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Old 15-09-2016, 04:20   #1
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Strange vibration feeling

New to me boat, 1984 full keel 35k lb 43 footer. Powered by a Perkins 4-236 with a aqua drive and a fixed 3 blade prop. The thing is butter smooth at any rpm on flat water but if it's choppy, every time the boat comes down a wave there's a slight vibration. Along with the vibration there's a slight audible noise. To me it sounds like the prop is being over run as the boat accelerates due to its mass. I've watched at the stuffing box, and both ends of the aqua drive, you can see the vibration ever so slightly in the engine, but it only lasts the duration of the down the wave, 3-5 seconds and then it's gone till the next one. Prop is tight, shaft is straight. Any thoughts, my first boat with a fixed prop maybe it's normal??
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Old 15-09-2016, 06:51   #2
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Re: Strange vibration feeling

I think the shaft is very much like a gyroscope and resists being moved, and when you move it it flexes? My theory anyway, I would like an Aqua Drive myself
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Old 16-09-2016, 01:45   #3
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Re: Strange vibration feeling

maybe worn cutless bearing or even loose cutless bearing holder. That little extra speed gets the harmonics going and it starts vibrating,
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Old 16-09-2016, 03:28   #4
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Re: Strange vibration feeling

Either the cutlass bearing or a bad motor mount.
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Old 16-09-2016, 03:41   #5
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Re: Strange vibration feeling

I did try to move the prop in the cutlass on the hard and was not able, don't know if I could if it was bad as its a 2 in shaft with a heavy prop on it. Motor mounts are suspect to me as well. Also an easy and relatively cheap fix.
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Old 16-09-2016, 04:26   #6
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Re: Strange vibration feeling

Loose strut our worn cutlass bearing?
Not likely the motor mounts since you have Aquadrive.
Is the Aquadrive bearing mounted securely?
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Old 16-09-2016, 08:25   #7
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Re: Strange vibration feeling

I just moved the boat 400 miles under power, it only makes the noise on waves, and I spent hours with the floor boards up listening and watching. There is no vibration or wobble in the shaft, the aqua drive, it does have a short cv shaft between the engine and aqua drive but mo vibration in it either. When you feel and hear it you see the engine wiggle.
Also no strut the shaft is though the back of the keel and done.
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Old 16-09-2016, 13:20   #8
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Re: Strange vibration feeling

Usually these vibrations are because something is loose. They only occur over a small engine range. Things like a bent or out of balance propeller usually vibrate constantly, but it might be a possibility. Maybe there's a zinc anode on the shaft that has eroded irregularly and this is putting the system out of balance. Other things to consider are misalignment of engine and drive shaft or something out of balance or loose inside the motor. It might be that going down a wave moves something in the sump enough to allow it to rub.
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Old 16-09-2016, 13:45   #9
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Re: Strange vibration feeling

That is quite common with nothing wrong. It's the prop cavitation changing as it's moved thru the water with the rocking of the boat.
Of course there could be something wrong also. But if it's only doing it when the boat pitches thru waves I wouldn't worry.
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Old 16-09-2016, 13:54   #10
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Re: Strange vibration feeling

When you accelerate down the face of a wave the prop sees a different load and turbulence, which sets up a harmonic that only last till the load normalizes again. If all your components are in good shape, the only place you would see motion is with the engine. Nothing else is supposed to move around. I wonder if softer more modern mounts would soak it up ?
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Old 16-09-2016, 13:58   #11
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Re: Strange vibration feeling

Quote:
Originally Posted by Guy View Post
When you accelerate down the face of a wave the prop sees a different load and turbulence, which sets up a harmonic that only last till the load normalizes again. If all your components are in good shape, the only place you would see motion is with the engine. Nothing else is supposed to move around. I wonder if softer more modern mounts would soak it up ?
.... or softer mounts may allow engine movement with the boat motion, flexing the shaft and causing more! Hard to say.
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