Vibration in propeller shaft

[rockycoastcruises]

I have a 1987 Albin 27 Sport with the 6BD1 Isuzu engine and 630A Hurth transmission. The other day as I was getting the boat ready for the winter up on stands in the boat yard, I was idling my engine way back around 600 rpm+/- and when I noticed my propeller shaft I could see that it had developed an harmonic vibration and was wobbling an inch or so back and forth in the middle. Once I brought the idle up to about 700 rpm this strange vibration stopped. Has anybody else experienced this? Could it be because it is up on stands? Is there a cause for this that I should be concerned about? This boat is new to me and I have not had a chance to observe this in the water.

[honza]

Not sure about the wobble, but it might not be the best idea to spin the shaft whet the cutless bearing is dry.

[TorreyWP]

Engine and shaft alignment are directly affected when the boat is on land and supported by stands.

I would not worry about this too much for now, re-evaluate when you launch the boat in the spring.

Something you could check now with the boat out of the water is your cutlass bearing. Put some blocks under the end of the prop shaft and gently pry up. if there is more than a couple thousands of an inch of movement of the shaft in that bearing, its time to replace it. Generally, expect to replace that cutlass bearing between 5-10 years. Going much longer than that can make the job more difficult.

[rockycoastcruises]

Thanks for your responses. The wobble happened when the engine was in neutral. I did not have it in gear and was advised to wait until it hits the water for this. The boat was surveyed and the cutlass bearing was said to be ok but I will go back and try this test. If all is ok I'll wait until it's in the water next season to see if this still happens. I did not notice this when I did the sea trial so fingers crossed.

[Great Lakes CK]

Natural Frequency of anything is based upon mass and stiffness. Changing the stiffness of a shaft can only really be done by making it a bigger/smaller diameter. (Read: this is the hard option), however you should be able to change the natural frequency by adding mass. Adding mass will lower the natural frequency/ move the shake to a lower RPM. And that's the goal, to move the primary resonance to a lower RPM outside of the normal operating range of the engine. You might want think about installing a zinc split prop shaft anode right where you saw the highest amplitude/motion of the prop shaft.

[honza]

Other remedies to try could be new/stiffer mounting rubber blocks supporting the engine and a semi-flexible, or just different style, shaft coupling. Should it be a shaft mass problem, more boats of the same model would probably experience it. But the shaft may not do it at all, or much less, when it is spinning and under load.

[dkirsop]

I wouldn't try anything until you have it back in the water except to check for play in the cutlass bearing. If there is play in the bearing replace it. The wobble you witnessed could be a direct result of friction from turning the prop on a dry bearing. If the bearing is good then wait for a sea trial. The results could be quite different with the prop under load and the bearing lubricated with water.