Catapulting and Compression
All surfers have fallen hard into water. When you impact water with speed, your body pauses on the water's surface for a second. Water resists displacement and will not immediately move out of your way, due to the speed you carry. Water does not compress, instead; your body compresses. You must absorb the impact with your body, which feels more like hitting the ground than liquid.
After a hard numbing collision with water, your body penetrates and decelerates. The collision possibly compressed your neck and vertebrate, this will hurt later. Your tissues and organs also took a jolting blow, when the water's surface did not give in. You probably feel a bit dizzy from a numbing bump to your head. As you come to your senses, you yearn and crave air, as your lungs compressed. Just when things start to calm, there is a roar and a thundering explosion. The wave breaks on you with a brutal and violent force that compresses your sinuses and ears. They feel like they will burst. Water twists and bends your body into unimaginable shapes. You survived and feel like you wrestled a WWE contender. You experienced the forces of hydrodynamic resistance.
You can break the water's surface with a dive or a pin drop entry. This may reduce some of the beating. Sometimes you are out of control, flying and spinning erratically in the air. The flight is often caused by catapulting, when a surfboard abruptly stops and launches you. Surfers cannot remain standing with extreme fluctuations in speed. The catapult often occurs at high speed when the surfboard is nearly airborne or free falling into a hard flat landing.
Whether the launch is anticipated or not, the surfer is helpless. A few seconds after gaining speed, he can be flying through the air. This is very similar to how passengers in a car or boat are tossed around, if they are not securely buckled in. Instead of seat belts, a surfer may grab a rail and crouch for stability. This may not be enough security.
All boats and ships must slow down in rough seas or passengers will be pitched. Flat bottom boats must slow more and sooner, hence they move slower than other hulls in turbulence.
Fast moving surfboards with flat bottoms stop abruptly. A speeding flat surfboard cannot decelerate because it cannot transition. Water is unable to move out of its path fast enough, much like your body in a catapult landing. Water moves forward in the board's path before streaming around its sides. A swell or wave builds up ahead of the surfboard (wave making resistance). The flat surfboard pushes against this mass or wake, which can be large due to cohesion. The surfboard attempts to ride on top of the wave due to its flat design. A flat bottom generates lift by resisting penetration. High nose rocker may keep the tip above water or the board may pearl or dive underwater. In both instances the end result is identical, the board reverses. The flat underside generates lift or pressure on the nose rocker bend (bow)and pushes the surfboard back. Instead of upward and forward lift, the pressure produces a reverse thrust. A flat bottom surfboard abruptly stops when it impacts water at high speed, much like your speeding body stopped on the surface in a catapult. The flat bottom surfboard stops with hydrodynamic resistance.
A dive or a pin drop entry parts water so your body can enter, a round or Vee hulled surfboard also displaces water. Water must be displaced for a vessel to move. Water is pushed out of the board's path with a round or Vee bow. At high speed a surfboard can transition and decelerate. The shapes cushion impact; the rider remains on his feet. The surfer keeps moving and may possibly finish his ride successfully. The catapult may be avoided.
The following trailer is from an era when surfboards were heavy with round bottoms and surfers were as brave and daring as today.
Tips from the old guys: Adding a little weight for wind and water penetration, reducing nose rocker, eliminating flatness in the bow and driving the board with weight forward of center may result in safer surfing. Surfers have progressed and are riding waves larger than anything ridden before. There will always be waves which cannot be entered by a man paddling with two little hands. Those who attempt these waves risk their lives.