Here's one I never fully understood... How does my fin know what size sail I'm using? (It can't see it you know).
The fin that works with my 6.5 is way too big when I'm powered with a 4.5 on the same board. It seems reasonable to assume that the force transferred to the board via the mast base and my feets, is pretty much the same for either rig. And, I would bet that I'm generally going faster on the 6.5 (flatter water) so more lift would be generated by the fin than with the 4.5.
Anybody?
cv
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I'm not sure I have the whole answer, but I have thought about this somewhat. My take is exactly as you say - you are going faster with a 6.5 than a 4.5 so the apparent wind is moved farther forward and you may be sheeted in harder. This means that there is more lift and a larger sideways (rather than forward) component of the sails lift for the fin to counter.
The largest fins are needed with the largest sails. With say a 10 meter you may be planning in 8 mph, and sailing maybe more than 2x windspeed (say 18-20 mph). Apparent wind really far forward, lots of lift.
With a 6.5 maybe you plane in 13 mph and go 20-25 mph. With a 4.5 maybe its plane in 18 mph and sail 18-22 mph. (Numbers are just guess, but the ratio of windspeed to boat speed is definitely higher with bigger sails and lighter winds).
Also with a bigger fin you can sail upwind more and use that to control you power - i.e. in a gust you head upwind.
Finally, when the water has more chop there is more tendency to tail walk as the chop creates a problem in keeping the board flat. This is when the big fin becomes way too big. In flat water I can use a much bigger fin than I can in choppier water, even one much bigger than the tail width of the board.
Randy
What happens in a black hole stays in a black hole.
.... it just knows,
signed,
the fin whisperer
I think some guys in suits sit around in Monday moring meetings and try to decide how to inhance their bottom line, by convincing the windurfing public that a whole array of fins at $125 a pop are necessary to be the biggest and best on the water. Sure there is a need to change fins from a 12.5 to a 4.5 sail but how many are needed inbetween.
The suits, described earlier, are engaging in a pratice called, Incrimental Income Production. It goes on everywhere, every Monday moring from 9:30 to 11.
How many fins from 12.5 to 4.5? Probably less than the number of boards most people would have for that range and almost certainly less than the number of sails.
I figure for my Techno Formula I need 3 fins - 65 cm, 51 cm and 40 cm weed fin. (May not be optimal but those are the ones I've got.) For 9.6 I star with the 65, for 5.8-7.2 the 51 and for 5.2 the weedfin. If I'm underpowered, I'll go to the next bigger fin. If tailwalking, I go down a size. I've seen a lot of times when I was planning a lot for an hour or 2, but then the wind dropped. Instead of going to a bigger sail, I go to a bigger fin and start planning again. If that doesn't work, I go home or rig a bigger sail.
So you can get by on one board and 3 fins (probably 2 w/o the weedfin.) So maybe that makes it all cheaper since you need fewer boards and sails.
If you have lots of boards, you probably need a fin for each one anyway, but still 3 or so should cover it. (Of course, they probably all have different boxes.)
As for the suits, I figure that they work at it a lot more than 2 hours on Monday on selling us stuff we don't need, but then if everyone only bought what they really needed, our economy would only be about 10% its current size. However, good fins don't have to be expensive. I got 2 really great fins (39 and 34) cm for my Explosion from Gene's consignment for about $25. At Gene's you could get a great shortobard and all the fins you would ever need for less than $200 - less than half the cost of most new sails.
Randy
(A paid endorser for Gene's Consigment - the Ebay of East Lanier.)
What happens in a black hole stays in a black hole.