Singing Prop

[Jobs a good un]

Hi All

After antifouling boat and propeller we have a high pitch noise coming from the propeller , anyone got any idea what this is caused by ?
It wasnt happening before we cleaned off prop and put antifoul on ?
Thanks
Mark

 

[vyv_cox]

Singing props are fairly common when first fitted, solved by filing a small flat on the sharp trailing edges of the blades. Not knowing exactly what you had before or have done it is difficult to say but it seems you have changed the hydrodynamic characteristics somehow.

 

[38mess]

My prop sings at low revs, a different note depending on revs. I have been told it's the edges of the blades cavitating. If I round them off the singing should go. Maybe you did something to a blade.

 

[rotrax]

I used to turn out for the Old Vasectomists extra 'B' XV.

Our tight head prop was a really good singer, especially when pissed.............................

 

[Praxinoscope]

Had exactly the same problem when I fitted a new three blade prop ( the old one was past saving),
talked to the nice people up at Castle Marine in Caernarfon (they are prop specialists) and they said not unusual and to do as vyv_cox has said flatten off the trailing edges, I am doing this in small stages as I don't want to overdo anything, and have reduced the singing at various revs, but need to go a bit further to lose it completely.

 

[bignick]

Does it happen at a particular RPM? Or happen above a particular RPM?

 

[QBhoy]

Worst case, would be that the cutlass bearing has been damaged and the shaft is rubbing on the p bracket (if she’s shaft drive).

 

[geem]

On foiling kite boards, whistling foils is a common thing. Lots of foils come razor sharp on the trailing edge. Sanding them round stops the issue. It sounds like a similar problem on your prop

 

[Daydream believer]

What is the song? Ideally you want to hone the edges so that get something like Nickleback when at speed & then a nice love song as you slow down for that quiet anchorage with the mistress

 

[seumask]

I like the noise on my foiling gear as it changes note at different speeds and indicates how harsh a crash landing could be if I get it wrong again!

 

[vyv_cox]

On foiling kite boards, whistling foils is a common thing. Lots of foils come razor sharp on the trailing edge. Sanding them round stops the issue. It sounds like a similar problem on your prop

Hobie 16 catamarans were just the same. More of a howling than a whistle but solved the same way.

 

[geem]

I like the noise on my foiling gear as it changes note at different speeds and indicates how harsh a crash landing could be if I get it wrong again!

In racing kite foils is associated with inefficiency these days

 

[tross]

I wonder if this is a "vortex shedding" problem where the vortex flip-flops from oneside to the other- this was a common problem with rudder vibration on the Hunter Horizon 23. It was cured by planning the trailing edge (woodern rudder) at 45 degrees

 

[Ian_Edwards]

It's known as vortex induced vibration, (VIV), it's caused by vortices flipping from side to side, as Tross says. The vibration excites the structure and finds one or more natural frequencies in the structure, causing it to resonate.
Filling the trailing edge of the prop' blade square, changes the natural frequency of the prop, and, with luck, moves it away from the frequencies generated by the VIV.
It probably reduces the efficiency of the prop' slightly, by increasing the drag, but I doubt that you'd notice the difference.
the foiling cats and the foiling Americas Cup boats have razer sharp trailing edges to the foils to minimize drag.
It's also the same phenomenon which causes rigging to sing of howl in a gale.

 

[Daydream believer]

It's known as vortex induced vibration, (VIV), it's caused by vortices flipping from side to side, as Tross says. The vibration excites the structure and finds one or more natural frequencies in the structure, causing it to resonate.
Filling the trailing edge of the prop' blade square, changes the natural frequency of the prop, and, with luck, moves it away from the frequencies generated by the VIV.
It probably reduces the efficiency of the prop' slightly, by increasing the drag, but I doubt that you'd notice the difference.
the foiling cats and the foiling Americas Cup boats have razer sharp trailing edges to the foils to minimize drag.
It's also the same phenomenon which causes rigging to sing of howl in a gale.

You have reminded me of a joint meeting our branch of the Chartered Institute of Building had with the local branch of Structural Engineers. One of their members had been involved in studying the cause of the (then) recent collapse of the cooling towers at Ferrybridge.(I think it was Ferrybridge)
He gave a fantastic talk. Starting from how a withy wavers in a stream. Then to how the walls of the cooling towers were seen to be flexing by several feet as the wind passed between the towers, leading to the collapse. Then they suddenly hit the right frequency needed to bring them down & which no one had foreseen
He spoke about other aspects of wind & frequency of structures & I recalled it when I had the contract to replace 50-60 windows in a tower block in Melish st on the Isle of Dogs after the IRA bomb. Every window was on the opposite side of the building to the blast & the building was .5 miles from the bomb.

 

[Ian_Edwards]

Yes, It's a fascinating subject which I studies and researched over many decades, everthing thing from singing props' to deep water risers, running between wellheads and Floating Production and Storage, in the Gulf of Mexico and off Brazil, to chimney stacks.
My boat has a Dyneema back stay which, when I first installed it would sing, quite loudly, and the vibrations would propagate through the hull of the boat, making sleep impossible in the aft cabins. I stopped it resonating by wrapping a spiral of 3mm cord around the the backstay with a pitch of about 250mm, holding it in place with a few stitches of twine every 250mm. It more of less eliminated the resonance, it now only occurs if I forget to release the backstay tension, which I wind on to about 2 tons when beating in strong wind to bend the mast and help flatten the mainsail and keep the forestay from sagging.