TOPIC: There's a whole lotta shakin' goin' on

I have a 1958 Larson ThunderHawk Jr. I installed a new Yamaha 8HP motor. (Lake restriction...) The motor shakes/vibrates the boat aweful at any powere setting above idle. My thought is to install a rubber transom "sheath" upon which I would re-install the motor.
Is there a prefered gauge/thickness/material for such an application?

Is the Yamaha 8hp a single cylinder or a twin?

The Yamaha is a 2020, 4 stroke, one-cylinder, 8 hp. Our lake requires 4 stroke or equivalent electric and 8hp max.

Could it be prop cavitation?

No. The hull of the larson is quite thin. It's a mechanical energy/resonance thing. I can't imagine this is a unique issue. Hoping for a McGuiver type solution to dampen the motor's vibration thruout the hull.

Maybe just a few pieces of surgical rubber tubing. that stuff is pretty spongy and you could just cut short strips to put between the clamps and transom on both sides.
www.amazon.com/surgical-rubber-tubing/s?k=surgical+rubber+tubing

Do you have this motor bolted through the transom or just clamped on? If only clamped, I would worry about you losing your motor to the lake bottom if you tried to use any rubber mount between the motor and the mounting surface. Even if you do have through bolts, your motor mount will move and the bolts will move with it causing the holes to ware with it along with any other surfaces being touched by the mount. I would wonder if you have a running issue with the motor, or issue in the lower unit or prop. Does it shake bad when in neutral at idle or high idle or only in gear? if only in gear, does it do it in forward and reverse? I found this online talking about Yahama outboard common issues. If there is no obvious damage to the prop, I suggest running it on muffs, in gear and see if you can see any movement like a bent prop shaft or a blade out of place. You should see it easier when in motion.

One more idea here. Seems that is WAY too small a motor for that boat, I understand if you have restrictions on your lake, but what is the pitch on that prop? I am thinking that maybe since that motor is designed for a much smaller lighter boat, it may have too large a pitch on it. Wouldnt this cause this little motor to lug as it is trying to move this ol boat? Can you get your outboard to optimal RPM at WOT?

Hate to say it but a single cylinder 8hp engine shaking is probably the nature of the beast. Not many single cylinder engines are smooth runners, vibration wise.

Pappy wrote: Hate to say it but a single cylinder 8hp engine shaking is probably the nature of the beast. Not many single cylinder engines are smooth runners, vibration wise.

Agreed Pappy, But it sounds like it more than a vibration. It really shouldn't shake. Maybe he can post a video of his problem in action.

(Apparently)There are SOLAS Amita propellers for that engine in both 5" and 7" pitch and are 4-blade propellers. I would go with the 5" 4-blade. That should change the vibration signature substantially. Rubber mat on the transom will also help. Check and make sure the prop is correct for your style propshaft and lower unit. Not exactly an engine I am intimately familiar with.

It's a brand-new motor, so that is not likely.

The vibration is nil in neutral, idle and higher. In forward, the vibration in nil at low settings (idle to 1/3 throttle) but progressively worse at higher settings (above 1/3 throttle.)
On the question of "too small," the boat is 14' and weighs less than 400 lbs. This same motor on 50 other boats on the lake is perfectly adequate. My focus is on the transmission of the vibration, endemic in one-cylinder motors, to the transom - then throughout the fairly thin hull.
I have the boat out of the (frozen) lake. Testing will happen in the spring. I am aware of the potential "loss" of the motor if I use too soft a rubber "transom sleeve." I very much appreciate the input and discussion from all.

Has the transom been redone, or are you confident it is sound? If it's rotten, I would suspect that would transfer vibration through the fiberglass excessively.
Cheers
Bill

I will say again to first try the propellers I mentioned above. By getting the engine up to its max allowable RPM for your particular application you will lessen the load on the engine to turn that propeller. This should noticeably lessen the felt vibration. A 4-blade propeller also smooths out drivetrain vibrations vs a 3-blade or 2-blade.