What force turns a twintip?

[sergei Scotland]

2. Rail engagement with relationship to primarily sidecut shape (rail), and secondarily, the amount of rocker.



For #2, a diagram of standard wakeboard sidecut vs reverse/snowboard sidecut.
Sidecut.jpg

So you can see that when you engage a rail that is curved, the average of that curve acts as a keel (not foiled). This is the primary turning force you can feel even when you do not try to induce a moment (by twisting your body), but rather lean back and to one side (don't lean back as much on a reverse/snowboard sidecut - you stay more centered).

1)In other words when I am edging with weight centered I have a keel or fin along the whole length of the board. When I edge and move body weight on my back foot more, nose goes up and I get a big keel at the back of the board but not on the front. Because of the pull downwind we have the force of water pushing on the keel horizontally away from the kite /upwind. This force rotates the back of the board upwind turning the board downwind. Or I could say the keel stops the back slipping while the nose is slipping downwind a lot because of the general downwind pull of the kite.
Correct?
2)So it looks that I need to edge more with weight on the back foot as soon as I am on the board. This should stop me rotating upwind. Correct?
3)And opposite is - to rotate upwind I need wait more centered over the board to allow rocker and kite pull on my fixed hook to rotate me and board upwind. Correct?

[sergei Scotland]

Basically my question is : both hook pull and rocker are trying to rotate me and my board to go upwind (too much).

It sounds like you are fighting the fins. The further the footpad is from the fin, the more leverage the fin will have. Try widening your stance width or moving the fins closer to the footpads. Or use smaller or no Fins.

Front or back? Or both? Why are fins rotating me upwind?
Could you explain what you mean? Thanks!

[Matteo V]

So it looks that I need to edge more with weight on the back foot as soon as I am on the board. This should stop me rotating upwind. Correct?

NO! Only edge hard when you have the speed.

When you are trying to get up out of the water, you edge hard to resist the kite and stop from reducing the apparent wind by going in the direction of the wind.

As your body clears the water, you want to NOT EDGE SO HARD so you can build up speed. Yes, you go downwind a bit, but the best way to build speed is to bear off slightly (downwind) of a line perpendicular to the wind.

Once you have speed (usually within one stroke of the kite), then you edge back to a line perpendicular to the wind (and thus not losing any more ground).

Then you start edging hard. BUT NOT TOO HARD! If you edge too hard, you head upwind and actually lose speed and power. That is how we used to depower non-depower kites (or c-kites with poor depower) back in the day. Edge hard enough to where you do not lose speed. If you do start to lose speed, you need to point back on a line perpendicular to the wind, or possibly a tiny bit downwind, then try again.

[sergei Scotland]

OK how about - as soon as I am on the board I need weight on the back foot to stop kite from rotating me upwind?
I am probably too centered on the board as soon as I am up and it makes it easy job for the hook/kite pull to rotate me and the board upwind. Keeping weight on the back foot more should create the anti-slip force on the back of the board (keel) allowing board to go without turning upwind too soon?

[sergei Scotland]

Basically from practical point of view-do I load back of the board to go downwind and center my weight more to gradually go upwind? Correct? Is this how it works?
I kind of deliberately ignore any body movements like throwing board around wave surfing style. I want board to turn under me "by itself" by using water induced force on different parts of the board smartly. After all it should all be effortless once I get it working, correct?

[tkaraszewski]

I’m not sure at this point what you actually want to accomplish. Are you having trouble getting the board to turn - I.e. it goes in a straight line and won’t rotate. Or are you having trouble riding downwind, but not turning - I.e. you can make the board point the direction you want, but not keep riding that way? Or something else entirely?

You’ve asked for a physics lesson but I think you actually want a technique for riding downwind effectively.

[Matteo V]

OK how about - as soon as I am on the board I need weight on the back foot to stop kite from rotating me upwind?

No, you want to use front foot weight as it will help you head downwind more. Temper this with the idea that you can start "pearling (sinking the nose of the board)" if you have TOO MUCH front foot weight. My assessment is that on a TT, you should never have more than 50% of your body weight on the front foot. For comparison/range, the back foot can go from 50% to 95% of your body weight. But that is just body weight, you also need to use toe-side pressure, and heel-side pressure to engage the upwind or downwind rail.

I am probably too centered on the board as soon as I am up and it makes it easy job for the hook/kite pull to rotate me and the board upwind.

Too centered (weight fully forward at 50/50) will allow you to go downwind better. But if you have too much front foot pressure, you will "pearl" the board - resulting in a faceplant. If you have not face planted by sinking the nose of the board, you have not been too far forward on the board.

Keeping weight on the back foot more should create the anti-slip force on the back of the board (keel) allowing board to go without turning upwind too soon?

Backfoot pressure with heel-side pressure will round the board upwind. Backfoot pressure with toe-side pressure can still make you go down wind. It may be that you are not getting the heel to toe-side pressure part of the equation.

Work on not edging with your heel side so much, and not leaning back on the back foot so much. Get a feel for this, but you won't necessarily feel success. It is not until you add in "pointing the board" via the rotational moment, that you can experience what different angles of movement to the wind do for you.

[sergei Scotland]

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?
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)

[sergei Scotland]

I’m not sure at this point what you actually want to accomplish. Are you having trouble getting the board to turn - I.e. it goes in a straight line and won’t rotate. Or are you having trouble riding downwind, but not turning - I.e. you can make the board point the direction you want, but not keep riding that way? Or something else entirely?

You’ve asked for a physics lesson but I think you actually want a technique for riding downwind effectively.

No I just want to know how you change direction on a TT, preferably with an explanation of why and how this works.
I understand that strictly speaking millions of riders just do it. I guess I just like to understand things not just use them. I am sure a good board designer understand this things and can achieve better results because of this.
For example - 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?

[Matteo V]

....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.

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.

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.