Planing lift and displacement lift in surfboards

 what makes a full nose fish catch waves better than a 5-10 glass slipper ?



A combination of greater displacement lift ( due to greater volume) and greater planing lift ( due to more area)



The two kinds of lift are planing and displacement lift, these occur in varying proportions but both are always present whenever the board is moving.



The board does not have to be 'planing' ie.e leaving the stern wave behind in order to produce planing lift, planing lift will be produced even at paddle in speeds.



Also the amount of planing lift produced by the board will not only depend upon speed but also on the angle of attack, thus more nose rocker for example will produce more lift at a given speed. It will also produce more drag in order to produce that lift, but in reality the greater lift can mean that the board takes off earlier, thus lifting the nose area out of the water, thereby reducing  drag.



So a board with greater area and/or rocker in the nose will have greater accleration on takeoffin many cases. Often we hear about low rocker making the board faster on the takeoff but that's only true in some situations.

Surfboard flex and pressure distribution in planing hulls

Pressure distribution under the hull is a very important topic when discussing surfboard flex.



The simple entry level scenario for planing hulls is shown below. It's  implications include those related to  full hull flex vs tail tip only flex.



In the water pressure distribution is obviously different from the typical land test scenario whereby the board is supported at two points, with all the upwards 'beneath the hull' pressures located around those points, with none in between.



The diagram below applies to a steady state planing situation, pumping the surfboard and turning it will alter the pressure distribution.

Those who advocate tail tip flex (as opposed to flexing the entire hull ) should not that there is very little pressure at the tail of the board ( at least for a typical surfboard, I'l explain exceptions to this rule later ) thus there is little force available for tail deflection.



If on the other hand the entire board flexes then much more of the water pressure can be used to flex the board, thus increasing efficiency dramatically.



Rider stance and board length complicate the issue somewhat, particularly with shortboards, but the basic analysis is correct, and is a good place to start.  





http://www.bluejacketboats.com/planing_boat_theory1.htm