How fish swim very fast despite water being 800 times denser than air?




How fast fish swim?


Whether a fish gets enough food or not depends to a great extent on speed. So, as expected, many fish are very fast swimmers.

It has been extremely difficult to check accurately the top speed of the fast swimmers. 

This is because such speed is often not sustained. 

Usually it is only a quick, lightning dart or sudden spurt that is required to capture an alert prey.

But fish-speeds over distances have been measured, though absolute accuracy is hard to achieve. 

The active tuna, the only fish whose body temperature is higher than sea temperature, swims constantly, because its body is heavier than seawater. 

Tuna seem to be able to swim at  75 kilometers per hour. 

One report says that the sailfish can reach 110  kilometers an hour. 

Even the giant manta ray, which swims by flapping its “wings,” can achieve enough speed to jump a good distance out of the water.

Truly such fish are “bundles” of energy and muscle. 

But this is not enough to explain their speed. 

The problem is that water is about 800 times as dense as air. 

It is also about fifty times as viscous, causing far more resistance. 

On ships the drag caused by water resistance and turbulence is a major factor, requiring a great expenditure of energy to “plow” through the water. 

Ship designers have tried to devise means of overcoming the problem. 

They have researched such questions as: 

How is it that fast fish like the tuna actually achieve greater speed than mathematicians say they should be able to? 

How do the tuna and the shark slip through the water so smoothly and without turbulence?

Some answers are known.


Fish body design 


First of all, such fish are highly streamlined. 

This, submarine designers have copied. 

Fast-swimming fish also can fold their fins against their bodies. 

Scales evidently adapt to water pressure to eliminate turbulence. 

But the primary secret of their speed, for a long time a mystery, lies in the construction of their skin, which is elastic and flexible. 

The dolphin’s tough, leathery skin appears to lie on a cushion of oil, making it yield to turbulent currents, thereby offsetting them. 

Additionally, the skin of many fast marine swimmers is porous and coated with mucus, which forms filaments that let the fish glide through the water, leaving it smooth and almost still. 

Experimenters trying to apply these principles to shipbuilding have used filament-forming substances and found that they were able to cut water resistance by as much as 70 percent! 

The cost of this method, however, is prohibitive.