Bad Prop Shaft

[Steve]

I recently purchased a 1992 H28. During sea trials, I noticed a vibration when running the engine in gear (forward and reverse). Since the purchase, I have had the propeller reconditioned and pulled the the prop shaft in preparation for replacing the cutlass bearing. The shaft is bronze and is showing a little wear in the stuffing box and cutlass bearing areas. I had to drop the rudder to get the shaft out and I don't want to put a "bad" shaft back in. The engine alignment seems to be OK - or at least isn't noticably out of alignmnet to my eye.My question is, how do you know when the wear is excessive and the shaft needs to be replaced?? Is it possible for a shaft to be perfectly straight but be out of balance and cause vibration??Thanks.

 

[Alex]

May want to check engine alignment again

If the shaft is out of alignment (not straight) it would cause the problem that you indicate. I am not certain however I think the only way to tell is to pull it and take it in to make sure that it is within tolerance.We had our 1992 35.5 out late last summer for similar reasons (knocking and vibration). After much review it turned out that the engine was in fact in need of re-alignment. You may want to take a closer look at that since if you were to just look at where our engine is mounted today (after the alignment) you would swear that it is absolutely not where it should be. The engine mounts are not centered on the mounting platform.The mis-alignment was the cause of the vibration and knocking and damaged the cutless bearing and shaft. We decided to replace the shaft with a Stainless shaft and have the bronze shaft straightened and stowed as a spare (see archives and other resources to determine if this is best for you and your situation).To help determine if the alignment is bad you should take a good look at the shaft and see if there is wear on it where it might be striking the shaft tube exiting the hull. You can also take a look at the shaft as it exits the hull (with the cutless bearing removed and see if it is laying aginst the shaft tube. Also make sure that the shaft support strut is also aligned as it may be bent and pressing the saft against the shaft tube and may have just been forced in place at the last service.Good luck

 

[Alex]

Photo

Here is a photo that might help

 

[Michael]

Shat

The shaft should be steel not brass. Brass is softer that steel and will wear much quicker. Since you have the shaft out lay it on a flat surface and look at it from the side. if You can see light under it, it is bent, probably due to the soft metal. Replace it with a new one and be sure the cutlass bearing is new and of the same size. The brass probably shows wear at the gland packing and that is not good. If you know how to align the shaft to transmission coupling go ahead and do it, however this is where problems develop if the alignment is not correct. While you are at it replace the packing gland with a PSS one. you won't regret it.The shaft could be out of balance but that is very rare. More likely the prop could be. In my experience the cutlass bearing is the culprit. Other than keeping the prop clean you might want to consider replacing it with one that folds. it'll get you a half a knot and will be clean and balanced.Fair Winds

 

[John]

motor mounts

Might look at the motor mounts as-well, if you are going to redue everthing else.