Shaft Vibration in Forward but not reverse - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 09-30-2003
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Shaft Vibration in Forward but not reverse

I''m wondering if anyone out there has any ideas on this. I have 30'' C&C that I purchased this summer. Under power I get vibration beginning around 1400 rpm and it gets worse with increased RPM''s. Sounds simple enough. However, in reverse, under full power, there is no vibration. The boat has a direct drive Atomic 4. I do not believe there is a reduction gear for reverse; so the prop should spin at the same rate for a given RPM as it does in forward -correct?

I had the boat surveyed and the shaft alignment checked. All seemed fine. The only issue the shaft log seemed a bit tired -understandable on a boat that is 31 years old. But nothing leaked and the stuffing box had just been repacked.

What I don''t understand is why in forward I get vibration, but in reverse I don''t. My thought is it has something to do with the 2 blade propeller.

Any thoughts are greatly appreciated.

Bill Strebel
C&C 30 #50
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Old 09-30-2003
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Shaft Vibration in Forward but not reverse

It could be a bent or out of balance prop or it could also be a worn out cutlass bearing.
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Shaft Vibration in Forward but not reverse

It could be a bent or out of balance prop or it could also be a worn out cutlass bearing.
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Old 09-30-2003
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Shaft Vibration in Forward but not reverse

One reason you may not experience vibration in reverse is I believe reverse doesn''t build the load on the engine that forward does, due to the prop blades slipping when turning backwards.
Two other things to check would be the motor mounts and the cutlass bearing.
Good luck - that''s a great boat (A former C& C 30 owner).
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Old 09-30-2003
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Shaft Vibration in Forward but not reverse

A4''s do have a reverse gear reduction.

That in conjunction with the previously mentioned inefficiency of the prop in reverse are likely responsible for smoothing the vibration, especially if it is the result of inadequate tip clearance. Less clearance than 25 percent of the blade''s diameter and you''re likely to get prop vibration.

I''ve got about an inch and a half clearance on my 2-blade 12x8 and I get the same vibration you mention.

Best,
-Chad
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Old 10-24-2003
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Shaft Vibration in Forward but not reverse

I think what you will find is the cutlass bearing is worn out. The boat in foward drives the shaft hard aginst one surface of the cutlass and it wears it down enough that it begins to get out of ballance or alignment and causes the viberation. The oposite is true in reverse. The cutlass is not worn on the other side and the shaft is pressed into a still tight bearing surface and no viberation.
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Old 10-28-2003
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Shaft Vibration in Forward but not reverse

Thanks for all your replies. I just had the boat hauled for the winter and sure enough the cutlass bearing is quite worn (despite what the surveryor said) to the point that the shaft will make a "clunk" sound when moved. Another interesting point is that the shaft is not centered in the shaft log, it is displaced to port. There is about a 1/4" of space on the starboard side. I assume this amount space is by design, especially since when I rotated the shaft by hand there is a very subtle "wobble" such that the shaft will brush the top of the shaft log causing the top part of the shaft to be clean of bottom paint. I may have a combination of problems here though. One tip of the prop appears to have some imperfections, the cutlass bearing is clearly worn and either the shaft is bent or the engine is out of alignment - or both. Plus there is a serious issue with the starbord rear engine mount. This mount is under the water pump which was allowed to drip causing severe corrosion. I suspect this mount may be the culprit. I haven''t seperated the shaft cupling from the output flange - it''s badly rusted and currently enjoying a soak in penetrating fluid. So I haven''t been able check deflection at that point. Although I''m not sure how reliable that will be considering the boat is not in the water.

Has anyone out there replaced mounts for an A-4 on a C&C 30? The other three are in remarkable condition, it is just this one that is so badly rusted, I''m not sure I will be able to get the adjusting stud out to replace it.

Any suggestions are most welcome!

Thanks,
Bill
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