The flow of water under a surfboard has a large rail-to-rail component. This is somewhat contrary to how most envision the flow of water under a surfboard to be, which is roughly a nose-to-tail flow. This nose-to-tail view of flow is reflected in what many shapers and surfers believe can be accomplished with bottom contours. Few look at a surfboard and ask what might happen if water is flowing at an angle across the bottom –i.e. with a rail-to-rail directional component.
Asymmetric designs, with respect to the stringer, have been and continue to be made, but remain rare. This is understandable given that few surfers want to lock themselves into going left or right, or any one thing in particular on a wave. Locking oneself into a given flow pattern would seem to require to the same considerations. Given the dynamic nature of the sport, flow patterns under the board can be expected to, and do change quite often (during a ride) and optimizing the bottom contours of a board (or any other aspect of board design) to fit one particular flow pattern requires caution. Though this is not to say that all bottom contours should be avoided.
Under the Board
Though there are maneuvers in surfing which are comparable to those of a sled or that of downhill skiing, most surfing maneuvers are at a angle to the face of the wave – i.e. are somewhat perpendicular to the flow of water up the face of the wave; a sled maneuver is parallel to the surface. Gravity is, of course important; but not so much as a source of energy while surfing. Gravity and the fin system allows the surfer to present the bottom of his board to the flow of water in a wave in a manner roughly analogous to the way a mast and keel allows the sailor to present his sail to the wind. Or to put it another way, gravity is used as a counter force against the force generated by the impact of the flow of water up the face of the wave. And like sailboats, more often than not, the real value of the sail/keel system is being able to tack – i.e. run in directions other than ‘with the wind’.
Propulsion in surfing comes from the flow of water rushing up the face of the wave. In particular, the kinetic energy of surfing comes from the transformation of flow energy of the water rushing up the face of the wave, and the transformation occurs at the bottom of the board. The bottom of the board redirects the flow and in doing so taps some of the kinetic energy of the flow, which is transferred to the kinetic of the surfboard (and surfer.)
Bottom contours, by their very design will modify the characteristics of the flow under the board - its what they do. But bottom contours are fixed, so to make them work they must be presented to the flow in the correct way. This is especially true for extreme bottom contours like deep triple concaves. In fact, if extreme enough, the bottom contours will do the ‘surfing’ for you, that is you will have to fight to do anything other than whatever it was the bottom contour was designed to do. (The Bonzer is a good example of extreme bottom contours, and their consequences –i.e. a Bonzer has a way of surfing you, as opposed to you surfing it.)
Planing is about the best dynamic description of what is going on under a surfboard when surfing. The water rushing up the face impacts the bottom and is then be redirected in some way. This is similar to the more familiar notion of planing in motorboats. In motorboats however, the means of forward propulsion is via an engine, nevertheless, the water impacting the bottom lifts the boat. When surfing, because of the unique relationship (angles) between the flow and the bottom of the surfboard, part of this lift can be translated into ’forward’ propulsion. The impact force is usually quite palpable as a peak area or point of maximum force. This is especially true for shortboards. Shortboarders usually place their forward foot on or a little ahead of it, their rear foot behind it. In a way, shortboards literally ‘surf’ it – they definitely know when they’ve lost it.
In longboards, the peak area is much more diffuse –i.e. spread out over a larger area, and it also migrates more than in shortboarding. The actual position of this peak area being a function of the flow and bottom presentation. Longboarders, if they’re so inclined, can often follow this area up and down their surfboard by ‘walking the board.’ In shortboards, if the center migrates, the general strategy is to shift weight, resulting in some maneuver to recapture the center of force, though the occasional (small) change of position often helps. Reorienting the board’s direction with respect to the flow also helps to capture or recapture flow, in both shortboarding and longboarding. The reorientation, or re presenting the bottom, usually amounts to exposing less surface area to the flow. (Fin presentation aside.)
Unlike shortboards, which have relatively limited amount of surface area to expose to the flow, a longboard has far more, and more surface area means more impact, which can mean more power generation. Though shortboards accelerate better than longboards, longboards can generate far more power, e.g. achieve a greater velocity.
Once the flow has impacted the bottom of the board, its has to be removed to make way for new fresh flow. The motion of the board helps, but chances are if not removed fast enough, some of the energy of the new flow will go towards removing the ‘spent’ flow, or exhausted flow. Simply put, you want to get rid of the exhausted flow as fast as possible, in most cases –i.e. you don’t want anything getting in its way, and just about anything other than nothing will.
Good surfboard design is about creating a platform for the surfer that will enable him to capture new flow and shed exhausted flow as rapidly as possible. and in a controlled way. Not surprisingly, the less there is on the bottom, the more this seems to be the case.
