Vibration in drive train at speed

[TheJudge]

In a recent survey we encountered a vibation from 2000 rpm to 2600 rpm which is the max on this Cummins 450 hp engine. The yard says it could be many things from the prop to the shaft to the engine alignment to the V-drive. It could be minor or major. The vibration is slight but definately discernable like a tire out of balance. Has anyone had any experience with this on a 36?

[jcollins]

Judge,
Your yard is correct. The most likely culprint is a prop that needs tuning. I had the same issue after I ran aground with my 28. I had the prop pulled and tuned at the end of the season. The yard also checked shaft alignment. After the tuning, no vibration. Hopefully that's your issue.
Your in a good position to negociate those repairs, whatever they end up being, with the seller.

[joreyn]

I agree with John. It's most likely the prop. I hit a submerged log summer before last, and was having a slight vibration which I narrowed down to the port drive train. Sure enough, when I had it hauled last winter, the port prop had a slight cupping on one of the blades. You could barely see it. We found it with the aid of some strong work lights. The prop shop tuned both props, and last summer there was zero vibration.

Another likely cuprit is motor mounts which can cause the engine to be slightly misaligned. But I feel that the boats is too new to have motor mount problems.

One other possibility occurs when the boat has been stored on land for a length of time. If the blocks and stands are not set up just right, the boat can actually distort its shape. Remember that even steel boats are engineered to be evenly supported in the water over the entire hull surface. If they are not carefully supported while out of the water, the hull can develop subtle distortions, which means that you could have alignment issues. How to fix it? Let it sit in the water for a week. The boat will gradually lose the distortion in the hull, and the vibration will be gone. This happened to me on my previous boat, a Grand Banks 32. It was really disturbing. But one week later, the vibration was gone.

One thing is for sure. The 36ET is a really smooth-running boat. Any vibrations should be tracked down and repaired. You should be able toi ask the seller to accomplish that.

[TheJudge]

The owner is having the problem evaluated and hopefully repaired. Then I will have my surveyor take her out for a run to verify all is well and we will be back at the table. This boat has been in the water so it should not be hull distortion. My guess is engine alignment. Thanks for all the comments.

[Mariner]

It's pretty unlikely that it's the prop. The exposed shaft on the 36ET is so short, it's nearly impossible to bend it. It's much more likely that the shaft just needs to be aligned.

When we first took delivery of our boat, we were surprised when it had somehow developed a wicked vibration. We finally discovered that, when the yard had painted and put new zincs on it before launching, they had installed a somewhat oversized collar zinc on the shaft (at my request) that was causing the vibration. After a few weeks of use, it vibrated loose and came off, and the vibration was gone. On the next haul-out, we found a more suitable zinc for this application that did not produce this vibration.

[Balboita]

I went thru all the above and now have a very smooth boat. The problem is, I didn't check engine alignment until last. Surprisingly it was out of alignment (why? Doesn’t matter). I would start there. If it is “out” the cutlass probably needs replacing because of that. (Cummins 1400 hours). Then on to checking the other downhill stuff.. shaft- prop etc.
Happy New Year All.

[RobS]

Check the motor mounts. They can be loose and/or vibration can loosen them.