Bob Arledge wrote:
Why not put a strain gauge on the paddle shaft just below the paddler's
hand. This would give you the moment at that point, so the force would be
the moment divided by the distance between the strain gauge and the centroid
of the paddle blade.



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. 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.

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.

Thanks,
-Kieran

"Kieran" wrote in message
news:j1tUd.66306$8a6.13749@trndny09...
Bob Arledge wrote:
Why not put a strain gauge on the paddle shaft just below the paddler's
hand. This would give you the moment at that point, so the force would be
the moment divided by the distance between the strain gauge and the
centroid of the paddle blade.

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.
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.

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.


Actually, it should be quite managable, seeing as how the paddle is a line.
You only need two points to track all the motions of the paddle shaft, and
two more on the sides of the blade to track the feather. Everything else
follows.

Seems like someone out there must have some sort of pressure plate: two
sheets of material with a lor of sensor points between. Put one on the blade
and get a readout of the water pressure against all points of the blade at
all times. If all you are interested in is the resultant force, put a
potentiometer on the bow and brace it against a wall.

--riverman
(I love trying to sound like I know what I'm talking about)

riverman writes
"Kieran" wrote
Bob Arledge wrote:
Why not put a strain gauge on the paddle shaft just below the paddler's
hand. This would give you the moment at that point, so the force would be
the moment divided by the distance between the strain gauge and the
centroid of the paddle blade.

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.
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.

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.


Actually, it should be quite managable, seeing as how the paddle is a line.
You only need two points to track all the motions of the paddle shaft, and
two more on the sides of the blade to track the feather. Everything else
follows.

Seems like someone out there must have some sort of pressure plate: two
sheets of material with a lor of sensor points between. Put one on the blade
and get a readout of the water pressure against all points of the blade at
all times. If all you are interested in is the resultant force, put a
potentiometer on the bow and brace it against a wall.


Since it is more the reduction in pressure on the convex back of the
paddle which moves the boat than the increase on concave face, you'll
need a sensing surface on both faces. Even then you'll measure pressure
but not shear (frictional) forces.

Might be better to measure the forces at the paddle neck directly by
means of strain-gauge rosettes. Then there's angle of shaft, direction
& velocity of blade motion & (variable) location of centre of pressure
to consider.

Sounds a nice easy problem, only slightly more difficult than the one
about life, the Universe & everything. Have fun, Keiran!

Cheers -
Carl

--
Carl Douglas Racing Shells -
Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories
Write: The Boathouse, Timsway, Chertsey Lane, Staines TW18 3JY, UK
Email: Tel: +44(0)1784-456344 Fax: -466550
URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)

"Carl Douglas" wrote in message
...
Since it is more the reduction in pressure on the convex back of the
paddle which moves the boat than the increase on concave face...

Huh?

Isn't it the force of the paddler's butt, feet, or whatever other
parts are in contact with the boat that propel it?

Wolfgang

In article , Carl Douglas
wrote:


Sounds a nice easy problem, only slightly more difficult than the one
about life, the Universe & everything. Have fun, Keiran!


So, there's the answer!

42



Allan Bennett
Not a fan of fish

--

Ah, we have a D. A. fan. Yes of course 42. I should have thought of that.

Ken

"Allan Bennett" wrote in message
...
In article , Carl Douglas
wrote:


Sounds a nice easy problem, only slightly more difficult than the one
about life, the Universe & everything. Have fun, Keiran!


So, there's the answer!

42



Allan Bennett
Not a fan of fish

--

On 27-Feb-2005, "riverman" wrote:

Actually, it should be quite managable, seeing as how the paddle is a line.
You only need two points to track all the motions of the paddle shaft, and
two more on the sides of the blade to track the feather. Everything else
follows.

You need three points in total to measure all the motions of the paddle.
If you have two on the shaft and one off the shaft (say, normal to the
mid-point) you can determine what's going on.

I thought about this and figure that measuring the motion of these
points is more trouble than it's worth. I'd be inclined to try the
following:

A six-degree-of-freedom accelerometer system (all in a box costs a
few hundred US$) attached to the paddle shaft. This will measure
all accelerations of the paddle in every direction. A six-channel
A-D converter actually costs more than the accelerometer.

A computer that integrates the above data from a starting point
(say, a paddle "saddle" as a zero-reference point on the deck
in front of the paddler). This can be used to determine the
position and orientation of the paddle at every point in the
experiment. Can be done in real time or after the fact.

A seat mounted on a three-support frame. This would be statically
determinate and will allow all paddler forces to be measured
directly from strain guages in the seat supports. The "seat"
would in fact be a frame that includes foot support, since
foot forces can be a considerable component of the paddler's
actions. The problem with this may be reinforcing the kayak
to allow all forces to be transmitted thru only three points
to the hull. You'd have to also design the seat frame to
have very little friction for certain degrees of freedom
at each support in order to have it determinate.

The geometry of the seat relative to the paddle starting point
can be measured accurately and then all paddle positions are
known in time. Forces at the three seat supports can then be
resolved into the forces exerted by the paddle.

Mike

"Michael Daly" wrote in message
...
On 27-Feb-2005, "riverman" wrote:

Actually, it should be quite managable, seeing as how the paddle
is a line.
You only need two points to track all the motions of the paddle
shaft, and
two more on the sides of the blade to track the feather.
Everything else
follows.

You need three points in total to measure all the motions of the
paddle.
If you have two on the shaft and one off the shaft (say, normal to
the
mid-point) you can determine what's going on.

I thought about this and figure that measuring the motion of these
points is more trouble than it's worth. I'd be inclined to try the
following:

A six-degree-of-freedom accelerometer system (all in a box costs a
few hundred US$) attached to the paddle shaft. This will measure
all accelerations of the paddle in every direction. A six-channel
A-D converter actually costs more than the accelerometer.

A computer that integrates the above data from a starting point
(say, a paddle "saddle" as a zero-reference point on the deck
in front of the paddler). This can be used to determine the
position and orientation of the paddle at every point in the
experiment. Can be done in real time or after the fact.

A seat mounted on a three-support frame. This would be statically
determinate and will allow all paddler forces to be measured
directly from strain guages in the seat supports. The "seat"
would in fact be a frame that includes foot support, since
foot forces can be a considerable component of the paddler's
actions. The problem with this may be reinforcing the kayak
to allow all forces to be transmitted thru only three points
to the hull. You'd have to also design the seat frame to
have very little friction for certain degrees of freedom
at each support in order to have it determinate.

The geometry of the seat relative to the paddle starting point
can be measured accurately and then all paddle positions are
known in time. Forces at the three seat supports can then be
resolved into the forces exerted by the paddle.

All of this sounds terribly complicated to me. Why not just tether
the stern of the boat to the measuring device of choice anchored to a
dock?

Wolfgang

On 28-Feb-2005, "Wolfgang" wrote:

All of this sounds terribly complicated to me. Why not just tether
the stern of the boat to the measuring device of choice anchored to a
dock?

Good idea - unless of course you want to measure something useful.

Mike

"Michael Daly" wrote in message
...
On 28-Feb-2005, "Wolfgang" wrote:

All of this sounds terribly complicated to me. Why not just tether
the stern of the boat to the measuring device of choice anchored to a
dock?

Good idea - unless of course you want to measure something useful.

Refresh my memory. What is it you wish to measure?

Wolfgang