RE: no reply ....
I moderate the Starboard Windsurfing School Forum, but I'm not a member of the Starboard Professional sailors "Team".
Perhaps the reason none of the "Pros" answered your question is becasue most of them do not sail the GO board. They are into wavesailing (Scotty and Ola H.); Formula and slalom racing (Kevin, Sam, Ian, and R??mi), and Taty Frans is a world class free style sailor.
They may have sailed the GO 155 for a few minutes at the Photo Shoot on Maui (to get some great pictures) but they would not normally spend enough time on the GO 155 to really get dialed in on the tuning of this board. I looked at the website and I'm not sure who the sailor is on the GO board and Gaastra sail, but it's not one of the pro team above.
Nice shot from the helicopter tho!
Here's a copy of your question about Fins for the GO 155:
Posted on 10-12-2006
"Hello from Sunshine Coast Queensland Australia. I have just purchased a Go 155 with a 56 Drake fin. I have 2 smaller fins, one a 48 cm and a weed fin. I have 3 sails 6sq. metre, 8.5 and a 10.5. I sail in mostly 10 to 15 knot winds, weigh 78 kgs. and am an intermediate skill sailor. What is the best combination of fins for my 3 sails or do I just use the 56 drake for all sails?"
In my experience, you would be using the 10.5 m2 rig to get the absolute best early planing, right? For this you probably would want a bigger (longer span and perhaps a wider chord on the foil) fin to balance the big 10.5 m2 sail.
Personally I think a 10.5 m2 is a little too large for the GO 155, but you are fairly light in weight (at 78 Kg./172 lbs.) so the smaller GO 155 (255 cm long x 85 cm wide) may respond quite well to the big sail and a larger 62-66 cm fin, and get you going very early indeed.
With your 8.5 m2 the stock Drake 56 cm should work quite well. You may have to stay off the fin a little right at first to get some water flowing by the fin (i.e. boardspeed) but once you get the board moving the 56 cm should work very well.
With your 6.0 m2 rig, the 48 cm may work OK, and it may also be a little small. A 6.0 m2 on a GO 155 would be what I would use in 22+ knots so you really don't need very much fin but 48 may be a little on the small side and not balance so well with the 85 cm wide board. 6.0 m2 sails are also about the smallest that an intermediate sailor would normally use on the GO 155.
You didn't state how big your weed fin is.
I'm sort of the "weed fin guru" to the world (somehow??) and weed fin performance can vary a great deal with the design of the weed fin and the board. If your weed fin is at least 40 cm it should work OK.
Hope this helps,