Speed + Water = Resistance

January 23, 2019

     Water is adhesive, it sticks to things.  Water sticks to almost anything.  More specifically for ocean enthusiasts, it sticks to ships, boats, kayaks, canoes, paddle boards and surfboards.  Water adhering to a vessel's surface is drag.  This is visible as turbulence.  Water attached to a surfboard moves independently of surrounding water molecules and generates resistance wakes or waves and spray.

 

     Water is cohesive, it bonds to itself. Inter molecular hydrogen bonding, holds water together. Bonding of water molecules leaves little space between molecules. Without space, molecules cannot be compacted or squeezed together. Water molecules push back when high force is applied. Water can be easily parted by slowly moving a finger through it.  Water cannot be parted by fast moving objects, especially with flat surfaces.  A fall into water from over a few hundred feet can be deadly.  Objects falling in water, slow by pushing it aside while sinking.  An object with speed, cannot move water fast enough.  The resistance causes an impact similar to hitting the ground.  This force is hydrodynamic resistance.  Hydrodynamic resistance also known as lift, supports a speeding surfboard.  Speed produces lift so a surfboard can ride on a wave's surface, an example of Bernoulli's Principle.

 

     Surfing has evolved with flight.  Surfers complete high aerial maneuvers in smaller waves and complete aerial drops in larger waves.  Surfboard bottoms are flat and assist flight due to a winglike underside.  A flat underside is not good for landing, especially when the board lands on the bent nose rocker bend, known as a bow.  The bow becomes a landing pad due to angle of attack and gravity.  High impact on the flat nose underside generates lift that stops a diving surfboard.  The rider cannot remain on his feet with sudden deceleration.  He is catapulted much like pearl dive or nose poke.   The only difference is the board stops on the surface without submerging.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

     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.

 

     In the following video, Garrett Mcnamara attempts to ride a very large and extremely steep wave.  He gets hung up at the top momentarily.  Garrett then free falls or air drops until the bow of his board impacts the face of the wave.  On impact his board abruptly decelerates.  

 

 

 

 

 

 

 

 

 

 

     The board decelerated as water could not move out of the way,  due to its cohesive properties.  The bottom of Garrett's board is flat and pushes water forward on impact.  A resistance wave forms directly in his path due to adhesion.  In cold water this phenomena is slightly amplified, due to heavier water density.  This is also what occurred in Mark Foo's fatal wipe out.   A vee or round bow may have helped both surfers ride further. Both shapes will part water and direct it out of the rider's path.  This is an ancient design used in boats.

 

     Garrett's surfboard stops but, he continues to move through the air with his kinetic energy.  He landed very hard on his arm, on water which can neither compress nor displace.  Water resistance causes him to plane hydro dynamically and body surf.  The impact shattered his arm, in nine pieces.  He wore a slightly inflated, buoyant wet suit. Penetrating the surface at high speed would be difficult in a normal wet suit.  It was impossible with a slightly inflated one.  He skipped across the wave for many meters.

 

     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 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.  A round or vee bow cushions a landing and parts water.  The surfboard may not abruptly stop and pitch its rider.

These are old designs from the past.  In the next video Greg Noll rides a round bottom surfboard and survives a free fall landing.

 

 

 

     Water Resistance may have caused a career ending injury for Garrett and a fatal wipe out for Mark Foo.   Surfboard design should allow movement through water resistance and efficiently utilize hydrodynamic lift.  Most surfboards have flat planning hulls which work a lower speeds and in clean conditions.  The flat planning area extending to the bow, can create a reverse thrust pushing a surfboard back, in a hard landing. Extreme conditions and aerial surfing require different bow designs to absorb and cushion hard landings.  

 

 

 

 

 

 

 

 

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