H31 Prop shaft knocking!

[Johnnie Poole]

Has anyone figured out a way to get rid of the RPM-specific knocking of the propshaft against the shaft tube. It seems to be a common problem with the 31 and 34 of this vintage. My engine mounts are in good shape and the shaft is aligned. The cutlass bearing has been changed without affecting the problem.As you bring the RPM up under load the shaft starts to hit the hull tube. If you rapidly increase the revs through the 'knock range' the shaft settles down and you can drop back to that RPM without the knock. My favorite mechanic has recommended that we come up with a 'top engine mount' to restrain the 2-3 inches of movement when it's knocking.I've been told that part of the problem is that the 1-inch shaft exits the hull through a 1 1/2 inch tube.Incidentally, I just put on a PSS dripless shaft seal and have had no problems with leakage, despite the knocking.Anybody found the secret?Johnnie Poole S/V 'Jazz'

 

[Bob Blackmann]

Knocking

Johnnie,We had the same problem with our Legend 37. I ended up 1. installing a flexible shaft coupling 2. Replacing the shaft with one of stainless steel, which is much stiffer 3. Replacing the 2 blade prop with a 3 blade. This really smoothed things out and eliminated all of the vibration through all rpm's.Hope this helps,Bob

 

[John B]

Shaft Knocking

I ended up with this problem in my '85 H31 when I had the stuffing box & 2 motor mounts replaced. Long after the yard that did this work called the shaft thumping "a Hunter Problem", I learned that the engine/shaft alignment is critical in this vintage of Hunters because the clearance between the shaft and the tube is so very close. Last January, I found a Yanmar/Hunter Dealer yard that knew of this critical adjustment & they took time to align it right. I am happy to report that I no longer have this annoying problem.