Message-Id: <9503220800.AA19122@st.unocal.com>
From: stssram@st.unocal.com (Bob Myers)
Date: Wed, 22 Mar 1995 00:00:52 -0800
In-Reply-To: Guillemot@aol.com
To: baidarka@imagelan.com
Subject: Re: Paddling...
On Mar 21, 21:56, Guillemot@aol.com wrote:
} Subject: Re: Paddling...
> Bob Myers:
>
> This is a good point and I can't see any holes in it. However I am still
> uncomfortable with the results. There must be something I am missing. Another
> experiment: paddle your kayak for 1 minute, once with a pencil, once with a
> pizza, applying the same force each time. In both cases the work done on the
> kayak is the same. I guess the question is did the paddler do any more work.
> I'm not sure. He applied the same force for the same time both times. Does
> the fact that the smaller paddle was moving faster really matter?
Yes, it matters. Force*time is impulse, or change in momentum.
Force*velocity is power, or change in energy per time. Moving the
paddle faster requires more power (and therefore more energy in a
given time period) if you use the same force.
> (... lots deleted ...)
> Therefore: The kinetic energy of the water is higher in the case of the
> smaller paddle. If you've added more KE to the water without increasing the
> KE of the boat you have wasted energy.
Right. Exactly what I said in my last post. The mailing list has a delay
and I think our posts crossed in the email.
> The lift can be broken into two components. I guess you are right "Lift" is
> defined as perpendicular to drag, but the "effective" lift on a wing is
> roughly perpendicular to the chord of the wing.
Yes, there are two different ways of defining lift and drag, but they are
always perpendicular. Method 1 is drag is along direction of motion, and
method 2 is drag is parallel to the chord of a wing. You're using method 2.
> The difference between the
> two (Lift and effective lift) is "induced" drag, which is in the same
> direction as the "form" drag, or the drag created by moving any form through
> a fluid. Therefore, the drag can be viewed as the sum of the form drag, skin
> friction, and induced drag, all acting in the same direction. The remaining
> force created by a wing is the effective lift. The induced drag can be
> eliminated, making the effective lift equal the Lift, but you can not
> eliminate the form drag and skin friction. If you want lift you must have
> drag and there is nothing you can do to make all the vectors add up to a
> 100% useful direction.
Why not? That's true for an aircraft, but I don't think it's true for a
paddle. Add the lift vector to the drag vector. That gives you the total
hydrodynamic force vector. It's going to point a bit to the rear of the
direction of motion, but the lift component should be bigger than the drag
component. I can imagine a backwards diving paddle with a horizontal
lift+drag vector.
However, I now think it would have to move backwards on a slant too much to
be practical, in light of the paddle slip analysis. A maximally efficient
vertical stroke (no horizontal movement through the water) definitely has a
drag vector in a non-useful direction. So on the whole I agree that this
isn't useful - if you do align the force in this manner you end up giving
too much KE to the water. I think the water flow would be more laminar and
less turbulent, but it would be moving too fast for efficient propulsion.
Interesting result. Lift isn't a magic solution, and aligning the forces
does not appear to be the best strategy. I didn't see that before.
> In light of the above KE solution I can see where by using both lift
> propulsion and drag propulsion you may end up imparting less KE to the water
> and end up with a more efficient paddle stroke.
Note that the key to more efficient energy transfer is less paddle slip.
If the paddle is slipping, you must be losing energy. I do think it
possible to use lift propulsion to reduce the paddle slip.
I found George's comment about the triangular paddle intriguing. It does
seem like there might be a way of using the horizontal perpendicular motion
to generate forward lift, but it sounds awfully tricky.
-- 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