Causes of prop vibration?

[snowleopard]

I did a trip on a friend's narrow boat. On the canals, pootling along at 1500 rpm it was fine but on the tideway more revs were needed and above 1900 it vibrated heavily. It has a 3-blade fixed prop with no noticeable missing lumps or distortions. Any ideas?

 

[sarabande]

loose engine / gearbox mountings
loose or misaligned shaft bearing / thrust coupling
loose P or other bracket if fitted
propeller too close to deadwood, so blades 'hit' still water every time they go round
propeller blade out of true

 

[Chrissie]

also look for snapped bolt inside engine foot, deteriated rubber in mount and too long a distance between the prop and the P bracket

 

[Deleted member 36384]

Vibration caused by cavitation perhaps? My boat is over propped and when I get to 1800 rpm the noise from the the propellor is loud and there is a vibration on the aft bulk head just above stern gland.

 

[pmagowan]

resonance? The prop is always vibrating but at some revs it will vibrate in sympathy with the shafts natural resonance. This sets up a quite upsettingly noticeable vibration. If it is this then increasing or decreasing the revs beyond this point should solve the problem.

 

[Daedelus]

A rope cutter fitted too close to the P bracket can do it. (Bitter experience).

 

[Tranona]

I did a trip on a friend's narrow boat. On the canals, pootling along at 1500 rpm it was fine but on the tideway more revs were needed and above 1900 it vibrated heavily. It has a 3-blade fixed prop with no noticeable missing lumps or distortions. Any ideas?

Narrow boats usually have a very large shaft (1 1/2") in relation to engine power. The stern gear is a steel tube with a cutless bearing. Props are large and run in a big aperture. They also frequently pick up rubbish which can damage the prop and get in the bearing. Damage to couplings and engine mounts is common following fouled props (think wire, bed springs, supermarket trolleys etc) so there is plenty of things to check.