Early designs, circa 1995, of the Extra Force Fin were peppered on the backside with small coned vortex generators.
I found that a fin of any length beyond that of the orginial Force Fin design created eddy sheading. You can witness this on other flat fin designs, by watching others as they kick. Their flat fins will swing side to side, or shudder and shake during the power down kick phase. The too many vortex generators of my earliest Extra Force Fin designs was overkill.
Through observation and experimentation, I learned to balance placment to Energy Saving Zones (ESZ).
