Mystery Song (Help please)

[Glyka]

Last year in an "improvement crisis" I had the following changes in the transmission system.

[*]Fitted an R & D flexible coupling between the gear's and the shaft's flanges. Shaft had to be (and was) shortened.
[*]Fitted the VOLVO shaft seal.
[*]Aligned the engine (out of the water).
[*]Slightly increased the propeller pitch. Althought done by the manufacturer, the way (by hand) and the means used (a gas torch, a handmade tool and a simple meter) don't make me feel very confident about the prop balance.
[*]Polished the propeller. Probably not a very good job as I didn't take into account the amount of bronze that was removed in the process. The looks, on the other hand, were impressive.

The facts:

a. While cruising, at 1600 rpm a loud, high pitch noise appears (like a slipping belt (sqeek?)). As the RPM increase so does the noise. At 2500 RPM the noise dissapears.
b. With the reverse gear disengaged there is no noise.
c. While under sail and the gear disengaged, the noise appears (and dissapears later), althought not that loud, but I cannot, of course, tell the RPM.
d. After checking with a tool that looks like a stethoscope (I don't know the name of it), it appears that the noise comes from the aft (prop) end of the shaft. Actually, after touching the Volvo seal with the tool's metal wire, I'm sure about it.
e. With the boat fast on its mooring and the gear engaged there is no noise!!!
f. When I lowered the shaft (reversing the alignment changes) just in case the shaft was touching the tube the noise remained the same.

Possible causes:

[*]Reverse gear (ball bearing - disks)? No, as the noise comes from astern (d).
[*]Shaft touching the tube? No (e, f).
[*]Volvo seal not getting enough water? Probable, but the dissapearence of the noise after 2500 RPM says no.
[*]Propeller singing? Probable but a: the RPM range is too wide and b: the noise is too loud.

Real problem:

SWMBO refuses to go on vacation until the problem is solved as she is worried that this is maybe an indication that something really bad is going on.

Please Help!

 

[Aja]

Good Grief!

... though it does sound like prop singing..

Others will tell how to radius the forward edges of the blades.

Donald

 

[Glyka]

Re: Good Grief!

Forward or trailing?

 

[tobble]

Re: Good Grief!

does the pitch of the noise increase with increacing revs? If so, it *could* be a shaft resonance. But then what do i know...

 

[MoodySabre]

Sounds like a singing prop - not that I've ever had one. The machining of the trailing edges seems to be the cure that others have applied although a build up of fouling can stop it.

You may find this interesting
http://www.hydrocompinc.com/knowledge/techexchange/tex0303.htm

 

[nedmin]

Re: Good Grief!

Had this prob. after fitting new prop.was informed that it was "prop singing" and that the leading edges were too sharp, on lift out found that they were like knife blades,as I had slight damage and wanted another 1inch of pitch sent them to a prop repairer,when they came back the knife edges had gone and prop did not "sing " anymore. Found out later if I had taken the sharp edges off with a file this would have stopped it.If you do do it yourself you want to "draw" file the edge off, ie use the file at right angles to the blade your filing.I would just take the sharp edge off.

 

[R32Stbrigid]

My prop and shaft ring at certain revs. Morte noticeable early in the season or after a scrub off! Suspect fouling damps the noise.

 

[Glyka]

Re: Good Grief!

Thank you very much for the info. Could you give me some detail? What was the RPM range that the singing occured and how loud was it? Was it louder under power than under sail?

As far as the fouling buid-up is concerned, I'm afraid that one improvement that I forgot to mention is that this year I used "Velox" prop antifouling that has proved to do the job very well!

 

[StugeronSteve]

When my boat was brand spanking I had this noise for some while, bloody annoying because it came on at the sort of revs that you use in rivers and harbours, and there was a woo-woo noise whilst sailing. Just disappeared eventually. Earlier this year I had the prop and rope cutter cleaned with an acid wash and guess what....

Relaunched and the noise was back, after a couple of weeks in the water it was gone again. Might be the prop, or the volvo seal not properly burped, but the yard engineer thinks it may well be the Ambassador rope cutter singing. Anyway, check that there is plenty of grease in the seal and no air, then live with it for a while.

 

[R32Stbrigid]

Re: Good Grief!

It sings at about 1800 (Engine) rpm - 5 ish knots. I lock the shaft under sail, so am not aware if it would sing under sail or not. After about 4 - 6 weeks afloat it gradually dissapears, but returns instantly after a scrub. I anm sure that growth and barnacles damp down whatever is causing this!

 

[whipper_snapper]

Re: Good Grief!

That suggests that a test might be to wrap a few turns of gaffer tape - or maybe self-amalgamating tape - around the blades. It should last long enough to help a diagnosis - if the noise stops until the tape falls off then it is a singing prop.

ah. just noticed the boat is in the Med, so drying out won't be such an easy option!

 

[nedmin]

Re: Good Grief!

I have a motor cruiser, cant remember revs when it sang, as you sail the prop can get fouled easier than a power boat,so it looks like the fouling is stopping the singing.suggests prop blade leading edge,s too sharp.

 

[Glyka]

Re: Good Grief!

I'm not sure. I will try to check next time. But if this is the case, what would the solution be?

 

[colvic987]

http://www.amazon.com/gp/recsradio/radio...agSuffix=dp_img

try this, /forums/images/graemlins/laugh.gif