RE: what fin size for a 6.8m on a S type104
I will emphasize again that the formula is not intended for experts. They do not need it. They could use it but it would have to be extended to a more complex matrix. I think that your example points out that the formula would work for you and Ian if you were to take the time to record enough data so that you could make the adjustments that work for you.
Now for the other 99% of us windsurfers: the formula was developed empirically using data from my friends with a range from beginner to very good (but I again emphasize not expert) with formula boards to wave boards and all kinds in between, with Drake formula fins and freeride fins to all kinds of brands of wave fins, from 9.8 to 3.7 sail sizes, over a 9 year period. I plotted the results and was astounded how consistent the data was given the huge range of parameters. That led to the formula, not the other way around.
It is easy to poke holes in any formula using extremes and exceptions, at the risk of loosing sight of the simple fact that this is a 9 year proven tool that provides excellent results for the vast majority of windsurfers in a huge range of applications. Again it would be fairly straightforward to extend the formula to cover the full range but that is not my intent The intent is to make life a lot easier for the huge majority of your customers and to keep it simple so that for those of us it is easy to use within our normal applications.
I could start poking holes in your arguments as it relates to my formula, but I think that it would be more effective if you kept track of the ?best? fin area for all your different conditions rather than looking at the possible ranges for the different set ups in conditions where you are not using your superior skills to make it work. An example is how you are able to use a terribly small wave fin that I would spin out like crazy, to get out to the waves, because that is the size that is ideal for surfing in, or in speed sailing when you bear off until you have sufficient speed to load the fin. Then if you find that there is not a consistent coefficient then we would genuinely have something to discuss. Or you could approach it from the other side and size the fins for your wife using the formula for a few months and see if she has any complaints.
You have also brought up a related but separate issue. A huge part of selecting a fin is deciding on the type of fin. And within that criteria there are a gigantic number of variables that are all very important on the performance. These issues are very well covered on some of the fin manufacturers? web sites and you have given some excellent examples. It is a related but discreetly separate issue from selecting the fin area. As my experience is that this is where most of us have the most trouble, this is what I have focused on.
Regarding your PS, this is relatively speaking what I did within my resources, so what is your point?
Now my PS, This has become the wrong place to discuss this and it is way off the original subject. It should have been under its own heading in the Bulletin board, but we have gotten to this point so we may as well finish it here unless you want to move it.