RE: [Paddlewise] Paddling in Space

At 12:56 a.m. 26/07/02 -0700, Matt Broze wrote:
>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.

That dummy Matt sure got this wrong! As hard as it is for me to "get off the
rocket" and believe that the center of mass remains the same it looks like
Raphael is correct here. I'm sorry I went off half-cocked, but even though I
know now it must be true it is still hard for me to imagine that enough
gases would be shot far enough out the back of the rocket to keep the center
of mass of the whole system the same. When I was thinking about a rocket
lifting off the earth against gravity I also neglected to consider that the
earth was moved back with as much momentum as the rocket was moved forward.
Things seem more clear in empty space. The lightweight exhaust gas particles
will be traveling backward much further than the heavier rocket moves
forward and the center of mass will stay in the same position. Thanks for
making me think about this some more Raphael.

I still think Raphael is missing his own well taken point when he wrote:
<SNIP>>>>>>On the other hand the system kayak-paddler-paddle move with
respect to the
media (called water) by interacting with it. Therefore that is an external
force. The system must get in touch with the external environment to gain
any speed.<<<<<<snip>

First off, even with this limited system size "kayak-paddler-paddle" (one
that doesn't even include water yet) you would not need to get in touch with
an external environment to gain some speed. You could simply throw the
paddle to gain some speed in the opposite direction (without having to have
the paddle interacting with water). But, why limit the size of the system to
"kayak-paddler-paddle"? If you make the system:
"kayak-paddler-paddle-water-earth" (and this could go on out
to -galaxy-universe, but don't need to for this example) and you are not
adding any new external force (one that isn't already operating consistently
inside the system anyhow). Every action still will have an equal and
opposite reaction. If you move the water (or earth) one way you will also
move the kayak the opposite way and the center of mass will be conserved
just like with the rocket ship. Something has to move one way for you to go
the other. In a paddlers case it is some water. The faster and further you
move the water one way the faster and further you will go the other.

Nick wrote (in regards to my saying a faster stroke rate was more efficient
because it lessened the accelerations/decelerations of the kayak):
<snip>>>>>>It is not only the stroke rate itself that gives the smoothing
benefit. It is the shorter "down-time" between strokes that typically
comes with a higher stroke rate.<<<<<

I agree with this completely but thought I had already made that point when
I quoted from an article I did on paddles:
"Reducing the swing weight of the paddle allows a faster transition between
strokes.  This reduces the amount the kayak must be accelerated during each
stroke to recover the speed lost to friction during the interval between
strokes.  It takes more energy to accelerate a kayak than it does to
maintain its speed because now inertia (as well as the already present
forces of friction and wavemaking) must also be overcome.
The frictional and wavemaking resistances (drag) also are higher with
greater variations in speed.  The energy savings from the time spent
traveling at less than the average speed does not balance the extra energy
used travelling at more than average speed.  This is partly because
frictional resistance increases at nearly the square of the speed.   Also,
when travelling at or accelerating to the speed where wavemaking resistance
becomes a significant factor (around 4 knots for a typical sea kayak)
resistance increases at near the fourth power in relation to the speed
increase.  A more even speed is consistent with the higher paddling cadence
that less blade weight allows."

Nick continued:
>>>>.Imagine two kayaks paddling side by side at the same speed. Guy A has
his paddle in the water for 1 second and then out for one second, the
Guy B is paddling with in the water for 2 seconds and out for 1.
They both will decelerate similarly between strokes, but Guy B will
not need to accelerate as much during the time his paddle in the
water. As a result he won't need to apply as much force and his
stroke will be more efficient.<<<<<,

I don't think I can agree with that. Given--for the sake of simplicity--that
each paddler keeps his own paddling force equal throughout their stroking
time (but not equal between paddlers). They will both need to accelerate up
to the same speed again after each one second of rest and deceleration, but
guy B will have twice as long to do it so his rate of acceleration will be
only half that of guy A's. Guy A will make 6 strokes in 12 seconds while guy
B will have only taken 4 strokes to work against the kayaks (same) drag over
the same distance at the same speed. Therefore Guy B will be working against
the kayaks drag for a total of 8 seconds and have had 4 seconds of rest
while guy A will only be working against that same drag for 6 seconds of the
12 and have had 6 seconds of rest between strokes. Guy B only has to
accelerate back up to speed 4 times and guy A 6 times in the 12 seconds they
traveled the same distance at the same speed. The questions are who is
working harder and which strategy is most efficient? I don't know, and
unfortunately I'm not up to trying to figure out the math right now, Any of
the mathematically adept out there want to give it a shot?
More thoughts: Guy A supplies more energy over less time with more rest time
and B works longer but at a lower energy rate while he is working. Since it
requires the same amount of total energy to work against the same drag for
the same distance (if the speed were held constant and if the paddles and
strokes have the same efficiency) the differences will have to come from how
the acceleration/deceleration factor plays out or possibly whether a short
strong stroke is more or less efficient than a longer stroke. There are also
limits to how long a stroke can last and still be efficient (personally I
think 2 seconds of in water time may be stretching these limits). A one
second stroke will be in the most efficient (near perpendicular to the push)
position a greater percentage of the stroke time and a shorter harder stroke
also seems to me to be more efficient as there isn't so much time for water
to "leak" off the paddle into directions other than straight back.
I don't know but if I had to bet I'd put my money on the one second stroke
time rather than the two second stroke time to take less energy. Also in
reality, with a longer stroke time the actual work may be having to be done
during a small part of the in water time. In that case the rest of the time
the paddle is hanging out in the water is really rest time and deceleration
will be occurring then as well.
The ideal amount of time to have the paddle in the water (working at a force
the paddler can tolerate) will be determined by the time it is operating at
high efficiency (moving water straight back rather at some angle-as during
the start and finish of a long held stroke such as a sweep stroke for
example). Perhaps to avoid this potential confounding we should restate the
question so that the longer and shorter stokes would be such that they would
be equally distant (in efficiency) from the most efficient stroke rate. That
way we would be testing the effect of the ratio (of in water to out of water
time) more directly.

Nick continued:
>>>>>The goal should be to minimize the time your paddle is out of the
water while maximizing the "duty-cycle" or the percentage of time the
paddle is in the water relative to the full stroke time.<<<<<<<

I would agree with the first part of this but would guess that the less time
between strokes is much more important than the percentage of time the
paddle is in the water vs. in the air. An ergonomic stroke rate that applies
the most power during the time the paddle is perpendicular (to the direction
of boat travel) with as little time as is practical coasting between the
strokes should be the goal (at least if your purpose is to maximize
efficiency at fast speeds).

"Misisco, David J" <David.Misisco_at_chomp.org> pointed out that there were a
lot of variables to control and made that seem to be an insurmountable task
to the testing of differences that stroke rates might cause.

I disagree. It will depend on the accuracy you need. Many variables must
certainly be controlled or corrected for but I think most can be, at least
to within a level of accuracy to still allow us to see the differences
created by the "tested for" variables that would have any significant effect
(such that even a racer would care about them).  Ship model testers have
been controlling for many of these variables for years (as well as several
other variables that wouldn't apply in full size kayak tests). During my own
timed (kayak turning and sprint) tests I am careful to record data and note
possible confoundings (including the date and location of the tests so I can
have a pretty good idea of water temperature and I record if the test was
done on salt or fresh water-so I can correct for them. I also note any
significant wind (if I couldn't find a place to get out of it to do a test)
so I know those "windy" results are less reliable.

What I'd really like to do (or induce someone else to do) is to make an
electronic paddle shaft (with interchangeable blade capability too) that can
measure the force the paddler puts into it accurately and convert this and
the data from an accurate knot meter into digital signals that can be
recorded synchronistically into a small on-board computer (small
analog/digital converter/computer modules exist and are quite low priced
now-18 years ago when I first proposed this approach to Sea Kayaker and
started to research it they were much more expensive and not integrated into
one unit). After controlling most of the variables I think such a device
could answer many of our present questions as well as be a great learning
aid to help teach paddlers to develop a more efficient stroke. It could also
be used to compare kayaks much more realistically than in a towing tank test
or the even more removed from reality simulations of towing tank results I
do for Sea Kayaker magazine now. For one thing, by measuring the forces
going into the paddle we would automatically control for the potentially
important variable of yaw (due to the off center propulsion of paddling)
which towing tanks totally ignore. It would also account for the speed
variation factor discussed earlier. With interchangeable blades we could
test and compare the efficiency of different paddle blades as well. I find
it hard to believe that the U.S. Kayak Sprint team hasn't done this (or
attempted to do this) already but the last time I checked (several years
ago) Greg Barton told me they did video motion studies but hadn't yet done a
paddle rigged with sensors.
If anyone with an engineering bent is interested in developing such a device
I would be willing to share what I've researched about the problem already
and consult with them on the project. With all the kayaking schools (and
tech-weenie kayakers) out there now it might even become a marketable
product (if it could be produced for a reasonable price--which I think it
could).

Matt Broze
http://www.marinerkayaks.com


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