Resonance coming from an outboard ???

[Fish Chris]

I'm actually asking about this for both my buddies boat, a 17 1/2 foot fiberglass Kenner, with a 75hp 4 stroke Merc, and my new boat, a 14 1/2 ft aluminum with 25hp 4 stroke Yamaha.

Now my motor is basically brand new, and runs great, but his has very low hours, and runs perfectly also.

However, coincidentally, we both are having the same problem, and in both of our cases, it was after getting a new prop. Ironically, we both feel like we have finally got just the right prop, for the correct RPM's, and peak performance.

But here's the deal..... When I throttle up to about 2000 to 2300 rpm's, I start getting a whining sound, reminiscent of a tuning fork (except its off key.... so its hard to sing along with I'm not sure what rpm's my buddies starts doing this. But either way, as soon as the rpm's climb above this range, it goes away...... but it does the same thing again as we are slowing down, and the rpm's pass through this range again.

I'm just about positive that this is not a mechanical problem, but on the other hand, it is annoying, and I have no idea what could be resonating like this, in either mine, or my buddies motors ???

Anybody have any experience with this type of thing, or ideas to eliminate it ?

Thanks in advance.

Peace,
Fish

[RichThiel]

Hub may against the huge Washer on the back side slippin maybe. See if you have the correct washer (I don''t know what they call that piece). They are differect ones for certain Prop's.

Just my thoughts

Rich Thiel

[Fish Chris]

I think that is certainly worth looking into....

Especially since the motor is so new, but has already had three different props on it. I could have gotten a washer swapped around..... (a regular washer that is..... not one of the large brass ones, that niether of us can remember the name of

Peace,
Fish

[tommypV2]

Hye Chris -
Are both props repaired? or straight from the factory? or maybe you swithced from alum props to stainless? sometimes, repaired props are out of balance and will cause the pinging you describe.. I put on a demonstrator prop once and it sounded like a bell at certain RPMs.
Put your old props back on and see if you still get the sound.

[Ray Vanacore]

There called thrust washers

[Fish Chris]

The first prop was the factory prop, and the boat was completely under propped (way too high of rpm's).

The second two were also both brand new, and also aluminum.... just trying to get my propping dialed in, and the third one was the charm.... as far as rpm's go. But yes.... "like a bell" at 2000 to 2300 rpm's, would be another way to describe it.

And hey Ray, yes ! Thrust washers ! I'm getting senile I guess

In any case, whether the sound is coming "directly from the prop", or "the prop is causing a resonance, which is coming from somewhere else in the motor", I'm almost sure the prop has something to do with it.

Will start working on this soon.

Peace,
Fish