The Forces of Water Catapult Surfers
by Donn Ito September 15, 2020
A very common and painful wipeout for surfers is the catapult. All surfers know the feeling. Surfers are catapulted when their surfboards stick into water and abruptly stop. They hit water so hard that their lungs deflate the little air they hold. Breathless and winded they seek the surface as the wave breaks, pushing them deeper to darker depths. These wipeouts may be reduced by surfboard design. First we must learn and understand the cause.
Density and shape affect the buoyancy of an object. An increase in surface area increases water displacement. A small surfboard displaces little water and is difficult to stand on when it is not moving. A long stationary surfboard will offer more stability and support. A floating object balances the weight force it generates on water with the upward force water exerts. The object will sink if its weight force is higher than water's force.
In the photo above, Andy Irons overcomes water's force, submerging his surfboard. The upward force of water is called upthrust or buoyancy. The upward force can be overcome with gradual pressure. The resistance is known as hydrodynamic pressure. Exerting pressure gradually, allows water time to move thus releasing hydrodynamic resistance. Water needs time to move from under the surfboard and out of the way. Generally this is how most objects move in water. Objects move by moving or displacing water out of the way. Examples of this are ships and sailboats. Their hulls extend deep below the surface. Deep keels and hulls must cut through water by pushing it aside as they move. They displace a volume of water equal in weight to itself. This takes time and restricts the vessel's speed. A surfer paddling in a waveless and windless ocean is also bound by this restriction. A surfer cannot paddle a surfboard fast enough to plane, he needs waves or swells for added momentum. This momentum is limited because surfers have no power source. An abrupt stop may end their ride.
The speed restriction does not limit surfboards and speed boats which can accelerate on the surface of water. They are not supported by buoyancy alone when accelerating. Speed increases pressure on water. As a surfboard accelerates its pressure on water increases. When water does not have time to move, it becomes somewhat stationary. The upthrust force of water increases to a point where it will support a surfer on a smaller and smaller surfboard. This is hydrodynamic lift or Bernoulli's Principle.
Gradual acceleration allows transition from buoyancy to hydrodynamic lift. When speed and water contact is sudden or instantaneous as in a fall, there is no transition. Transition gives water time to move. Without time for water to flow, hydrodynamic force is strong enough to stop movement abruptly on impact. Surfboards stop in nose first free falls, pearl dives or nose pokes. All stops are due to hydrodynamic pressure on a surfboard's flat underside, and nose rocker bend. The bottom nose rocker bend is called a bow in boats and is where a bow wave is generated. The bow can catch the wave face, wave bottom or chop at high speed. A flat underside pushes water forward in its path, requiring more time for water to flow aside. Pushed water is a wake or bow wave that blocks the surfboard, and stops it abruptly. The surfer is catapulted and hits water very hard, his speed prevents him from initially submerging.. His body will momentarily stop on the surface with hydrodynamic force. He will absorb the impact with his body, as water can not slowly move to cushion him. The next video is an example of nose rocker bend sticking in a very hollow wave at high speed. Niccolo Porcella almost makes this wave. He does not give up and tries again with a near fatal result. His second effort won him Wipeout of the Year. Click on the first photo to youtube for the award winning wipeout. The wipeout is not due to a lack of skill but a failure of equipment and perhaps judgement.
If the forward underside or bow of surfboards were more boat like in shape, being Vee or round, water would be directed aside and out of the way. This relieves the hydrodynamic pressure that blocks the surfboard. Only the bow needs modification. Less nose rocker will also allow a surfboard to skim through a shallow poke. The following is an old video from our past, featuring Greg Noll riding a round bottom surfboard with less nose rocker. In the days of single fin surfboards, the wide point of a surfboard was forward of center. This allowed surfers to lean and ride forward in a dive and drive through water. Weight forward increases gravity force that helps push water out of the surfer's path on impact. Modern surfers in a dive, ride on their heels to keep the front bow from hitting. Single fin surfboards did not hold the wave as well as today's multi-fin boards. The rail edge created by a flat bottom helped hold the wave with one fin. Greg Noll's board had belly extending to the mid-section of his board.
Success is dependent on the skill of the surfer. A round or Veed bow will ride further through a shallow nose poke or through irregular chop, The surfer has an opportunity at success. Any surfboard which deeply submerges in a pearl dive will not recover. A Pearl is when the surfboard nose dives under water. In many wipe outs the nose does not submerge. The flat bow and nose bottom bend generates a bow wave that stops movement on the water's surface. The board is visible motionless, after the surfer is pitched. Catapults can be prevented with surface stops only, not in stops underwater. The board progresses further by directing water out of the board's path with a Vee or round shape. A rider who is not pitched, may recover and has a chance of success. I have built several proto-types which were tested in medium surf with success. Only the bow or forward underside needs modification. The surfboard will be a semi-displacement board and will still plane on its aft half. Success in larger waves can be seen in older videos.