Approaching Terminal Velocity, Water is Difficult to Compress

January 8, 2018

Water resists compression.  Water is difficult to compress because of inter molecular hydrogen bonding.  This is known as cohesion.  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.  

 When someone falls in water, they displace water by pushing it aside.  As the person sinks, water moves out of his path.  Sink rate or descent speed gradually decreases to zero.  Water displacement cushions the fall.  Water is displaced and not really compressed. in a fall with lower impact and lower descent speed, water is displaced by the gravitational  force of the person's weight.

 

Falls from heights above two thousand feet into water, are usually fatal.  At these heights objects attain terminal velocity, which is the maximum speed reached by falling objects.  Due to velocity of the object, water cannot move out the way fast enough.  Very little water is displaced by fast moving, objects.  This causes a very hard landing.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In the video above, 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.  Garrett is launched through the air at a very high speed.  His board decelerated because water could not move out of the way fast enough.  The bottom of his board is flat and pushes water forward on impact.  A resistance wave forms directly in his path and interrupts his ride.  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 bow may have helped both surfers to ride further. A vee 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 out his way fast enough.  Water resistance causes him to plane hydro dynamically and body surf.  The impact shattered his arm, in nine pieces.  He wore an 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.

 

Water Resistance may have caused a career ending injury for Garret and a fatal wipe out for Mark Foo.  Water resistance also provides hydro dynamic lift which allows surfboards to plane.  Surfboard design should allow movement through water resistance and utilize hydro dynamic lift.  Most surfboards have flat planning hulls which work a lower speeds and in clean conditions.  Extreme conditions and aerial surfing require different hull designs to absorb and cushion hard landings.

 

 

 

 

 

 

 

 

 

 

 Garrett gets hung up and is nearly airborne when he drops in.

 

 His board decelerates twice as the bow hits water.  A bow wave sprays water above his nose in the photo above.  

 In this last photo the nose of Garrett's board is not submerged, however; the bend of nose rocker on the under side is stuck.  The bow is flat and cannot push water aside.

Garrett is launched with tremendous force.

 

In a wind tunnel test, the vapor flow separates from the bow of a flat bottom surfboard. The nose rocker curve  creates a bump in the flow.  In water this bump is a bow wave, which can restrict movement.  Wind tunnel test by Garrett Mcnamara at Mercedes' lab.

Mark Foo is stopped by water accumulating under the flat bow of his surfboard.  Note the similarity with Garrett's last photo.  Both riders get stuck by the nose rocker bend and flat bow.  Speed and high impact on a flat surface, resists penetration in water.  This is hydro dynamic lift working against forward movement.  Reducing rocker and adding a vee in the bow will allow the surfboard to progress further.

photo by Bob Bobour.

 

Garrett Macnamara video and Photos from YouTube.

 

 

 

 

Please reload

Featured Posts

I'm busy working on my blog posts. Watch this space!

Please reload

Recent Posts

July 6, 2019

Please reload

Follow Us
Please reload

Search By Tags
Please reload