A while back I sold my formula windsurfing board. I did it because I wasn't in love with it and because I had too many light-wind watersports options between racing paddleboards, windsurfable paddleboards, and even windsurfable racing paddleboards.
The remaining board that comes closest to filling the early-planing niche of the absent formula board is the Exocet WindSUP 11'8, which is 360 cm long by 80 cm wide, 220 liters volume, and 16 kg (heavy). That is far from the optimal early-planing stats of a formula board (~235 cm long x 100 cm wide, 165 liters, 9 kg), but the WindSUP is not as far behind in planing ability as you might think. With my 9.5 Ezzy Cheetah sail, I could get my formula board "over the hump" to planing in just about 10 knots of wind, whereas on the WindSUP there's a gradual entry to planing between 10 and 12 knots. The type of planing is a little different, though. On a formula board you're really zipping along "riding the fin" and you can go upwind easily. But on the WindSUP, at least with the 44 cm stock fin, you don't have much upwind drive when using a big sail, like the fin is not quite balancing out the weight and pressure of the board and sail so you don't quite get the flying over the water feeling. In my experience the 44 cm fin is plenty for sails up to 8.0 meters squared, but not for the 9.5.
SO, I ordered a 50 cm Hydrotech fin to test with the 9.5 sail. The first time out was one of those frustrating days where the wind was good when I left my house but had sunk to just below the planing threshold by the time I got to the beach. So I didn't plane that time and of course couldn't tell much difference between the new 50 cm fin and the old 44 cm one. Yesterday, though, I got a good session in 10-15 knot winds, and the 50 cm fin really showed its worth. I could put tons of pressure on it with my feet in the outboard footstraps, and translate that pressure into upwind sailing angle. I think it also helped a little bit with early planing and with maintaining a plane through lulls. The next step is to try it with an 11.0 sail to try to find the ultimate early planing limits of the board. :)
Here's the big fin in the board.
Here you can see the stock 44cm fin and its sheath next to the sheath for the new fin. They say the maximum useful fin size for a board is approximately equal to the width of the board underneath the fin. The old fin was considerably less than that, while the new one is close to right on. Of course, with smaller sails and for waveriding and stuff the WindSUP handles best with smaller fins. The whopper fin is just for early-planing with the biggest sails possible.
PS- After I got this fin, which I ordered online from Sandy Point Progressive Sports, I realized I probably could have gotten something just as good from the more local Ace Performer shop. Next time I'll try to remember to order from them because I like what they're doing up in Fort Myers, teaching lots of people to windsurf and sup at the Sanibel Causeway, keeping windsurfing somewhat alive in Southwest Florida.