From my understanding of theory, a forward sweep is inherently unstable and will try to rotate itself 180 degrees. Interesting concept for a fin. It would be interesting to see what kind of twisting forces it is putting on the base as compared to a similar fin that is swept back.Maybe a compromise between the two.
Here’s another take on fins.
I do not think the curve is essential for drive. I make fins with a straight leading edge and they work just fine.
The rake angle, however, is a very sensitive component of fin design. Tilt a fin up 5 degrees, and it changes a lot.
Forward swept fins are not inherently unstable if very stiff and mounted as part of a system. The tip of a forward swept fin is incredibly prone to stalling, which allows the boomerang concept in which the tip turns rearward.
The boomerang moves most of the fin area away from the board, where the difference in speed between the board and the water is greatest. So, aspects of the fin action are maximized because the foil acts on water with a greater velocity. This, I believe, is the key to the Mental template (and the boomerang), and not relieving oneself of the 3/4" turbulence layer. This point is also achieved in the starfin, which has a long chord length near the tip - and also an end-plate.
The fin construction is thus a balance. More fin area further from the board gives better action (for fixed fin area and leading edge rake). It does this because water flows with the board, both in the 3/4" turbulent layer, and also in the next inch or two in which laminar flow moves with the board. So the relative speed difference between board and water is greater further from the board. However, fin area closer to the tip promotes tip stalling. One solution to this is to have a wide tip with an end-plate (starfin). Another is the Mental template in which the fin chord length is longest in the middle, and tapers at the tip to reduce stalling.
99% of all thruster fins do not attempt to optimize in this respect. Chord length is longest at the base and tapered at the tip. I’d be willing to be money this will have a larger impact on fin performance than concaving the inner surfaces of rail fins (instead of keeping them flat).