Hydrodynamics · Fin setup
Fin cant, toe-in, and twin-keel geometry
A fin is a small wing working in water that would rather be elsewhere. Two angles and one cross-section decide what that wing does: toe-in, cant, and the foil itself.
01Toe-in and angle of attack
Toe-in points the front of each side fin slightly toward the stringer. Measured at the base, the leading edge sits an eighth to a quarter inch closer to the stringer than the trailing edge. On a 4 1/2" base that works out to about 1.6° at an eighth and 3.2° at a quarter.
It is a small angle, and the fin lives on it. Toe-in gives the foil a built-in angle of attack, so it produces side force the moment the board moves and starts working before you ask. Too little and the board skids. Too much and the fin works harder than the wave needs, scrubbing speed.
02Cant, the outward lean
Cant leans the fin tip away from the stringer, usually four to eight degrees. Our standard side fin runs 6.5°. When you roll the board onto its rail in a turn, a canted fin stands closer to upright in the water and keeps working at a useful angle.
More cant gives a looser, springier board that pivots quickly. Less cant gives hold and a longer, more drawn-out arc. Because cant and toe-in act together, we change only one at a time when we tune a board.
03Foil, camber, and Reynolds number
A fin section is a foil, flat on the inside face and curved on the outside. That curve is camber, and it builds lift without needing a large angle. How the foil behaves depends on the Reynolds number, the ratio of inertial to viscous forces.
For a side fin with a 0.09-meter mean chord moving at four meters per second, in 52°F water, that number is about 280,000. Water at 80°F is roughly 30 percent less viscous, so the same fin in the tropics runs near 410,000. Cold water is thicker, the flow clings less eagerly, and a fin that feels crisp in Costa Rica can feel a touch sticky here. We lean toward thinner tips and a little more sweep for Monterey Bay.
04Twin keels in flat sections
The twin-keel fish carries two large, long-based fins set near the rails, with about 4° of cant and a quarter inch of toe-in. That much area at the rail gives a lot of drive and very little pivot. In the flat sections of a wave, the shoulder and the slow apron of a long pointbreak, the board holds speed and draws long lines without pumping.
The price arrives when the wave stands up. With no trailing fin to steady the tail, a keel board tends to slide out in a steep pocket.
05Thruster clusters in steep pockets
The thruster adds a center fin with no toe and no cant, and sets the side fins at 6.5° with an eighth inch of toe. The side fins drive. The center fin stabilizes and gives the tail something to pivot against. In a steep, critical pocket you can swing the tail around and the center fin holds, so the board changes direction without losing its grip.
The quad trades that center fin for a second pair, gaining speed through the pocket and giving up a little of the pivot.
06Setting fins at the bay
We set toe-in and cant with a jig rather than by eye, and keep a notebook of every board we send out. If yours feels stuck, ask for less toe-in. If it skids, ask for more. Quarter-inch changes are visible in the water, so move one thing at a time and tell us what you felt.
Bring your old fins, too. A set you love tells us more about the foil and base length you want than any description, and we can match the template or improve on it.
| Setup | Toe-in | Cant | Feel |
|---|---|---|---|
| Twin keel fish | 1/4" | 4° | Drive in flat sections |
| Thruster side fins | 1/8" | 6.5° | Pivot, hold in steep pockets |
| Quad front / rear | 3/16" / 1/8" | 6° / 4° | Speed through the pocket |
- Side fin base
- 4 1/2"
- Reynolds number
- ≈ 280,000 @ 52°F
- Standard cant
- 6.5°