Tracing transmission whine
Q
Since recomissioning our Hunter 32 after the winter lay-up we’ve noticed a pronounced transmission whine which only occurs when throttling back after a period of motoring at normal cruising speed. We changed the KM 2P gearbox oil in the autumn with SAE 10/40 multigrade. I wondered whether some engine misalignment could be occurring while slowing down but the noise doesn’t resolve until the engine revs are either decreased or increased. If there were worn gearbox bearings why should this have developed during a period of disuse? The engine mounts look OK. There were no apparent problems before the boat was overwintered ashore. The prop has been coated with epoxy primer underneath antifouling and we’ve always had a shaft anode without any problems. I’d be very grateful for your thoughts. Richard Lewis, Pontefract, Yorkshire
STU DAVIES RESPONDS The Yanmar manual recommends a 20 or 30 single grade oil for your gearbox but as this has become difficult to find people now use 10/40 multigrade as a substitute. It might run thinner as it heats up and could possibly be causing the symptoms.
Alignment of the engine shouldn’t affect gearbox noises so I’d discount that.
Propellers do sometimes ‘sing’ and the paint you have put on the prop could be a cause by minutely altering the balance or shape.
I’d check by hand to make sure there’s no movement in the gearbox shaft, that there is no sign of overheating, and if you can find some 20 or 30 SAE oil change it to check if it makes a difference.