RE: min/max fin size for my isonics question for ian fox
no Ian here but I'd like to discuss the "one foot off" rule, and hear from Ian about it too.
It seems to me that the rough guide of fin being more or less as long as the OFO measure was born when board designs where not so variated as today. A medium wind slalom board would be about 9'x21.5"x12.5" back then, with not much variations. And a good size for the fin would be 12" to 13".
The OFO rule probably comes from the fact that the wider the tail, the more leverage you have to keep the board/fin flat at high speed. Today, board design has been revolutionized by wider designs and different shapers have different approaches at it: no more "classic" recipe with variations, but much different recipes. One may find wide boards with wide tails to be used in the same (well, actually wider) wind ranges of older "narrow" designs; so I think that the relationship between tail width and fin length may still be valid, but actually could be useless. I mean that a board could carry a very large fin because of its tail width, but actually be used in such a wind range in which large fins are not the call.
You can get the last issue of PlancheMag and look at the slalom boards they tested; if you look at the measured tail widths and stock fin lengths, you will understand what I mean.