Need help diagnosing strange noise - '99 21v

[mdk681]

Good, you have a modern "wet" shaft seal. The fact that's it's the wet style reduces the likelihood of what I suggested, but it's still worth checking.

Yes, you can get the parts and do not have to replace the whole thing. But the even better news is that you can often reverse the stainless ring to use its "fresh" face, essentially doubling its useful life. When mine got damaged that's what the dealer mechanic did, and it's worked great ever since. Obviously this won't help if the carbon ring is scored, but it's worth a look.

As for confirming this: I was probing around in the engine compartment with a mechanic's stethoscope and happened to touch the rubber bellows on the shaft seal - and the sound vanished! The compliance of the bellows allowed the ring(s) to oscillate just enough to make noise. Touching the bellows changed that relationship enough to silence it. I would try that... while the boat is in gear and making the sound, gently probe the bellows and see if the sound changes. Costs nothing to try, and might reveal the answer.

This sounds like it may be the ticket - it is very difficult to tell where the noise is coming from but this certainly seems like the best possibility. So I guess my next question would be, how did this thing wear out so soon if it is indeed the problem area? I figured that the new dripless shaft seal would be good for years to come...

[IDBoating]

They should last a very long time. But sometimes a foreign object gets between the machined faces and scores them. Or if they run dry, they can overheat and score or warp. Under normal circumstances they will last hundreds of hours.

[mdk681]

They should last a very long time. But sometimes a foreign object gets between the machined faces and scores them. Or if they run dry, they can overheat and score or warp. Under normal circumstances they will last hundreds of hours.

Yeah I suppose you are right. Something must have gone wrong to cause it. I may attempt to dig into it this weekend. Is there anything that I can take apart to visually inspect the components? Anything to look for? I'm gonna have someone else drive and open up the compartment to try and probe around a bit and see if the noise changes or goes away.

[Thegerman618]

Those shaft seal require the prop shaft to slid out for changing, since they don't usually require to be changed for several hundred hours it's not that big of a deal and usually a prop shaft alignment is needed every couple hundred hours as well anyway. Hopefully they don't have to remove the rudder to slide the shaft out far enough to do the change of that is the problem. Also may want to check the water pump bearings, they can get a whine when the bearings are going and only at certain RPMs. Did a rebuild of water pump bearings its not fun, but cheaper than a PSS and shaft alignment. Just another place to check.