WHY FISH SWIM SO WELL AND ITS RELEVANCE TO SWIMMING
Personal communication from Mojmir Knapek, National Coach of Finswimming, Czech Republic [March, 2002].
Barrett, D. S., Triantafyllou, M. S. Yue, D. K., Grosenbaugh, M. A., & Wolfgang, M. J. (1999). Drag reduction in fish-like locomotion. Journal of Fluid Mechanics, 392, 183-212.
This study related experimental force and power measurements that demonstrated the power required to propel an actively swimming, streamlined, fish-like body is significantly smaller than the power needed to tow the body straight and rigid at the same speed. [Therefore, swimming studies using towing are invalid for swimming.]
A wave-like lateral motion of a fish's body steadily increases in amplitude as it travels to the tail. That motion accelerates the speed of water along the body producing propulsion off the tail. The speed of the body wave has to be faster than the forward speed of the fish. The role played by the body's oscillations and wave development are more important than previously thought.
Other articles by the same authors on this topic can be accessed by clicking on the following links:
Davis, B. (March 4, 2000). Jet-propelled tuna. New Scientist, 165(2228), 36.
"Undulations in its body create differences in water pressure that help to pull the fish forward, much as the curves on the surfaces of an airplane's wing create a difference in air pressure that allows the plane to rise into the air. But these movements also stir the water around the fish, whipping up a series of large vortices, one after another. Depending on the way the fish wiggles, some of the vortices rotate clockwise, others anticlockwise. As the fish moves forward, these vortices roll along the fish's body and into the path of its tail.
That could spell trouble. Depending on their direction of spin, some of the vortices throw water back toward the fish. That might seem useful, like having a tailwind.
In reality, it creates chaos. If these vortices simply fell directly behind the fish, they'd send water crashing into the wake that the fish is creating with its tail. The result would be a drag-inducing backwash.
However, the researchers have shown that fish have evolved a clever trick to avoid this. As the vortices trail off the body at the tail, they roll up into bundles. "The vortices swirl around each other and wrap up together," says Grosenbaugh. The fish seems to sense the direction in which each is spinning. With flicks of its tail, it sweeps each bundle of vortices spinning clockwise towards the left and anticlockwise ones to the right. When these vortices meet behind the fish, they form a jet that sends water away from the fish, boosting thrust just as a jet engine sends hot gases backward to propel an airplane forward" (Daviss, 2000).
[It was noted that small vortices flowing along the body are also used to produce greater propulsion than that possible purely with the flap of a tail. It is reasonable to assert that a fish swims fast and efficiently, the bluefin tuna is the most efficient of all, because the propulsive force is the sum of the tail velocity and the velocity contained in the spinning vortices against which the tail pushes.]
Personal communication from Mojmir Knapek, National Coach of Finswimming, Czech Republic [March, 2002].
Some articles presented in the hydrodynamics section of the Swimming Science Journal, should now be qualified. Those dealing with towing on the surface (e.g., Jiskoot, J. & Clarys, J. P. (1988). Body resistance on and under the water surface. In J. Terauds & W. Bedingfield (Eds.), International series on sport sciences, SWIMMING III, Vol. 8. Baltimore: University Park Press.), should be revised because the measurements were done by towing. Being towed on a rope does not replicate swimming.
When resistance is measured by towing on rope, only profile drag is assessed. That will lead to a finding of lower resistance on the surface and higher resistance underwater. But, that would not be the same result as would be obtained with active swimming.
Experienced finswimmers can attest to underwater swimming being easier than swimming on the surface. The wave that increases in amplitude down a swimmer's body, to be released with the action of the fins, is a reasonably good copy of what happens with a swimming fish or aquatic mammal. As proof of this, consider the following world records for men in finswimming.
|
Distance |
Swimmer's Orientation |
WR Time |
|
50 m |
surface |
16.04 |
|
50 m |
underwater |
14.51 |
|
100 m |
surface |
35.86 |
|
100 m |
underwater |
31.84 |
|
400 m |
surface |
3:05.0 |
|
400 m |
underwater |
2:50.5 |
Perhaps this topic can explain why many of today's swimmers are faster underwater, performing just a double-leg kick with accompanying body wave, than they are on the surface when swimming full strokes employing arms and legs.
Implication.The findings of studies that tow inactive bodies through water are invalid for infering about the characteristics of active bodies moving in water.
Return to Table of Contents for Hydrodynamics of Swimming.