409 Dive Log Australasia December 24

This colony shows immature female Grey Nurse

Silent swimming The most important aspect of minimising turbulence is to do with finding food. Sharks are excellent at silent swimming. Stealth is vital to hunting prey! If sharks are creating big pressure waves, this will alert potential prey, which will escape. Their lateral line is designed to detect these waves of energy. The result is that if the fish has time to escape; the shark will go hungry! Therefore, active swimming sharks have a fineness ratio of 4 and ½ to one. The dimensions best suited to energy conservation and efficiency, as well as stealth. Humans copy nature in dimensions of ship design and in the size ratios of wings, fuselages in planes and surfboards. Bio-mimicry is used in many areas of human endeavour. Take a look at how nature does it and copy it. Is it cheating? No way! It is simply that nature has faced this problem and worked out the best design. You would be a fool to ignore the best.

The Heterocercal tail The heterocercal tail has been filmed and analysed in detail by Dr Lauder of Harvard University. He used a new ground breaking technology called Particle Image Velocimetry. (PIV) Swimming sharks are filmed with high speed digital cameras under sheets of laser light. The water contains millions of microscopic silver reflective balls; nanoballs, if you like. The digital imagery is analysed with advanced software to study eddies and vortexes and water movement that is normally not able to be seen. He verified that the tail pushes water downward, creating upward lift in the shark. The tail creates two vortex rings in water which are offset to direct an upward thrust. Tail fin strokes of sharks is a study in 4-D geometry. We can now “see” what the water is doing in detail. It shows where energy is lost and where it is directed. Some sharks have special radial muscles around the tail which can alter the angle to help it take off or change its angle of lift.

This long slender male Great White shark will bulk up as he reaches maturity

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DIVE LOG Australasia #409 - December ‘24

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