Kieran:

As a person who did considerable white water kayaking in the 60's
(since then it's been mostly C-1 and rafting), plus a combined-fields
background (B. of M.E. and Ph.D. physics), I hope I can offer some
constructive comments.

First, let us just consider measuring the forces in sufficient detail.
I agree with the suggestion of Carl Douglas on February 28 that strain
gaging the paddle shaft is probably the most effective way to go.

Strain gage arrays can be designed to do each of the following:

(1) Measure flexursl (bending) moment.

(2) Measure axial force.

(3) Measure perpendicular (shear) force.

(4) Measure torsional (twisting) moment.

Incidentally, I prefer using "arrays" instead of "rosettes" because a
rosette most often is used to denote two or more adjacent strain gages
mounted on the same backing sheet. "Array" is more general, as it can
also include strain gages mounted on opposite sides of the paddle
shaft.

Thus, a combination of strain gage arrays between the paddle blade and
the paddler's hand can measure all necessary force components. A
similar combination of arrays, rotated by 90 degrees with a feathered
paddle, would be mounted between the other paddle blade and the
paddler's corresponding hand.

This leaves the question of forces in the paddle shaft between the
paddler's hands. We should not assume that these are zero. Strain
gage arrays can be mounted at the middle of the shaft. Again, all of
the above (1-4) can be measured. Measuring flexural moments at the
midpoint can even resolve possible flexural moments exerted by the
paddler's hands.

Thus, we are talking about a total of 12 strain-gage-array measurement
channels. But with all of them, the forces and moments on the
paddler's hands become statically determined. Possible extrapolation
to forces at wrists, elbows and shoulders remain separate problems.

Using strain gages sounds deceptively simple. At risk of telling you
what you already know, let me recommend "Strain Gage Users's Handbook"
(1992) edited by Hannah and Reed, most highly. It is published by the
Society for Experimental Mechanics, Inc. Bethel, CT. Among other
things, it is not advisable to mount strain gages on plastic or
composite surfaces. This has to do with heat-sinking. Metal surfaces
are best. Thus, if the kayak paddle has an aluminum tube core (as many
do), suggest stripping the outer, plastic layers off before installing
the strain gages.

Regarding the problem of velocity measurements (to get the power), I
suspect that the video method which you proposed would be most
effective, especially as I got the impression that some people in your
department already have some experience with that. The alternative
idea of using 3-D arrays of six accelerometers is also intriguing.
Effects of error propagation in integrating acceleration can induce
serious inaccuracies, unless great care is exercised.

Overall, my reaction is the following:

(1) The project is certainly feasible, and has exciting potential.

(2) Considering its scope (if done thoruoghly) it may be too much for a
Master's thesis, and more appropriate for a Ph.D. thesis. You may wish
to talk with your professor about that.

Please feel free to contact me directly.

Andres Peekna
Innovative Mechanics, Inc.
5908 North River Bay Road
Waterford, WI 53185-3035




Kieran wrote:
Allan Bennett wrote:
In article j1tUd.66306$8a6.13749@trndny09, Kieran
wrote:

That's the general idea, but because the paddling motion is 3-d,
it's
not very easy to determine power just from the strain in the paddle

shaft.


The flex in a paddle-shaft will be a reflection of all the forces
acting upon
the blade in the water. Using the force profile: t v deflection)
and
suitable calibration, it will be possible to determine the power.

Hmmm... this seems to be the part I'm missing. How do you get power
without knowing the path of the force?

You need to know instantaneous velocity (direction and magnitude)
at every
moment. In a fixed-pivot environment like rowing, you can just
put a
potentiometer on the oar-lock. But the kayak/canoe paddle has no
fixed
pivot point. So, I imagine that a virtual pivot point would have
to be
derived via 3-d kinematic video analysis.


It seems there is a virtual point (see Plagenhoef, 1979 and
others), just as
there is a virtual point where all the forces that propel the boat
seem to
meet - a valuable tool for those athletes with adequate
imagination.

Thanks for the reference. I'll see if I can find that publication.
Would that be a book or a journal article?

I haven't yet sat down and done a free-body of the system, but in
my
head, it seems like it's going to be an indeterminant system... not
fun.


..and the ultimate purpose?

Trying to come up with a master's thesis for my degree in
biomechanics.
A research prof here has an ongoing project that considers at a
high
(systems) level the energetics of different forms of human locomotion

through/in/on water, including surface swimming with/without fins,
submerged (e.g. scuba) swimming, rowing, and kayaking. There's very
little published research that we can find on kayaking, so that's the

part I'm tackling.

Thanks for your input!
-Kieran