shaft vibration

[Walter McNeil]

I have a strange problem that I have been chasing for several years. Here's the background. When I aquired the boat, it had a vibration at cruising speeds (~2500) that would shake apart the stuffing box. The vibration is not really apparent in the hull of the boat, but the shaft moved visibly. I initially suspected the usual offenders; engine allignment, prop bent or out of ballance or shaft bent. None of these were the case. Since then I have sent the prop (15"x1" 2 blade max prop) back to PYI for overhaul, had the shaft checked/coupling faced and replaced the stock Yanmar mounts with one grade stiffer. Still the vibration. When the vibration killed the V-drive I replaced the transmission and added a dripless shaft seal and a flexible coupling to compensate for any misalignment resulting from shaft loading (there is no static misallignment). With the new trans came a new longer shaft (almost 6'). I have been told that is borderline too long for a 1" shaft supported only by the coupling and the cutless.After all of this I discovered a twist. The vibration does not occur when the boat is tied to the pier, only when underway. This would suggest that the vibration is due to some interaction between the hull and the prop that is only present when the boat is underway (there is about more than 2" between the skeg and prop tips). Apparently this disturbance is hitting a resonant frequency in the shaft.I recently changed the shaft rotation from left to right handed to see if that would change the dynamic enough to aleviate the problem. There has been a reduction in the vibration, but it still shakes the stuffing box enough to allow some water to get past the lip seal.Now finally here's my question.Do you think the shaft is just too small and flexible, or should I be looking for another solution like a 3 blade max? Would stiffening the shaft possibly put a greater load on the prop strut?I could go to a 1 1/8" shaft without redesigning the boat, 1 1/4" would be tough. Could increasing to 1 1/8" add enough stiffness to make a difference?I do not want to buy a 3 blade prop.Thanks,Walt

 

[harvey]

You don't describe the underbody configuration-full keel, fin keel, aperture for the prop shaft. Sailing magazine Oct 1002 issue describes the same problem. Bottm line of the article was that i the prop is in an aperture, you must use a 3 bladed prop.Check the article for more details.

 

[Ray Bowles]

Walter, While I'm not an engineer I did pick-up

on some facts you presented that might be causing and compounding your problem. My mechanical background is from having built and drag raced hotrods for over 40 years and the associated balancing and flexing of systems present in these cars.  Yes I do think that the 1" shaft is adding to your problem.To tie up this site would be unfair to the other users. If interested and if you think I might be able to help with advise contact me by e-mail.Good luck, it sounds very annoying and like it has been somewhat expensive.Ray BowlesS/V Speedy

 

[Walter McNeil]

underbody

the underbody is modified fin w/ skeg hung rudder in a semi-aperature. There is a bustle that starts just forward of the prop strut then the hull rises just aft of the prop and turns into the rudder skeg above the prop.See the url for a bad picture, it's the only one I have that shows the prop in place.

 

[Harvey]

Pardon my spelling, that was Oct 2001 issue of Sailing magazine.

 

[Kevin L. Woody]

Maybe tip speed?

Hello Walt, Thanks for the question. You bring up a great many ideas and thoughts, which all seem to have some impact to the problem. I find it interesting that minimum vibration when tied to dock and reduced vibration when going in reverse. It would seem to me that both tied and reverse will create greater load then your normal forward situation. When you switched propeller rotation how did you do it at the transmission? Did you simply reverse the linkage? If so, most reduction gears are smaller in reverse then forward. This will cause greater loads to the motor producing similar conditions to being tied to the dock. If the propeller has excessive tip speed, when compared to the propellers current pitch, this can cause cavitation, which can cause severe unbalancing of the propeller and may produce the noise you are now hearing and seeing. This condition can be caused by to little pitch and in some cases to much pitch. Have you and PYI talked about whether the prop is properly pitched? Give us a call and we will start the process with you. 800-523-7558. My guess is that since we can reduce and in some cases eliminate, the options of increasing shaft diameter and things like that may simply mask the real problem. Side note: If the transmission is being run in reverse to produce forward movement, be very careful that the make of transmission you have is capable of being run in reverse for extended period of times. Most transmissions have a maximum time limit in reverse before you have to run it in the opposite direction. One main reason for this is that the lubrication pump may not function in reverse but only in forward. Please let me know if you have any other questions. Hope this helps, Kevin