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Apart from the 'nature does it best' argument (that birds and fish don't
have propellers) there's a fairly fundemental reason that propellers should
be less efficient than (large) fins or flapping wings. Remember that
propulsion through any fluid depends on momentum transfer, not energy
expended. Action and re-action. Momentum transfer rate is proportional to
mv/t where m is the mass of the accelerated fluid, v is its velocity (wrt
the fish or bird) and t is the time it takes to accelerate said mass to
said velocity. Power, on the other hand, is proportional to mv**2/t where
the symbols have the same meaning. Therefore, lowering the exhaust
velocity and moving more fluid will provide the same momentum transfer rate
(hence propulsion) at a lower power level. How does this apply to fish and
birds? By swimming with their whole body, the fluid mass involved in
propulsion can be very large, i.e. their whole body functions as a sort of
propeller. The flapping action of a bird's wings is even more propeller
like, but again, with a very large reaction mass. This same argument would
suggest that waterjets (with a high exhaust velocity and low reaction mass)
will always be fairly inefficient.
This leads to another question which I suspect has a different
answer. Does a propeller provide more or less efficient propulsion than a
captive fish of the *same size*? Here, I would go with the propeller since
a rotating shaft tends to have less transmission loss than a system which
must reverse its direction regularly.
OTOH, it is also true that biological processes (e.g. muscles) tend
to operate at fairly low speed and at fairly low stress levels, also they
take up a lot of room (how much payload does a fish *have* to carry? Thus
there is a fairly good match between the power source and propulsion
mechanism for fish and birds, as there is for boats and propellers (power
source compact, high-speed, and with high-strength materials). The problem
of how to efficiently match a compact power source with a high power
density to a low-speed, low-power-density propulsion mechanism is obviously
a problem, hence boats don't flap their tails to move.
Apologies for the fairly long post, but I think (shall I say IMHO)
the fish-tail propulsion is the obvious way to head for overall efficiency,
but there are probably some significant power transmission problems to
overcome on the way.
Gene
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