I gotta chime in again since this thread is still alive.
Several posters have referred to fins that create “drag”. At the risk of being labeled an engineer (I are), I have to note that the purpose of a fin is NOT to create drag. Drag is a (mostly) unintended consequence of any disturbance in the way the water “wants” to flow. Water in any given situation will “want” to flow in a manner that loses the greatest amount of energy in the least time. If you try to modify or change that flow, it will cost energy, hence create what is felt as drag. “Strange” bottom contours modify water flow and thus create drag. Side note… those channels, grooves, golf balls, whatever are mostly within the boundary layer and thus not as significant as they would be if not with the BL.
The trick for a fin is to create the least amount of drag, while still having sufficient depth to get some effectiveness. None of this is useful if you want a board that sideslips, slides ass, whirlybirds, helicopters or whatever.
So much for fins with respect to drag. What then, are they “supposed” to do? Fins give the board a direction by allowing passing water to flow only parallel to the fin.
To create “lift” the fin has to penetrate the boundary layer. Recall that the BL is water traveling more or less with the board. Put another way, in the BL the water is not flowing past the board at the overall over-water speed the board is moving. Any proturbance in the BL will not be fully effective. Example: many small fins, or “strange” bottom contours which do not penetrate the BL will not be as effective as fewer fin of the same total area, penetrating the BL.
Again… an ideal control device (fin, bottom contour, whatever) would create minimum drag, yet create the greatest side force when called upon, in a location where it produced the greatest result. In simple terms, other things equal, the smallest-area single fin, placed farthest back, creates greatest leverage. Demands for other aspects of performance will modify this…
But don’t talk of fins in terms of drag unless you want to discuss loss of total available speed. That’s about the only place where discussion of drag is relevant. Fins are not intended to create drag.
