Propellor humming

[sgbernd]

Here is a new one to me. I wondered if anyone else has this problem and if so what they did to cure it.

I recently repowered (replaced Volvo MD7B with Beta 16) and since the prop direction reversed, had to replace the old 2 blade Michigan Wheel with a 3 blade Acme. It works fine and vibrates less but at certain rpms, it has an annoying whine which I am certain is coming from the prop itself. The noise doesn't hurt anything, and is high pitched, but makes the rpm bands where it resonates unusable due rapid deterioration in the captain's mood. Fortunately, cruising speed of 2,500 rpm is quiet.

I read on the internet about people trying other props, even the same model from the same manufacturer, and having some success due to variations in manufacture. This prop is one of those CNC machined ones, and very beautiful but has much more narrow and knife sharp blades than I am used too. They (ACME) claim this improves performance (which is logical) but I am wondering if the narrow, sharp knife blades are more likely to hum than the old, fat, wing like blades. Simply, a thicker blade would have a harder time resonating than a thin one. Also a larger diameter would have a lower resonant frequency (thus less annoying) than a smaller diameter. Am I thinking correctly?

Did anyone else have this problem and if so, what did you do? I am half inclined to run it for some hours and see if corrosion, barnacles, whiteworm, erosion, etc. don't change the resonance frequency but fear that at some point, I won't be able to return it for credit if I do it that way.

Regards,
Steve Bernd

[Jeff and Sarah]

I repropped my competition ski boat and spent quite a bit of time on the phone with Acme choosing the correct size/pitch. They also rebuilt my old prop that was bent pretty badly. I was consistently impressed with their customer service so you might just try giving them a call and asking for their advice. I'd be willing to bet they have the expertise to help solve that problem pretty quickly.

[Jim Walsh]

Now I know what people are talking about when they refer to a prop's "pitch"

[Northstar1014]

You're sure its not your cutlass bearing? That can cause vibration, and because its so close to the prop on these boat, might be worth checking.

[sgbernd]

No, it is not the cutlass bearing as that would be at the frequency of the prop shaft, which is relatively low. The hum is very high pitched and at both engine speeds it appears (just off idle and about 1,500 rpm), it is the same frequency.

Good idea to call ACME. I will give that one a try.

-sgb