As we know everything is a trade off. No matter what you do you gain somewhere and loose somewhere else. Without seeing the board it’s a little dicey to make a call but I share a few things that’ll give you something to think about.
A flex fin will let the board rails work nicely if it has enough base it’ll hold in well but will spill some power off as it flexes and lets the rail engadge. A TK flex about 8.25" would do you fine. Too much flex and too small a profile or too vertical a fin will spill off to much power for the board we’re talking about.
You won’t want too vertical of a fin unless you want to go with one of Cheyne Horan’s Starfins which can be a good call in sectioning conditions and will drive you through the tuff spots well as long as it suits you surf style. It’s a fin that should be in you quiver if you’re surfing beach breaks single. All you have to do is watch some films of Cheyne on one and you’ll see what I mean.
You could go with a real conservative fin about 8.0" too. It may suit you well. After all every thing you try is just another experiment.
My favorite for that board would be either a 7.75 inch or 8.0 inch “Mental” depending on how big you are. I attached a photo of one I made some time ago for a Virginia beach surfer whose a pretty big boy and surfs it on his 7’2" single.
I have a 7’0" single fin roundpin and have tried a variety of fins on it. Surprisingly this 9" stock injection molded plastic fin works the best so far.
