There are so many other factors involved in fin performance that it's typically not worth the hassle to compute area. And this is not because I'm afraid of numbers... (I am a mathematician after all...).
In particular, in any multifin windsurfing board the total fina area (ie adding up individual areas) does not say much at all.
If areas for all fins were available and commonly used as a reference point (like with sails) then it would be OK, but since they are not, it rather complicates things to start involving this extra variable.
In practice with fins, you more or less have to go by individual comparisons and word of mouth. Ie, if you have a certain board and fin combo and want to change something particular. Go out to a forum such as this one and ask around. This will typically give you very exact info about how certain other fins will behave in relation to the ones you already have. It sounds fuzzy, I know, but it actually works. In other words, use the communicative power of modern computing rather than the calculation power.
Select to expand quoteMark _australia said..
^^^ standard fins in my 100L are MFC QS350 (350cm2) which is 9cm and 15.5cm.
I am yet to try a set of 300cm2 that I have - which has 14.5cm rears.- but given how the board works when it is windier I reckon it will be fine.
But K4 are so damned flexy I don't rate your chances with 14cm rears. Unless you are pretty light I guess
I used the 14cm K4 rears for 4 consecutive sessions in head high to half mast cross to cross off last week and they were EPIC! transformed the board in turns, especially the top turn (SSD Keel quad has super wide tail)
Only negative i could establish was in sailing upwind - wasn't bad, just wasn't as good as previous bigger, stiffer rear fins.
Small fins in big boards is an underrated combination i think
