Surfboard flex and pressure distribution in planing hulls part 2 a question from 'Kayu'

kayu wrote:

Pressure distribution is mostly controlled by the rider and where he applies it. When the tail is buried , the pressure is on the tail if tapered will flex more"



Yes that is true  however the greatest pressure will still be near the leading edge of the wetted surface area even when the board is trimmed aft during a turn. Trimming aft during a turn has the effect of shortening the wetted surface area. By shortening the wetted surface area the length of the beam has been shortened, this stiffens the surfboard and reduces flex.



That is why boards which require a rider position extremely close to the tail in order to turn are not ideal for flex. Compensating for a shortened wetted surface area during turns by making a short section of tail very flexible is not efficient due to the fact that it makes a kink in the rocker, and also it creates a drag inducing reverse rocker.



In addition to this, making the board very flexible over a short distance is ineffective as the soft flex and resulting rocker curve when bent further reduces pressure on the part of the board which has the least pressure, thus reducing the ability of the rider to store and release energy via weighting and unweighting. It also reduces feedback to the rider by placing the stiffest part of the board beneath his feet.



For the greatest benefits from flex the board should be designed to turn from a more forward position, which should also be the trimming position. If the rider has to walk forward in order to trim the board, and aft to turn, the weighting and unweighting motions required for efficient energy transfer become time consuming, ungainly, and lacking in both subtlety and power.



Keeping the fore and aft movement of the rider to the minimum also prevents the problem explained above whereby the wetted surface area is shortened, reducing flex at exactly the moment when the board is required to bend in order to store energy. Long flexible tails are more efficient than very short flexible ones as they can collect more energy with less increase in rocker curve

Of course there is a lot more to it than this, I'm just covering the basics. http://olosurfer-woodensurfboardsatpipeline.blogspot.com/2010/12/surfboard-flex-and-pressure.html 

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