The reduction of the size of the fillet had more to do with vortex drag, the movement of fluid over the end of a wing section. In the case of keels and fins, where the end meets the hull/bottom , we're talking about the lack of this movement. The more perpendicular the joint the more efficient (less movement away from direct fore aft). The bigger the fillet, the more easily the airflow can deflect. This effect I believe would hold regardless of whether the hull is planing or not. In fact at planing speeds it might be more apparent. You see old boats and planes having big fillets (which are aesthtiically pleasing) which I think was more a nod to structural intergrity, but modern ones do not. Whether or not this can even be felt on a surfboard I don't know, but I kind of doubt it. If I recall correctly GG was adamant about the inferiority of glassed on fins.
I like glass ons, eg. I would never uses boxes for the runners.
In a complete different but realted topic, while googling to find a scholarly article on interference drag of sailbnoat appendages I came across this- LEX- leading edge extensions. Or more specifically LERT - leading edge root extensions. The concept sounds similar to the canard theory. I do not recollect anyone doing something like this with surfboard fins but I could be wrong.
Leading Edge Root Extensions were first used in Northrop F-5A/B Freedom Fighter, which first flew in 1959. After that, many fighter air-crafts have used it, some of which are mentioned after the qu...
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