Prop Shaft

[Tony Wilcox]

Kevin,As I left my boat all covered for the winter, and having a last look at winter precautions I noticed the prop shaft where it exits the hull. The shaft was right on one side of what I presume is the stern tube. I have not had any undue vibration etc. Is this likely to be normal or does it indicate the engine needs to be realigned ?Tony

 

[Kevin]

shaft location

Dear Tony,Thanks for the note. The designed location of the shaft, with reference to its position in the stern tube, is be centered in the stern tube. This position will help to reduce any damaging impacts of the shaft to the tube or any undo scoring of the shaft. In reality not many shafts are centered in the stern tube. It becomes a problem when the shaft is so far out of column that it makes contact with the stern tube wall. This contact can contribute to noise, vibration and possible shaft damage if left unattended. I would feel though the greatest concern would be to damage the stern tube in a way which could cause a possible hull leak. Our company was involved with a water leak on a boat which it was thought to be the PSS shaft seal. After further inspection it was found that the shaft was making contact to the stern tube wall when motoring and over time the shaft scored the glass enough that it caused it to leak. In order to center the shaft in the stern tube may require more then just a simple engine alignment. I would first check the existing alignment to see if it is out. If the alignment is off, hopefully, its off to the problem side. If the alignment is off, the shaft rubbing up against the tube can create support, which would keep vibrations at bay. If the current alignment is good and the shaft rubs, you may have to reposition the engine and strut because once you begin to move the engine, you begin to place the shaft out of column to the strut. Of coarse not being there to see it, advice from a trusted boat yard person is always a good idea. Hope this helps, Sincerely Kevin L. Woody

 

[Carl Dupre]

Another Possibility......

Tony: Something else to consider as a possibility (Kevin; please come back if what I say is total BS). If you were physically looking at the prop shaft exiting the stern tube (from outside the boat I presume), I must assume that the boat is out of the water and up on stands. If the boat is indeed out of the water, then the hull is likely not sitting the same way that it would in the water, and judgements on proper shaft alignment cannot be made. However, it does sound like something might be significantly out-of-whack, and the other possibility is that the boat is not quite supported in a well-balanced manner on the stands. If the boat is not quite properly supported it could easily distort the hull shape enough to displace the shaft/tube alignment. I would check more closely to see if the shaft is actually in contact with the stern tube and, if it is, try to determine if there is any real stress; can you rotate the shaft by hand? If prior to hauling the boat your engine ran well without noise or vibration, and now you have contact and can't rotate the shaft, then I would quickly have someone from the boat yard check the jack stand support. On the other hand, if there is not direct contact, or you can rotate the shaft easily by hand, then I would consider loosening or disconnecting the coupling for the winter to prevent cold temperatures from possibly making matters worse, and do a good and proper alignment in the water next spring.Good luck.Carl