REALLY LOUD buzzing vibration

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Jun 5, 2023
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Finger Lakes, NY
I just bought a really nice 1984 Grumman 14 ft. utility v-bottom that I put a new Tohatsu 9.8 on. It's a really sweet setup. Very simple, no carpet, just the aluminum bench seats.

BUT- at certain RPM ranges the hull vibrates like a buzz saw, incredibly loud. I'm pretty good at finding noises, and it does it in two different places that I have found so far at different RPM ranges.

The loudest is where the gunwale cover meets the side sheet metal in one area a few inches long. If I clamp that spot with my hand it helps a lot. This is at about 3800-4200 RPM. The other place is on the rear seat around 2500 RPM. It's not as loud as the gunwale noise but still really annoying. If I push down on the middle of the top sheet metal it stops it.

It is unbearably loud, to the point it sounds like the boat is rattling apart. It happens in a very useful RPM range, so it's not that easy to just adjust out of it.

Are there any cures for this? I have found very little on line for this specific issue. Most recommendations are to add carpeting or other sound-deadening sheets, which I don't think would solve this issue. This is not a boat that is generally noisy like an aluminum boat tends to be. This is very specific spots with very loud resonant vibrations at certain RPM ranges.

Thanks for any help.

Have had similiar issues, mine were caused by air bubbles trapped and vibrating under the hull, not sure that is the correct explanation. By adding some weight and changing the balance would change the amount and location of the buzz. We called it harmonic vibration. Cant say for sure if that is your issue, but it sounds familiar.
This is the hull itself vibrating in a couple of particular places.
I have no idea if this is the "right" thing to do or not, but my first action would be to take some silicone caulking and put it in that crack to see if it helps. Mind you, you're not sealing against water intrusion, you're just "padding" to reduce/eliminate the vibration. If you like, you can use a marine sealant to do the same thing.

As for the seat, are you saying that it's a piece of sheet aluminum with no structure underneath that just vibrates freely? If that's the case, then I would test it by putting a piece of soft material like carpet or even a computer mouse pad held down by something. Then try to recreate the vibration. If it goes away, then some material glued to that spot might be in order. If you don't want that on top, then perhaps you could add some structure underneath like a cross member that you attach to the underside; maybe a piece of aluminum angle epoxied to the underside to provide stiffness? Hope that makes sense.
Here is a video of the gunwale vibration. That sound isn't the motor, it's the buzzing
gunwale/hull side. At the end of the clip I change the RPMs and the vibration basically goes away.
View attachment
Try using some cedar shims. Stick them in between where you think it's loose and see if that stops it. If so, use a little JB Weld or silicone or whatever to make a permanent fix.
I wonder if it's caused by the way the OB is mounted.
Thanks for the suggestion, Stand Up. I was thinking shims, too. The JB Weld is a good idea. I hadn't thought of that. I have not used JB Weld, but my friend swears by it. He has actually used it to repair engine crankcases on lawn equipment and such.

I am also wondering if the transom is suspect. I think the plywood has been replaced in the past, and I'm not sure it was done right. I'm thinking maybe it could be the source for the vibration resonating into the hull. I will try to find out how the transom should be put together.

The motor itself is brand new, and appears to have some serious vibration damping.
Is your motor clamped onto wood on both sides of the transom?? Clamping directly to metal can cause vibration issues ! Also they make a hard rubber piece that gooes over the transom to clamp the motor onto...these have been known to help vibration issues. In the area where the loud vibration is....any loose rivets or does that area need more rivets closer together ??
Are the gunnels riveted on? I had an aluminum floor riveted in place on an alumacraft 1648 and when it got older I always knew I had another loose rivet in the floor when it made a similar sound to your video. I drilled out the loose ones and put in a slightly bigger one in its place.
I haven't found any obviously loose rivets yet. I will go over it carefully to see if I can find any. Is there a way to snug up existing rivets without replacing them?

I think the transom may be a big part of the problem. It looks like the previous owner slapped some plywood on there, and it is getting cruddy looking. There is a gap between the 2 sheets of plywood, and there is no top rail covering it. I'm trying to find pictures of an original one for reference.

There is vibration dampening material for transoms that supposedly work pretty well. I may try that.
Harmonic vibrations can be tricky. If everything is tight and your prop is in good condition (balanced) you might try dampening the vibration. Maybe try Hydro Turf or something similar on the bench seats or gunnel area to see if it helps.
WATCH OUT for silicone use on tin boats! In some tubes, the curing agent used ATTACKS tin.

I too would opt for a shim or other reinforcement to ‘prevent’ the piece or panel from
Loose rivets can be tightened up, it is called " rebucking" the rivet, works best if the tool on the head side has a radius to match the head of the rivet. Smack it good with a stout hammer. Many U tube vids on it....
I took it out again yesterday with someone else driving so I could move around and find spots vibrating.

The top gunwale rail where it meets the hull side is a somewhat loose fit along much of it. I had mixed results putting shims in the gap, and also tried clamping it in a couple of places. It was a bit like playing Whack-a Mole. I couldn't find or eliminate all the spots.

This has me thinking the vibration is intensifying somewhere, and spreading throughout the boat. In other words, the whole boat has a vibration problem, not just a couple of loose spots.

I think the transom is at fault. The previous owner apparently replaced it, with 2 thicknesses of regular 3/4" plywood. There is a gap between the two pieces where it looks a bit swollen and warped. There is some sort of rubber piece on the outside between the aluminum part of the transom and the engine. There is no top rail covering the top edge of the plywood.

I am trying to find pictures of this boat detailing how the transom pieces are assembled, but haven't had any luck so far.
Agree with transom being suspect...why not just replace the gaps this time...make it solid with various thicknesses built up, screwed and glued and sealed !! Where you have sloppy fits along the gunwale, just add some more rivets...or even some small SS screws and nuts. I have used them many times over the years where rivets were not availiable. Take a magnet and be sure you get real SS fasteners !! If the gaps are wide enough, squeeze some sealant up there to help take up space and dampen the vibration. Do not use silicone! I never use silicone on a boat ! Once vibration starts it can travel, called harmonics and can be a pain, but usually fixable !!

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