Hydrodynamic Resistance

February 17, 2019

     Speed on water generates resistance.  Water resists compression. Water does not compress due to inter molecular hydrogen bonding. Bonding of hydrogen leaves little space between molecules. Without space, molecules cannot be compacted or squeezed together. Water molecules push back when high force is applied.  This is hydrodynamic resistance or hydrodynamic lift.

 

     Water can be parted by slowly moving a finger through it.  When speed  is introduced water resists parting.  This resistance is strong enough to break speed boats apart on impact.  Water becomes as resistant as earth, and can shatter a boat into pieces.  High speed impact with water will also catapult passengers and cargo.,  A chop or wake feels like an asphalt speed bump.

 

 

 

 

 

 

 

 

 

 

 

     In the last clip of the previous video all passengers are launched.  Much like what happens in a car accident.  The passengers were neither, seated nor belted in.  Surfers also cannot remain standing when their surfboard hits water at high speed.  When a flat bottom surfboard hits water hard, it stops and pitches its rider.  This is the reason speed boat bottoms are not flat.  Even with high tech hull designs, water is difficult to part at high speeds.

 

     Water repells fast moving bodies.  The following video features water skiers hitting the surface at high speed.  They cannot penetrate water when they hit water hard.  Many skiers sustain injuries and have lost teeth with a hard landing.

 

     Surfing has evolved with flight.  The playing field has forever changed. In smaller waves, surfers take to the sky, performing high aerial tricks with successful landings. In large waves, the skills required to land high aerials has transferred. Surfers are successfully landing free falls in big air drops, using very small boards. This has come with a price, as many injuries occur in flat landings. Improved landings can improve surfing and reduce injuries.

 

 

 

 

 

 

 

 

 

     In flight, the flat bottom provides lift.  The board becomes a wing, air pressure or lift under the board exceeds pressure from the rider's weight, providing flight (Bernoulli's Principle).  The same principle applies when a surfboard planes on water.  Speed on water generates lift on a flat surfaces.   Speed and a flat surface are essential factors to planning on water, but; the factors can work against a surfboard with high nose rocker.

 

     The nose area does not provide much lift in flight, due to the nose rocker bend and reduced surface area.   Air spills around the rocker bend, therefore; the nose section can be modified to improve landing without affecting flight.  The nose area may not generate much lift in the air, however; it can provide hydrodynamic resistance on water, especially on a hard fast nose landing.  

 

     The nose bend or nose rocker creates a bow area under the nose of the board. The bow is usually out of water when a surfboard is planning.  When the bow enters the water, the surfboard is not planning at optimum speed.  The bow attempts to move water out of the board's path.  If water is not moved out of the way fast enough, the board decelerates. 

 

     The bow becomes a landing pad in air drops, due to the angle of attack and gravity.  With speed, high impact on the bent area of the nose can push the board back.  Water does not move out of the way and lift is created by the flat surface and speed.  The board may abruptly decelerate and pitch the rider.  A round or Vee bow can penetrate the surface and provide a safer landing.

 

 

 

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