RE: what fin size for a 6.8m on a S type104
I'm trying not to be insensitive since I'm a mathemathician and hence I like both numbers and structure. I would love a formula to help me understand these thing better and to give better advice. However, there has always been something about your linear formula that has been bugging me. Often when I see a formula or some other type of "explanation" I get this feeling "right - thats it". Since I've had no record of the areas of my fins I have not been able to "see though" your formula from an experience point of view and I neither really see why it would hold theoretically (maybe it does (within limits) but I just do see why).
So, I just took out a few fins and compared their area, the Slalom pro 34, Drake Natural 24 and Drake Style 20. They all pretty much have the same area (around 220cm2, +-2cm2 or so, the Style being a tad bigger).
I don't have very much experience with the slalom pro except that is worked pretty good with a powered up 7.5 freeride sail. OK, upwind was not excellet, but still oceans better than any wave board with any wave fin.
The Drake Natural 24 is a fin I use a lot, usually with 5.3 in onshore conditions or with 5.5-5.6. Its noth a nice wave fin but also a nice freeride/B&J fin for say a Kombat and 5.3. In that setting it could easily handle 4.5 too.
The Drake Style 20 can be used with 5.3 or even bigger, but it then feels very small and very freestyle specific. For general sailing (on freestyle, crossover or wave board), I would prefer the Style 22 for 5.3. With the style 20 you need a fair bit of technique to go upwind.
So, what's my point? Well, first, using the Style 20 would be totally useless with a 7.5. I would say its easier to get planing with the SlalomPro 34 and a 7.5 than with the Style 20 and a 5.3 (different winds and different boards, but you get the idea) and the difference would be even bigger if you less good a sailor.
Even if you compare the Style 20 and Natural 24 on the same board (Kombat, fx) with, say, a 5.3, I consider the Natural 24 to be very easy to use and friendly also to beginners with that sail size, while the Style 20 is demanding to sail and a beginner would not have a very good time on that fin.
Using the 0,53 coefficient with my 70 kilos puts the (max) sail size for both fins at just below 6.0. Thats "super expert freestyle only area" for the Style and personally, despite being quite good with small fins, I wouldn't be totally comfortable in most conditions on the Natural24 with a 6.0 either (confirming my thought that these wave fins sails small, not only given depth but also area).
So, whenever you don't stay with the same type of board, sail and fin, it seems to me it get kind of hard to use the formula to predict fin size. However, it may well be that the fomula is still excellet for predicting how much you shoul increase fin size when you get a new bigger sail (fin type, board and sail type staying fixed).
PS: The other day I read an article about scientists that had measured the activity level of each of the 30.000 humen genes on 100 patients with a certain form of cancer. By using mathematics, they could from this very sparse data get a relevant visual representation where you could indentify two discinct groups of gene expressions. This predicted that there was actually two distinct forms of cancer and htis turned out to be clinically relevant (one of the groups tunred out to live longer than the other, statistically speaking). My point with theis ps is that given enough data, it would be possible to come of with lots of results on which fin that works best. After all, we would be dealing with less than 30.000 variables and could probably fins more that 100 people willing to report data to us. Not sure I'm willing to coordinate this though...