While the width of the board plays a role in the fin size, the width of the tail probably plays a more important role as well as how the board is to be used. The slalom boards have wide tails which means that they can carry larger fins than freeride or wave boards with narrower tails. Wave sailors/boards generally are NOT interested in speed, but they are interested in quick turning, and a small swept back wave fin does it best.
Slalom boards jibing at speed and in chop turn best with a more vertical, longer style fin. The radius of the turns is pretty large compared to a wave board, in order to maintain a plane) Smaller fins would be faster, but if you go too small, then the likelihood of spin out increases. Also, quick acceleration and planing out of each jibe is critical and the larger fins help significantly.
I looked at the select charts and they seem like an excellent guide for what to buy for the various types of sailing, but as you can see, there are a lot of variables.
You are right, if 5 different sized boards have the same size fin (25cm), they all offer the same lateral resistance to keep the board from sliding sideways. Let's say the smallest of the 5 boards is a speed board that is very narrow and is only used in a straight line, off the wind. The 25 cm fin might be too big because the lift could be too great and cause the board to "turtle" or flip over. The largest board of the 5 might be formula board. Since formula boards are designed to go upwind and downwind, they need large fins (70cm) for maximum upwind performance. With a 25 cm fin, it may be fine off the wind but the huge sails and lateral force on the board would make it a huge challenge to get upwind with any great success.
As you can see, how the board is used plays a huge role on fin size and design. Also, sail size is equally important since a 4.0 sail will have limited lateral force on the fin compared to a 12.0 sail.
So you have to consider the following in determining fin size:
1. Board size (volume, length, width and tail width)
2. Board design (speed, wave, slalom, freeride, freestyle, formula)
3. Board use (the board my be designed for wave use, but many will use it as a freeride)
4. Sail size (depends on board in use, wind speeds, type of sailing and sailor's skill level)
5. Sail use (downwind slalom or beam reaching or upwind)
6. Sailor skill level (beginners and novices generally use smaller fins since planing is secondary to learning the basics, plus short fins work better in shallow water)
7. Weight of the sailor (heavy guys need large fins since they can produce more leverage)
All this is pretty simplified and there is a lot more when you look at each fin regarding it's cord length, foil, flex, length and how much is it swept back vs a vertical profile.
I don't know if I have helped or not, but maybe some others can help out with a better explanation.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.
Last edited by Ken; 15th June 2011 at 12:22 AM.