Fins Part Three: Why choose a thruster setup?
That subtle vibration through your board during a heavy bottom turn isn't just in your head. It's the result of complex fluid dynamics happening between the water and your fins. While this can cause some fin setups to stall, three fins tend to work in concert to keep you carving rather than careening.
Since Simon Anderson's newly-minted thruster design won the Bells Beach contest in 1981, the 3-fin setup has dominated surfboard design. German researchers recently did something that sounds like overkill: CT scan an actual surfboard with FCS Accelerator fins, then run the digital model through computational fluid dynamics simulations. Think of it as a virtual water tunnel where they could measure exactly how water flows around each fin at different angles.
The thruster's advantage lies in how the three fins interact as a system rather than operating independently. When you lean into a bottom turn, each fin experiences different flow conditions based on its position. The inside fin, closest to the wave face, works in relatively clean water and produces the most lift - that perpendicular force that drives your board through the turn. Maximum lift occurs around a 20-degree angle, which corresponds to moderately aggressive turning.
Beyond that 20-degree sweet spot, the flow starts separating from the fin surface in what aerodynamicists call "stall." You'd expect performance to drop off, but the thruster configuration has a built-in redundancy. The center and outside fins operate in the disturbed water created by the inside fin, and this interaction actually helps maintain control even as individual fins begin to lose their grip on the water.
The side fins use asymmetric profiles (curved on one side, flatter on the other) while the center fin is symmetric. The asymmetric side fins are optimized for generating lift in one direction, while the symmetric center fin provides stability and can work effectively regardless of turn direction.
The researchers discovered that the optimal balance between turning power and speed occurs at a much gentler 6.25-degree angle - those wide, flowing turns that carry you down the line rather than sharp pivots toward shore. As angles increase toward aggressive bottom turns (30-45 degrees), drag rises dramatically.
At steep angles, the fins start shedding alternating vortices - swirling packets of water that detach and create oscillating forces. This explains that vibration you feel during committed turns and why some boards feel less stable when you're really leaning into them. The center fin experiences the most dramatic force fluctuations, with lift varying by nearly 50% as these vortices pass by.
Anderson's thruster might have been born from intuition, but the math backs up what surfers have known for decades: three fins just work better than the alternatives when it comes to hard turns. Still, try to take twin fins from me, I dare you.
Further Reading:
Now let's consider a 3 fin (2 +1) setup on longboards 9 ft and longer v/s a single fin board.
Granted the "general" maneuverability of a longboard tends to be quite different from shortboards. Yet some of us "loggers" can and do crank hard sharp turns on our aircraft carriers.
While I'm at it, let's toss in the 5 fin combos offered on some Takayama longboards.
The fun never ends
Thrusters are the best.
What I want to know is (and maybe you already have an article planned on this) is how do shapers know where to place the fins? Is it just an intuitive feeling, or is there a science behind that, too? I've noticed it especially in quads, where you have the different setups. Some quads have the two back fins close together, and others have them far apart. And I get the drive, looseness aspect of that, but I also want to know how they figure out how close the back fin should be to the front fin, and how far from the tail, etc.