i have 2 nearly identical boards the only diff is that one has alot less double concave. vector foil fins work great in the one with lots of concave but seem to drag a bit in the one with subtle concave. same waves,got many years surfing so i can tell whats going i just cant figure out why. are vectors meant to work in tandem with more pronounced concave?
Personally, my opinion is that Vectors are a scam that developed when a batch of fins warped during the bagging process...but that's just me. Fans of the Vectors claim they turn better and I guess having a curved surface can be effienct through an arc, but then you're really counting on the outside fin(s) not grabbing water. I had them on a displacement hull and they absolutely sucked.
A (mildly) educated guess is that the deeper concaves create more low pressure in the center and providing some forgiveness of the fins lack of effiency. This amount of low pressure isn't present in flatter or bellied boards, placing the fin into higher pressure water and making the front and rear edges of the Vector drag.
Vector foiled fins create less lift than a flat foiled fin On a flat sided fin, you have a greater difference in pressure... about 8%... than on a Vector fin, so... 8% less lift. (I didn't do the calculations... somebody much smarter than me did).
OK... what does that mean for surfing performance? It means you don't want to use them in small or weak surf when you want as much lift as you can get, but you might want to use them in large, powerful surf when you want to control lift. And that's been my experience... they work good when the waves are good, and don't work good when the waves are aren't good.
Now consider concaves. Concaves create lift under the board, and create the most lift where the concave is deepest. Deeper concave, more lift under the board. As for the rail fins' relationship with concaves, deeper concaves reduce cant, and so they reduce the effect of fin-induced lift. Fin induced lift creates an upward force under your back foot. Concave induced lift creates lift between your feet. Imagine the relative forces and how they might be effecting the pitch of your board's bottom as a planing surface. What's going on? More concave forward, less under the tail? You're tail is dragging? Less concave forward, less fin lift under the tail? Flatter planing surface... more speed?
Put your fins in one board, measure the cant relative to horizontal, then do the same with the other board, and see if there's a difference in cant angle. Put a straight edge across the bottom of both boards and see where the deepest part of the concaves are. Think about the relative forces at work, and how they interact with each other, the board, and the wave.
Then again, maybe it's the board, and not the fins. You might think they're identical, but they might not be.
Is the above explanation a fact based on physics or a widely accepted theory?
I always believed concaves allowed air under the board and was more of a hydroplaning effect, providing instability that is described as looseness. I've always considered lift, on the other hand, something created by Bernoulli's Principle, that applies more to fins than the hull of surfboards.
I always though the highest pressure was at the rails.