I have a fanatic falcon 121 the 2011 that I put a 8.5 overdrive on what fin size would people recommend for downwind and figure eight with an 8.5?
I have 44cm t-43 but I find that I am spinning out all the time when heading upwind when marginally powered and keeping the board flat. If I pretend its a wave board and bank it over to windward or I leave my back foot out of the strap, its not a problem, it's especially bad when running the sail with negative out haul. I can't imagine any of things I am doing are making it fast.
Yesterday was particularly frustrating, the wind was 11-14knots (as recorded but the local station not sure what it really was but not enough to plane all the time).
Putting the boom up helped a bit.
The 44cm seems fine with 15knots and above, but I have not enough experience to tell if I could cope with more, or I may I just need to refine my technique? Maybe I just need to flatten the sail once up on the plane with the outhaul, in these conditions, but just guessing...
Would prefer to stick with one fin, one sail, and work on my technique, though could be convinced to get two one for normal use and one for very light winds, as this is my biggest sail/board combination.
I have a Falcon 2010 133 , using Talon 46 with 8.5 Koncept. I also found that it spins out much more easily than other boards, specially with a bit of chop. Moving the mast track a bit forward and being careful with not loading the back foot too much helped. It's not ideal and I am still trying to work out the best trims, because the board is very flat and tends to hit the chop going downwind...I tried with a bigger fin, but in powered up conditions it made the board very hard to control.
Hi RacerX
You will notice with the Falcon race boards that the fin box is placed about 10-15mm further back (towards the tail), compared to nearly all other slalom boards on the market.
(check next time you are down the beach)
I'm not saying this is a bad thing, - just a part of the design.
But it do require a bit more "refined" trim and definitely a good fin, that deliver a lot of control and can handle good pressure through the back leg. Other vice you can quite easily overload the fin and you can get spin outs. Or in the case of using a bigger fin, the board ride up and out of control.
I did try / test some never Fanatic slalom boards over the last few years and I quite liked the ride as they sit nice and free on the water (especially flat water).
The placement of the fin box further back, is adding a little more direct pressure on the fin, so there is a fine balance in riding the board free on the water and not over powering it to spin out or lift the board to much.
If you went to the other extreme and moved your finbox forward on the board you would notice the trim of the board to be very flat and "life less" (not enough lift).
It is a balance. Like Power Sloshin mentioned, try to move the mast track more forward (but keep the boom relative high), and use the bottom outhaul eyelet on the sail, - will all help to reduce the backhand power + back leg pressure and help on controlling the trim of the board better.
Also try a (or borrow from your mates) few different fins to see if this have an impact.
Thanks for all those tips,
I will take a note of fin box next time, I certainly like the feel of falcon, I also have the 99,
BTW I got a reply from Dennis Parton, he mention two things, the T-43 is an advanced fin (I got it 2nd hand) which I take is his polite way
of saying I don't have the technique 
, the Talon would suit me better, and other was that the 44cm is just two small for that sail/board combination, at least with the the fins he offers. His recommendation was a 46 talon for the 8.5/121.
If this helps I used a 44 C3 Sting II in my 121 with a 7.8.
Powerful fin and it worked well for light wind. Was definitely under-finned on the 8.6.
