ANSWER

Dear Jane

I assume that you mean 1500 and 2000 rpm because no modern engine would run as slow as 15 to 20 rpm.

If its as loud as a slipping fan belt, then no, I have no idea, but if its quieter it could well be shaft bearing trouble. Are you sure nothing has fallen against the shaft?

First use the stern gland greaser to push loads of grease into the shaft assembly and see if that makes a difference. Also check that the prop has not fouled.

Whilst you have the weed hatch off try tugging the prop up and down. It should not move, but if you can get some "lift" on it you may have worn shaft bearings.

If the boat also appears to have lost power a coupling could well have come loose on the shaft. I would expect the coupling to get hot and there to be "iron filings" around the area.

If the shaft has plumber blocks (housings with (usually) ball bearings inside) to support the shaft, one of these may have seized up.

If this narrowboat was fitted with rubber (cutless) shaft bearings these will squeak if the receive insufficient water, but its far quieter than a fan belt.

I think that you should get the noise happening and use a long stick against your ear to put against various parts - NOT A ROTATING SHAFT THOUGH. This acts as a kind of stethoscope and helps you localise the noise.

I have also heard of the anti-cavitation plate that hangs down from below the weed hatch vibrating and making very odd noises as it rubs against the weed-hatch sides. You could try wrapping the plate edges in lots of insulating tape and if this cures the problem either grind down the plate a bit or get the plate's "hangers" strengthened.

Hope this helps, but I fear it will not.

Tony Brooks