Removing the Noise/Vibration from Foils

Well tuned foils, and mast improve the experience.

You buy a new hydro foil, skeg, keel, centerboard, daggerboard, rudder, and it makes noise. It could be a low drone, humming, or a whistle. This is annoying and might be effecting the efficiency of the foil.

While studying Naval Architecture I use to meet with my favorite professor Ernest Brierley in his office. He would take time to help me further my knowledge on a subjects of my choice. These meeting were beyond the normal curriculum. One day I said something about keel/rudder humming since I was drawing the foils for a new 33’ design. His great response, “You have never heard of Donaldson Trailing Edge?” “Nope”

This was 1986, no internet, no cell phones, just good old books. When knowledge meant you read research papers. So I dove in at night, and we talk a few times more on the subject. Thanks Ernie for so many great lessons.

Donaldson trailing edge is the best I have found and thousands of others have found to remove the noise from the trailing edge of foils in water. Since all of this information is copyright I will add links so you can read the papers and see the images.

In 1956 Donaldson did the research and came up with this very simple but effective way. Does it stop 100% of the noise all the time, no, but it does let you fine tune it. And it is better than a 45 degree chamfer, or square trailing which most use. Great paper on the subject. There are some very smart people who wrote these paper, and it is a fascinating subject.

Since most foils are not built to a knife trailing edge which would just break or get damaged, unless it is a pro-program and keeps getting constant attention. The rest of us there needs to be a better solution, and for foiling boats and foiling boards a safer way. Knife edge on a recreational wing board foils is not a smart move.

Most foils are built with a round or squared off trailing edge for simplicity in construction. Some will come from the factory with no noise at all. Having now tuned a lot of different hydrofoils, plus decades of skegs/keels/daggerboards/rudders, some have low or high frequency noise from the same model. There are a lot of reasons that this happens and construction simplicity is just one of the reasons. I will cover another big reason in my next post.

Here is how I go about putting a Donaldson Trailing Edge on our beautiful Armstrong carbon wing foils.

If the trailing edge is rounded carefully bring it to a square edge from the angle of attack (AOA). You should have a nice edge mostly on the low pressure side. The high pressure side will get the Donaldson Trailing edge shape.
I tape off the low pressure side right to the trailing edge. You need to make sure this is a fair run off to the trailing edge you just set up.
Then use calipers and measure the trailing edge thickness. Take 1.8 times the trailing edge thickness, and put tape on the high pressure side at that distance. If it is a symmetrical keel, board, or rudder you just pick a side. Both of my A31 keels from France had Donaldson trailing edges in the lead from the factory.
This gives you the area to sand in the Donaldson Trailing Edge.
I also use a mirror and magnifying glass with our hydrofoils. This lets you look down the trailing edge with the mirror and magnify that image since you are dealing with millimeters on our hydrofoils
I take my time and try not to sand all the way on my first sanding. Remember this is a 45º with a 3 rd degree polynomial curve. Get it started and go use the foil. Listen for the change in sound. It should go higher pitch if you are doing it correctly. And it should start at higher speeds.
Normally it takes me three to four times to sand, sailing in between to hear the change. The last one usually is just a small touchup at 1000/2000 grit wet pads.
With our wing foil gear I start with 220-400 wet/dry paper depending on the thickness, then 600, 1000, 1500, with a final buff with 2000.
Take your time it needs to be very accurate work. Have great light on the sanding area, and check often.

I hope this helps you control the noise and vibration on your foils. We can all thank Donaldson and team for a simple effective way of solving the problem.

Important note: Some of the hydrofoils I have sanded this helps a lot but has not removed all the noise. In the next post I will show how I have made those foils silent with better control and speed.

Get out on the water and have fun every day,

See you soon, Paul