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The Bottom of the Board

  There are several bottom contour configurations found in contemporary surfboard design. They may be divided into three primary groups: flat, convex, and concave. All may be applied to any type of surfboard, although most are relevant to specific types of boards.

Flat is flat. Some shapers employ a flat bottom nose to tail in their designs, but a flat bottom is hard pressed to add any performance features to a surfboard - shortboard, hybrid, gun, or longboard. Flat provides no lateral stability, lift, or leverage. Bottom contours with flat areas combined with other contours are common. Combined with vee, tri planes, soft round surfaces, and concaves flat bottom contours move away from their safe, neutral, vanilla features and help develop acceleration, speed, and control.

Convex bottom contours are any combination of flats and curves that descend below the rail line of the surfboard. Convex bottoms may also be described as "displacement hulls." These designs provide great lateral stability and control, smooth transitions from rail to rail, and are very forgiving. They handle well at a full range of speeds. They are truly essential to big wave guns, hybrids, and longboards. And, they are functional, although somewhat conservative, in shortboards and specialty shortboards.

The following illustrations show two convex bottom configurations. Flat to vee is very simple and common to shortboards, hybrids, and longboards. The flat entry is fast enough and forgiving enough for surfers of all skills. The depth and length of the vee is varied to determine turning radius and provide control at higher speeds or in critical sections. When employed in hybrids and longboards, surfboards with lots of surface area and volume, the vee helps manage and control the boards' mass. Tri plane to vee is also a very simple bottom, and arguably a more versatile and effecient configuration than flat to vee. The displacement in front of the vee is faster than flat, as it planes in the face of a wave with less wet surface, it's very forgiving as it lifts the forward rails out of the water in critical sections, and it initiates turns with less effort as the surfer leads the surfboard onto rail or from rail to rail.
  XXL wave guns require a convex bottom configuration. These boards are designed to be functional and effecient in extreme conditions. Vee< in the entry transitions to nearly flat or tri plane in the mid section to panel vee in the tail section of the surfboard. The vee in entry can cut through the chop and turbulence of wind blown or riptide surfaces or slice into a glassy surface at the speeds developed dropping into a big wave. The flatter mid section keeps the hull at speed and accelerates the board into the shallow panel vee in the tail. The panel vee in the tail steers the board from rail to rail and holds the board in a line much like the keel of a sailboat transfers the force of the wind into forward motion.  
  Concave bottom contours are any combination of curves that ascend above the rail line "into" the surfboard. Variations of single to double concaves are the primary bottom contour configurations in the modern shortboard. Concaves are one of the most complicated and contradictory design components included in a shortboard. Consideration of other design variables of the board is essential to decisions about the arrangement, depth, and placement of concaves. Imagination and experimentation - trial and error - testing and observation yield efficient multiple concave bottoms.

Concaves produce lift with laminar flow (channeling water under the board) and surface area (a curved line is longer than a straight line side to side across a surface) as water passes under a surfboard. They produce additional lift when water runs into the aft section of the surfboard's template (where the template turns into the tail and crosses the path of the water flowing towards the tail) and lifts the tail under the surfers back foot. (Try placing the concave side of a spoon under a faucet of flowing water !)

Working effectively with lift and drag is key to designing concaves into the bottom of a surfboard. Efficient multiple concaves feed water under a surfboard to the surfer's stance then release water through the fins and tail of a surfboard behind the surfer's back foot. When a surfer weights the rail and bottom of a surfboard he compresses the water, channeling it through the concave array. The rocker, template, rail, and fin arrangement provides this compressed water with an avenue of escape - out through the tail section of the board. This phenomena excentuates the acceleration of the surfboard through it's turns. Tuning each variable and their interaction with the other variables determines the performance characteristics of the surfboard.

Most concave bottoms will follow one or two designs: (1) Single to double concave: Flat or slight vee in the entry, shallow single concave increasing in depth to maximum depth just in front of the rail fins, double concave begins at or near this point and carries through rail fins, double concave transitions to vee behind the rail fins, and (2) Single concave: Flat or slight vee in the entry, shallow single concave increasing in depth to maximum depth just in front of the rail fins, concave decreases in depth through the trailing fin to the tail.

Holding the template and rocker constant, single concaves tend to be looser than single to double concaves and have the potential for greater variation in surfer / rider dependent turning radius. Single to double concaves tend to hold a consistent turning radius without much variation and will favor easier, less surfer / rider depedent projection out of turns.
  Link to May 2019 essay regarding single to double and single concaves  
  Traditional longboards, and reproductions of these classic surfboards of the '50s and '60s, have a convex bottom. The longboard bottom design is a displacement hull, moderately rolled from rail to rail from nose to tail. These boards are designed to glide, trim, and roll from side to side to turn and adjust trim. The convex features hold the board firmly in the face of a wave.

Contemporary Longboards also have a convex bottom. Unlike the rolled bottom nose to tail of traditional LBs, contemporary LB's convex bottom often features a tri plane to rolled vee design. The tri plane offers the opportunity to introduce concave in the center plane in the nose to enhance nose riding.
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