A single concave is a hollow channel scooped into the bottom of the board, running roughly from under the front foot to the back. Water flowing under a moving board accelerates as it passes through the narrower channel — the same physics as wind picking up speed through a canyon. The result is lift and forward speed.
What it does on a surfboard
Single concave is the most common front-half bottom contour on modern shortboards. It generates speed through fast sections, lifts the board onto a plane earlier, and gives a clean, drawn-out feel through long-line turns. The deeper the channel, the more dramatic the speed-and-lift effect — but very deep concaves can feel stuck-to-rail in tight pockets.
Most HPSBs run single concave through the front 2/3 of the bottom and transition to double concave (sometimes via a flat-blend zone or a vee) in the back third where you want release rather than lift.
What to look for
Phrases like "single concave deep" or "single concave through the middle" mean the same hollow shape; the depth and length vary per model. Lundquist HPSBs run single concave under the front foot transitioning to double in the back third — a common contour family but tuned for the line's flatter rocker signature.