Vibration at 1900 - 2100 rpms

[Capt. Rick]

Question for someone here who may have run into this before. My boat is relatively new, 600 hours - she is a RP Willis Beal 31 with a Volvo 435 D6. I am getting a slight vibration between 1600 to 1800 (7-8 Knots). Above and below that she is smooth. The shaft and prop are perfect, they were pulled and checked twice. Seems to be a motor mount issue. My builder installed the Volvo "recreational" style of mounts. They seem to be pretty "cushy". They are bolted to heavy steel angles, everything is tight. I do not like having this vibration, and would like to address the problem. Has any of you ever run into this same issue with the Volvo motor mounts?

 

[F/V First Team]

Your engine is out of alignment for the rest of your running gear at that rpm range, think of it this way, when you rev your car engine, does it not torque that part of the car to the direction of the engine rotation? Hard mount that baby or find similar soft mounts that are a higher density. Should solve your issue.

Your hull could also be resonating at that particular frequency during that rpm range, if your vessel doesn't have outriggers on the stringers you might consider them as well. Easy way to check is to have someone run your vessel at that range, and go into the engine room for a visual inspection, you can also put your hand on the stringers to get a feel as to what's going on.

 

[Capt. Rick]

Hey Travis, Thanks for that..... would you mind if i called you for clarification?
Boat Shop #?
Rick

 

[Blitzen]

I would check the alignment first but another thing to check is to see if the engine is vibrating in neutral at those rpms you are seeing the problem in gear. If it still vibrates in neutral than it is not an alignment issue but could be a mount problem. Also where are the mounts on the engine, are the rear two mounts on the gear or are the rear two on the engine?
Barry Controls makes many mounts for engine mfgs, and they would be a good source of information as to the proper mount you should have for your engine weight and application.

 

[tunaorlater]

Just a thought. Last year I installed a brand new shaft and coupling in my nb. I had a vibration from day one. Checked everything aligned three times and same result. In the long run it was the brand new shaft coupling that was out. I brought it to the local prop guy and had it faced, now the boat is running smoother then ever.

 

[Downeaster]

You've received some good advice here and I'd start with the easiest and cheapest of them.

If they don't solve the issue you may want to consider whether or not you had a flexible doughnut installed in your drive line - if so, you should also have soft (e.g., not rigid) engine mounts and vice versa. If you decide on new mounts, I can recommend the 700 series by Ace.

Also, you mentioned "pulled and checked". Have you checked alignment with a dial indicator and feeler blades with the boat in the water?

Another quick question/answer that might be it. What's the diameter of your wheel and what is the size of your propeller aperture?

 

[F/V First Team]

easy peasy

 

[traditions]

I would reface the coupling on the shaft

 

[F/V First Team]

If the engine is wobbling, checking your coupling a thousand times over wont solve your issue. Since your engine mounts are stiffer, try checking what they are on, either your engine beds or the stringers they are attached to. Any way to slip a large bar clamp down in there? You could try tying the two stringers together and see if that helps any. I'd still suggest outriggers.

 

[MDI45]

Rick,Did you have a issue with the coupling pulling away from the tranny not to long ago??

 

[Downeaster]

Can you post a photo of the propeller and the entire aperture? What is your shaft diameter?