Name this prop noise...

[freyr]

My experience is all with pumps, fans, motors etc, but in industry a vibration at 1/2 shaft speed is kind of unusual. Most misalignment or mechanical looseness will show up at 1 or 2X shaft rpm, and there will be multiples upwards of that set up by different parts of the machine, for example blade pass frequencies in a fan or pump, or even gear tooth frequencies in a gearbox. I'm not sure if, in an open environment like a prop in the ocean, you'd get a blade pass type reading, but I'm pretty sure unbalance would show up at 1x rpm.

As telling as the vibration frequency is it's direction and location.... you should be able to feel with your finger where it's worst, and maybe it's direction. Don't touch the moving parts!

An alignment that's .040" is unacceptable in my field, our standards are .002". I'd likely bite the bullet and get the machining done if it were mine.
Mike

[clyde]

a thot just raced thru my mind while reading...when we had noise problems with large trucks, etc, i used a stethascope to try and isolate the noise/vibration! 5ft shaft? on our 41ft skookum, we have a 14ft x 2" shaft, with a center bearing behind the pss shaft seal...good luck, will be waiting for the solution...clyde

[leftbrainstuff]

Originally Posted by delmarrey

If the sound is half the rpm, I'd say the shaft is whipping. I had the same problem when I upgraded to a new motor with more torque. I had a 1" shaft at 5' long, which started whipping and slapping the stern tube.

I tried all kinds of things. I cheanged to a standard stuffing box, put in a Pythondrive and even had the new prop balanced with very little results. I went to a 1/8" larger shaft and it purrs like a kitten now. I think the Pythondrive is helping quite a bit since the new motor dances around but the larger shaft did the trick. I even went back to the PSS (packing).

Shaft bent is a static problem. Straighten if necessary.

If the shaft is whipping you have a dynamic problem.

You have 3 options to deal with the whipping.

1 change shaft stiffness by increasing diameter.

2 change rotational mass. This is an easy one to test. Clamp a mass around the shaft and move it up and down the shaft to tune out the resonance. It needs to be close to static balanced and not just a lump hanging off the shaft. This is less desirable than changing shaft thickness but it is easy to do in place.

3 avoid that rpm to avoid the resonance.

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[donradcliffe]

I agree, 0.004" alignment might be OK, but 0.040" could be the problem.

[AndDavRos]

Our shaft is 7" long with no bearing at the midpoint. Looking over the ABYC specs, I should be good with the shaft dia and material out to a little over 9' with just the stern cutlass and transmission mount. I am still thrown off by the frequency of the vibration. There were multiple harmonics in the audio file, but the highest amplitude was in the 350hz range, well above background noise, which seemed to match what we were hearing.

I'm adding the machining of the mounts to our list of items to do while we are hard standing in the next couple of weeks. Any recommendations on good machine shops in the hampton roads area of Virginia?


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[AndDavRos]

Originally Posted by leftbrainstuff

Shaft bent is a static problem. Straighten if necessary.

If the shaft is whipping you have a dynamic problem.

You have 3 options to deal with the whipping.

1 change shaft stiffness by increasing diameter.

2 change rotational mass. This is an easy one to test. Clamp a mass around the shaft and move it up and down the shaft to tune out the resonance. It needs to be close to static balanced and not just a lump hanging off the shaft. This is less desirable than changing shaft thickness but it is easy to do in place.

3 avoid that rpm to avoid the resonance.

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We ruled out the resonant RPM theory due to the vibration being present in at all RPMs and in reverse.


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[delmarrey]

Originally Posted by freyr

An alignment that's .040" is unacceptable in my field, our standards are .002". I'd likely bite the bullet and get the machining done if it were mine.
Mike

And yes, I forgot to mention. .015" is minimum, with <.005" being desirable. .040 would cause enough stress to crack couplers or shaft, or work the clearance enough to hourglass the coupler &/or shaft.

If you have to remount the motor then that's the price one has to pay for quality.

[Cheechako]

any possibility the travelift strap bent the prop or shaft when you launched?

[SkiprJohn]

I think sorting out the alignment is going to cure your noise problem. Is your engine hard mounted or flexibly mounted?

[CLady]

Is it a fixed prop or a feathering/folding prop? If its the latter and not fully opening up, it can really vibrate!

[Wind River]

I have a similar noise/vibration that started last week in my boat and seems to be most pronounced at the prop. Nothing has been changed or modified in my boat before this noise started and nothing that I know of has hit or damaged my prop (full keel boat). A visual inspection found the cone zinc aft of the prop to be loose but after removing it completely there was no change in the noise/vibration.

I would be in the "engine out of alignment" camp and that is where I am going start to resolve my issue.

Good luck with yours. I will be watching to see what your outcome is.

[AndDavRos]

Just got the word... I have a gremlin in my stern tube. With the boat out of the water the prop and shaft look fine. The yard took another look at the alignment and said it looked perfect (after some more adjusting). Nothing loose and everything spins freely. But if you ever so lightly tap on a blade on the prop you hear the shaft hitting 'something' about 8" up in the stern tube. Whe we did the install, I noticed a LOT of rust scaling in the tube. This MAY be it, but the yard doesn't think so. There is also a possibility that it's the end of a screw on one of the two intake scoops that is poking into the tube.

Their verdict, we can move the boat, but if we keep it there it will be at least two months before they can pull the shaft.

So the answer is we don't have an answer yet, just a vague assurance that it's not any of the usual suspects.


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[AndDavRos]

The prop is protected inside the full keel and rudder shoe. It's a fixed prop.


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[AndDavRos]

So it looks like we are going to bring the boat the 90mi south to where it's closer to us and I have more options regarding machine shops and yards and will have to give the stern tube a close look when we haul her back out next week.


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[delmarrey]

That's what was happening with mine. The shaft was hammering the start tube. I only had 1/16" clearance per side. I ended up replacing my stern tube to accommodate the new shaft size. I even went the max for the PSS available.