Most surfboards have flat undersides from nose tip to fin area. Tail designs vary with flat, Vee or concave shapes. The forward section of the surfboard under the nose bend, is called the bow in boats. In boats the bow parts water pushes out of its path. In surfing, the bow takes impact in air drops, dives and vertical aerial maneuvers. High impact on the nose rocker bend produces an abrupt stall, that pitches surfers through the air. The flat bow section produces lift by resisting water penetration. The stall, which occurs both on the surface and below, is called a pearl dive. The resistance is hydrodynamic force, which supports a surfer on the water and abruptly stops him with pressure under the nose bend.
Abrupt stalls can be reduced with a modified surfboard shape from the past. The design is the Semi- Displacement Surfboard. This shape planes on its aft half like most modern surfboards today and moves water out of the rider's path when the bow hits.
The Displacement rule: "Vessels move through water by displacing water." Vessels push water aside, taking the place of water it displaces. Speed boats and surfboards initially follow this rule, but; have the ability to rise on the surface and move with hydrodynamic lift. Water hinders movement and restricts speed on hulls that extend below the surface. A speeding vessel cannot move through water optimally, but can move on it very efficiently. The water line differentiates movement in water from movement on water. Increasing speed, increases the pressure applied on water. When speed is increased, water does not have time to move from under the speeding vessel’s path. When water cannot move or be displaced, it supports vessels with increasing pressure. Pressure builds as water blocks the speeding vessel on the surface. Water's resistance exceeds pressure from weight combined with the force from speed. This force is called Gravitational Force. When pressure on the hull bottom exceeds Gravitational Force, It rides on the surface. The bow lifts above water and no longer pushes water or a bow wave. Hydrodynamic lift is generated by pressure exerted on water by a fast moving objects. This is Bernoulli's principle, speed boats and surfboards plane by generating pressure on water.
Rough water affects speed and performance for all vessels, especially those with flat bottoms. Vessels get airborne climbing swells and land hard on their bows. This produces a porpoise like bounce when going over multiple swells. The bows of boats are designed to part water and disperse gravitational force. Surfboard can benefit from this feature with a semi-displacement design. Surfboards hit its bows like boats but usually cannot continue. One bounce normally kills its kinetic energy. Surfboards have the speed and power to push water out of its path, but; a flat bow restricts movement in water. Surfboards can ride through a shallow dive with a bow shape that parts water.
A surfboard cannot transition out of a stall because the flat surface is blocked by water. Water cannot move out of its path because of the speed it carries. It must sink below the water line and displace water to move. It is unable to move on water and must move in it. The displacement rule applies, it must go through the initial phase of movement. In aerial landings, riders shift their weight over the nose to increase gravity force. Increasing gravity force displaces water with impact. It is not possible to execute a weighted nose landing in a free fall air drop. In shallow dives reducing impact with a round or Vee bow shape produces a similar effect. A board must decelerate slowly to transition, so water can move out of the way. This can be done with a bottom shape that parts water. A round or Vee bow cushions impact with water. This allows the board to transition without stopping, and without pitching the rider.
Evolution in surfing by flight and air drops is dictating a more complex bottom shape. Flatness from end to end works if the rider can keep the nose above water through aerial transitions. Pros are adept in this, but; normal average surfers can benefit from shapes which absorb impact, such as round belly or Vee’d bows.