Subject: Re: baidarka high aspect ratio paddle blades; lanceolate shapes; drag, inerti...
From: RCH3149@aol.com
Date: Mon Mar 13 2000 - 19:19:21 EST
In a message dated 00-03-13 07:06:20 EST, you write:
<< Ralph wrote a rather lengthy post on paddles and rather than trying to
deal with a large number of issues I would like to simplify the discussion to
basic principles...
John Winters >>
Yep, again apologies for that!
What's worse, in my misguided attempt at "brevity" (would you believe it?), I
assumed and implied certain parts of the various premises and arguments
involved, which were obviously not obvious unless you happened to be in my
head (read: clairvoyant). John puts some of these issues up for further
debate. I will sit on the topic for a while and try to put something together
that will read more easily, explains my thoughts better etc.
This complicated subject deserves more than a few hasty thoughts between
bites of breakfast cereal -- whole books have been written on the various
aspects involved. As far as I know, none in the context of "traditional"
paddling and paddling equipment and its particular perspective (due to lack
of commercial interest perhaps?)
Let me just run through the following description of a stroke, which relies
on a large lift component to produce forward thrust, BRIEFLY! (I take full
responsibility for potential misinterpretations of Doug van Duren's
instructional video as far as body motions are concerned. He does not touch
upon fluid dynamics, so I have no-one else to blame for that in any case):
-1-
Step 1:
Procedure: Starting position with forearms parallel to water surface, elbows
"in your lap"; the torso is already twisted towards impending stroke side,
let's say left blade forward; upper blade edge tilted forward; left arm
slices down, right arm lifts (somewhat to allow paddle to stay clear of the
deck), torso begins to unwind in anti-clockwise direction.
Argument: Tilting the upper edge of your blade forward during the catch
forces your blade to TEND to slice downwards once you hit the water. It is
impossible to just "drag" the blade backwards with that angle of attack. The
lower edge of the blade becomes the leading edge of a foil, which produces
lift (by whatever means). Yes, pulling the blade backward will introuce a
drag component, one vector of the reaction of which will be thrust in the
direction of travel of the boat.
-2-
Interlude: If you follow Inuit paddling practice (please, forgive the gross
generalization and bear with me for the sake of the argument) and keep the
arms low, the stroke short, i.e. cadence relatively high (cf Doug van Duren's
training video), the stroke consists of only catch and release / finish, with
no real "pull" phase inbetween.
-3-
Step 2:
Procedure (continuation from -1-): Torso continues to twist anti-clockwise;
left forearm switches from downward motion to upward motion, righ arm punches
donwards (yes and somewhat forward) and thereby aids in levering the blade
out of the water; we have reached the beginning of the next stroke ...
Argument: At the point of change from catch to finish / release, the still
forward tilting upper edge of the blade becomes the leading edge of the foil
-- creating lift.
Should I introduce the concept of the tip vortices created at the point of
change from downward to upward slicing and their importance in producing
thrust, in a badly limping analogy to shark tails :-), John? (Very pointy
things, those shark tails, by the way!) Perhaps some aspect of the movement
and flexibility of dolphin tails would help general understanding. Then
again, the shark's body must compensate unbalanced forces eminating from the
differing areas of the respective upper and lower portions of its tail. Is
this a valid consideration for traditional paddles? Perhaps the more
symetrically loaded dolphin tail would be better ... Alright, I'll spare you
further frivolity for the sake of brevity.
Ralph Hoehn
Stamford CT
-
Baidarka Mailing List - All postings copyright the author and not to be
reproduced outside Baidarka or Baidarka archives without author's permission
Submissions: baidarka@lists.intelenet.net
Subscriptions: baidarka-request@lists.intelenet.net
Searchable archive: http://rtpnet.org/robroy/baidarka
This archive was generated by hypermail 2b27 : Sat Apr 01 2000 - 01:00:05 EST