Caution, here is what I found out:
- A single fin is most efficient in "displacement" mode, i.e. before planing. And as the planing speed is higher as the water line is longer, this means that long boards will be often in displacement mode, and as small waves are often of a short period, they are also slower, so you go slower.
- At slow speeds, if you roll the board to the right, the water reaction on the hull will pull it to the right at its center, and then the fin(s) act as a weathervane tail, so its lift will be to the left. At fast speeds, it is the opposite, the rider use the rails to turn, and the fin(s) adds lift to the right to counter the centrifugal force, a quite different situation. Multi-fins are mostly designed to work at fast speeds (hence the flat inner side of fins, they are designed to provide lift into the turn) and feel awkward at slow speed, while symmetrical setups (single fins, twin setups like on Simmons) can work in both modes and do not feel weird in between them.
- Single fins lose grip and efficiency when the board rolls significantly, in carving turns. Side fins keep holding.
So I guess is what you found out is that a single fin is much more pleasant at slow speeds.
Note that you can compromise the setups to increase their range:
- 2+1 will add small twins to a single to help it handle more speed. But by keeping the side fins small and often symmetrically foiled, you avoid the "lock" you can feel with a thruster setup at slow speed. This is why it is such a popular all-around setup.
- Jackson close in his 10ft video has a thruster setup, but with the center fin moved all the way forward in the box: this way you reduce the "lock" a normal thruster setup has at low speed. I used this setup on a 9'11", the looseness was exhilarating, but you lacked drive if the wave power increased.
- Harley Ingleby rides bigger side fins and a smaller center, I guess for the same effect of reducing the "thruster lock" at slow speeds. I must say I like this combo best: you get the looseness of a small center, but when noseriding the side fin digs deep in the face and provide a nice hold if you do not go too slow. And in fast waves, the central fin is aft enough to provide decent stability. Its drawback is that it does not have the solid feel underfoot of a big single when the board is not on its rail. Drop knee turns are harder for instance.
- "Simmons" twins (big fins with symmetrical foils and a low toe-in angle, on the rear) may be also a very interesting noseriding design: you get the efficiency of 2 single fins at slow speed, that maintain their grip when on the rail, and no "thruster lock"