Ding-a-ling

[Praxinoscope]

I recently fitted a new prop (12x8 three blade) to replace the old one (same spec) which was getting a bit worse for wear, the new prop drives the boat (Sadler 25 Beta 13hp engine) fine but at some speeds it is starting to ring like a CofE church on Sunday.
As the specs are identical and there is no unexpected vibration or loss of speed at various revs' any ideas why this new prop should ring when the old one was silent.

 

[sarabande]

A "singing" tone ? High-pitched meeeeeeeeeeeeeeeep ?

If so, it's cavitation.

Did you paint or treat the new prop ? If the boat dries put you may be able to see on the blades where the cavitation is taking place. First task is to polish the blade, and if the ;leading edge is very sharp, to take the edge of it with some sandpaper; it's the trailing edge that should be sharp.

 

[Praxinoscope]

A "singing" tone ? High-pitched meeeeeeeeeeeeeeeep ?

If so, it's cavitation.

Did you paint or treat the new prop ? If the boat dries put you may be able to see on the blades where the cavitation is taking place. First task is to polish the blade, and if the ;leading edge is very sharp, to take the edge of it with some sandpaper; it's the trailing edge that should be sharp.

Thanks, going down at low tide this afternoon so will have grovel around the prop, it's Quite possible leading edge is still sharp.

 

[seumask]

Hmmm are you sure its the leading edge that needs a little blunting, my understanding is that its the cavitation at the training edge and that is the edge that needs a blunter square profile . I stand to be corrected.

 

[reeac]

I looked into this phenomenon about 10 years ago on behalf of a friend who had encountered it with a new propellor. I spoke to a propellor designer and as far as I recall he said that it was little understood and with no one method of curing but, as already said, associated with the edge geometry.

 

[vyv_cox]

Yes, its the trailing edge and not actually cavitation. File a flat on the trailing edges (the sharp ones) about 1-2 mm wide and the sound will go. The same thing happens on dinghy catamaran rudders, where the same treatment fixes it.

 

[anoccasionalyachtsman]

It's called 'prop sing' and it if it's a nice square trailing edge then take 30 degrees off it.

 

[bikedaft]

it appeared with same prop but with new shaft anode. quick swim and randomly moved it a bit on the shaft fixed it.

a bit of growth should "sort" it?