Name this prop noise...

[AndDavRos]

Here is our scenario. New prop shaft, rebuilt and reinstalled stuffing box, re worked prop, new cutlass bearing, and engine re-aligned to within 0.04" (closest we could get without machining new engine mounts) and we have a vibration coming from either the prop, shaft, or rudder. Here is the trick: I recorded the sound while we were at 2000 engine rpm (about 690 shaft) and the frequency of the vibration is half of shaft rpm (340-360hz). And it is LOUD! You can feel it in the deck. The sound is present in forward, and reverse. There are rpm bands where the intensity changes, but it's always there. Frequency stays about 1/2 shaft rpm throughout the range as best I can tell.

The prop clearances all check out, and the vibration is present at all rpm ranges (so we know it's not 'singing').


Here is a recording of the sound (on my Dropbox)

https://www.dropbox.com/sh/gc4ml78q3...LrzlfKhQa?dl=0

Any suggestions would be of help. We are paying for a haul today to see if there is anything goofy going on, but this is driving us nuts.

Thanks for any suggestions you can send our way!

-Shawn
S/V Firefly


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[malbert73]

Couldnt listen to the file. Did you put prop on shaft without key, advance all the way onto taper, and mark that point? That ensures that the key isn't preventing prop from getting all the way onto the shaft- it could be off center on shaft if key is preventing this and certainly could cause vibration.

Otherwise, assuming you trust the alignment, it's either a poor prop or the shaft has a bow in it. You could remove both and have a prop shop quickly check for trueness.


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[malbert73]

Forgot to mention. If you are hauling, you really need to check shaft and prop, since they both have changed. Even minor misalignment wouldnt cause all of that I believe. Assuming that because they are new or reworked that they are okay could be a mistake. Shaft could be misaligned in coupling as well.


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[AndDavRos]

Sounds like a place to start. Thank you!


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[AndDavRos]

Here is another attempt at posting a link to the sound:

https://www.dropbox.com/s/fdt8rj0n9p...Noise.m4a?dl=0


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[AndDavRos]

As for installing prop, I fit checked the prop without the key while the shaft was in the shop, marked and cleaned all the burrs, and made sure things lined up prior to installation. If there is nothing obvious on the haul out I'll have the yard go ahead and pull the prop and shaft again and check everything over.


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[a64pilot]

First link to sound worked for me, as an old helicopter test pilot, I can tell you it's extremely common to get harmonics at multiples of RPM, matter of fact we would chase vibrations by first installing an accelerometer and recording the vibrations and looking at frequencies of the peaks and see what turned at that RPM, it was real common to get 2 per rev and even 4 per rev vibration peaks.
Rudder you could feel in the rudder shaft couldn't you? Should be easy to isolate.
Since it is in fwd and reverse I think that usually eliminates the rudder?
let us know what you find

[bsurvey]

was the coupling refaced.

you don't say what kind of a boat this is. you may have had some flexing and the coupling should be broken and re aligned in the water

[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).

[AndDavRos]

If the shaft is whipping it may be a trick to change sizes. The shaft is 1.25" in dia and we are only using a 72HP motor. The hull is steel, so I would have to go after the stern tube with a plasma cutter to make room for a larger cutlass bearing.


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[AndDavRos]

The coupling was refaced with the shaft installed. We did the alignment with the boat in the water.

Sorry for the oversight. The boat is a 46' Bruce Roberts Mauritius steel cutter.


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[delmarrey]

Can you see the shaft as it's turning?

I'd try to locate the source of the noise from the engine room. It may not even be the shaft. Could be the motor banging into something, broken motor mount, something on the motor loose.

Just need to get down there and become part of the system while it's running. Just be very carful of moving parts!

[AndDavRos]

I spent some time both in the engine room and steering/ aft birth. Sounds loudest aft near the prop.


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[delmarrey]

Then I'd be looking at the prop.

While its out of the water run the system with water running over the cutless bearing and into stern tube (stuffing box) and watch the shaft.

If the noise is that far back I'd suspect the prop being out of balance, loose or even bent where it presses on.

[AndDavRos]

Thanks for the ideas! We are waiting to hear back from the yard this afternoon. They are hauling it out and checking things over. I'll pass on what I find out.


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