Paddlewheels versus Propellers

On Nov 9, 2:39*pm, wrote:
Ron Wanttaja wrote:
Years ago, when there was a controversy as to whether paddles or
propellers were most efficient for ships, the British came up with a
simple test: *They built two identical ships, with identical engines,
one with paddles and one with a prop. *They tied a rope between the
sterns, and had the captains go to full power to see which had more
thrust.
Wikipedia says that "In 1848 the British Admiralty held a tug of war
contest between a propeller driven ship, Rattler, and a paddle wheel ship,
Alecto. Rattler won, towing Alecto astern at 2.5 knots (4.6 km/h)...."
http://en.wikipedia.org/wiki/Propeller
Jim Logajan wrote:
However, it is probable that the paddle wheel ship simply didn't have the
right size paddles. Paddle wheels should be capable of efficiencies similar
to propellers - but it takes very large wheels.

I used to think so, too, but an article a couple of years ago in
Flying, by Peter Garrison, set me straight. It was about Lift to Drag
ratios and *Coefficients of Lift, and laid out what those mean not
only in terms of wings, but any foil, including propellers. A
paddlewheel cannot generate any more thrust than the torque applied to
its shaft can produce at the arm-length of the paddle, while a
propeller can produce more thrust because its blades can cut through
the medium with a minimum of effort and produce considerably more lift
or thrust for a given torque than a paddle will.

So we don't see paddlewheels on boats anymore.

I wish I could find Garrison's article again.

Dan

That's a more insightful argument than the ones I'd have made, eg (1)
no matter how large the paddle wheel, at least some of the energy
would be spent pushing down, than up, on the water, rather than
throwing it aft, and (2) a screw can be placed deep in the water with
less rooster tail losses. You can't get more power out than you put
in, but any effort that makes the thrust more efficient pays off.

Thanks for the tip.

As an aside, someone pointed out that a windmilling prop is a lot like
a wing flying upside down -- not nearly as effective, camber is on the
wrong side. Once mentioned it was obvious.