Last summer I bought a standup paddleboard- a 10'4" Angulo Surfa. The Surfa has a mast track, and it's big and wide enough to use as a beginner windsurf board. The only thing is that, unlike a real beginner windsurf board, it doesn't have a daggerboard or center fin. As a result, a beginner windsurfer would have trouble getting it to sail in a straight line without drifting downwind.
I mentioned that to Josh Angulo, and he offered to help me with an "experiment" to install a second fin box on the board, in the center where a daggerboard would go. We chose to use a "US Box" (aka "A Box") because it's the most simple to install and it gives some forward - backward adjustability to the fin. Anyway, after a long time fiddling around starting and stopping the project this winter, I finally installed the box. For the fin to go with it, I found a biggest, oldest, fattest US box fin I could get.
This appalling picture shows the new fin in place. It also shows the self-destruction of my wetsuit. After this day I cut it completely into two pieces and I've just been wearing the "pants" with a neoprene top because it's warm enough now to do that.
Anyway, the maiden voyage with the new fin took place last weekend at Lake Quannapowitt in Wakefield, MA. (My designated board tester, Lady Notorious, had requested the lake to avoid the choppy water and offshore winds that we might have encountered at Nahant.) Wind conditions were typical lake conditions: shifty, gusty, 0 - 15 mph. We rigged a 4.2. While her bulldog and I watched from shore, Lady Notorious went out there and kicked butt. In contrast with her previous outing at Nahant, she had no trouble staying upwind, and she even managed to return to where she'd started after the wind shifted to put the launch directly upwind.
In paddleboard mode on flat water, I can't tell much difference with the center fin in or out. Of course when sailing in waves on the ocean I remove the center fin.