Hi Crazy Chemical,
Sorry for the delayed reply here, but I was "off the net" for a few days an having oversize valve installed in my heart and getting the other valves cleaned and inspected.
So, now I'm back and I will be working my way back up to speed.
Ok, glad to hear you found the conditions to get your 113 liter board going with the 6.2 m2 rig.
Here are your 3 questions (pasted in) and I will deal with each one in turn:
"1) Jibing is not my most fluent move and i found myself in the water quite a few times when i tried to turn the board around (i did manage to do the laydown part of a laydownjibe and felt so proud :d)"
Wow, working on your laydown jibes! Very cool!
Now, the biggest problem i faced when waterstarting (i have to waterstart on a board of such volume because of my weight (90Kg)) i tended to drift with the current and with the wind as i caught it with my sail. Also my board automatically turned itself with it's tail towards me making my job not at all easier. Even though i never get into any trouble with it, it's rather annoying."
If you are sailing where there is wind and current, and they both go in the same direction, staying upwind can be a problem. Mostly (if you are just sailing back and forth recreationally) you need to figure out which tack will keep you upwind the most, and then concentrate on really getting "on the fin" on that tack so you stay upwind.
As far as not falling in your jibes, that's pretty much "practice makes best" so spending lots of time on the water (TOW) and doing lots of jibes is the best way to get better at them. Maybe you are trying to advance to the laydown jibe a little too soon and need to work more on getting your "all the way around" carve jibe mastered.
As far as your board turning it's tail toward you on your waterstarts, that's a waterstarting/ how to steer the board with the mast foot issue and something you will get straightened out with more TOW on the new board.
"2) i, as many others, have problems going upwind with this board. I never had any such difficulties with the Go139 or at least i could compensate any drifting by just going upwind without planning. I can't on this board because i weigh to much and upwind without planning is a big nono if i want to keep this board floating. I can get the board planning but it starts going downwind with a smashing speed and by the time i get into my footstraps i'm blasting downwind and i can't alter the course enough to compensate for the lost terrain. I thus get pushed away from beginpoint A by X meters depending on the wind and on how much i managed to correct and when then going back to shore i lose another few meters and i end up at least 100 meters from point A whereas the other surfers just seem to go up and down without to much hasle."
I see this as 2 issues.........
#1 Staying upwind at sub planing speeds (it can be done by a 90 kg. sailor on a 113 liter board, but it's going to take a bit of a learning curve for you to figure out how much to tip your board (upwind rail down or lower than the lee rail) and how much you need to keep the rig raked forward to keep your sail powered up. Once again, TOW on this little board will have you sailing upwind with all the other sailors very soon.
#2 Staying upwind in fully planing conditions....you need to carefully consider how much power you have in your rig, how quickly you can get going out of your jibes, how much downwind you need to go to get your board "lit up" enough to sail upwind "on the fin".
Once again more TOW is going to give you better skills to match this smaller board to the conditions.
It will become pretty easy to head off, lite up your board onto a full plane, then gradually increase the fin pressure and railing of your board to take you upwind.
The technique here is going to be a little different than your GO139.
""3) That just about covers the problems i have during windsurfing. Now to another more specific issue: my dad, during our stay in Sicily wrecked two fins, one of which is was mine so he owes me one (haha!). Now for the Flow284 (113L board) which i used to gather the stories of the two problems above i was thinking of buying an extra fin. The current one is a G10 crossover fin, 31 cm long. I found that in the chop that fin was absolutely the right choice because i later discovered that fins that are to straight get too much pushed aside by the current (the Go139 with its 48 cm fin does not work well in chop at all, even with an overpowering sail i could barely get it planning whereas my dad with that same sail and a crossover fin on his big board kicked ass). However, my local spot is on a lake where there is like no current so i was thinking of a slalom/ride fin of 36 cm, would that be too much for rigs of 6.2 > ? or could i make it work you think? "
Well, good luck on "fixing" your dad. I could never fix my dad, and I doubt you'll have much success fixing yours. But, I agree, he does owe you a new fin.
As far as what fin to use, first remember that similar to Starboards the Mistrals are normally "slightly underfinned" with the stock fin.
You've been used to your GO 139 which actually has almost the perfect "all around" fin size with the stock 48 cm race fin).
So when you select another fin for your 113 L Flow, you can get better upwind (and early planing to some degree) performance by increasing the fin size/span or changing to a fin design (outline shape here) that's known to be better upwind, or both.
I'm not sure what to say about your statement that "fins that are to straight get too much pushed aside by the current (the Go139 with its 48 cm fin does not work well in chop at all". With proper technique, your can make any sort of fin work to keep you upwind, but the techniqes do change depending on if you are fully planing or not.
To say that the vertical race blade on your GO 139 gets pushed too much by the current suggests that you haven't quite figured out how to sail that board "on the fin"
I think an upgrade to a 36 cm (from a 31 cm) would be pretty good!
Hope this helps,