In Dangerous Dives, Hydrodynamic Force Stops Ships, Boats, and Surfboards
In the following video, surfers bravely battle strong off shore wind and steep lurching cavernous waves at Mavericks. All make daring attempts to set their fins into tall cylindrical walls held up by strong gusts. The brisk wind holds surfers back, delaying their entry until they are in-flight to a dark abyss. A few are successful, but; many free fall and land flat on the wave face. They hit water hard with the surfboard flat underside, on unforgiving cold water that will not cushion impact. The force of their landing generates hydro-dynamic resistance which stops the surfboard and catapults the surfer.
Boats and ships riding an ocean swell may encounter a very similar and dangerous condition known as a broach. A broach is where hydrodynamic forces against the vessel"s bow counter the speed and power of the ship or boat. This can happen when the vessel surfs down a wave. Its front points downward as a wave lifts the boat's aft section. The boat accelerates down the wave and impacts the wave in front of it, in a close interval swell. It may also impact the bottom of the wave as surfers do. A large portion of the bow is submerged and exposed to hydrodynamic resistance. As the boat attempts to push through water, it builds a large impassable bow wave in its path. Due to boat speed, water does not have time to move out of its way. Forward momentum is blocked by hydrodynamic resistance, which may cause the boat to turn sideways and rollover. The boat may also pitch pole over its mast or cab and land upside down.
The same thing happens to a speeding surfer flying down a wave. He can suddenly hit a chop or the wave bottom with flat nose underside. The surfboard will either suddenly stop on the surface or submerge underwater. The result is both cases is identical. The surfer is catapulted off his surfboard.
It is possible to recover out of a shallow dive, as the video below show. The right bow design and timing is critical.
Surfers can also recover from a shallow dive. Moderate rocker will keep a surfboard moving in a nose poke. If high rocker is preferred, the bow should not be flat. Flat bows push water forward before it moves aside, taking valuable time. Vee or round bow will push water aside, out of the rider's path to perpetuate momentum. Water cannot be penetrated by speeding flat objects. Speed generates resistance, requiring more time for water to move. A speeding flat bottom surfboard stops when water cannot move out of its path in the given time interval. This is the same force or lift that allows a surfer to plane on a wave. When the force is applied to the aft half of the surfboard, the surfer planes. When the force is abruptly applied to the forward half of a diving surfboard, the surfer is catapulted.