vibration through the boat at 6 knots

[dnej]

When travelling in 6 knot areas, I get a shudder through the hull of my aluminium cuddy cab. I can get rid of it by trimming out the motor.. Of course it disappears when revs are applied, but then you get over the six knots.
Is this like driving a motor vehicle in the wrong gear?
David.

[scottar]

I assume you have a two stroke and that the problem has always been there or is it a new issue?

[dnej]

Scottar, yep 90 HP 2 stroke. The slow speed makes items in the boat rattle, or shudder. Nothing to do with the motor per say,
Its just the low revs, and I guess the prop trying to do its job, at those low revs. As I mentioned, trimming the motor up, stops the shudder immediately, as the prop is not doing so much work.
David

[scottar]

It is an engine vibration. By trimming the motor up you are removing load on the engine by reducing back pressure on the exhaust and possibly load off the prop as well. I used to do the same thing a bit when slow trolling with my old two banger. Sometimes they can be made a bit smoother by playing with the idle mixture on the carbs but you need to know how to set your engine up or have a copy of the service manual for your engine.

[Noelm]

Pretty common "feature" of carby 2 strokes, I used to have a tiller steer 20 Metc, and in the speed limited zone near the ramp, it would start a kind of vibration, that would result in a loud almost "buzz" noise and shake stuff around, a bit slower, or faster and it was OK, my theory, and I have nothing to base this on, but guess, is it is the change over from the idle jets to the mid range jets, and the motor kind of gets stuck between the two, so, a slight increase in throttle gets you over the change over point, possible? Maybe....

[dnej]

The hull is the old banana shape at the stern, and up wards curve. ( 1980's)
When the motor was installed , we had to fit a wedge, to compensate for the porpoiseing.
Great fix , by the way.( Yatala Yamaha at that time)
So off the plane, the motor can be very much " tucked in", probably unnecessarily.
Trimming out , seems to take the work load off the prop, and all is sweet..
As I mentioned, a bit like driving a vehicle in the wrong gear.
David

[The Woo]

It's just the engine mid section vibrating in the transom assembly of the motor when you are fully trimmed down. Little bit of wear here, some there, and that causes a loss of refinement over time. When trimmed all the way down it's exacerbated.
Unless its extreme there's no dramas.

[WildFishExpeditions]

this can happen to my boat if i have the engine trimmed up too high, like if i leave the ramp and forget to put it down low enough. it doesnt bother me. my boat is pretty much brand new so I wouldn't worry about it.