Vibration, suspect engine mounts

[Joef]

Men:

My boat has developed an odd vibration - and i suspect motor mounts. the boat has 3208Ts - boat and motors are 1991 vintage.

Really noticed the issue last trip out, running in a beam sea. Waves were coming from STBD. As each wave approached, the boat would list to port as the wave approached the boat - at the peak of the list (the highest angle of list) the boat had NO vibration, very smooth. As the wave would pass under the boat, and the boat would then begin to list to STBD, i would feel a noticeable vibration. The vibration seems like its coming from the STBD side. At the bottom of the trough of the wave, things would feel OK again...then the process would repeat with the next wave.

All running gear is new (props shafts, stuffing boxes) 2 years ago. While running in flat water, at any speed, i don't get this vibration -

2 years ago, i had the PORT engine out of the boat to repair the tranny - and while it was out i had the guys at CAT install new engine mounts...but the STBD engine has original mounts. I took a close look at each mount and they seem OK - but not sure if i'd be able to "see" an issue just by looking.

I can't imagine any other cause for this...i'm guessing a weak or damaged mount could allow the engine to get out of alignment when listing to stbd?

Boat is coming out of the water in 2 weeks or so - will address this - but is there a way to "test" engine mounts visually or some other way to confirm what is going on?

Thanks
Joe

[Brewster Minton]

I would take a 2x4 and see if I could make it move at all to start with. Then I would run it in some waves and open the hatch and watch the engine and the tranny and the shaft to see if you can see anything going on. Just my two cents.

[Bruce]

Anyway it could have been prop cavitation?

I've seen it before in a beam sea.

Easiest way to check mounts is take the boat out in calm water, open the box and have someone put the boat in fwd and then rev the engine to see if there is any movement. Leave port out of gear.

Stop the boat and do the same for reverse. This is how I check for mounts in a sea trial.

Also watch the shaft and gear for any large or jumping movements.

[JP Dalik]

Look with a tool.

Make sure mounts are tight into the stringers, mounts are tight into the block and mounts are tight on the studs. Do this with a wrench or socket, eyeballs suck for checking torque.

Your positive it was not cavitation caused by air getting under the boat due to sea conditions. We'll shudder and shimmy along in a big quatrtering beam sea and then occasionally "run over a bear" (the growly noise of the wheels coming out of the water)

A bad mount should show up somewhere in your rpm curve, same with pushed alignment. Open a hatch and take a look at the coupling spinning. Were the couplings faces machined to match when the running gear was redone.

Lots of questions. Vibrations are funny things.

Good luck.

[Joef]

pretty certain it was not a capitation issue...i'm familiar with that sound (running over a bear...exactly!) When i replaced the shafts, couplers, and props, i had the new couplers machined/faced to ensure a flat surface that was 100% perpendicular to the shaft...then carefully installed it so i didn't get warped by tightening the pinch-bolts incorrectly.

I'll put myself down in there and watch as someone throttles up and down, forward and reverse...if a mount is bad, should be able to see something going on.

Will breakout my MOAR...mother of all wrenches and check tightness on everything.

Thanks
Joe