outboard isolation?

[brettmc]

i just bought a new 2011 Mercury 15 hp 4-stroke for my little 14 ft aluminum semi-v fishing boat. I had it out for the first time last weekend and the only thing I don't like about this motor is the vibration. At a few points on the rpm scale, the motor vibrates the boat and whatever is touching the hull (just about everything) makes an ungodly noise.

I was wondering if I could safely isolate the motor from the boat by placing thin rubber wafers between it and the transom? Maybe some cut up and flattened out auto heater hose placed over the top of the transom, a couple of pucks of the hose under the transom tightening screws? What do you think?

Or should I ignore the vibration for now and in time I won't notice it anymore?

 

[Philster]

Re: outboard isolation?

Did a new prop go on the motor, too?

 

[lncoop]

Re: outboard isolation?

Small outboards on small boats can vibrate quite a bit, but before I did anything I think I'd check with the dealer to be sure it's not vibrating more than it should. All sorts of things can get whomperjawed (technical term) on these little guys during shipment. Once you've eliminated that possibility I'd say either or both of your prescribed remedies would be fine.

 

[Home Cookin']

Re: outboard isolation?

typical problem with small tin boats.

check prop if it's really bad (borrow a substitute; get a spare)

hang a rubber mat on the transom.

Is there a piece of wood on the outside of the transom where the motor rests? that helps.
of course you always tie a motor to the boat

 

[brettmc]

Re: outboard isolation?

yes, it came with a new prop.

no wood on the outside of the transom, only on the inside.

Thanks for the replies, i'll put a few pieces of hose on transom.

Also, I just remembered I bought a used pickup bed mat last fall to cut up and line the bottom of the boat. That should help out a lot.

 

[JimS123]

Re: outboard isolation?

Old 2-strokes tend to vibrate. A new 4-stroke should be smoothe as silk.

You got a problem! Either the prop is bent, the motor isn't mounted correctly or the transom is toast.

Bolt the motor on - tightly. Forget the rubber hose - that might just make it fall off!

If that doesn't help, have a pro look at the transom for signs of rot, and / or take the prop to a prop shop and have it checked out.

 

[Mi duckdown]

Re: outboard isolation?

+ Jim

 

[brettmc]

Re: outboard isolation?

I have never mounted a motor on a boat before. I simply put the motor in the center of the transom and tightened her down. Could something be wrong there?

 

[Ned L]

Re: outboard isolation?

As Jim said, anything soft enough to absorb vibration will also allow the outboard to flip right off the transom the first time you make a hard turn.

 

[Philster]

Re: outboard isolation?

Four-stroke, small outboards vibrate sometimes. It's just four pistons and they are naturally prone to vibration because they don't naturally counterbalance eachother, but this is often offset by proper counterbalancing more than the mere presence of valve-train sitting above.

You could get all squared away and get harmonic vibration due to how to vibrations work off various surfaces. Could just be a musical combination of boat and engine.

All you can do is ensure she is mounted according to all specs and ensure everything top to bottom is in fine working order.

Then you consider dampening techniques (others make suggestions; I'm not qualified to comment on some). I've got a little four stroke on an alum boat, and it's very utilitarian by nature, so I don't even notice the vibration until someone else barks about it. Comes and goes at different RPM.

I guess it depends on how bad it is.

 

[Captain Caveman]

Re: outboard isolation?

Try a different 4-stroke outboard on your boat. Try your motor on a different small boat. That should help you figure out if it's the boat, the motor, or the combination of the two.