Here's something we were developing for the Fischer ski but due to time and budget constraints, it was never introduced. Basically it's a low drag adjustable wing. It's low drag as the discs that the wing is attached to are less than half the thickness of the fin. When tightened together, the discs are the same thickness as the fin thus insuring a very unrestricted water flow. As for the flat top adjustable fin that's illustrated, please disregard! ;-)
My "EXTEND A SKI" fin, early to mid 80's. Didn't quite catch on but sure looks neat! Tubes turned out to be a fad. The extend a ski bracket screwed onto the rear of the fin could be applied today to the ski allowing the skier to set to change the distance from tail by moving the rear of the ski back or forward instead of the fin. Additionally, this bracket could be hinged to allow up or down adjustment thereby gving the skier the ability to raise or lower the tail/tip of the ski without touching the fin.
One of my first fin clamps leading up to the invention of the adjustable fin. Note the pencil marks on the front of the fin drawing out a shorter length! History was in the making here!
Schnitz Adjustable Fin/Wing Prototype - Circa Mid 1980's