Message-Id: <9503202022.AA16302@st.unocal.com>
From: stssram@st.unocal.com (Bob Myers)
Date: Mon, 20 Mar 1995 12:22:06 -0800
In-Reply-To: MRPK81A@prodigy.com (MR TOM L CLARKE)
To: baidarka@imagelan.com, "Bob Myers" <stssram@st.unocal.com>
Subject: Re: Paddling...
On Mar 19, 21:23, MR TOM L CLARKE wrote:
} Subject: Paddling...
> OK... let's say I rig up a contraption on my kayak that
> provides for a 5 lb weight to be suspended by a line and set
> of pulleys such that when the weight is released, it moves a
> 'clamp' back, in a straight line, toward the stern... I can
> attach anything to the clamp... a pencil, a
> pizza, a paddle, whatever...
> Are you saying that if I attach a pencil to my clamp, the
> kayak will move forward as far and as fast as it would if I
> used a paddle in the clamp?
Not exactly. This would not be true on a stroke for stroke basis, but only
on a *time average* basis. If you had the pencil on the end, you'd need to
make a lot more strokes - but the pencil would move proportionally faster
through the water - *exactly* in proportion in the ideal case.
> My intuition says that the paddle version would move the
> kayak further... the pencil version would create a lot of
> turbulence in the water, but not move the kayak (much)...
You're right that each stroke with the pencil would not move the kayak as
much - but you could make proportionally more strokes than with the paddle
version.
------------------
But you're also right about something else - there *is* a real problem with
this force-only analysis. We also need to look at power and energy, and
this is where smaller paddles show their inefficiency. In terms of work
done (I'm referring to the physics definition of work here), each paddle
stroke using the same force pulled through the same arc does the same amount
of work. Work = Force x distance. So if you make twice as many paddle
strokes, but only result in the same average force on the kayak, you've done
twice as much work to do that!
To get back to your dropping weight thought experiment, if we have to drop
the weight more often to get the same time-average force, we have to raise
the weight more often - and that takes more energy, without using any more
force.
Another way to look at it is the power situation. The power you expend in
paddling is equal to the force x velocity -- so if you paddle faster, you're
expending more power. Power is also the energy/time, so if your average
power expenditure is twice as high, your energy expenditure over that time
will also be twice as high.
I think what's happening if you get cavitation & turbulence is that the
paddle drag coefficients are reduced, so if you're pulling just as hard, you
have to pull faster to create the same drag - and therefore are using more
energy to create the same force.
Caveat: all of this ignores the biological aspects of power generation.
-- Bob Myers Unocal Tech. & Ops. I. S. Support Internet: Bob.Myers@st.unocal.com P. O. Box 68076 Phone: [714] 693-6951 Anaheim, California 92817-8076