Reducing Engine Vibration

[Blain Albin]

I have a steel pontoon boat with a Volvo-Penta AQAD 31 with 290 sterndrive mounted inside the centre pontoon. I am building new pontoons and will be reconstructing the engine well.

I'm pondering on the motor mount setup in order to reduce engine vibration being tranmitted through the rest of the boat. Currently the transom is just the 14 ga. steel built up with plywood to give it thickness. The forward engine mounts are bolted to two u-channels that extend from the transom to a fairly light cross-member that's welded to the sides of the pontoon just forward of the engine.

My theory is the heavier the structural members are that the engine mounts to the better. Is this theory correct or are there other things I should consider? I was considering using 1/4" plate for the transom and beefing up the area that the u channels are welded to so that things are "stiffer".

Any thoughts?
Blain Albin

[marshmat]

Stiffening a structure doesn't always reduce the amplitude of vibrations, but it will usually increase the frequency at which the structure will tend to resonate. If the natural frequency of the structure near the motor is a lot higher than the vibration frequency of the motor, vibrations will be a lot easier to damp out.
I tend to be strongly in favour of good beefy structures when it comes to this sort of thing, not only to reduce vibration but also to improve safety factors in the event that something goes wrong.

[alaskatrawler]

R&D Makes soft mounts just to solve this problem. Go to www.pyiinc.com to get specific sizes. Cost is very reasonable. I use them and they work great.

Dan Walsh

[stonebreaker]

+1 on the softer motor mounts.

[marshmat]

+2 on the good mounts.... very rare to see an installation without good mounts that lasts very long. It's sort of a given, that vibration damping mounts will be used. You still need a structure under them that is good and rigid though.