RE: [Paddlewise] Paddling in Space

Ron Dunnington writes:
>
>"A rocket engine does not push against anything
>except the rocket body and that is why it can operate with, or without an
>atmosphere, or, even in water. A paddle must have something to push against
>(more like a jet engine than a rocket), to be effective. "

No, a jet is propelled in the same Newtonian manner as a rocket and does not
need air to push against to work (rather it needs the air to supply the
oxygen for combustion-a rocket carries its oxygen with it to do the same
thing-i.e. push against the front of the inside of the combustion chamber
where there is no equal push against the opposite side to nullify it (as is
the case with the explosive expansion due to combustion pushing in all other
directions inside the combustion chamber) because the combustion chamber is
open on the back end where the expanding combustion gasses are free to
escape.

Raphael wrote:
<SNIP>>>>>>If large amounts of combustion
particles come out fast, the rocket gains speed in the opposite direction
but the center of mass stays as it was.<<<<<<<<<

No Way! The fuel in the rocket is a huge store of potential energy that when
released will propel the rocket far beyond it keeping the same center of
mass for it and its exhaust particles. This potential energy constitutes an
outside force. The laws of motion you are talking about concern collisions
between moving objects when no force beyond their existing masses and
momentums is present or introduced.

>>>>>> On the other hand paddling is like sticking a pole in the mud and
pulling
ourselves from it. There is an external force, or reference that the system
uses to pull itself from.<<<<<<SNIP>

Equal and opposite, just the same as with a rocket. You pull the mass of the
world just a little bit towards you and just to keep things equal the much
smaller mass of you and the kayak moves in the opposite direction, but a
whole lot further than the earth did (to conserve energy and mass).

>>>>>>> Paddles are not ideal and if they were would be too difficult to use
when
starting to accelerate. Since paddles leak some water and suffer some
displacement from the entrance point, some energy is lost, but also may
help the paddler if he doesn't want to put all his strength or if he wants
to save some energy in long tours. Stronger paddlers will want better grip
in the water. Weaker fast stroking paddlers will want some leaking to allow
them to give more strokes with less force, even at the expense of some
energy loss.<<<<<<<<<<

I disagree. When starting to accelerate with the (impossible) "ideal" 100%
efficient paddle you would just not pull as hard at first (or use a shorter
length shaft). Energy is always lost (for doing useful work) to
randomization (entropy) not necessarily by the blades displacement from the
entry point straight back (although water moving in direction other than
straight back (leakage?) will contribute to the displacement of the blade).
The "ideal" paddle would still pull something back (even if it was the
earth) in order to move the kayak forward. There would just be no loss in
the system due to entropy (impossible).
The differences between the paddle preferences of stronger and weaker
paddlers will have much to do with the amount of muscle and joint strain
they are willing to endure and the efficiency of their own bodies at
different stroke rates. The length of the paddle (leverage) is most likely
the best way to control the strain to muscle and joint (rather than
purposefully making the blade less efficient). As pointed out earlier the
higher stroke rate will be more efficient because it minimizes the losses
due to greater acceleration during and deceleration between strokes with a
slower stroke rate. Our coordination also puts limits on how fast we can
efficiently stroke. I suspect that not only is Greg Barton especially strong
and in excellent physical condition, but he is most likely extraordinarily
quick of reflex and inordinately coordinated in order to place his paddle in
the water in such a way that he doesn't get ventilation behind his blades
during the powerful strokes (where most of us would be digging divots out of
the water if generating that kind of power) while at the same time doing his
imitation of a hummingbird.

Matt Broze
http://www.marinerkayaks.com


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