Prop Shaft Vibration

Apr 24, 2020
40
Hunter Cheribini 37 Portland
I just ran my engine on the hard and noticed that the prop shaft was vibrating about an eighth inch behind the stuffing box outside of the boat at around 600 rpm. The shaft was not spinning just vibrating. I had put a slightly heavier folding prop on. The shaft is 1 in and seems undersized to me gut feel. I can pry the shaft left to right about a 3/16 of an inch just behind the stuffing box from the outside. at higher speeds it calms down. this doesn't seem good to me. Thoughts?
 
Jun 11, 2004
1,221
Oday 31 Redondo Beach
By outside of the boat I might think you mean the cutlass bearing and not the stuffing box?
If so, Nigel Calder's book "Boatowners Electrical and Mechanical Manual" says that anything more than 1/16" play per 1" shaft diameter should be considered for replacement. And "excessive vibration under low RPMS may be an indicator that it's time uless it is an extremely cold day when you start up".
 
Apr 24, 2020
40
Hunter Cheribini 37 Portland
By outside of the boat I might think you mean the cutlass bearing and not the stuffing box?
If so, Nigel Calder's book "Boatowners Electrical and Mechanical Manual" says that anything more than 1/16" play per 1" shaft diameter should be considered for replacement. And "excessive vibration under low RPMS may be an indicator that it's time uless it is an extremely cold day when you start up".
no. I have a new Cutlass bearing. The forward end of the shaft can be moved quite a bit with a pry bar. it seems the shaft was vibrating at a resonant frequency.
 

dmax

.
Jul 29, 2018
508
O'Day 35 Buzzards Bay
no. I have a new Cutlass bearing. The forward end of the shaft can be moved quite a bit with a pry bar. it seems the shaft was vibrating at a resonant frequency.
I'm assuming your engine/transmission are moving along with the shaft when you use the pry bar - if that's the case, it's the give in your motor mounts. Some mounts are stiffer than others - I have Vetus mounts which are pretty soft, I've never tried the pry bar thing but I bet I can get 3/16" if I tried. At 600 RPM a diesel is going to be shaking a lot - not sure what engine you have but most diesels should idle in the 800 - 1100 range with the engine typically smoothing out as you move into the higher end of the range. Bring your idle up and see how it behaves. Take a good look at your motor mounts and make sure they are sound.
 
Feb 26, 2004
21,960
Catalina 34 224 Maple Bay, BC, Canada
no. I have a new Cutlass bearing.
no. I HAD a new Cutlass bearing. Maybe. Running the engine in gear on the hard is not good, because the CB requires water for cooling. It's kinda hard to tell if you did from your OP.

A 1" shaft is not unreasonable nor generally undersized for a boat like yours. Mine's 1" and I'll bet we have similar displacements. Perhaps others with the same or slightly larger boats could chime in with the size of theirs, just for a reality check.

Might be helpful if you told us what engine you have, too.

Good luck.