Having been a member here since the last century as Dunk I’d pretty much given up on Sways for pretty much the same reasons as so many others, I rejoined a couple years back but struggled to muster much ethusiasm until a recent l2/l3 fusion means I have rather more time on my hands than I’m used to.
So with thanks to Stoneburner for reassuring me that all things Sway are not going to result in a flaming I’d like to offer the following for discussion.
Essentially my thoughts are that lift and drag as traditionally defined, and I’m not going to suggest redefining them, are not particularly useful concepts in surfboard design and that focussing on just two very specific and derived factors can make it difficult to see the wood for the trees.
I’m going to suggest that there are much more intuitive ways of thinking that are equally valid from scientific point of view yet correlate much better with our experiences as surfers.
My suggestion is that it is easier to think of the pressures that develop over the hull/foil surfaces and how these change either by design or by circumstance.
Can you give some examples that would demonstrate various effects of design and pressure change?
Take two boards and riders of the same weight trimming across the wave at the same speed. Lift and drag are going to be pretty much equal but these are merely the resultant forces of a great deal more that is going on.
Pressure is largely a function of the speed and angle at which the water hits the hull/foil surfaces. So if in this example the boards are very different, say an HPSB vs a longboard, I’d expect the short board with higher rocker etc to have a small intense pressure zone aiding manoeuvrability while the longboard should have a larger less intense pressure map giving a smoother more stable ride.
Pressure acts perpendicular to the surface so I would expect vee or a convex surface to focus the foces closer to the rider requiring less of reaction moment to roll the board than a concave surface which tend to focus the forces away from the rider.
Regarding short vs. long, it seems like it would be more about available planing surface.
It has always seemed to me that a concave bottom is focusing the water/pressure much the same as a parabolic mirror focuses light. Seems like there would alos be an air cushion developing as velocity increases.
If the planing surface is larger but the total force remains equal, the force per unit area must on average be less.
I think the turbulence that would accompany any kind of air cushion would make a board instantly unrideable.