Nose rocker aids performance, by keeping the surfboard from diving underwater. As a board moves faster, it generates gravity force. This force produces resistance when a flat bow impacts water. The bow is the underside of surfboard, where the nose rocker bends. The bow becomes a landing pad when a surfboard free falls. The more nose rocker the board has, the more bow is exposed to water impact. Niccolo almost makes this wave, but his nose underside meets fast rising water.
There is a limit to the amount of nose rocker a board can have. The limit is governed by gravity force, which is a function of speed and weight. Gravity force can be calculated using an incline plane, with trigonometry and geometry. This is not necessary, just knowing that impact momentum is generated by speed and weight suffices. High gravity force at the perfect rocker bend to water surface impact, will produce a force that pushes the board backward. Water will not move when the flat bottom impacts the surface hard and flatly.
An object slows when moving through water, by pushing water aside as it sinks. When a flat object is moving fast, it cannot push enough water out of its path. Very little water moves and with high impact, the water surface becomes hard like the ground. This is hydro dynamic force or lift. It is not possible to add power and increase speed like a boat. Therefore, the best alternative is to modify the bow contour to a Vee or round shape. Both of these shapes can penetrate water at high speed.
A surfboard can perform perfectly on a wave, then on the next; fail miserably. This is due to the fact that extreme rocker can function at high speed, if the bow is kept above water. When the bow contacts water, it generates a bow wave or friction (f in the fig. above). This is due to adhesion, water sticks to the surfboard. The bow wave is under the board and not visible to the rider. The only indication is spray shooting up from under the nose of the surfboard. This is not a problem at lower speeds. At high speeds, if a flat bow impacts chop or free falls on to wave face, the board stops. Hydro dynamic lift on the bent underside of nose rocker or bow, pushes the board back. The flat bottom is doing what it is designed to do, generate lift. This is Bernoulli's principle, pressure from under the board exceeds pressure from the top.
The above photo is of a surfboard pushing against its rocker bend. The surfboard is distorted due to light refraction. The arrow indicates the resistance holding the board back. The surfers weight and speed push against hydro dynamic lift. Water over the top of the board curls over in an eddy creating a bow wave. The bow wave is known as wave making resistance (friction). It slows surfboards, boats, and anything that moves through water. Water over the top does not add much resistance. The resistance is generated on the underside of the surfboard.
Surfboard designers increased rocker to keep the nose out of water. This only increased a protruding bow bend, which may extend below the water line with impact. High rocker only works at low speeds or if the bow remains above water line at high speeds. Alternatively, high rocker combined with a round or vee bow can work at higher speeds.
Boats move through water rising up and down in seas. When moving fast, the boat's occupants or pay load gets tossed about. A boat may come close to stalling while climbing a large swell, but; may use a power source to progress. A surfboard can stall when hitting a chop or wave bottom. Unlike a boat there is no additional power source. The surfboard can lose all its momentum or kinetic energy hitting water. An impact at high speed will launch the rider off his board. This can happen with the nose above or below the surface.
A flat bottom produces a higher impact with water because of hydro dynamic resistance. Water is cohesive and bonds to itself. It cannot be parted with a flat surface at high speed. A round or vee bow will cushion the impact with chop and the wave bottom. The bow will push water aside and the surfboard can keep moving.
The next video demonstrate how rocker and bottom contour can negatively affect a ride. Water moving up the wave face fuels the bow wave.
Niccolo doesn't give up and tries again on what looks like the same surfboard.
The last video shows how a good board will keep moving when the bow meets water moving up the wave face. The rider's skill is a large factor.
Tim Pruvost rider Tikanui Smith at Teahupoo