Date: Mon, 20 Mar 1995 20:47:54 -0800 (PST)
From: David White <dwhite@u.washington.edu>
To: baidarka@imagelan.com
Subject: Re: Paddling...
In-Reply-To: <9503202006.AA16192@st.unocal.com>
Message-Id: <Pine.A32.3.91c.950320200815.78656B-100000@mead2.u.washington.edu>
On Mon, 20 Mar 1995, Bob Myers wrote:
> > The presence of turbulence, in and of it self, means absolutely nothing.
> > Turbulence is nothing more than a fluid (water) rushing to fill the "hole"
> > left by the paddle. "HOW"
> > the water was moved, resulting in turbulence, is the point. The width,
> > length, area, shape, edge, and speed of a paddle all contribute to the
> > amount of turbulence because they first are responsible for the initial
> > movement of the water. Turbulence is a byproduct, like smoke from a fire.
>
> I think this is wrong. The amount of turbulence should be relevant - it
> takes energy to stir up the water, energy that can't be used to propel the
> boat. To propel the boat, you need to move water backwards - turbulent
> water has a lot of motion in a lot of different directions.
I agree.
I think the key concept here is "equal and opposite reaction" ... a
fundamental law at best. Conservation of energy is not a valid arguement
in this case. Energy can freely change from kinetic to rotational to
thermal. And without a good model of this transformation the resulting
modes of energy become very difficult to quantify.
If we're talking about *moving* a kayak then we must discuss momentum.
Turbulence is rotational motion. The kayak (we hope) is in translational
motion. Each type of motion has its own conservation constraints:
conservation of angular and linear momentum, respectively. Rotational
motion cannot transform purely into translational motion; that would
violate conservation of angular momentum. Pure turbulence (pure
rotational motion) therefore cannot result in forward motion. On the
other hand, using our "equal and opposite reactions" anecdote, moving a
piece of water straight back can only move a boat forward. Therefore it
seems pretty apparent that minimizing turbulence would increase forward
momentum. Seem reasonable?
OK, I'm ready for the bashing from the fluid dynamics crew now. :)
-----------------------------
David White
mail: dwhite@u.washington.edu
Dept. of Oceanography
University of Washington
-----------------------------