Re: [Paddlewise] Paddling efficency

Once again I have been guilty of not communicating precisely, something my wife
accuses me of from time to time...  Let me try to clarify:

Nick Schade wrote:

> >David Carlson wrote:
> >
> >>  If the boats have the same drag and are going the same speed, then the power
> >  > output from the paddles is the same.
>
> Actually no. The power output by the paddle is the shown by how fast
> you move the boat, plus how fast you move the water. You are moving
> both, so your power output must account for both. If all you had to
> look at was how fast you moved the boat then everything would be 100%
> efficient.

I obviously didn't mean that.  I think I meant to say that the useful bower output,
which is that required to overcome the resistance of the boats through the water,
must be the same.  Given paddles generating the same force on the paddle blade, the
PADDLE with the highest speed is generating more power, and if these paddles are
being used to propel the boats mentioned above, the paddle with the fastest speed is
wasting more power, and is the less efficient.


Michael Edelman wrote:

> David Carlson wrote:
> >
> ...
> > My discussion was considering only  how much work is being done by the
> > paddle vs. how much work goes to propelling the boat.  The paddle puts
> > out exactly as much power as was put into it. There is no significant
> > friction or other energy dissipating mechanism internal to the
> > paddle.  So the difference between effort (paddler output) and paddle
> > output is nil.  The only effort I am neglecting to consider is
> > physiological, and internal to the paddler.
>
> You're making two contradictory claims. One, that power output is equal
> to power input for all paddles. Two, that some paddles are more
> efficient than others. Since efficiency is a measure of the ratio
> between power input and output, you've sort of disproved your claim via
> a classic reductio argument.

Yes, it appears that I have been imprecise.  I am claiming that the effort, or power
put into the paddle by the paddler is the same as that put into the water by the
paddle.  Not all of it provides propulsion for the boat.  This last point is where
paddles and paddlers/paddle technique are different.  For the efficiency
calculation, power input would be power input by the paddle to the water, or effort
put into the paddle by the paddler, which is the same.  Power output would be that
power which is actually required to overcome the resistance of the boat.

>
>
> If in fact some paddles are indeed more efficient than others- and that
> is not a claim I have ever made- then you have consider how much useful
> work is done by the paddle, and how much just goes into heating the
> water. That's why I stressed boat speed as a measure of useful work
> done. Since we're not paddling in a calorimeter our only measure of
> power output is boat speed.
>
> So while the energy coming out of the paddle is equal to that going in,
> minus some negligible hysteresis losses, that's still not the entire
> story.

I think we're in agreement here, if the boats and their loads are the same, except
that I claim that both paddles and paddling techniques can have different
efficiencies.

>
>
> But we're really still arguing two separate issues. My position was that
> a human is more efficient at producing lower output over a long period
> of time than at producing high output for a short time. Biomechanical
> studies bear that out, and that is why narrow paddles tend to be better
> for distance paddling.

I agree with your assertions about humans and their power output.  I don't think
that we have proved that narrow paddles are more efficient at any power output.  I
have asserted that for a given propulsive power requirement, if the paddle providing
it is moving faster, it is less efficient and the power used to drive the paddle
must be more.  This is because the forces of the water on the paddle in the
direction of boat travel must be the same as the boat resistence (i.e., the same for
any paddle providing the required propulsive power).  If power output from the
paddle is force on the paddle times velocity of the paddle and the force on the
paddle is determined by the boat characteristics and speed, then the only thing the
paddle characteristics determine is the paddle velocity required to generate that
force.  If more paddle velocity is required, the paddle is less efficient.  Note
that with some minor changes, this analysis could be made to accomodate the case
where significant "lift" is involved, although Nick has addressed this in a previous
post.

Regards,

Dave


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