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Single concave, double concave,… on a kitesurfing board? We explain everything to you!

Almost all kitesurfing boards are nowadays designed with a single, double or even quad concave as well as sometimes with channels. Besides the design/marketing aspect of the kitesurfing board, is there any interest technically speaking?



Concave definition

The concave corresponds to the base curvature of the rail to rail board. It can be observed by looking at the board in the length.

There are different types of concave, and each one has its own interest, we explain why.

 

 

Comparison to the nautical world

Given that boats were appeared well before kiteboard and a Research and Development budget that is completely different from those of kitesurfing brands, it is interesting to draw inspiration from the results of the nautical world.  

We observe 2 different types of shapes on boat hulls:  

Spray rails– triangular shape, midway between the top and bottom of the boat’s hull

 

 

 

 

Scratches – triangular shape, base parallel to the water surface

 

Each of these two forms has a specific importance.

The spray rails have an influence at low speed: the boat is still quite immersed, and these rails act as a dynamic support on the water surface to start reducing the volume of water displaced by the hull.  They contribute to the elevation of the boat at low speed. These latter elements are also involved in the hull stabilization process, especially in curves, where they maintain a grip by directing the water flow.

The strakes can be assimilated to the concave of a kitesurfing board, allowing to channel the water flow. It is the Venturi effect that is at work in the use of these strakes.

So we could immediately think, why don’t we do even more concave on the kitesurfing boards? According to Bernoulli’s theorem, a local increase in the velocity of a fluid is accompanied by a pressure reduction. The kitesurfing board would “stick” to the water. So we just have to find the right balance.

It should be noted that these phenomena occur at boat “cruising” speeds, i.e. from 17-20 knots… It remains to be compared if you can go so fast in kitesurfing? =) =)


In conclusion

From a scientific point of view, a kitesurfing board with a simple concave or without has less grip on the water. On the other hand, it has less drag because less suction on the water, which can give the feeling of “floating” above the water. Less drag also means better speed on the water. So they are kitesurfing boards oriented for freeriding.

On the other hand, a kitesurfing board with a double concave or quad concave will have a suction effect on the water and therefore a better grip. The channels only accentuate this effect, even playing the role of mini fins. These will mainly bekitesurfing boards oriented for freestyle/wakestyle. 

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