The RPM that happens at depends on the engine load. We go black at at 3000 plus but it does depend on fouling. For the time being the solution will be another clean as you've lost the effectiveness of the anti-fouling `plus water temp is high plus sunshine equals much fouling. Other than that rev up till it goes black and back off and live with it. Long term you may have to go a size down on your prop.

 

[jimg]

It is OK if you have reached a "sensible"speed, what speed did you acheive and what is the boat length?

 

[Dave_Rolfe]

I achieved about five and a half knots on a 30ft boat with a 27ft water line before the black smoke started to appear from the exhaust. but was expecting a 6+kt speed. My main concern is that I have to cover about 100 miles on my delivery trip and just want reassurance that the problem is probably nothing more serious than a dirty bottom or similar. Of course this is all assuming that a favovourable wind will not exist and I will need to motor or motorsail.

 

[andy_wilson]

If the a/f is shot and she has been afloat for 5 weeks, that is the most likely answer. Average water temp. is high just now, so fouling is rampant.

Going round the waterline in the dinghy with a scraper (I have a brush with a scraping edge) might help. You should be able to clean all of the forward sections and a good 3-4 feet byond the waterline elswhere, plus aft sections and rudder.

 

[savageseadog]

As I say, have the same setup on a heavy 30 footer, with the boat clean we would get 7 Knots before smoke, dirtyish 6 Knots. So you'd probably be looking at 6 and 5 knots respectively.

 

[kengill]

The engine should run at its maximum speed when doing maximum hull speed.

If you are acheiveing hull speed but only the low revs then you should look to the prop. When we re-engined the prop diameter was increasedd by 1" the pitch by 2" and the blade area by 30%.

Finally after much argument and many lifts and modifications, wringing, clipping, chamfering we settled on a prop with +10% blade area and the original diameter and pitch.
Its perfectly matched and does 3600 rpm at 7.2 knots and not a whisker of black smoke.

You may think you will save fuel by having as it were a higher gear but what will happen is that the engine will not operate correctly and will wear more quickly and may even tend to glaze the bores because it can't develop maximum power.