The Yanmar 3GM30 in the boat we were having surveyed was producing large amounts of thick black smoke when we went out for the sea trial. The engine would not go above 2600 RPM and produced a max speed of 4 knots on a Hunter 340. The smoke was thick and black and visibly mixed with the water in the prop wash. The surveyor suggested that a badly fouled prop could be the problem but that there was more smoke than could be explained by the prop. When the boat was hauled to survey and inspect the hull, the two bladed prop was indeed completely covered by a single layer of large barnacles. We removed the barnacles but could not go for another sea trial. The surveyor suggested and I agreed that the motor should be looked at by a competent mechanic. The local Yanmar dealer proptly sent over a mechanic who pulled and tested the injectors, tested the compression and took the boat for another sea trial. The injectors all tested good and the compression tested as good and equal in all cylinders. At sea trail the engine ran up above 3400 RPM and did not smoke. The mechanic confirmed that a fouled prop will produce lots and lots of black smoke but everyone was surprised at the amount. Since we are buying the boat I am glad that we had the engine checked. My advice to those with a black smoking diesel with low RPM's and no other symptoms is to check the prop before anything else.
