Calling all Mech E's: Modal analysis on prop shaft

[Tom Boles]

It is Rocket science!Has anyone ever done or thought about doing some experimental modal analysis on a sailboat prop shaft? It seems to me that because of distributed mass, damped bearings, sprung bearings, Etc., the whole lash-up is a fairly complex system. Placement of the zink could effect 1st & 2nd torsional modes such that some folks have "vibration" at certain speeds, while others with identical boats do not. Any thoughts?

 

[Ray Bowles]

Tom, Your way over my head but I do wonder...

With the small shaft diameters you are speaking of and the rpm's they operate at, what would be the weights required of the zinks to cause these vibrations. Also, where on the shaft would be the worst place to place these zinks?RayS/V Speedy

 

[Tom Boles]

You have the same basic question I have....

Hi ray-We've got the same question. I am hoping to get some comments from somebody who really knows modal analysis. I've done a little of it, but really confined myself to the insturmention side of things.I *think* the best place for the zink would be close to the strut & cutless bearing on the engine side of the strut. Everyone's set-up is a little different, but the basic idea is to put the mass of the zink as close to a shaft support as possibe. The middle of the exposed shaft, between the engine and the strut would be the worst, in terms of accentuating the vibrational modes....I can't say much about size or weight. Football shaped seems to make sense, and the smallest size you can get away with would *seem* to be best.

 

[cole]

I put mine

I usually place my zinks very close to the strut for one good reason: It helps keep the shaft on incase of losing the whole thing.