July 4, 2022

Concave and convex

The bottom of the board can be flat, concave, and convex. Next, I will tell you how its contours affect the board’s behavior

The characteristics of a surfboard are determined by the design of its elements – nose, tail, rocker, and rails.

But one more important detail is not striking – the bottom contour. Instead, it is pure physics and engineering, and today I want to detail how the bulges and concavities of the bottom of a surfboard work.

At a cursory glance at any board, it seems that its bottom is flat, but it is not.

Through experimentation and trial and error, the shapers have calculated that a slight curvature of the bottom contours strongly affects the board’s behavior in the water.

As you might guess, the bottom can be flat, concave, or convex, which is scientifically called flat, concave and convex, respectively.

However, there is often a combination of bends when the contour changes from the nose to the tail.


Why does board behavior change? Because the curved lines guide and define the trajectory of the water under the board.

The faster the laminar (that is, homogeneous) fluid flow moves, the less pressure it exerts on the vessel wall, and in our case, on the board surface.

If the bottom is flat, the tension is the same over the entire surface, and the fun begins when it is curved!

How concave works

A concave is a concave bottom, that is, in fact, squeezed channels through which water moves.

The hollow reaches its maximum depth in the back of the board, from the middle to the fin.

Then, at the very tail, it levels out relatively sharply (by rather sharp, I mean that it does not go a step, but the depth difference is more sensitive than at the beginning of the concave).

This difference in depth provides an abrupt change in pressure from low to high; it ensures that the board’s tail is pushed out of the water.

Try to place the concave side of the spoon under the tap water; you will see how a powerful broad fountain forms at the edge.

Approximately the same is formed when driving behind the tail of a board with a concave. Water is forcefully pushed out from under the board, giving it acceleration and buoyancy.

Why do you need a concave

Concave is needed to make the board faster and more forcefully pushed out of the water, so it found its use in the design of modern shortboards.

The two most popular shortboard bottom configurations are single concave and single to double concave.


In the first case, we have one channel from the front third of the board to the side fin and then fades out under the back center fin.

Such a bottom contour provides high speed when driving in a straight line, especially on large, even waves, and allows you to make sharp turns.

Still, under poor conditions, it loses in stability and maneuverability.

Single concave is suitable for surfers of all skill levels (shortboard ready, of course), but the quality of the rides will significantly depend on the conditions in the ocean.


Single-to-double concave – starts at the front of the board and then bifurcates, bringing the flow of water in two directions – between the center and side fin.

The presence of one channel in the center of the board allows you to maintain speed characteristics, and two outlets for water flow give maneuverability and acceleration when cornering.

These boards work well on a wide range of waves, but only experienced surfers can unleash their potential.


In addition, there are especially high-speed boards on the tail of which the so-called channels are visible.

Almost the same as concave, but deep, with a sharp transition and going to the very edge of the board.

These water flow guides transform the board into an ultra-high-speed rocket with a gas pedal under the rear foot: the harder you press on the tail, the more influential the water flows from each channel will be, and, accordingly, the greater the acceleration.

Again, the number of channels varies; four, six, or even eight can be.

The flip side of the coin is this: Deep channels mean less foam in the board’s tail, which means less buoyancy.

Moreover, this shape of the bottom works well under perfectly smooth conditions.

During a chop, air can enter the channels because acceleration begins to be given at the wrong time and in the wrong direction.

As they say, the more radical the solution, the narrower its application spectrum.

How convex works

As a result, it makes convex surfboards very stable and forgiving

A convex bottom works precisely the opposite of a concave one – changing the pressure from more to less, as it were, pulls the board inward, preventing it from breaking away from the water.

In addition, it is easy to transfer weight from rail to rail on such a board; it is easy to control and gently turns in large arcs.

Why do you need convex

The convex bottom is most often used in longboards and hybrids (from Malibu to shortboard)

Convex is rounded and called the belly, and sometimes with an angle (V), they behave about the same way.

Convex is a must-have for no-ride longs, as the tail, which is attracted to the water, compensates for the surfer’s weight on the board’s nose.

In addition, all ghans for big waves are made with the obligatory convex, in extreme conditions, on enormous waves, surfers are not up to cutbacks and snaps; they need a stable board that will turn and sweeps them away from the collapsing freight car of foam in time.


A flat bottom can be safely called the last century, yes, the first models of boards were made without any tricks, but they were also intended for imposing driving in a straight line.

Nowadays, only student soft tops are made with a flat bottom (and not always), but they often use the flat part to transition from one bend to another

So, for example, everyone in the same nosed longboards makes a concave under the board’s nose and convex under the tail; respectively, the middle has nothing left but flat.

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