Singing Prop - What to Do?

[markpj23]

Just had the prop re-pitched to match the engine and now she sings quite loudly at around 1300 RPM. Higher or lower RPM and all is fine.

What causes this (I know it has to be related to the pitch angle) and what damage can it cause (if any)?

Other than re-pitch, what can be done about it?

[svHyLyte]

Cavitation (as in cavity)—the formation and collapse of little bubbles on the prop-surface that slowly but surely eat your prop and resonate your prop quite merrily as they do their evil deed!

I'm going to disagree with HyLite here. Cavitation rarely gets better as the prop loading (i.e., speed) gets higher. And I have never heard cavitation described as "singing" Rumbling, roaring, maybe.

You didn't say if you raised or lower the pitch. An idea of the specifics of the boat, prop and engine would allow a calculation to give an idea of it cavitation was possible.

If you are hearing the music over a narrow range of rpm's it is likely you are seeing a simple case of resonance. Changing almost anything can shift the resonance frequency enough that it is not a problem. Even something as simple as adding an extra zinc to the shaft.

It is not unusual for very large drive trains in very big ships to have a speed where they resonate, and if it is at a low frequency it can be bad for the parts. It is avoided by not running at those speeds.

Bill

[MidLandOne]

As GreatKetch says it is likely a resonance and not unusual to find it on a new boat or one repropped.

The usual cure is to grind a little off the trailing edge of the prop blades but I would recommend that you get some advice on this from a prop specialist appropriate to your prop type (sail boat optimised, conventional, etc).

I have come across it while on new 100 foot plus cats during commissioning and the resonance on larger props can be very loud, especially as they are twin engined of course - but a touch with the grinder in the right hands kills it completely.

If it is not too bad you may find it goes away once the prop is fouled a little. Our own prop (a narrow bladed sail boat optimised one) sings a little for a while just after it has been polished until it has fouled a little. Interestingly, it was polished a few weeks ago but for the first time was Propspeed coated, and I had noticed that the Propspeed coating alone seems was enough to prevent the singing this time. So a very little change can make the difference.

[Alan Wheeler]

The issue of "prop-singing" is a harmonic, set up by the leading edge of the blade vibrating as it bites the water. The cause is often because someone has sharpened the leading edge. It could be that the edge was always sharp, but the re-pitching has placed the sharp edge at a different angle to the water. What is happening is not unlike a person playing a saw with a bow. A sharp edge can cause all sorts of issues, but believe it or not, Balance is one of them. Sharp leading edges can cause the blades to "bite" at different rates and cause issues that feel like unbalanced props, inducing vibration, exccessive noise and cavitation. The singing is not cavitation, but cavitation may or maynot also be an additional issue. Remember, a prop is a rotary wing. It is the surface of the prop that gives the propulsion, not the bite into the water.
The remedy is simple. Round the leading edge. It is what actually should be in the first place. It is easy to do with emery cloth. Just remove any edge and ensure you have a round'ish edge.
trailing edges are not usually the issue, but I am not saying never. However, it is best to also have a round edge on the trailing edge. Or the same issue in reverse can occur. However, due the shape of some blades, cavitation can be a major issue when the prop is swinging hard in reverse, creating a heck of a vibration noise.

[MidLandOne]

This link Marine Boat Propellers and propulsion Terminology (go to the very bottom of the page) describes the phenomena well and the usual method of curing it.

This is what I have seen done to cure it on the few occasions I have heard it on new builds. Personally, I would get someone experienced with fixing such problems if it comes to grinding the actual chamfering on an expensive prop so that the minimum metal is removed.

The link mentions removing metal from the prop as being a last resort - as I posted before, in our own case the singing was not very loud so I never did anything about it and fouling on the prop soon quietened it in any event. PropSpeeding the prop seems to have done the same.

[markpj23]

Many thanks for the links & replies. In our case the singing is VERY loud and only through a narrow RPM range. Just hauled out / relaunched so the prop is clean and new shaft zinc went on in the exact same spot as the old one.

As an aside we also discovered (surprise!) that our shaft coupling bolts were loose and the alignment was way off. Now that we've fixed that I'll have to see if the singing has remained - I suspect it will still be there.

Never a dull moment....

[psteele235]

Join in in harmony?

[Alan Wheeler]

Just never name your boat "Fat Lady". Then you have no worries about anything being over.

[Elie]

Interesting that tread on priops that behave like violins. I have a different problem: My prop is mute while under power or free wheeling under sail. But under sail it starts singing loudly at speed over 5.2 knts. It drives me nuts and really spoil my fun... Any explanation?

[bobconnie]

If your prop free wheels think about adding a Vee belt and a small alternator, easy way to add to the electrical needs Its worked for me ! Just a thought

[Elie]

My engine compartment is quite crowded and I my engine is already equipped with 2 alternators. In fact I don't like the idea of a free spinning prop, and what it can do to the transmission bearings. It must be some sort of harmonic generated by the water passing on my prop. But what a salor to do?