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> > > I saw an item about the artificial fish tail compared with a propeller, on a > > UK TV program called `Tomorrow's World'. It seems that the fishtail sends more > > water backwards and less sideways than the standard propeller. But it seems to > > me that what you gain in efficiency this way you lose in payload and motor > > space by the space occupied by the tail-waving mechanism. I suspect that most > > research will have to be towards more efficient screws and waterjets etc. > > I would think that an artificial tail would be less efficient than a propeller. > Remember, $billions$ has been spent on fluid flow research, and the mechanics > of propulsion. Why aren't airplanes propelled by tails? A fluid, only thinner. > Incidently, I'm not sure that the wash of tails would be narrower than the > propwash of a well-designed propeller. > If I remember correctly, the article in New Scientist (that started this thread) was basically considering this question. The author noted that all propulsion relies on the movement of mass in the opposite direction. Basically you can either move a large mass slowly or a small mass quickly and you will get the same theoretical propulsion. It appears that moving a large mass slowly is far more efficient but only at slow (relatively) speed of forward motion. I would guess that a tail falls in this category. The fish moves a relatively large mass (compared to the mass of the fish) but the top swimming speed is limited (even if it doesn't like like it from a divers perspective). On the other hand, a boat propeller moves a relatively small mass and a comparatively high speed --less efficient but allowing higher top speeds. My gut feeling is that no fish could keep up with a Formula 1 boat :). Airplanes require a high airflow over their wings and thus have small fast props. Helicopters a case of moving large bodies of air at fairly high speed and they aren't exactly cheap on fuel (compare to a plane of similar mass). Somehow I think the article may be just a little clearer...... (lots of diagrams:) Cheers
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