Prop shaft "twanging" sound

[Set2sea]

Toward the end of my New England season this year, I was getting a pretty bad twanging sound from the prop shaft. In August it started as intermittent, by September it had got much worse. When I pulled the boat in late September I did have about 3 inches of growth on the shaft which could obviously be adding to the issue.
After I cleaned the shaft, I started looking for play. It is tight at the cutlass bearing/strut but the play where it goes into the hull is greater than another C34 and C30 in my yard. I seem to have a good tight bolt up at the transmission flange. So my question is: Does the stuffing box/shaft log give the shaft bearing support?

[Jim Fitch]

Hi Paul -

No, it should not.  When you do your engine alignment, which is done every year (a week or two after you launch so the hull has time to settle back to shape), you separate that flange near the transmission.  The stuffing box is then "supporting" that end of the shaft but there shouldn't be any load on it.  Then you align your transmission half of the flange with the shaft half of the flange so that the load is carried by the transmission and the cutlass bearing and not the stuffing box.

This is where I have to admit that I don't really do the alignment every year but we are supposed to.

I'm guessing the imbalance of that growth probably caused your "twang".

Jim

[scgunner]

Paul,

Three inches of growth! Seriously? With a bottom fouled that badly not only could that throw your prop/shaft out of balance but also you're not getting full RPMs and your cruising speed (sail or motor) could be down by 2 or 3 knots. You should think about finding a good bottom service when you're in the water.

Unfortunately since you're on the hard there's no way to check to see if this the cause of the problem at least not until you're back in the water. Although we can at least draw a conclusion. The problem started intermittently in August and got progressively worse into September until you pulled the boat. Makes sense it could be a growth problem, you drop a clean bottom into the water in the Spring but the days are still short and the water is still cold so growth is retarded. As Summer approaches days get longer the water gets warmer and growth accelerates. The growth rate aligns with the appearance and worsening of you problem. Other than check everything (which it sounds like you've already done) and of course clean the bottom I don't think I'd do anything until it's back in the water.

[Jim Hardesty]

I started looking for play. It is tight at the cutlass bearing/strut but the play where it goes into the hull is greater than another C34 and C30 in my yard. I seem to have a good tight bolt up at the transmission flange. So my question is: Does the stuffing box/shaft log give the shaft bearing support?

Hi Paul,
The stuffing box/shaft log only gives what support there is with the shaft log hose, not much support there. You said the bolts at the transmission flange were tight.  Did you check the shaft to coupling?  Did you try, or have someone rotate the prop and shaft while you were inside and able to see or hear the twang?
That's just my thoughts.  Have no experience with any sound like that.
Jim