Just been thinking about the concave in a longboard. Should it follow the curve of the rocker or should the bottom of the concave be a straight line between it’s starting point and it’s finish point?. Both produce a concave but which would be most effective?
IMHO all foil contours should flow with the board rocker.
i’m glad you brought this up, because i’ve often wondered if concaves and channels increase speed by actually cutting through the board so that part of the board has a reduced or flatter rocker and less drag. That is, if the concave is flatter than the rocker it is cut into. ( whether LB or short).
Halcyon’s reply noted.
I think that some “well known” shapes have a different rocker in the concave. Isn’t it the speed dialer whose concave increases the rocker in the center of the board as compared to the rail lines? As for longboards, there are some photos in the resources section where you see channels in the bottoms which have much more rocker than the rail lines.
Greg Griffin has some beauties:
If as the concave runs for and aft this concavity increases in depth in the center of its run and goes shallower at the board ends the rocker that this concave produces will have a wider arc, that is less rocker, than the rocker it is cut into. If you cut concave into either end of a board then where the concave begins the rocker will increase relative to the rail rocker. When this concave disappears toward the end of the board the rocker from where it begins to where it ends the rocker it carries will be more gradual than the rail rocker.
This sort of mixed rocker treatment, were and how much of it is introduced and its application in tuning surfboard performance is a most tightly held secret that only top custom shapers use with great facility.
Study a Colletta high performance thruster or a Goin twinzer
closely and you’ll see some beautiful subtle examples of this art.
yeah, it is probably the best secret in the art of boardmaking. In sailboards (which I have much more experience with) I know that some speed shapes actually have “negative rocker” in the last were few inches of the board (like a subtle v gone wrong). This is supposed to help balance the forces the extremely long fins generate and keep the board “in pitch”. I have seen a couple with this and they are fast. The shapers were world cup class and so were the boards and fins. Nothing like what you see on production boards.