Silly prop question

Relax

Relax

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I have had a minor cavitation prob for some time - only does it when I'm doing between about 5 and 6 knots - under engine or sail. No vibration just a very annoying whine. Putting the engine in gear when sailing stops this - trying to remember you've left the engine in reverse when you start it is sometimes more difficult . Still does over 6 knots with a bit of a puff - Yanmar 1GM10.

Anyway ............. as I've just extracted the thing from the oggin I've noticed that the prop has the curvy thin bit leading (forward) and the rounded, thicker, straight bit trailing (aft). This does'nt look right to me. When I bought the boat (Sadler 25) it was up the road and off the trailer immediately so I did'nt have time to check and probably would'nt have noticed anyway.

Does anybody think someones put the wheel on inside out?

Sorry, no piccies.
 
Well, I had my new 2-bladed prop delivered to me this morning (RH, 2GM20F ). Its form is exactly as you describe, so is likely to be right. What probably should be done is cutting or improving the anti-singing edge on the prop.
 
That is not cavitation. The prop is driving the shaft and you are hearing bearing and gearbox whine. Perfectly normal. Put it in gear to stop it if it annoys you. It is also possible that you have what is called a "singing prop" This is not cavitation either and is usually caused by a good prop in perfect balance resonating at a sympathetic harmonic frequency (a bit like ringing a bell) This can be corrected by gently filing the leading edge, but I would get a prop expert on the job rather than attacking it yourself if you have never done it before.
 
Forgot it would be on a taper.................. /forums/images/graemlins/tongue.gif

Not wild about attacking my prop with the angle grinder either .........

Has anyone done this??

Its an annoying noise - but maybe I could tune it to B Flat?
 
Definitely not the gearbox or bearings.

This is a harmonic (to my ears) noise - should I do anything about it?

Its a bit like the boat breaking wind in some respects - you're bowling along on a beam reach spray flying and suddenly this hum/whine spoils the moment.
 
No problem leaving in gear, if you start it in gear so what, I would be doing that anyway, forget the "is it faster in or out of gear " argument. Think about the wear on stern gland and gear box if it's turning all the time you are sailing.
 
This happens with my prop, I have found that it is one blade only that sings. I have now marked my shaft and ensure that it is stopped with the singing blade behind the woodwork - that has cured the high pitched ringing completely.

Only other solution that I know of is to alter the leading edge of the blade very slightly with a grinder.
 
I agree that it doesn't matter much what you choose to do when sailing. My last boat had a Volvo 2002 and the instructions from the maker were to let the prop rotate when sailing, but I was unable to determine any difference in speed. I now have a saildrive & the gears whine if I forget to lock the prop. The instructions for some non-saildrive models include the advice to run the engine after every 10 hours sailing, which sounds ridiculous.
 
Took a file to mine - problem solved. Can't remember if it was leading or trailing edge but it was easy and it worked.
 
Does the noise appear only at a certain number of revs and fade out of hearing above a higher level - eg appears at 1000 engine revs and gone by 1400? Almost certainly prop whine caused by the passage of water over the free rotating prop. You can get to work with a grinder etc and blunder about trying to change this, but you would be well advised to leave it alone and eat it. The likelihood is you will just move the range of vibration, not eliminate it. And be left with an out of balance prop - not recommended!

If the noise appears at say, 3/4 hull speed under sail and becomes more pronounced as you sail faster, it's gearbox whine. Proof is, lock the prop in reverse gear, engine off, and it's gone. Yup - gearbox whine. Nothing you can do about that one, even with a grinder! Install a bigger HiFi system....?

Almost 100% sure it's not cavitation...

PWG
 
Now you have said that I think it's a "singing" prop. As I said before it can be a slight modification to the leading edge that will cure it but don't just take a file to it or you may well unbalance the prop. If indeed this is what it is it usually sounds like a high pitched ringing noise at a particular RPM and goes away above por below this. A hydrodynamicist once told me that he was asked to go to sea in a warship to hear the noise that the engineering officer thought was a "screaming" bearing, but it only occurred at about 28 knots. Unfortunately it was so loud it told every submarine within the northern hemisphere where they were! The "fix" there was to change the leading edge of the props to present a different pressure signature over the blades. All totally beyond me but I know it can be very loud when it happens. My advice is to take the prop off and take it to a reputable "prop shop" like Bruntons, explain the problem and get them to mod it. Shouldn't be mega quids and they should understand. If they look at you blankly and scratch their gonads, go somewhere else!
 
We have quite a deep pitched whine (?) hum(?) which occurs under sail at over 7 kts. I dont think it is the prop (which folds) although it could be flapping a bit in turbulence behind the saildrive. I think you can feel it through the floor almost as much as hear it . - Any thoughts ?
 
[ QUOTE ]
The "fix" there was to change the leading edge of the props to present a different pressure signature over the blades. All totally beyond me but I know it can be very loud when it happens.

[/ QUOTE ]

Looked through my old papers, but when I had this job done some years ago, the TRAILING edge of my prop had the anti-singing chamfering done. And as it did its job (the noise had been absolutely annoying), they must have done something right. Mind you, the workshop foreman considered it as something of a black art, as only one -the oldest- of his twelve employees was trusted with this mission!
Also, I found this:
web page
 
Yeah! That's the kind of gobbledegook that made me say it's all beyond me!
/forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif
 
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