How Windsurfing fins work pt. 2: the kite conundrum!

[rigitrite]

Kiters effectively have no fin. so.......how do kiters go upwind?

discuss.

[isobars]

Clear evidence that we don't need foils, just a blunt surface and viscous forces, to go upwind.

[swchandler]

Ever followed behind kiters, particularly those with hard edged twin tips? They cut the crap out of the water with their windward edge. It must be remembered that many of their board designs lack any real volume, and as a result, they deeply cut into the water to provide directional drive.

[Sailboarder]

And kite racers use boards with fins...

I would guess because fins provide better sideways force to drag ratios than just using the viscous forces ?!

[U2U2U2]

[swchandler]

Actually, Naish makes a wooden kiteboard without any fins. A friend of mine has one that I checked out. Mind you, I'm not a kiter, but I still try to notice what's going on with our brothers in the wind.

[boardsurfr]

Good point! When Caesar Finies visited us a couple of month ago, he never put a fin into his windsurf board in light wind, either. Occasionally, he has sailed without a fin in planing conditions, too.

You don't need fins (or lift from fins) to windsurf or kite surf. But kite racing illustrated wonderfully that you will go faster if you use big fins. Check the size of kite racing fins - 36.5 to 42 cm at ristafins, to give an example. And they usually use 3 fins. Check this article in the ksurf magazine for pictures and more examples.

The big racing fins have made kites as fast as formula and slalom boards around a triangle course, and sometimes faster. But they are left in the dust by hydrofoil kites - check the "Kiteboarding has become Kite Foilboarding" article in the Scuttlebutt Sailing News - here's a picture from the article:


While kite foilboarding relies on the lift from a hydrofoil, I think a large part of the speed increase comes from eliminating all contact between the board and the water, since the board is about 2 feet above the water surface. Anyone who has ever speedsurfed in chop and in perfectly flat water will know what I mean.

[DanWeiss]

Perhaps an additional difference may also prove significant: direction of pull from the kite lines versus the boom. The boom pulls in a rotational manner that spirals as the mast wants to slam down to windward. This mast base pressure pushes the board's nose down and too leeward to counteract our mass that drives the tail and windward rail into the water and also to leeward. The fin provides torque on fin box (rolling the windward rail up) and our mass counteracts that torque, ideally providing a balanced vector to what's going on up front with the rig.

The dynamics of a kite board include similar components but lacks an important aspect of rig force that differentiates the fin requirements of the respective gear. There is no mast base pressure on a kite board and thus little force to drive the kite's nose off the wind unless the kite is too low and forward in the window. Without the leeward yaw component, simply sailing a freeride kite on its edge or with tiny fins provides enough grip for stability. A race board works differently. Like FW boards, it is a combination of the high aspect planing surface and very deep fin(s) that produce massive power upwind that can only be contained by a sailor located far from the fin. While some volume of a race board can be a function of sub-planing ability, I rather suspect the tail thickness is there to seat the deep fin boxes, to provide stability from rail volume and link the rails structurally at low weight. And since race boards do run the kites low and forward to optimize the pulling angle, most race boards are equipped with multiple fins, some forward than the others to counteract the yaw of the board when driving upwind and to avoid instability by locating a single fin between the footsteps.

[boardsurfr]

[dvCali]

Main reason Kites, other than racing Kites, are so slow across/upwind: they use the board as a fin. Same way you can go upwind with a big Wind-sup without a daggerboard ... very inefficient but it works.