In a 1993 issue of Windsport (the Canadian magazine), there was an
article called "Fin Magic" which, in short, stated that there is a simple
was to calculate the optimum size of fin for a given sail (someone else
posted on this earlier). For the purposes of the article "size" is the
surface area of the fin. This measure of size is claimed to be best as
it allows aspect ratio to be ignored. The fin sizing method assumes that
there should be a constant ratio of fin to sail area which will result in
a "magic number". A survey of good sailors was conducted to determine the
magic number. The result was that for a givensail size, in m, the optimum
cross-sectional area of the fin, in square cm, can be obtained by multiplying
by 41. For a 5.5 m sail this yields 225 square cm. They claim a magic number
of 41 is applicable for sails of 4.0 to 6.0 m size and 45 is applicable for
sails 6.5 to 7.5 m.
Intrigued by the simplicity of the concept, I traced 6 of the
fins I was familiar with and calculated their area. I was then able to test
the formula using my experience using a range of sails with these fins.
The good news is that, yes, I obtained a fairly consistent ratio, however,
my personal "magic number" was different and ranged from 52 to 55 for 4.5 to
6.2 m sails. This is a significant difference, for example, for a 5.5 I would
be using a 290 square cm fin rather than a 225. This is the difference between
a 12 in blade and a 13.5 in blade!! A quick look at recommended fin sizes in
brochures and articles also suggests 41 is too small.
Drag out some graph paper and try it for your sails and fins. I
would be interested in your findings. For reference; I use 8'6" and 9'
dedicated slalom boards, fully cambered slalom sails and a mix of pointer
and blade fins for mostly flat water sailing in chop. I weigh 168 pounds
John Broome, Ottawa, Canada
(saw first snow today, how depressing)