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#21
05-08-2012, 12:31 PM
 Gray Ghost Guest Join Date: Apr 2008 Posts: 2,978
Adding another article to the pile; this one about a research team examining the role that the fish's body plays in interacting with and generating vortices that increase propulsion efficiency. In the particular velocity regime they were interested in, they found that fins were more efficient that propellers:
Quote:
 By multiplying the width of an object's wake by the frequency with which vortices form inside it, then dividing the result by the speed of the flow, the Strouhal number gives a measure of how fast vortices are being created and how close together they are. Working with Mark Grosenbaugh, a marine engineer at the Woods Hole Oceanographic Institution in Massachusetts, the researchers decided to apply the equation to swimming fish. They adapted the Strouhal number to express the frequency of tail swishes multiplied by the width of the jet, with the product divided by the fish's forward speed. Then they retreated to their lab and flapped their foils at a range of speeds and amplitudes. Measuring the results and refining the combinations, they found that the foils moved the most water using the least amount of energy when the Strouhal number was between 0.25 and 0.35. In that range, their fabricated fishtails delivered efficiencies of up to 86 per cent, compared with a maximum of about 80 per cent for the peak efficiency of ships' propellers. Later, they discovered that most fish swim within that same range-far above the 0.1 or less that the Triantafyllous' earlier foils had struggled to achieve.
I suspect the answer to "which is more efficient" is going to depend a whole lot on how you set up the problem. Anyway, there's a few cites within the article for further reading.

I vaguely remember in the past, that Discover had an article talking about fins vs propellers and legs vs wheels. I can't seem to google it up, but IIRC, they mentioned the fins trumping propellers but that wheels were much more efficient than legs.