Nose rocker is an important aspect of a surfboard, it keeps the board from diving under. The down side of nose rocker is that it creates drag.The nose section is usually out of water, how can it possibly cause drag? The nose area usually enters the water at lower speeds. Water contact is made under the nose rocker bend, in an area called the bow in boating. The red arrow points to the bow area in the following illustration.
The bow area pushes water out of the surfboard's path at lower speeds. When the bow is flat, as with most boards today, water is first pushed forward, then around the board. With a round or Vee bow, water is pushed directly out of the board's path. Drag is created with all bows, but; the resistance is less with round and Vee bows. When the nose hits water at high speed, a flat bottom cannot move water out of the way fast enough. Water accumulates in the form of a bow wave and pushes the board back. This is hydrodynamic lift producing a reverse force. Lift is created with the flat bow by speed. This is Bernoulli's Principle, pressure on the bottom exceeds pressure on the top, generating lift.
The following video does not feature fatalities, many of the wipe outs are caused by the bow stopping the board or the board free falling uncontrollably.
At low speeds water may be parted by flat objects, not so at higher speeds. Inter molecular hydrogen bonding, holds water molecules together. Water cannot compress due to the lack of space between molecules. Water actually pushes back on high impact compression. Water can be parted with a thin speeding fin cutting through it. Turn the fin sideways and add high speed; turbulence is created, control is lost. Pressure on the fin drops due to water whirlpools or vortices. The flat bow combined with the rocker bend forms a wall, similar to a fin mounted sideways. At the perfect angle of impact, water blocks movement and spills over the board in a vortex. The flat bow is effective at low speed, but may stall at high. This is similar to an airplane wing with an angle of attack too high for available thrust or speed.
Surfboards were not originally designed for flying, nonetheless; flight is a permanent part of surfing. Flat bottoms act like a wing, producing lift in the air, Bernoulli's Principle. In free falls on large waves, surfers often land on the bow area with high impact. Off the lips and floaters have changed with modern vertical surfing. Surfers break their fins free out the back of the wave and fly to the bottom. Again landings are often completed on the bow section of their boards due to gravity and angle of attack. When the impact is hard, a flat surfboard may stop. A sudden stop can pitch the rider off his surfboard. Round or Vee bows can cushion the landing without stopping movement and catapulting riders. Weighted landing on the tail or nose will penetrate the surface. Surfers land on the bow with their weight fully on the nose, to flip the board around in a reverse. Their weight penetrates water so the board keeps moving fluidly without abruptly stopping.
Is the planing hull fastest hull? Compared to a Semi-Displacement hull, they are equal, if the planing hull completes a ride free of bow contact. When a bow contacts water a semi-displacement hull is faster. A semi-displacement hull is a board with a round or Vee bow and flat or concave aft half.
The drawing below is a cross cut section of the nose area with a displacement hull superimposed over a planing hull.
There are many factors affecting speed such as rail shape and rocker. The general rule is, we trade speed for control.