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Francone said..
1 Are there any disadvantages in using too long a fin, aside from damaging it in very shallow water?
Too big a fin will develop a lot of power in windy conditions and demand a fair amount of back foot pressure to keep it under control. The board will become unruly more quickly with too big a fin.
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2. What determiners the appropriate length of a fin ? Is it the sailor's weight, the sail's size? the wind speed? is a 48 cm fin a better all-around fin for light winds than a 40 cm fin ?
Both the board width at the rear footstraps - which determines to a good extent how much leverage we can exert on the fin; and wind speed/expected board speed. In light wind conditions, we need the fin to develop more lift since the board can only move so fast through the water (I'm assuming 12-16 kts). When it's windier, and the sail has little trouble giving you enough power to move quickly, the board is traveling fast enough that a smaller fin will develop enough lift for you.
Naturally, a heavier rider will exert more force on the fin so can tolerate a slightly larger fin. Sail size is related to wind and board speed. You'll typically be using big sails in light winds where just getting and staying on a plane is your primary goal. When it's windier, you have, usually, sufficient power to getting and staying on plane.
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3.. I have been windsurfing for a number of years ( without claiming any proficiency beyond the strictly intermediate level,) , but I was spoiled for having always used an " unsinkable" (!) board like a 12 ft Bic Windsup. You never fall with it!
Now with a shortboard like the Bic 160 , my balance is severely put to test, even more so that I am 80 and age affect the sense of balance .
I can still manage, but I wonder if a longer fin would help stabilizing the board,, so that I don't have to..dance my way around" to keep balance , like when I used it for the first time last fall. It would be a welcome added bonus.
Not in a significant or noticeable way. A foil, however would as they are significantly longer and, more importantly, heavier. I see all the time 85-95kg guys on 135-145 l. foil boards. For most people with a normal sense of balance, it's a technique issue. I will speak in generalities here since you may already be doing all of the following things. One most be more mindful of (when not planing) keeping their feet on the centerline and not pressing unnecessarily on the heels especially when trying to counter force on the sail. They should remind themselves to press down with the balls of the feet which are towards the middle of the board. Make sure they aren't in the habit of looking down, at the water, at feet, athands, etc but instead looking up and out at horizon level. This is a big part of "balance". Your board is 82cm wide which is a good comfortable width.
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4. I was also wondering if the center fin interacts with the longer rear fin. Should it be kept on or removed, if I use a 40 or 48 cm rear fin?
Not in any way different than it does a shorter rear fin. We used to race longboards all the time with 44-48cm fins. However, a longer rear fin should be sufficient for going upwind. Unless you are going out on a day when you are sure you aren't going to plane, I'd leave off the front fin as, imho, it will make board handling a bit strange, add drag and not really improve things overall in any conditions where there is a chance you'll plane. Most of us sailing shortboards, even smaller ones, are fine without any sort of center fin.
A last note - it seems in looking at the specs of your board that the center and rear fin were intended for pretty early in the learning curve and they assumed, as the user increased skill level, they would be transitioning to a windfoil. Similar width Bic/Tahe boards such as the Techno 148 and 293OD came with 46-48 cm fins.