Complex subject I know but assuming that everything is tight and structurally sound. What can cause humming?
All foil/fins will humm at some point. Generally for our foils it is a poor release finish causing vortex and mismatches of water flow at release (a thicker foil will humm earlier).
The easiest way to alleviate humming is to very slightly neaten the trailing edge of your foils. Be careful here as you can actually also sharpen your foils trailing edge here too.
There is actually as few threads already on Seabreeze that cover this so I'll be brief. What works 99.9% of the time for me is,
1. Understand which side of what foil is essentially flat (i.e. the bottom of the front wing and the top of the rear wing).
2. First, using a cork block ensure that the release of these flat sides is just that = flat. Maybe just one or two passes from the fuselage out to the tips, ensure that the release finished inline/flat with the wings surface (no little production bubbles from clear coats drying or rounding). I use 800 grit paper wet for this.
3. Test once this is done. It is usually your rear wing, so you could even just try this one first.
If you're still getting humm, follow the same approach to the curved side of the wings. Ensuring you follow the contour to the finish. I try to keep some flat to the absolute edge so it if not sharp. Sometimes if it ends up too sharp, I sand at 90 degrees to the "FLAT" side of the wing just to dull it a tiny bit.
4. Test this. You should get results with only one or two passes done smoothly and in line with the according trailing surface.
5. If humming reducing but not go, then repeat with just one pass.
6. If humming is still aparrent, then it may be your mast. Same theory, clean up the trailing edge using the cork block ensuring you are continuing the flow of that surface. Do Not Sharpen the edge by increasing the angle!
7. All this should work. But keep in mind, depending on what foils you're using, at some point they will humm or whistle once you get to a certain speed.
Enjoy, good luck,
Ride safe,
JB
Great answer JB. When I got my NP windsurfing foil, I was told it was the symmetrical nature of the mast and the flow of water it created. I put a very thin strip of tape on one side of the trailing edge which (on the whole) got rid of the hum.
Made it easier to sneak up on my mates silently as they were plodding along waiting for a gust.
Finding out the cause of the hum can be difficult. If it varies with your height above the water it is probably the mast. Sometimes when I have tried light sanding of a trailing edge it has actually made things worse (before I did the Donaldson edge - see video)
I'd recommend identifying the cause first. If you have spare wings / stabilizers try them first. Also trying putting scotch tape along the trailing edges of the mast, wing and stabiliser with a 1 - 2 mm overhang. That normally kills any hum and allows you to find the source by a process of elimination (although the tape only stays on for a few minutes at normally foiling speed).
The best way to then get rid of the hum seems to be the Donaldsons trailing edge;
Thanks for the info guys.
I will try that Donaldsons edge sanding and see what happens.
I will report back when I get a chance to try it
Select to expand quotesimonp123 said..
Finding out the cause of the hum can be difficult. If it varies with your height above the water it is probably the mast. Sometimes when I have tried light sanding of a trailing edge it has actually made things worse (before I did the Donaldson edge - see video)
I'd recommend identifying the cause first. If you have spare wings / stabilizers try them first. Also trying putting scotch tape along the trailing edges of the mast, wing and stabiliser with a 1 - 2 mm overhang. That normally kills any hum and allows you to find the source by a process of elimination (although the tape only stays on for a few minutes at normally foiling speed).
The best way to then get rid of the hum seems to be the Donaldsons trailing edge;
If the answer is just Scotch tape, why not just do that.
Would it add drag (or actually even decrease drag).
Birds don't have stiff trailing edges
I'm not sure it is feasible to identify the cause of a noisy foil unless there's some damage to see. There are multiple causes and it can come and go depending on how much power you're carrying and how much speed and load you're putting into the foil.
I have never done any work on my foil masts. They seem impervious to any wear and tear. I did fix a little groove in the leading edge after getting tangled in fishing line. I recently scrubbed the mast with Scotch Brite. It came up nice and clean with a pleasing matte finish.
One of my foils had a series of micro pits in the edge. They could only be seen through very high magnification. That foil screamed like a banshee. I rubbed some epoxy paste into the holes, sanded and painted. The foil is now quiet.
Two of my foils developed a bit of noise after use. It was simply wear and tear on the tips. Small scratches and chips can be sanded out. Bigger scratches are filled with epoxy paste then sanded and painted.
A quiet foil developed a squeal after an impact with a reef. There was a small split in the leading edge. I injected epoxy with a syringe, clamped then sanded. Noise gone.
My Gofoils were quite noisy. A slight bevel on the trailing edge of the curved edge fixed that. As JB says, you need to make sure the trailing edge is not too sharp. I left a little sharp line on the trailing edge of a stabiliser. Unfortunately I can't fill the ding in my finger with epoxy paste. 
My kite foils make a noise like skateboard wheels at high speed in smooth water. I quite like that. It sounds fast.
My Naish 2019 1600 did not hum at all until I left it in the car fully assembled for a few days and it got torqued somehow. Now it hums. Doesn't seem to affect performance really, but I think it is about 1 cm out of alignment. Doesn't take much.
