Transom soot puzzler

[Beaufort Rob]

Here is a puzzler for you guys: 

The last trip I took to Cape Lookout involved about 3 hours of motoring and another 3 hours of motor sailing.  When I returned there was a massive amount of soot on the stern.  There was no unusual black smoke from the engine while motoring.  I would suspect I'm over-propped with a 17x11 two blade.  However, I've been using the boat for over 3 years with no problem.  The engine is somewhat unique in that the block is a D1402, slightly larger bore version of the D1102 used in the M30.  All the external parts are interchangeable.  I'm guessing somewhere between 25 and 30hp.  I don't know if it was ordered this way or the block was upgraded.  The engine hour meter says just over 200.  I did the usual addition of Biobor additive to the diesel in the Fall.  My guess is that my recent swap of a 160 degree thermostat to a 180 degree is burning the carbon/crud out of the engine and exhaust.  I made the swap based on discussion about problems with running too cool, one of which was carbon buildup if I remember correctly.  There is a blower for the engine box which I have rarely used before.  I started using it on a short trip to see if the engine box was too hot and starving the engine of O2.  It was a sailing day so the engine got only about an hours use but still had soot.  I have been trying to cure fan belt alignment issues which cause some dust in the engine box without much luck.  There is no filter element on the engine but it seems hard to believe that much soot could be created.  Also, I have had the belt issue since I've owned her without any issue.  Maybe that buildup is what is burning off.  A friend believes it is a result of too much back pressure on the exhaust caused by the exhaust dipping below the waterline when motoring. 

Any other ideas as to the cause or method of cleaning?

Thanks,

Rob

[Mary Connaughton]

I had a huge amount of soot on the transom too, but I have a fairly new yanmar. .the issue was the mixing elbow was clogged and it blew put a lot of soot at once...I changed out the mixing elbow and all is good.  You might want to check that..good luck

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[Beaufort Rob]

Thanks, Mary.  I am overdue to replace the homemade mixing elbow after more than 3 years.  I'm still getting good water pressure but I was warned that they wouldn't last more than a few years.  I bought enough pipe and fittings for two elbows.  Now I just have to remember where they are stored.

[Bill Cornpropst]

I agree with Mary on the exhaust elbow, worth checking into. Is your prop clean? A load on the engine will cause smoke as well.For years I used Valvetec diesel additive in addition to a biocide additive and never got a dirty transom.

Let us know what you find.

Bill

[Beaufort Rob]

The prop is clean but the strut has some growth that is stubborn.  Fear of the line cutter learned the hard way doesn't help either.  I'll replace the exhaust elbow this weekend get some decent gloves for cleaning the strut.

Thanks for the advice,

Rob

[Beaufort Rob]

The riser broke just above the water inlet with just a little pressure to break the seal to the manifold.  The corrosion/carbon deposits were pretty bad where it broke.  I found the spare to install after I paint and seal joints.  The exhaust manifold looks pretty clean to me.  I'll let you know if replacing the riser fixes the problem.  I read that soot problems are sometimes caused by replacing the riser.  That would make it hard to tell if the blockage was causing soot.