Got this sent from AHPC, may be helpful to others
Noisy Foils
The following is my understanding of the causes and solutions for noisy centreboards and rudders. Although I am an engineer my knowledge of fluid dynamics is limited, so I do not pretend to be an expert, but I supply this information to help anyone trying to tune (no pun intended) a noisy foil.
Firstly some of my experience. We manufacture many foils, from a number of different moulds. Some of our products are never noisy eg our Tornado centreboards, and tornado/5.7/ F18 rudders. Other foils we manufacture have a proportion that is noisy. The frustrating thing is that these are often the best quality foils, often absolutely fault free. I have taken particularly noisy foils and checked them for symmetry and straightness and found them to be perfect within the limits I can measure to, yet a chipped, scratched dented and twisted foil from the same mould will be silent. Making the first foil a chipped, scratched dented does not necessarily fix the problem.
Generally a foil will not be noisy when it is under load, unless it has an obvious leading edge fault.
As best I can understand is the noise is generated by an oscillation setting up in the water exiting the foil. Minor random variations in flow across the foil will cause the eddy on the trailing edge to spin to one side, if the foil has a high degree of symmetry, another variation in flow can cause it to suddenly switch to the other side. As the flow swaps back and forward it sets up a harmonic on the blade, causing severe vibration. Hence when under load foils are generally quiet as the flow is forced to on side or the other by the lift it is generating.
I did not appreciate the power of such harmonics until I saw a demonstration of sound waves on a wineglass. At a quite low sound level, the technician was able to set up a vibration that was moving the top of the glass by what I estimated to be 1cm- the glass broke of course. Water is a denser medium by 1000’s of times so has the potential to set up vibrations 1000’s of times stronger, little wonder that harmonics are so troublesome.
In aircraft this problem is called flutter and has caused the death of may a test pilot. The problem is at its worst when the foil is flawless. Flaws tend to drive the flow one way or the other, and prevent it from switching back and forth across the surfaces. The solution that is usually used is that the flow is “spoilt” in some way, on yacht foils the most common practice is to put a 45 degree chamfer on the trailing edge so the foil is no longer symmetrical. This is not always successful, and I will give the following example to explain the difficulty of dealing with the situation.
A customer phoned me to report he had noisy rudders on his boat, after some testing it became clear that both rudders were noisy. I had him return the foils to the factory and I checked the rudders, made sure that the trailing edges were clean and free of imperfections. The foils were perfect as we could make them. The customer reported the slightest improvement, but claimed he still needed ear plugs to sail the boat. I then sailed the boat myself, and took a file with me. I chamfered the starboard rudder slightly on the outside trailing edge, and then test sailed the boat- the noise was worse. This was encouraging as I had at least caused a change. So I chamfered the inside- this resulted in an improvement, so then I did more, eventually I had that blade completely quiet. I repeated the process for the other rudder, this blade too needed to be chamfered on the inside (ie the opposite side of the blade). I thought that the 2 blades must have been asymmetrical on opposite sides, so I took them back to the workshop- they proved to be identical, symmetrical and with no twist. So why did they need the opposite solution to fix them? Next, I had the customer swap the foils from one hull to the other, the foils remained quiet. I still do not have any explanation why each foil required the opposite solution, despite being identical foils.
Be warned though- if it ain’t broke, don’t fix it. I was testing a foil that was slightly noisy at some speeds, I chamfered the port side slightly, the noise got worse, so I then chamfered the port side, the noise became even worse. Eventually I took the foil back to work and squared off the trailing edge, it then went back to the slight noise it originally had.
The solution:
1 Determine which foil it is;
a. Sail up wind, if the boat is quiet, the problem is probably one of the centreboards, confirm this by removing the centreboards, one at a time while reaching down wind.
b. If it’s a rudder the noise will be present most of the time, and less pronounced up wind. The noise will disappear when the rudders are turned. Find out which one by sailing down wind with one rudder up, then the other.
2.Check for the obvious;
a. Look for faults, mould marks, or hard edges on the leading edge.
b. Sand the trailing edge with 1500 rubbing paper, so the exit is very clean and flat.
3. TEST
4. Spoil the shape, or change the flow characteristic.
slightly round the trailing edge
Chamfer the trailing edge by the method I described above.
Change the leading edge to a different radius- note small changes make big differences.
