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James Mitchell wrote:

> 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).

I think that the definition of "efficiency" is the stumbling point.  It
is the optimization of a task (or system) to a particular performance
index, given a set of constraints.  Like you said above, the boat is
optimized for speed (minimize time), and the fish is optimized for
distance (minimize fuel consumption).  For their respective performance
indices, both are efficient.

This reminds me of a story a few years ago when I was in grad school.
One of my instructors had previously worked for GE in their nuclear
propulsion division on a project to develop a nuclear aircraft engine.
It was super fuel efficient (it could continuously run for years), but
it was so heavy that the aircraft could never generate enough lift to
get it off the ground!  This is also why nuclear engines work great
in space (zero-g environment), but you'll never see them in booster
rockets.

Christina

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