"Two minor questions about the board:"
"(1) I don't have a clear idea when to use the drop-down centerboard. We used it all the time in schlogging mode, and often in third gear (because I didn't know the gust was coming). Very occasionally in fourth gear the big extra centerboard became a bit much to handle, but for the most part two sailors could muscle it down."
I never really used the centerboard unless it was really sub planing conditions.
In clear water (no weeds) the stock fin does a good job and adding the CB just seemed to cause alot of extra drag and slow things down.
You can tip the Gemini upwind rail down a little (both sailors step a little off center toward the upwind side) and it goes upwind in nearly planing speeds, and once you are planing, if you put the footstraps on, you can crank upwind off the fin really well.
Balancing the front and rear sails (smaller in front for sure) can help you stay upwind as well.
"(2) We didn't install the straps, so I didn't have a chance to try them, but they seem further back then I usually wanted my feet. Any experience with the straps? (I should say that it was surprising to me we could get the Gemini into fourth gear without the straps, and without it feeling scary most of the time. I think it's because the board is just so huge that it planed early and easy, and was very controllable even at high speed. On my single I would have been terrified at that speed out of the straps.)"
I used all the straps on the Gemini I had since day one
They seem really far back, but that's where your weight needs to be.
Get your board strapped up, and you will find it's real comfortable back there.
And, for your crew up forward, having their feet in the straps always seemed to
get them to settle down so you could really get the board cranking.
Bill (from NC/NH (I think??) and I took the Gemini out with 2 4.8 m2 Sailworks Huckers one day.
I have to tell you that in high winds the Gemini really get rolling, even with 2 very small rigs.
Only issue that day was we needed a big steam whistle to get the guys on 80-100 liter shortboards out of our way, cause when the Gemini gets rolling that fast, it steers about like an aircraft carrier or super tanker.
Hope this helps,