Introduction
The hissing sound of a foil often comes from square or chipped trailing edges.
Once in the water and with speed, this creates turbulent flows along the surfaces of your foil. (small vortices after the trailing edge causing annoying noises and potentially vibrations at high speeds. The shape of this trailing edge is one of the main factors in this noise problem.
Karmen vortex
To dig a little deeper and understand whistling: It’s a fluid dynamics phenomenon called the “Karmen vortex”.
As the fluid flows around the profile, two vortices are generated by the two square corners of the trailing edge. Each vortex is a low-pressure vortex, and they tend to follow one another.
A multitude of vortices, some coming from the top of the trailing edge, others from the bottom, follow one another, creating an oscillating force acting on the trailing edge of the airfoil and thus a whistling sound.
It’s the same phenomenon that makes overhead power cables whistle, or that caused the Tacoma Bridge to vibrate and then collapse in 1940.
To reduce noise, a commonly used solution is a “Donaldson” trailing edge. This solution was “discovered” in the 1950s by the aeronautical engineer of the same name.
How can I solve the problem with my foil?
If your foil whistles, the first thing to do is check the trailing edge. If you find any rough spots, you can sand them down with fine sandpaper.
If the trailing edge is smooth, you can check the wing assembly. Make sure all parts are tight and that there is no play.
If you have checked the trailing edge and wing assembly and the whistling persists, the wing may be defective. In this case, you will need to contact the manufacturer or dealer for a replacement.
Here are the steps for sanding the trailing edge of a foil
To solve a whistling problem, simply sand the trailing edges very lightly using this method, to obtain laminar flows thanks to a fine, regular trailing edge.
- Gather the necessary tools. You will need :
- Water sandpaper (p220 and p1000)
- A sanding block
- Wet the sandpaper. This step is important because wet sandpaper does not work if it is dry.
- Place the foil on a stable surface.
- Start sanding the trailing edge.
Be sure to identify the bottom surface of your wing or stab, as this is where you’ll be sanding the trailing edge.
- Use wide, even strokes. It is important not to sand too hard to avoid distorting the wing profile. Carefully use wet sandpaper on the front wing and stabilizer, following specific angles and steps.
- Start with the coarsest sandpaper (p220) and end with the finest sandpaper (p1000). The aim is not to make scratches, but to slightly reduce the thickness of the trailing edge. Sanding should be very light and even. The trailing edge should then be trimmed with a shim to ensure the sanding is as even as possible.
- Sand the trailing edge at 45° using p240 and a rigid shim, to obtain a smooth, sloping trailing edge. Move slowly and evenly to avoid distorting the trailing edge. Usually 2 to 6 round trips are enough.
- Repeat the operation with a p400, going back and forth 2 times, and finish by rounding off the last small angle between the surface of the profile and the trailing edge.
- Repeat operation 2- with p600-p800 or p1000
- Slightly round off the trailing edge at the end of sanding. This will help reduce vibrations.
Results
Once you’ve properly sanded the trailing edge of your foil, you should notice a reduction or disappearance of the whistling.
Warnings
It is important to be careful when sanding the trailing edge of a foil. Edges can be sharp and dangerous.
If you are unsure about the procedure to follow, it is best to consult a professional.
Once finished, you’ll probably notice a reduction in vibrations and a quieter foil. Caution is essential, as sharp edges can be dangerous.
Always wear work gloves when sanding, and be aware of the associated risks. any person deciding to carry out these procedures does so with full knowledge of the risks and under his or her own responsibility.
Before you begin, please read the warranty policy carefully.