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Old 12-10-2010, 15:42   #16
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I strongly agree with those above who suggest a look for debris on the prop or shaft. 95%+ of the time when when vibrations are present as a sudden change from smooth running lines caught on the prop are responsible.
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Old 12-10-2010, 19:42   #17
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I had a similar problem with one of my boats and found it was due to an imbalance in the engine mounts. Made a sound at various rpms that sounded like a "whap whap", which ended up being the exhaust hoses vibrating against the fiberglass. Fixed the motor mounts and aligned the shaft which took most of the problem away. Then had to replace the cutlass bearing to remove the rest of the vibration. If you can dectect play in the shaft at the cutlass bearing (by shaking the shaft) then you need to replace it. All went to normal after those repairs.

Hope that helps!
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Old 12-10-2010, 19:55   #18
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From the OP and his further comments is appears that most everything from engine mounts to the propeller are shot. So little can be done until the boat is hauled. Fixing one thing now with all the other problems still there simply will not result in a lasting fix.
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Old 12-10-2010, 20:11   #19
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Negative osiris sail, they aren't shot. However, I am replacing at least the prop because I am changing from 2 to 3 blade and changing the pitch. I figure I might as well replace the whole back end while I'm at it out of an abundance of caution and since it all is approaching at least a decade of use. I have a diver looking at it tomorrow and I'll post the results here. I'm still going for the loose shaft zinc theory, although CaptainBW's experience sounds fairly similar, so I may try some more alignment tweaks.
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Old 12-10-2010, 21:31   #20
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No. It's strange and hard to describe. The motor is fairly lightly mounted so the whole motor mount twists up and down slightly (rotates perpendicular to the boat's beam). This amount decreases at higher rpm and as I said, goes away if I turn by hand and leave it parked.
Only going by what you posted. A "whole motor mount that twists up and down . . ." - usually indicates bad motor mounts. Movement of the motor will put the prop shaft out of alignment with the shaft log, cutlass bearings and other parts in the line.
- - Remember the propeller does not push the boat.
- - The propeller pushes the prop shaft that pushes the transmission that pushes the engine that pushes the motor mounts and then they push the boat through the water. (except if you have a thrust bearing system on the prop shaft.) Motor mounts are critical in the propulsion line and do wear out which will cause misalignment and extra wear on the other parts down stream.
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Old 13-10-2010, 07:16   #21
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CaptainBW's experience sounds fairly similar, so I may try some more alignment tweaks.
Since my experience sounds similar I will expand on the motor mount imbalance: There were 4 motor mounts on the engine and 2 of them, opposite diagonally, were sitting properly on the motor mount pads. The other 2 had a gap between the bottom of the motor mount and the pad. I am not sure to this day how they got that far out of adjustment. I adjusted all 4 so that they were sitting properly on the pads, keeping the shaft alignment to spec as I went.

The vibration drastically improved after that (as you can imagine). Replacement of the cutlass bearing took care of the little vibration that was left.
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Old 13-10-2010, 08:57   #22
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Just got a call from the diver. Loose shaft zinc, check. Barnacle growth, also check. Cutlass bearing was good though.

Thanks everyone for the input!
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Old 13-10-2010, 09:25   #23
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. Barnacle growth, also check.
YES!!!!!!!!!!!!

The boy wins!

Good to hear its not something serious
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Old 18-10-2010, 22:42   #24
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Just one last word on the situation. The loose zinc was apparently having quite a bit of fun. Take a look at the grooves that got worn into it either by the prop or the cutlass bearing (attached).

Everything ran great last weekend, so the loose zinc was apparently the problem. Looks like it's time to get a little zinc fishy to attach to the prop shaft when the boat is going to be idle for a while....
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