RE: what fin size for a 6.8m on a S type104
Following is my entry for your challenge to provide a formula. These should all be added to your new Windsurfing 101 section of your website. If my entry wins please send the prize c/o HSM, Maui. Thank you.
REQUIRED FIN AREA
A general guideline for required fin area in normal sailing conditions is:
Required fin area (cm2) = 0.53 x sail size (m2) x sailor weight (kg.)
A smaller fin will result in spinning out and difficulty to point upwind.
Larger fins are unnecessarily slower from additional drag and if much larger result in instability and lack of control.
You can modify the coefficient to make it suit your personal preferences, which will then give you a reference for different types of boards, sail sizes and conditions.
As fin area is very sensitive to speed, experts and racers can use smaller fin areas with a coefficient as low as 0.36*. (Ian please feel free to modify and add to.)
REQUIRED SAIL SIZE FOR WIND CONDITIONS
Your sail size for a given wind speed depends on the type of sail construction, your ability, preference to being over or under powered or if you are racing, but a good rule of thumb is:
Sail size (m2) = 2.47 x sailor weight (kg) / wind speed (kmph)
Sail size (m2) = 0.7 x sailor weight (lbs) / wind speed (mph)
Sail size (m2) = 1.34 x sailor weight (kg) / wind speed (knots)
REQUIRED BOARD SIZE TO FLOAT
For a board to float you in stagnant conditions your board volume (litres) needs to be your weight (kg) plus 25.
This allows 10kg for the board, 10 kg for your rig and 5 kg for your personal gear.
Any forward motion of the board will significantly aid in keeping you floating which allows you to reduce board size.
SAIL QUIVER SIZING
There is no specific correct answer to the right sail spacing as it is a personal choice of the compromise between bigger gaps that give a wider overall range of sails for more time on the water or a narrower range so you can size more precisely for the wind speed.
Generally a ratio of sail sizes of less than 13% is not significant enough to justify the extra expense of having more sails. On the other hand a ratio of more than 20%* can make it difficult to pick the right size.
(*This is our preference but Ola seems to think this should be a bit larger, feel free to change it based on your real data.)
MAXIMUM SAIL SIZE FOR A BOARD SIZE (This is an iffy one)
There are too many factors such as board type, shape, width, tail width to provide a formula for the maximum sail size for a board volume. On average this may provide an estimate:
Max sail size (m2) = 5.9 x Board size (litres) / sailor weight (kg).
Minimum Board size (litres) = .17 x sail size (m2) x sailor weight (kg).
Add the info from your catalogue