Subject: Re: [baidarka] Lift and Drag
From: Peter Chopelas (pac@premier1.net)
Date: Wed Oct 13 2004 - 14:31:34 EDT
From: "James Mitchell" <baidarka@earthlink.net>
> Peter, let's see if I understand you correctly. You are saying that the
> flow from the ridge outboard to the edge is generating significant
> lift? If that is what you are claiming, you are free to claim it but I
> still disagree.
No you misunderstood, perhaps I was not clear. The flow goes from the ridge
to the edge, and then wraps around the edge generating the vortex on the
backside (the side opposite the ridge). The two vortexes formed this way
are curling in towards each other, pulling the fluid flow around the back of
the blade in a half arc on each side. This acceleration of the fluid around
these two vortexes is what generates the lift in the forward direction (on
the side away from the ridge side). This is known as vortex lift, and if
you watch turbid or water with bubbles in it you can see how the "far field"
flow is curved around the blade (that is the flow not right against the
paddle surface). There are actually two lift vectors, one from each vortex,
that are at about 45 degrees to eachother (I suspect), both forward. Then
net effect is a forward thrust. Note that if the two vortexes are not of
equal size the blade will pull one way, and then the other and you get
flutter. This is why the ridge is important.
> I do agree that a Greenland paddle held as you describe is generating
> lift, in much the same way as a propeller: a rapid sweeping motion with
> the paddle at a slight downward angle. I find this stroke interesting,
> and one of my kayaking friends in Japan uses this stroke successfully
> for long distances. It does not fit my paddling style well, although I
> have used it as a break during long paddles. I hope you agree with me
> that an Aleut paddle is not well suited for this stroke.
exactly correct.
> The low-speed
> foil shape of the traditional Greenland paddle is well suited for this
> stroke, but not as well suited for the typical "resistance-based" (for
> lack of a better term) stroke that we do with the Aleut paddle.
> I design my paddles for smooth power over long distances. I optimize
> them for ease of paddling, for greatest ease on the paddler's muscles
> and joints, for the greatest efficiency in a relaxing long distance
> touring stroke. To my knowledge, everyone who has paddled with one of
> my paddles agrees that the paddle accomplishes this goal. In
> accomplishing this design, I have started with the best traditional
> design knowledge I could find, and continued to optimize it for power
> and smoothness -- and for weight and "feel". These discussions are
> interesting to the extent that they contribute to the design.
And you have done a good job of it too. There is no reason you need to know
a lot of technical stuff to experiment with paddle shapes and find what
feels good and what works good. That does not mean that you have a clear
understanding of how or why it happens, nor do you need to know to make a
paddle that you like.
>
> Given that you have a different understanding of the physics involved
> in accomplishing my goal, please tell us all: what have you learned
> from your study? How are your paddles shaped? What experience do you
> have in paddling your ideas? How do you apply what you have learned?
This understanding has made me realize why high aspect ratio native paddles
are better for long distance cruising, it has helped me develop some of the
finer details of the paddle shape (which are not obvious from printed
drawings or pictures) and it has helped me fine tune my stroke mechanics and
rolling technique. I have also developed a paddle shape of a Greenland type
paddle that tapers toward the tip (rather than the more common reveres
taper) that I really like. After I had made it I have learned that this
exact shape is illustrated in Peterson's "Contributions..." book in the
chapter on paddles. How about that?
Peter
-
Baidarka Mailing List - All postings copyright the author and not to be
reproduced outside Baidarka or Baidarka archives without author's permission
Submissions: baidarka@paddlewise.net
Subscriptions: baidarka-request@paddlewise.net
Searchable archive: http://rtpnet.org/robroy/baidarka
This archive was generated by hypermail 2b27 : Mon Nov 01 2004 - 01:30:03 EST