My only regret about having to switch from surfing to SUP was no longer having a twin-fin in my quiver, even though I’d loved twin-fins when I was younger.

There are quite a few different styles of twin-fin surfboards:
- the short, “competitive” ones with wide tails, popularized by Mark Richards
- the retro “fish” boards focused on speed
- and now the “mid-length” round-pin-tail boards, often with channels (e.g Gong’s Fury)

Since I already have a SUP thruster that’s very much geared toward smooth glide (my custom 8'1" hybrid shape between a Curve SP and an Alley) with a fast rocker, which I’d been riding with small fins to maximize its glide, I decided to have two fin boxes installed to convert it into a twin. This will serve as a test to see if I need to (even more) pester the Gong shaper for a future genuine Twin Fin SUP.

It cost me €140 at my local shaper (Spoutnik Surfboards, 200 meters from my house). Great work (FCS II boxes in Airex PVC foam cassettes), 220 g of weight added total. What I asked was for maximizing glide, speed and hold:
- the rear of fin base set 28 cm (11") from the tail, aft enough for drive and grip, as I planned to use Quobba L fins, smaller than usual Twin Fin fins for the less drag. I thought that the thin rails would provide enough bite to not require big fins.
- very little toe-in—about 1°— speed at the expense of looseness
- not too close to the rail, to mitigate the risk of spin-outs due to ventilation

Before, a thruster 3-boxes setup:



After:



As you can see, the twin fin boxes are just between the side and central thruster boxes. 1" more to the rear than most 8'-ish twin fins.





First session, I rode some tiny but fairly hollow waves: not enough to attempt turns, but the speed was definitely there. Getting into the wave was noticeably easier than with its thruster setup.



Second day, the waves were a bit bigger but pretty mushy. I was really surprised by the grip: the fins set further back provide insane drive (and also less toe-in/fin pinch), and excellent stability in turns—despite their small size for twins: L —. That said, since the Quobba fins have both less drag and more stability than normal fins, it makes sense.
This board, already fast thanks to its rocker, is going to become a speed monster. Can’t wait to test it tomorrow in bigger waves (head high).

Third day: I’m blown away by this transformation:

- The board has perfect grip; the tail is anchored in the water like a single fin, with no wobble, no sloppiness, and no skidding that twin fins can get. I think this is due to the fin placement and their slight toe-in, combined with the thin rear rails.
- The board is still easy to turn by its pulled-in tail, but you must use the rear rails in turns (roll the board into turns).
- The natural speed gain is noticeable compared to its previous thruster setup—even with small fins—and especially the acceleration at the bottom turn, the “drive,” is on par with the best thrusters, plus it has the low drag of a twin fin.
- No issues on the backside—that’s always my concern with twin fins.
- The ride feels more like a mid-length Simmons than a Mark Richards-style twin.
- No need for big “twin” fins with all their drag. Good, standard yet effective fins are enough, and they’re significantly faster (less drag).

The board setup is more technical and less playful than a "pro" model like the Gong Alley; you have to set up your turns more carefully, push into them, and follow through—a bit like riding a single fin... but with the acceleration out of turns of a multi-fin setup..
It combines the teachable and reliable nature of a single fin with the drive and speed of a twin fin.

So, converting a board to a twin fin works.

I'd recommend however to put the fins 1" from the tail, with standard (similar to a thruster sides) toe-in, if you want a more balanced compromise speed/looseness, and use real (bigger) Twin Fin fins however. Also plan a box to add a central trailer if your rear rails are not very thin.