about 1/2 way down this page shows 3 surfboard lengths and where they plane/perform as the wave rises up and spills over.  longer for further out and mushy, shorter for steeper.  


http://www.surfresearch.com.au/a_surfboard_dynamics.html


yeah and so for total speed relative to land we have the speed of the wave plus the speed down the line plus (at times) speed down the wave itself.  i would believe we often hit 15 mph on fast days?