Yacht Forum UK

I share a boat with a friend, and last month he ran out of diesel whilst motor sailing. The result was that he sucked up a lot of sludge into the primary fuel filter. We filled the tank, but there was a big difference in revs when in neutral and in gear. I guess that sludge had been sucked into the filter. Since then we have changed that filter and even cleaned out the tank. Finally, we got ready to go out again. However, when in neutral we get about 3,500rpm, but when in gear under load we are still only getting 2,500 rpm. The engine sounds fine, and the power does seem to be there - not like it was before we changed the fuel filter. We live in Spain and the weather has been very warm recently. I am thinking that the problem 'now' has nothing to do with the filters, but more to do with the huge amount of growth on the hull and probably the prop. I think I already know the answer, but I just want a second opinion. Basically, can a very dirty hull and prop have such a big effect on the revs you'll get when under power. Thanks.

I don't guarantee that it's the answer but certainly having lots of growth on the hull increases te power required to push the boat through the water. That in turn can limit the engine revs (it depends in complicated ways on the propellor match to work being done). The connection between prop and water is tighter than you might at first expect. So pushing the boat with lots of weed is like driving with the brake on, limiting the engine's max speed as it reaches its max power output.
On the other hand, engine power can be restricted by limiting the fuel flow so it's possible that there is still some gunge from the bottom of the tank in the pipework somewhere.
Derek

Last time I moved mine with the engine it was fine for ages then slowly dropped in speed despite increasing power, we ended up crawling along and when we finally reached the pontoon we found we had picked up so much weed it was like an island, so yes, excess growth can certainly have that effect