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In a message dated 9/23/2009 2:37:44 P.M. Pacific Daylight Time,
ojaivalleybeefarm@dslextreme.com writes:
http://www.newscientist.com/article/dn17836-knifefish-robot-takes-to-the-wat Hi Brian. The "knifefish" mechanism looks interesting.
Early on in my submarine "dream" designs I had contemplated a
propulsion system with a similar movement.
My design was inspired by the "wing" movements of a Manta Ray.
The concept was to have rubber skinned wings attached to flexible
fiberglass struts that would move up and down like a bird's wings.
The struts would be hinged perpendicular to the hull, one on each
side, with thick rubber "skin" shaped like a Manta Ray wing trailing after the
strut. The rubber wings would have run lengthwise down the side of the hull,
like it is on the bottom of the video sub "Knifefish".
My thought was to power the struts movement up and down with 2 way
hydraulic cylinders, driven by a pump and using shuttle valves. The simple
up-down movement could be operated quite slowly and by using one side or the
other perhaps some turning movements could be generated.
I never got to the "model" stage with that one, but it still intrigues me.
I think that a high speed rotating propeller is probably not very
efficient at converting mechanical energy into thrust. Vance has often
pointed out that the very large propellers turning slowly were better at pushing
a sub than a smaller prop turning fast.
The concept of the water passing over a moving "foil" much like wind flows
over a sail is at work here.
A large sail with light wind is more efficient than a small sail with
higher wind speed.
Therefore a large propeller "foil" turning slowly could possibly generate
more thrust for a given amount of applied energy than a smaller "foil" rotating
very quickly.
Like any thrust device, the higher the speed of fluid travel across the
foil, the greater the friction losses and dynamic resistance encountered at the
foil's leading edge.
A very interesting concept.
Like many things though, the "devil's in the details".
A Manta Ray "wing" would stick out to the sides too far, making maneuvering
in tight quarters problematic.
The mechanisms required to move the wings up and down are considerably more
complicated than just running some wires to a high speed motor and prop.
Although a large "foil" type device may be more efficient in overall
energy usage, it's complicated mechanism would require extensive fabrication
which ultimately would lead to higher maintenance.
Looking at the twin cam drive mechanism on the "Knifefish" illustrates the
complexity of such a system. Lots of parts to break there.
One advantage to having smaller props in several locations is the ability
to provide thrust forces at that local area so maneuverability is improved even
though overall energy efficiency is decreased.
Like on my sub, the two side thrusters should make it possible to rotate
the sub on it's own horizontal axis by running one forward/one
reversed. The "vectored" thrust of the dive plane rotation ( and rudder
rotation ) adds additional maneuvering capabilities.
Still, a very interesting concept. It would be fun to make a model of that
and see what developed. It should be possible to measure the total energy
applied and make a comparison of each design's efficiency in terms of speed
developed per watt used.
Frank D.
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