“As for what is “hydrodynamically smooth” aft of the transition point, when sailing at 2 knots, it’s any scratch smaller than 4 mils (thousandths of an inch). At 12 knots, the “admissible roughness” reduces to under 1 mil. A human hair is approximately 2 to 3 mils in diameter, and a bottom finished with 400-grit sandpaper should have a hydrodynamically smooth finish aft of the transition point for speeds up to 7 knots. So, for most keelboats, a bottom which is finished with 400-grit sandpaper in the aft sections is adequate. For planing dinghies, which sail faster, the aft sections of the bottom need to be smoother.” From what I’ve read the laminar sub-layer occurs only within the turbulent zone (which would not include the very front of the board) and is very thin - a few hundredths of a millimeter. The laminar sub-layer appears to be partially caused by the fact that in that area random movements of the fluid are to some degree prevented by the presence of the “wall” (surfboard, hull, pipe wall, whatever) So any roughness in the bottom of a surfboard needs to not exceed that depth in order to avoid destruction of the laminar sub layer. – The math to calculate such things appears to be known but over my head. I suppose it’s arguable that the entry point of a surfboard might need to be so smooth as to be polished, while the remainder of the board could (or should)be sanded… but to what grit? How fast ARE surfboards anyway? 15-20 knots? tow-in boards are even faster…
so there you go. 400 plus grit, dimples a la golf balls don’t work, and I can relax about the elusive gloss coat.
and how fast are surfboards anyway?