Hydrodynamics · Bottom shape
Bottom contours and planing hydrodynamics
Under your feet a surfboard is a planing hull. Once it is moving, the water does not simply flow around the board; the bottom shape redirects it, and where that water goes decides how the board drives, holds, and turns.
01From displacing to planing
Paddling is displacement: the board pushes water aside and its speed is capped by its length. A 5'10" hull has a classic hull speed near 3.2 knots. A surfer trimming along a clean wall moves at ten knots or more, three times past that limit, and the hull gets there only because it climbs onto the water and skims. Planing turns forward speed into dynamic lift under the board, so the wetted area shrinks and drag falls.
A shaper cannot change the water, only the shape it meets. I read a bottom like a map of where the flow will be asked to go: toward the rails, off the tail, or down the stringer. Every contour below is a way of managing that skim.
02Coandă attachment at the rail
Water leaving the bottom meets the rail, and what happens next depends on the edge. A soft, rounded rail lets the flow cling and curl around the radius, the Coandă effect, which gives the rail a held, sticky feel and a gentle release. A hard edge breaks that attachment and sends the flow cleanly away, so the board runs freer and faster.
We carry a full, soft 50/50 rail through the front half for forgiveness, then sharpen to a hard edge from the fin cluster to the tail. Hold up front, release at the back.
03Single concave and the venturi picture
A shallow single concave, usually an eighth to three-sixteenths of an inch deep, scoops the bottom between the nose and the front fin. The working model is a venturi: the flow is squeezed into a narrowing channel, speeds up, and loses pressure, which seats the board on the face and then releases it forward.
I treat that as a useful picture, not a laboratory proof. What I can say from the water is consistent with it. A single concave makes a board feel lively off the bottom turn and quick to get going in small, soft waves.
04Double concave and vee out the tail
Through the center we deepen into a double concave, two channels either side of the stringer, which carries the accelerated flow toward both rails at once. Then the tail resolves into a vee, a quarter to a half inch of depth by six inches from the tail.
The vee sets the roll rate. A deeper vee rolls from rail to rail faster, so turns come with less effort, but the board gives up some straight-line stability. A flatter tail sits down and drives, and asks more of your back foot.
05Step-deck foil distribution
Volume does not have to live in the rails. In a step-deck, the deck drops away along the outer third of the board, so foam stays thick along the stringer and the rails fall to about one and three-eighths inches. The same liters, thinner edges.
Thin rails sink into the face and bite in a turn. Thick centerline foam keeps you floating in the paddle and eases the duck dive, because there is less rail to push under. In a thick wetsuit you need that float, and you still want the bite.
06Putting it together
None of these features works alone. A hard-edged vee tail on flat rocker will be fast and stiff. A deep double concave on steep rocker will be quick and a little nervous. We choose the combination from the wave you chase: more single concave and flatter rocker for long pointbreak walls, more vee and a lively tail for punchy beachbreaks.
Walk through the bay and you will see five different bottoms on the rack. None of them is the best one. Each is a bet about a wave, and the homepage calculator gives you the starting numbers for yours.
| Station | Contour | Depth | Job |
|---|---|---|---|
| Nose to front fin | Single concave | 1/8"–3/16" | Gather and accelerate |
| Center | Double concave | 1/4" | Spread flow to rails |
| Tail, 6" out | Vee exit | 1/4"–1/2" | Set the roll rate |
- Reference hull
- 5'10" × 19 3/4" × 2 7/16" · 31.4L
- Rail
- Soft 50/50 to hard edge
- Rail depth
- 1 3/8" step-deck