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

Wolfgang wrote:
"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


Force on the paddle shaft, at the handgrip.

"Kieran" wrote in message
news:_R9Vd.73792$g16.11180@trndny08...
Wolfgang wrote:
"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
Force on the paddle shaft, at the handgrip.

Extrapolate. Look at the tables. Call the company. Do the math.

Wolfgang
who is no rocket scientist.

Kieran wrote:
Wolfgang wrote:
"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


Force on the paddle shaft, at the handgrip.

Makes me think of a big torque wrench. Do you get any deflection of the
paddle shaft while paddling? Use a smaller shaft until you do, Take
video, or measure the deflection of the needle! Then in the lab,
measure the force needed to duplicate the deflection. You should then
have an idea of what the possible force exerted on the shaft would be
for a particular paddler.

The potential force would be based on as wolfgang points out the
effectiveness of the engine mount, the paddlers seat and feet, the
grip, and other loss of efficiency factors that could be isolated for
significance. TnT

Tinkerntom wrote:

Force on the paddle shaft, at the handgrip.


Makes me think of a big torque wrench. Do you get any deflection of the
paddle shaft while paddling? Use a smaller shaft until you do, Take
video, or measure the deflection of the needle! Then in the lab,
measure the force needed to duplicate the deflection. You should then
have an idea of what the possible force exerted on the shaft would be
for a particular paddler.

The potential force would be based on as wolfgang points out the
effectiveness of the engine mount, the paddlers seat and feet, the
grip, and other loss of efficiency factors that could be isolated for
significance. TnT


The problem is not how to measure the moment (torque) on the shaft.
Strain guages have been around for ages that will allow me to do that,
and I'm well familiar with how to implement them. The problem is
determining power from that force.

The force balance in the kayak system is weird, as there is no fixed
pivot point on the paddle. So, the pivot point is a "virtual" one.

I'm making progress, but still wonder if anyone has done this already.

The only way I can see to determine power at the hand grip is to record
3D kinematic video of the motion, so that the actual 3D vector of the
handgrip velocity is known. Then Power=FxV. But I wonder if there's a
better/simpler way to do it.

I did find a paper (Aitken, 1992) that measured paddle shaft torque
(bending) with strain guages, then used the hull velocity through the
water to get power. I don't see how this is valid, though, since hand
velocity is not equal to hull velocity. But then I suppose it would
depend on what your frame of reference was... Hmmmm....

Any other bright ideas out there? :-)

-Kieran

Kieran writes
Tinkerntom wrote:

Force on the paddle shaft, at the handgrip.
Makes me think of a big torque wrench. Do you get any deflection of
the
paddle shaft while paddling? Use a smaller shaft until you do, Take
video, or measure the deflection of the needle! Then in the lab,
measure the force needed to duplicate the deflection. You should then
have an idea of what the possible force exerted on the shaft would be
for a particular paddler.
The potential force would be based on as wolfgang points out the
effectiveness of the engine mount, the paddlers seat and feet, the
grip, and other loss of efficiency factors that could be isolated for
significance. TnT


The problem is not how to measure the moment (torque) on the shaft.
Strain guages have been around for ages that will allow me to do that,
and I'm well familiar with how to implement them. The problem is
determining power from that force.

The force balance in the kayak system is weird, as there is no fixed
pivot point on the paddle. So, the pivot point is a "virtual" one.

I'm making progress, but still wonder if anyone has done this already.

The only way I can see to determine power at the hand grip is to record
3D kinematic video of the motion, so that the actual 3D vector of the
handgrip velocity is known. Then Power=FxV. But I wonder if there's a
better/simpler way to do it.

I did find a paper (Aitken, 1992) that measured paddle shaft torque
(bending) with strain guages, then used the hull velocity through the
water to get power. I don't see how this is valid, though, since hand
velocity is not equal to hull velocity. But then I suppose it would
depend on what your frame of reference was... Hmmmm....

Any other bright ideas out there? :-)


Keiran -

Your getting lots of feedback, but the complexity of the problem is vast
and the simplifications on offer may be too simplistic, although you've
made that point in some you've answered already.

Trying to assess fluid drag on the boat from towing measurements is not
going to give a great answer, since no kayak goes in straight lines.
And even if you could measure a more accurate power loss for the hull
that gives you no handle on the power losses around the immersed paddle.
A paddle is probably more efficient than an oar, but how efficient is
it, & how does its propulsive efficiency vary through the stroke?

Could one "catch" all the energy added to a finite but significant
volume of free water surrounding the path of the paddle stroke? Are
there ways to track the 3-D motion within that volume over time (it
sounds like a real-time tomography problem, perhaps done by laser scans
using suspended reflective particles), & feed that back into a CFD
program to sum up momentum transfers and frictional losses.

Now, if you could strain gauge a paddler........ It'd be great if the
means existed. Then you'd be able to measure the forces & speeds of
action at every bodily joint. Does that mean you'd have to build a
robotic paddler & tune him until he imposed the same loads & speeds of
action as a human on real paddles in a real moving boat? Or is there
any feasible way to take such direct measurements?

The answer may still be 42, of course.

Good luck there -
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)