who can tell me what it meansThe bottom contour is a single

[kitesurfer]

https://surfing-waves.com/board/surfboard1.htm


KS

[kitesurfer]

:?: The bottom contour is a single to double concave

The answer is exactly what you have already written. It means the shape of the under side of the board goes from a single concave (as shown in the pictures in the link i posted) to a double concave (as shown in the pictures in the link i posted) with the single towards the nose blending into the double at the back.
And its called a single to double concave and its very common in shortboard design.

And as for a flat vee, well in my books a flat vee is a flat!

KS