Surfboard fin guide part 3: The dreaded flat spot, fin flex.

This example of the typical flat spot showed up today on magicseaweed.. . a great example of how not to foil a fin:

The flat spot shown above extends over three quarters of the fin base and half the height of the fin, it will cause flow separation, a decreased ability to handle high angles of attack, stalling,  and unnecessary drag.

With care and a sanding block the fin could be fixed in an hour or two. .  something most people, even the supposedly 'best' fin makers usually can't be bothered to do.

Flexy or stiff ?

The main factor which will increase stiffness is the thickness to height ratio of the fin ( regardless of the type of glass orcore used). If one prefers the fin to be stiffer just make it thicker, and keep greater thickness into the tip, tapering it less.  Extending the fore and aft length of the fin will prevent the chord ratio from becoming too low. Bringing the position of maximum chord thickness back to about 40 to 45% aft will help lower drag on a thicker fin also.

Fin flex on an upright pivot fin as shown below is not beneficial, it only works to advantage on a low, highly raked fin where the flex gives a motion like a fish tail.  Even the standard Greenough/Liddle dolphin fin shape is insufficiently raked for beneficial flex. We prefer to use a fore and aft rake to height ratio of less than 1:1, the pivot  fin is closer to 1:3.