Hi Paul C,
Well, I've used weed fins down to 40 cm, but I don't think I every used an SR6B on a board as wide as the FT138.
So, I'd encourage you to give it a try, to satisfy both of our curiosities.
The SR6B is a very slalom oriented fin with not too much lift, but also with very
Not sure how that combo will work on the FT, but with a small rig (maybe even down to a 6.5 m2) it may indeed give you a magic carpet ride.
Let me know what you find out.
It will either amaze you with it's speed and how smooth the board rides, or it will spin out
from you putting a little too much pressure on it.
I'd make sure you get the board well off the wind and really cranked up speedwise before putting any real presure (upwind) on the SR6B.
Hope this helps,