OK, I couldn't find the 1 foot off measurement for the F2 Stoke 145 liter either, but from the look of the board it's certainly in the 55-65 cm range I would think.
So, you have a fairly narrow tailed board so the 1 foot off length to fin length ratio probably is not ciritical in your situation.
You have also "introduced" something that may be more liked to technique or the type of fin you are using than to an actual fin quality issue.
If you are "spinning out" (i.e. "spinning" in your terms, pushing the fin loose by overloading it, in my terms) it could be a fin quality issue, but since it seems to happen when you get slightly up off the water after hitting a bit of chop, it very well may be that your "technique" has you pushing a little too hard on the fin when going upwind.
The fin works well when all of it is engaged fully in the water, but it "spins" when you continue to push on it with only part of the the fin in the water because your board has "gotten some air" due to hitting chop.
You could get a larger fin, but that might solve one problem (the spin outs) but create another (control issues because your fin is simply too big for the conditions.
You could change your stance a little (stand up a little straighter and don't push quite so hard on the fin, by changing your boom height and harness line length.
You could change to a more forgiving fin design ( I couldn't find a photo of the stock F2 Stoke 145 liter fin either).
If your stock fin is a swept pointer design (large curve radius on both the leading and trailing edges) you might want to try a more vertical slalom type fin to eliminate the spinning, but this may alter your turning and jibing performance a little.
Getting a 50 cm fin to replace a 48 cm fin is probably not going to change much (unless your stock fin has some quality issues).
If you want earlier planing, I'd suggest either a 52 cm or 54 cm for the fin size to be really noticeable.
BUT, what you gain on the bottom end, you may lose is control issues on the top end, so if you are going to be fully powered up, you probably want to use the 48 cm fin.
I think a Tectonics Tomcat 48 might solve your spinning problems, and it's an excellent fin, for sure.
Hope this helps,
Last edited by Roger; 20th May 2008 at 11:32 PM.