Twist and why should you need to adjust it during windsurfing and foiling.
Before explaining twist or loose leach we first have to look at relation between lift and drag. Drag and resistance are different words in general for the same natural phenomenon.
There are mainly three kinds of drag for all lifting bodies like wings, fins and sails, either in the air or in the water.
- Friction drag
- Form drag
- Induced drag
Naturally there is also something very specific drag like “wave drag” and “laminar drag and or turbulent drag“ however we leave this out for now.
For a lifting body with given dimensions these sorts of drag; all have different values at different speeds, angles of attack and generated lift.
Friction drag depends mainly on the surface area and increases with the square of speed.
Form drag depends on the width/thickness that slices through the air or water and also increases with the square of speed. So; thinner RDM masts have noticeable lesser form drag than SDM masts!
Induced drag depends primary on generated lift and the relative length of the lifting body. Induced drag is high at high lift forces especially at low speeds. However induced drag decreases with speed and even with the square of the length of a lifting body. Induced drag is visible in the tip vortex. For example when you see planes touching ground during landing, the tip vortex becomes visible as big curls at the end of the wing tip produces by the smoke of the tires. You could also state that the induced drag is the result or equal to the unwillingly agitated air volume at the tip left behind by the wing. So an infinite length of a lifting body indeed has no induced drag at all.
Twist or loose leach
Twist or loose leach is the opportunity available to designers and users of lifting bodies for wings-/fins- and sails to generate highest lift possible at the minimum cost of induced drag. In the evolution of birds like eagles/condors also bats and insects they have developed different ways to optimize wing efficiency at every different speeds. Eagles for example have slotted curved wing tips to lower induced drag at high lift and low speed due to their limited aspect ratios. Seagulls have developed long slender wings to lower induced drag and therefore they fly miles seemingly without any effort. Examples of human development to optimize wings can be seen in airplane wings, the sails of sailing vessels, surfboards, foiling etc. They all have a certain twist to lower the tip vortex especially designed at crossing speed. It is important to remember that lift/drag ratio or wing efficiency is actually different at every speed and generated lift. This means; for every speed and lift angle there is a different optimal twist.
The right twist
For windsurfers and foilers in particular; As explained earlier, twist in lifting bodies help to generate more lift at lower induced drag at the same time. The right twist makes the sail also more stable and more comfortable to use. For a smaller part, however especially for fin surfers and foilers, the right twist improves also the load distribution below the boom due to lower wind speed (shear) just above the water. The wind speed at the tip of the sail and beneath the boom are quite different. The difference between the top and beneath the boom due to wind shear will also be different at different board speed. You could say that apparent wind speed and wind shear are two side of the same coin that makes it also necessary to change twist during a session.
No planning no top speed
As we all know, sail producers indicate the right downhaul dimensions on their sails. However these indications are valuable for situations when there is enough and constant wind. Furthermore, these dimensions are designed for highest top speed in optimal conditions. In low wind conditions these specific downhaul dimensions make it very difficult to get planning! So no planning means no top speed. Getting planning as soon as possible is important.
Changing wind conditions from average and high wind conditions, make it difficult to have the best trim on the water all the time. With a low downhaul in high wind conditions just opening your sail is not enough to be fast and comfortable on your board. Again a reason to trim your sail during a session on the water.
