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seanseamour 01-08-2009 20:46

Prop Shaft Rattle
I have recently developed a knocking in the prop-shaft which is coming either from the hull passage (let's say skeg) or the strut support bracket before the prop. This only happens engine idling when the drive is not engaged or engaged at engine speeds below 1000rpm (if I increase engine speed without engaging drive rattle ceases). If I shake the shaft from inside before the shaft seal I can effect the knocking sound. I suspect the BTR Silverline cutlass bearing is going out and wondered if anyone has replaced one of these himself and how urgently I should address this (does bearing degradation accelerate quickly?)
1978 nc33 Sean Seamour III

neilpride 01-08-2009 21:54

Hi sean, are you check the bolts in the coupling? Knock in idle and low revs sound loose coupling.

seanseamour 02-08-2009 02:31

Thanx for the quick response, just back from the boat, not the coupling that is tight, when I lift on the shaft I can hear a knock definitely under hull, what I cannot ascertain is if the knock is at the strut or thru-hull.

GordMay 02-08-2009 03:26

Sounds like it might be the cutless bearing.

Check out the manual

neilpride 02-08-2009 07:39

Hi sean, ok if the coupling is tight, is the cutlass bearing, check if you have a P bracket loose suporting the bearing in the case the boat dont have a stud extension,
hey good luck in the next haul out.Cheers.

Fishspearit 02-08-2009 08:40

Easy to check the cutlass bearing, jump in the water and see if you can shake the shaft by grabbing the prop. Always a good idea to check your engine alignment if you are wearing out your cutlass bearing quickly.

senormechanico 02-08-2009 10:59

Engine mounts might be sagging. This could cause it as the engine moves about more at idle than when at higher RPM. Check engine alignment.

Steve B.

Chief Engineer 02-08-2009 19:27

Check the whole drivetrain, engine mounts......I recently had a knock...Allt he engine mounts were loose and the engine was way out of alignment....The stress on the shaft caused the knocking

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