sergei Scotland wrote: ↑Tue Jun 04, 2019 12:07 am
....people say bigger fins make board rotate upwind more. Would be nice to understand why
Actually I'd think it would be other way round given the fact there is slippage downwind?
Bigger fins do not make the board round upwind. My experience is quite the opposite from when I experimented with large and very large fins on TT's. What I experienced with very large fins (and large fins at high speeds with a small kite) is what I would describe as "tail lock". This occurs only at high speeds, and there seems to be no way to turn the board with your body (you cant generate enough of a moment) as the fins seem "locked". Thus you cannot change course (turn) the board in a different direction. I have heard this referred to as "overspeed" sometimes.
But fins do provide a foiling force (sideways, not vertical like a hydrofoil) to stop what you could call sideways slippage on a given angle of attack (AOA) or linear direction of travel. The larger the fin, the more the force for a given windspeed. But again, too much fin, and what I have experienced is being unable to turn the board. The primary function of fins is to allow a more effecient planing angle (less edging) to the water's surface by reducing the amount you need to edge. Edging has it's benefits, but the efficiency of a foiled fin cannot be beat.
Just a side note: Fins are not very effecient at lows speeds, but edging is. Edging is not very efficient at high speeds, but fins are.
sergei Scotland wrote: ↑Mon Jun 03, 2019 11:58 pm
Thanks Matteo. So it is other way round : given the same edging angle, balance over the center of the board rotates the board downwind and loading back foot causes the board to rotate upwind? Correct?
Is this what you mean?
I think you have just stumbled on something that directly links windsurfing to kitesurfing. Leaning the sail forward turns you downwind, leaning the sail back turns you upwind. But do not confuse turning upwind or downwind with being on a tack with an upwind or downwind tack. Turning is a state of change, while being on a upwind or downwind path of travel is more steady. Think of a car. If you turn the steering wheel and keep it there, you are going to be constantly going in circles. When you turn a car wheel, then correct once you have turned in the direction you want, you are now traveling along a path in a straight line. If you are having issues with rounding upwind, you are doing so because you are turning the wheel and keeping it turned.
sergei Scotland wrote: ↑Mon Jun 03, 2019 11:58 pm
For clarity - we are considering a rider going for a while in the same direction roughly perpendicular to the wind and the only thing he/she is doing is changing weight distribution (more or less weight on the front foot)
If a board is flat on the water and not edged, the kite will just pull you downwind faster and faster until you reduce the apparent wind speed to the threshold of where your kite stops pulling on you. So weight distribution cannot be used alone with the board flat. But when weight distribution is used with a rail engaged to some fixed degree, then yes, some turning upwind or downwind should occur (with a wakeboard sidecut). I would suggest that this is not how you should think about it.
The first 3 principles all work together and affect each other. Changing one type of application of force changes the other, so that you cannot really control for one while varying the other. Even though the moment applied by the rider is distinguishable and separate from edging, changing that moment will change the entire system. Same with if you are edging to some force (and angle to the water) and you suddenly turn the board sideways with your body (moment) the edging forces change and it would be very difficult to maintain the previous angle of the board edge to the water.