Terminal Velocity

December 9, 2018

Terminal Velocity is the maximum velocity of a falling body through a medium.  Years ago, I lost a friend, who fell in a failed helicopter rescue.  I wondered what happened when he hit water at Terminal Velocity.  With ignorance and curiosity, I did some research.  Initially, your body stops on impact.  Much like hitting the ground.  Bones splinter, then tissue and organs compress.  Water cannot compress, therefore; the body compresses.

 

 

 

 

 

 

 

 

 

 

 

 In a lower fall at lower velocities a body will push water out of the way.  A body slows as it pushes water.  At high speeds, water cannot move out of the way fast enough.  Water does not move and creates resistance.  This is fluid dynamics or hydro dynamic lift blocking the falling body.  The body cannot penetrate the surface because water asserts a higher pressure, Bernoulli's Principle.  Ross Clarke-Jones explains this in the following video.

 

 

 

 

 

 

 

 

In the next video from 2016, Garrett McNamara free falls is a vertical drop.  Surfboards are supported by hydrodynamic lift.  Garrett's flat bottom board, hits water at high speed and stops.  Hydrodynamic pressure on the nose rocker bend pushes his board back.  This is similar to a wipe out endured by a less fortunate surfer, Mark Foo.  Both Mark and Garrett are catapulted.  The impact with water, dislocated Garrett's shoulder and shattered his arm in 9 places.  He is wearing a safety vest and is rescued by his jet ski partner.  At the time of the video was made, his diagnosis was optimistic.  Garrett suffered more complications which may end his surfing career.  Wishing Garrett a full recovery.

 

 

Water is adhesive, sticking to things.  Water is also cohesive, sticking to itself.  You can  break cohesion and part water by slowly moving your finger through it.  If you add speed and more fingers, more water moves due to its cohesive properties.  If you spread your fingers apart and move them through water rapidly, you create a powerful paddling stroke.  Open fingers increase the surface area of your hand.  Water is cohesive and with speed, and will not easily slip between your fingers.  When you paddle slowly water can move through your fingers.  

 

Water is cohesive due to inter molecular hydrogen bonding.  The Hydrogen bond is stronger at low temperatures.  Cold water will have more resistance than warm and is therefore more difficult to move out of the way. 

 

 When the bow of a surfboard pushes water out of a surfboard's path, it creates a bow wave.  Water is displaced and the board moves.  The  board follows behind the bow wave and creates a second trailing wake.  The board moves between two wakes until it is fully planning.  Once on a plane the bow is out of the water, and the bow wave disappears.  The board does not push water with its bow on a full plane, increasing speed potential. 

 

In flight, a surfboard often lands hard on its bow due to the angle of attack and gravity.  On landing, cohesion can prevent water from parting with high impact.  When water does not move out of the way, a board may stop, much like a falling body.   Mark Foo sticks his bow in the photo below.

 

 RIP Mark Foo's Last Ride Mavericks    by Bob Babour

 

 

 

 

In a wind tunnel test the vapor stream lifts at the bow.  This indicates a disruption of flow by a flat bow.  The stream would flow around a vee or round bow.  A surfboard with less rocker and a convex or vee bow will part water and avoid abrupt catapulting stops.

 

 

 

 

 

 

 

Flat bow boards stop when hydro dynamic lift pushes against the nose rocker bend.   Hydro dynamic lift can abruptly stop a board by pushing it backward, launching the rider.  Less rocker  with a Vee bow or round bow may avoid a catapult.

 

 

 

 

 

 

 

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