Mechanics of fish swimming...

[Externet]

Is the mechanics of swimming by fish more energy efficient than boat propellers ?

Was it ever proved/disproved/calculated ? (As per comparative construction of artificial caudal fin propulsion)

[alpha2cen]

This is a comparing method.

 

[latex]drag \quad coefficient; \quad \eta [/latex]

 

[latex]\eta \propto 2F/AV^{2}[/latex]

 

[latex]1; \quad fish, \quad 2; \quad boat

[/latex]

 

[latex]\eta _{1}/\eta _{2}=\frac{F_{1}/(A_{1}V_{1}^{2})}{F_{2}/(A_{2}V_{2}^{2})}

[/latex]

 

[latex]F; \quad drag force, \quad V;\quad moving \quad speed [/latex]

 

[latex]A_{1};\quad fish \quad surface \quad area,\quad A_{2};\quad boat\quad surface \quad area \quad contacting\quad with\quad a \quad water.

[/latex]

Edited November 29, 2012 by alpha2cen

[Enthalpy]

The power used by a fish is hard to evaluate and this has led to many wrong conclusions. One example is the corrugated skin supposed to improve the water flow. For long also, the speed of sharks and swordfish was wrongly reported as unexplainable.

 

Caudal propulsion has advantages:

- It doesn't catch algae as easily as a propeller

- It can be bigger than a propeller, hence potentially more efficient, by taking more water and accelerating it less

- More subtle, it can be more effective at limiting the tip vortices.

 

That is, an upward part of the tail can accelerate water downward and inevitably to one side, but a downward part of the tail can keep or improve the water's downward speed and a the same time brake its side motion. This saves power from unnecessary lateral speed and the associated vortices.

 

A propeller could do nearly the same... It would need to be enclose in a cylindre and be followed by stator blades that elimiante the unnecessary water rotation. Not done because all these steps introduce their own losses and complexity.

 

Also: once the side speed is reduced or suppressed, the tail doesn't need to move laterally much faster than the water does to the aft - a propeller need to so it produces downward speed rather than rotation speed in water. The slower tail is a big advantage to avoid cavitation. This would be one more benefit to boats if they adopted caudal propulsion.