thruster fin physics

[mac]

I'm not positive about any of this, because my hydrodynamics courses are over 30 years in my rear view mirror, and my experience with tri-tips nearly as far. But many of the factors pointed out here--rocker and mast track placement for example have a great effect on board handling regardless of fin characterization. Most of the past 30 years I have been on go-fast fins, formula or slalom. They turn fine if you have good technique, and other factors like board width and rocker have more to do with the predictability of turning than the number of fins--in go fast mode.

The fundamental purpose of fins is to give you lift to point the board up, or downwind. It is pretty straightforward--lift is a factor of the shape of the foil (wider chord = more lift and more drag) and the area of the foil. So a tri fin with the same area and profile as a single fin will give you roughly the same lift. The primary issue, and a disadvantage to short fins, is that air gets sucked under a board in really rough conditions, and reduces lift. On the other hand, shorter fins are easier to manipulate to re-attach the flow.

I don't think fins have that much to do with how easy a board is to turn, or how to initiate a turn, when you first start. All the force to turn a board is provided by the rail, and the rake of the sail (bend your knees) determines the angle of attack. Wave boards turn more readily than slalom boards because they're built that way, and it is rocker and tail width that do most of that work. But the advantage of multiple fins comes when the board heels over, the water is turbulent, fluid hydrodynamics doesn't really predict behavior, and so forth. At that point you have more fins providing some counter, or tracking force, as the board turns. If one spins out, chances are one or two will still provide some balance. Thus, I buy the "stickier" argument.

As zirtaeb points out, three fins that are not aligned in a straight line are a different matter, they cause divergence or convergence of flow lines. But that is liable to increase drag--that is, slow you down.

Last note. Most small fins not build by talented custom fin makers have more drag for a given amount of lift than slalom or formula fins. Thus, they slow you down. That can be exactly what you want when you are working a wave or swell. All designs involve a series of compromises, and wave boards are much easier to turn quickly--in part because they are slower than slalom boards. No matter what fin or rider is involved.

[U2U2U2]

Some interesting observations on this thread, I would think some replies from Cobra built Tri fins would have made this thread.

Many fins can be used for side fins, dependent on the fin box installed or potentially installed.

The fins by MAUI ULTRA FIN, BLACK PROJECT FIN, ARE CRAFTED nicely.
K4 being a pioneer in plastic composite fins which offer toe in and asymmetrical on Graham Ezzy signature fins are all leading edge technology.

Witchcraft and Flikka make their own fins and compare to the best made.

[scottwerden]

Most if not all of the R&D into fin/foil design and fin configuration in windsurfing is done as a seat of the pants operation. It is expensive and complicated to do a thorough design based on computer modeling and tank-testing like they do with America's Cup yachts. Windsurfing is a not exactly a growing, lucrative sport so it has to rely on creative thinking rather than science and detailed engineering. That is fine, there is nothing wrong with that. The problem is, the boards and sails are changing at the same time as the fins so when something feels right it is impossible to know if it is the board, the sail, or the fins. Basically you have to be happy with the combo you get and not worry too much about what exactly it was that all of a sudden made you into a Robbie Naish type sailor.

[U2U2U2]

Above post:

The surfing industry does seem to do more technical testing.

Windsurfing, Witchcraft have used various methods to shape foils,
What and how is probably proprietary, all the fins I have from them are great.

I agree that seat of the pants testing has many variables, but some locations have consistent conditions, and testing can be very informative , to what works or doesn't work, doesn't work is easy to gauge, it's how efficient is harder.

I don't feel you need to be a world class sailor to judge what work for yourself, however it may not transfer to a broad spectrum .

[isobars]