concept

Froude efficiency

The share of an animal's hydrodynamic power output that actually moves the animal rather than the water. For a jet the expression is startlingly compact: twice the swimming speed divided by the sum of the swimming speed and the jet speed. Read it aloud and the conclusion is immediate — efficiency approaches one only as jet velocity approaches swimming velocity, which is to say only as you stop jetting and start gently sliding water rearwards. Measured jetting squid land at 0.38–0.55; flapping rays exceed 0.85. Harder jetting always means more waste, and that is the fundamental ceiling on jet propulsion.