Cavitating Rudders that Sing to you

i recall a discussion on the old forum that this may have been caused by the modified trailing edge of the rudders. you mentioned that you

flattened

it, meaning you dulled the edge?

s.

If your rudders are cavitating at high speed then you just need to make sure of the trailing edge you flattened that it is consistant and no more than 10/100's of an inch, a flat line about the width of a sharp pencil line on paper...and make sure it is your rudders and not you dagger boards too...

You mean it's a bad thing when your boat sings to you? (Just kidding)

I was able to take the noise out of my P19 rudders just by re-setting the alignment...

A run down of what happens and what I have tried from an article of foil noise

1) A foil dagger board or rudder should be perfectly symmetrical.

2) The trailing edge should be razor sharp in reality this is not achievable.

3) The why a foil works water is split by the leading edge.The water passes over the foil at the same speed and rejoins at the trailing edge the trialing edge. This is where the noise comes from.

The noise is the oscillations developed as the waters slips into the flow of water from the opposite side of the foil turbulence is caused a low pressure point is created and the water then switches across the back of the foil. the water then starts to join on the new oppersit side of the foil. A low pressure zone is now created so puling the water back across the trailing edge this happens many times a second as the low pressure skips from side to side of the trailing edge.

4) to stop this noise a permanent imbalance has to be introduced. Squire the trailing edge using fine wet and dry 800 grade raped around a file should do. Make the trailing edge about 0.25 mm wide. Then with one stroke of the 800 grade bound file remove one of the corners between the trailing edge and the side of the foil.

5) What happens now. The foil will split the water as before. The water will now rejoin at the trailing with the low pressure zone held in one place. The water from one side of the foil will continually slipping in a linier fashion into the flow on the opposite side.

there is no reason to have symetric boards. with asymetric foils, you can have the low pressure and the lift where you want it.

Mark,

I just sand the side trailing edges with wet sand paper (600 or higher) til you can just see the glass on each side. look at the glass on both sides and when it is even, about 1/16 to 1/8" wide all the way down the trailing edge you're pretty much there. Start your sanding taper about an inch in so the angle isn't so great at the trailing edge. This is with boards and rudders. Use a wood block or whatever is hard that will give you evenness if you want. I use my fingers because it gives it a kind of natural roundness and you can feel the finish (there is a technique to it) Finish it with the highest grit you can find horizontally, the way the water runs. Try not to sand the very trailing edge of the foil until done and do it very very lightly, just to even it out. you want the trailing edge to be as sharp and thin as class specifications. Polish the whole thing and you are done.

JC

Thanks everybody for your input, I will look at the setup again this coming weekend. Hopefully, to success, even if she is a little quieter I will be happier, don't want to go upsetting any neighbouring sailors!!

When conditions are good and my H14 is moving fast, she hums. I thought this was neat. Does this necessarilly mean my rudders have problems or could something else be wrong (or right)? My wife likes it when it hums.

Thanks,

Greg H14, H16

This weekend I took the wet and dry paper again to my rudders to make the trailing edge sharper. There has now been a reduction in the noise, or it happens less frequently and with less ferociousness. However I cannot take any more back else I will be running onto the glass itself. It was quite interesting to actually here the trailing wash instead.

I hope it lasts!