Roger Jackson fra Starboard om finner:

Roger Jackson Starboards technical Guru
Nowadays with an increase in technology the developments in windsurfing are becoming more advanced and evident in the performance of the boards each year. Even with all this improved technology it is important not to forget about the basic concepts that are very often overlooked. Windsurfing as a sport is very technical that is why we caught up with Roger Jackson to see if he can explain a bit more in detail the effects of fins, fin sizes, and what we should expect to feel if we are sailing with an over sized fin or alternatively a fin that is too small. Roger offers technical support on our forum and has many years of windsurfing experience.

Firstly what effect does the fin have on the windsurfing Board?
Although the fin may seem like a relatively small component, it is perhaps the most important. The water flowing around the fin provides the same kind of lift as the air that is flowing past the sail. Modern windsurfing rigs develop 2 basic force components. One force (the helpful one) drives the board forward and the other (less helpful) force pushes the board sideways. The fin provides the "resistance" that counteracts the sideways force, provides some lift of its own, and allows us to steer the board. Without a fin, the tail of the board would "slide off" sideways and downwind.

The goal is to have these forces "balance" in order to achieve the desired level of speed, upwind angle, and/or looseness for whatever style of board is being sailed. A sail has a much larger surface area than a fin, but because water is roughly 1000 times denser than air, the small size of the fin is able achieve this balance. Important factors to consider when selecting a fin are sail size, the sailor’s own size and weight, tail width of the board, and the type of sailing (racing, slalom, waves, freestyle, etc.) I recommend trying the stock fin first, and then getting a few other fins to give you a wider range of use. This might include a fin that is 4–6 cm larger (for light winds and early planning), a fin that is 4–6 cm smaller (for good control in higher winds), and possibly a weed fin (for shedding weeds, and/or shallow water). Larger, heavier sailors would probably prefer a larger fin, while lighter, smaller sailors will often prefer a smaller fin.

What effect do different shaped fins have on different boards? If you were to compare the fin on an Acid with a Futura for example.

The outline shape (planform) of different fins are developed to do specific things or more clearly, to give the best performance in specific disciplines of windsurfing.
The master craftsmen who designs fins do lots of testing to develop the optimum planforms and foils for these specific disciplines:

Wave Sailing	Very "curvy" planform, soft "twisty" tip; Sizes: 18 –28 cm
Bump and Jump	Curvy planform, less tip twist, better upwind; Sizes: 24–34 cm
Freestyle	Special planform, enabling tail first sailing and slides, for freestyle tricks; Sizes: 18–26 cm (This picture is a Flare production fin that has been cut down which is what many of the freestylers do!)
Free riding	An all round fin, goes upwind well, jibes well, can be fast in the right conditions. Not the best at anything, but a compromise that is pretty good everywhere. Sizes: 28–52 cm
Slalom racing	Emphasis on speed and keeping the sailor upwind. Jibes OK. Sizes 26–52 cm
Formula Racing	Emphasis on upwind speed and maximum VMG, technical jibes; Sizes: 60–70

So to answer the question, a smaller, curvier, looser fin would work better on an Acid. A "loose" fin is designed to allow the fin to "release" its hold deliberately in order to slide on a wave, or over the water for tricks. A larger, more vertical fin would be better suited for one of the larger Futuras or iSonics, and will provide a better lift/drag for increased speed and better high–speed control.

How would your board react if your fin were too small?
If your fin is too small to properly "balance" with your sail size, you will experience too much sideslip. If you push too hard on the fin and exceed its ability to provide lift and drive, you will experience the dreaded phenomenon we call "spin out". The fin seems to release its "hold" on the water and you slide, or "spin" the tail of the board downwind. Properly designed small fins can be fast (less drag) but you need to be able to get up to a speed where the fin has the ability (more speed = more lift) to resist all the forces being applied to it by the rig and by the back foot’s pressure across the top of the fin. If you push too hard before you have the necessary speed, you will spin out.

How would your board react if your fin were too big?
If you have a fin that is too big, it may cause an excessive amount of drag and you will run into some control issues. Fins are all about the "lift Vs drag" ratio. Too much drag will limit your speed and too much lift will affect your control. The board can begin to "tail walk" where it seems to ride up off the water on the fin and the board becomes increasingly difficult to steer. The board wants to kind of "shake its head" (heading here and there) and you cannot control the direction.

Spinout can also occur. In this case, it is due to the sailor’s inability to control the attitude and direction of the fin, rather than from overloading the foil. Balancing the length of the fin with the tail width of the board is important too. A wide board tail helps to balance the lift of a large fin, but the same fin on a narrower board will not be balanced and will cause problems.