Say you're being pushed by a friend while you're on your skateboard. At some point your friend begins to tire and slows down. Unless he holds on to you, you're likely to pull away from him, if only momentarily until you (on you're skateboard) start to slow down (due to friction and air resistance) and match your friends speed once again. The point being that it takes a force to change an objects velocity.
In the open ocean there is generally little net flow of water associated with the passing of a wave. But the waveform itself is associated with a flow. As the wave passes, the water first begins to flow up and forward and then at some point begins to flow down and back. This cyclic motion extends from the crest to a great depth.
Shoaling waves, that is waves that are moving into shallower water, at some point start to 'feel' the bottom (the bottom being the sandbar, reef, etc.) The water molecules literally start to rub along the bottom, and in doing so exchange momentum with the bottom and start to slow down. This slowing down is transmitted to the layers directly above, and eventually right to the crest, but not instantaneously. There is a lag in time as it takes some time for the slowing down to propagate up through the waveform. The result is a differential profile of forward velocity – at any instant, a lower layer is moving slower than the layer directly above it. The wave as whole is decelerating, but not all regions of the wave are doing so at the same rate.
This is why some shoaling waves break. The differential forward velocity profile exists along the whole face of the wave, from bottom to top – the layer beneath any given layer decelerating, or traveling that much slower than the layer immediately above. As the wave continues to shoal, the upper layers, will literally start to slide up upon the lower layers – the wave increasing in height as a result. When the difference in forward velocity between two layers becomes sufficiently great the upper level begins to shear off in a dramatic fashion – it breaks – the shearing upper layer pulls away from layers beneath – 'you on your skateboard just keep traveling as you tired friend falls behind.'
This differential (forward) velocity profile of the wave face is not only important during the breaking process. It plays a role in virtually all aspects of surfing from catching waves to surfing and, I would argue, to surfboard design itself.
Remember, an object traveling at a given velocity will continue to travel at that velocity unless acted upon by a force. As a surfer 'drops' down the face of a wave, the wave is slowing down in its direction of propagation, or decelerating, but the surfer is not slowing down as fast -i.e. there's a bit of a lag. Its as if the water on the face is trying to pull away from him, or in this case the bottom of his board. In fact its quit common for a surfer to lose contact the face during a drop – sometimes for just a moment, sometimes more. If you surf, its likely, when it has happened to you in the past, you've just shrugged it off as a hop or a bounce after hitting a little chop, which it might have been, but then again, maybe not. Or perhaps you've been lucky enough to have experienced, on those bigger days, when after taking off, have found yourself momentarily in free-fall.
This curious effect works to keep your board from partially submerging – that is you'll ride higher in the water – at least on the dropping maneuver. When you start to climb the situation reverses. As you climb you are counting on the wave to accelerate you both in the forward and up directions. But once again there is a lag – so you appear to sink or ride lower in the water. These effects are in play all the time while you're on the wave.
In fact, its why you can catch shoaling waves. As you 'paddle for the wave' it eventually starts to overtake you. Since its shoaling the wave is slowing down, and because the upper layers are starting slide up upon one another, the wave itself is continuously changing shape. When if finally catches up to you its upper layers try and drag you along for the ride – both up and forward. But not instantaneously, as it take time to accelerate you. As a result you appear to sink a little, but you actually don't drop vertically, the water is rising or flowing over you as it tries to move past you. In the meantime the wave continues to shoal and change its profile – walling up even more – the lower layers decelerating even more. You however, continuing to paddle and be accelerated or dragged and pushed along by the wave's upper layers until you begin to feel as if the wave has 'captured' you. But by then the layers beneath you are now moving slower than you are and as you stand, you seemingly 'pop out' in front of the face, or drop onto to a receding face, as it continues to decelerate in the forward direction (faster than you) - 'you on your skateboard just keep traveling as you tired friend falls behind.'
The same mechanism is involved when you attempt to score some 'air time'. When you leave the wave you are traveling forward at a speed close to that of crest or lip. The rest of the wave is slowing down. So if your 'time in the air' isn't too great, that is you aren't slowed down by air resistance, when you come back down you land on the face, which was continuing to slow while you where in the air.
How might this differential forward flow impact design?
I'm inclined to believe it forces a 'rethink' about the role of everything from fin configuration (including toe and cant) to bottom contours to rail profiles, as well as rail curvature, rocker, and even its role in planing, the primary means of propulsion in surfing. And by rethink I mean how they might be functioning. Its not that a given current design is necessarily wrong, just that it might not be working the way you think. If you understand how a given design component is interacting with this aspect of the wave, it might lead to some interesting innovation.
This curious aspect of interaction, that of the wetted parts of a surfboard and a constantly decelerating wave face is not easy to get one's head around. Especially if you view surfing a one big toboggan or sleigh ride. For one thing it gives new meaning to all that spray that you can generate when you 'hit' a lip – is it due to you carving off a section of water traveling at a slower speed, or is it you colliding with water traveling at a faster speed (as you enter a faster zone of forward flow.)
I'm inclined to believe this much is true – that those design features which assist you in staying in sync with the decelerating wave face are likely to be among the most beneficial, at least they are likely to reveal themselves as such over time - that is, they'll tend to be among those design features that manage to stand the test of time.
This last statement doesn't sound all that unreasonable, until you realize the only way you're likely to manage to continually keep connected with a decelerating wave face is by some clever use of 'drag'.
Surfboard design is all about strategically implementing a mix of passive and controllable drag, as opposed to an approach that is driven by 'eliminating drag wherever you can find it'. Well, at least that's my take.
kc