Choosing the type of fins to use on a surfboard can be challenging, especially if you listen to what shapers and professional surfers say on the internet. Honestly, if you're riding a standard thruster shortboard, just go with the stock fins the shaper sells you. You'll save money and you won't notice the difference given they're the size/shape you're looking for. I realized this one time in Panama when I broke an expensive glass fin and used my backups which were those hard plastic stock fins you get for cheap at any surf shop. I had amazing sessions on those cheaper plastic fins and I remember laughing at myself for spending the extra hundred bucks on the expensive glass versions... I guess they WERE blue and they DID look cool. I think that's what I was really paying for. I never learned my lesson either because I still buy expensive glass fins for my boards. I blame fashion.
But fin SHAPE and SIZE make a lot of difference in how a board rides. The NUMBER of fins, and where they are placed on the board have a big impact on performance as well. But once you start adventuring into single fin surfboards, you'll find that there are a ton of options out there depending on the shape and length of your rails. Figuring out a good size for a single fin, and the placement in the fin box, isn't always easy. And any adjustments to the variables can really change the way a board feels when it's planing and turning. Sometimes you want to have options depending on the type of waves you're surfing. But it often takes a while to figure out what's best for your board from a fin's perspective. And obviously I mean best in the sense of what I like best. I ride the board, so that's what matters.
One thing I've found riding single fin surfboards is to never count a fin out. There have been many times where I thought a fin would never work and it totally did. Sometimes I figured this out by accident or by weird circumstance. Other times I was just bored and wanted to try something different. If you really want to go scientific method you can join a "fin club" where you pay a membership and you swap fins out at your leisure. Check out Finatic at https://finatic.com. In the end, the reward of finding the right configiration for your single fin board is worth the time it takes to figure out. But here are some tips to help move you along...
For single fin longboards, more in the high performance range, the general rule is an inch of fin for every foot of board. But this rule is no fun. Stick a really big fin in there if you want a lot of hold and drive, smaller to loosen things up. Also, you can take a smaller fin and move it to the back of the box to make it feel bigger. But there are some problems with that because your rail and fin work together and too far back can really impact turning. Honestly, there's a really big window for finding a good spot in the fin box on any board 9ft or bigger... so no need to get all paranoid.
I really like using cutaway fins in my longboards, they have good enough hold, but mostly I like the way they pivot and draw sharper lines. I also feel like I can wiggle into better spots on the wave with a cutaway fin. But on the contrary, I do enjoy the feel of a good flex fin with traditional size and longer rake. I like the way it sort've slings you out of turns. So big days on a point, I'd probably go more traditional flex fin. Right now I rotate two fins through my high performance longboard... a 9in Rainbow Flex Fin and a 9in glass FCS II Connect GF. I had an 8in Futures cutaway, but I broke the cutaway off on a Horseneck sandbar last fall and have yet to replace it. Here are the fins I use in my Gee Rainbow 9'2 pin tail high performance board:
Now, finding the right fin for single fin shortboards and midsizes is a bit more complex. I've sacrificed many sessions and spent many dollars trying various setups over the years. I am encouraged by FCS II's ability to change a fin position in the water, or change a fin completely without any tools. The only thing there is you will lose a fin from time to time. Their system is far from flawless. On more than one occasion I've knocked an FCS II fin out of the box while surfing. One time it happened in Costa Rica on a new thruster. I was extra pissed because I had just splurged on Matt Biolas Lost fins and had no replacements for the rest of my trip. So if I were you, add a bolt to your long box, or if it's a quad/thruster use the FCS box screws just to be safe.
Right now, I use the same 9in Rainbow Flex Fin in my 6'2 double wing single fin smurfboard. But I've used a ton of different sizes and shapes in there. I've had this board for over a decade so it's been through quite a bit of experimenting. A 7in performance style fin works well in there, in the center of the box. But the bigger flex fin definitely turns that board on... so there's really no reason to switch it out. It's just a perfect marriage. It took a while to find the sweet spot in the fin box, but once I did I marked it with a sharpie and never looked back. My recommendation on finding the best spot in the box is to start in the middle and adjust from there.
For my brand new Vec 7'6 single fin, I'm thinking of going with one of these True Ames templates...
I like the long rake of the yellow Norm Flex and my shaper says that's the way to go. If I don't have luck with one of these, I can always throw my 9in rainbow flex fin in there. It's sort've my go-to single fin. I'l let you now how it all plays out.