Re: [Paddlewise] PaddleWise [wing theory]

Steve brown wrote:


> Here are some observations drawn from airplane wings I have seen or flown
> and paddles I have built and/or used:
>
> -A paddle is used like a wing used at high angle of attack, making it
> susceptible to stall (cavitation in water world). Stalling a wing results
in
> a dramatic reduction of lift. In our case that is a dramatic reduction in
> propulsive force

You mean ventilation (cavitation is when the water is vaporized, which I do
not think the pressure gradients are quite large enough to occur in a
paddle).  but a high AR shape, whether wing or paddle, will be more
susceptible to stall, and therefore not desirable for the rough water
conditions of WW or surf kayaking.  More thrust can be developed for short
burst of power with less risk of stalling the paddle with a large area, low
AR paddle, which is why they work best for WW and surf kayaking.  But they
are not as efficient for LOW POWER, LOW SPEED, LONG DISTANCE CRUISING,
assuming the shape is the same.


>
> -Putting camber on a wing helps it to achieve higher angles of attack
> without stalling. That's putting a curve into the blade. A simple
> symmetrical blade has some advantages, but delayed stall is not one. Thus,
> those who use Greenland paddles must incorporate motion into their stroke
> that reduces or eliminates cavitation by reducing the angle of attack.
There
> is an article that addresses this in Sea Kayaker which I am too lazy to
look
> up at the moment.
>

Actually for the short burst of power when necessary, the cant of the
Greenland stick, it traps a strong vortex on the low pressure side of the
blade which prevents stalling, allowing very high angles of attack and
thrust.  It is costly in terms of drag, but effective for short burst of
thrust.  During low power, low speed, long distance cruising, the blade is
sliced through the water with little vortex formation (except at the tip),
which is where the high AR blade has its advantage.

> -A "wing" paddle has a huge camber, just like the wing of an airplane used
> for low speed work, or like the wings of an airliner with leading and
train
> edge flaps deployed for landing.
>
wing paddles are an outgrowth of developing paddles for racing, which is an
attempt to maximize SPEED (i.e. min time over a distance) requiring MAX
thrust (right at the stall), not efficiency at low speed (i.e. min energy
output over a given distance).  If you where to have two identical wing
shapes except for AR, and use them both at low speeds to measure energy
consumption over the same distance, the high AR will be superior.


> -Higher aspect ratio wings have a higher lift-to-drag ratio. That implies
> greater efficiency when paddling a high aspect ratio paddle. On the other
> hand, Greenland paddles are symmetrical which reduces wing efficiency.

Irrelevant, we do a lot more with a paddle than just go forward, any paddle
design in a compromise between its various functions.  If you were to
optimize the profile and blade shape for simply moving forward AT LOW SPEED,
you would end up with a shape that would be good for little else.  I like a
fully symmetrical paddle in both profile and planform so no matter HOW I am
holding the paddle, I always will have it in the CORRECT position,
especially for emergency maneuvers.  This compromises its efficiency for
cruising, but makes it more suitable for all the other functions a paddle
must perform.

Also,
> lift-to-drag theory applies to a wing that is designed to move parallel to
> the wing surface, not perpendicular to it. This is confusing to translate
> into paddling, but the velocity of the paddle through the water parallel
to
> the blade surface creates drag which is not related to the velocity of the
> boat through the water. That may disconnect the lift-to-drag ratio from
> having a positive bearing on paddling efficiency.

You are confusing yourself.  You need to go back and carefully reread the
posting.  There is no useful drag on the paddle, the thrust, (or "lift") is
the USEFUL force coming off the blade for propelling yourself forward, the
drag is defined as the force FELT at the handle of the blade.  Minimum force
at the handle, and maximum thrust at the blade is HOW the efficiency is
measured, efficiency reduces to simply Thrust/Drag(at the handle).  So 100
percent would be where you get the same amount of trust as the force at the
handle you put into it (it is that simple!). The drag of the hull is
unrelated to anything to do with the efficiency of the stroke.

>
> -Lower aspect ratio wings have more gentle transitions into stall
> (cavitation). That's why trainer airplanes have wings like a Hershey bar.
> Lower aspect ratio paddles should not break into cavitation as abruptly
and
> my observation from lousy paddles I have built is that they do not.
> Conversely, my Greenland paddle required careful technique to prevent
> cavitation.

Low AR are not just more gentle, but also more stall resistant (i.e. they
can go to much high angles of attack relative to the flow direction before
stall).  And yes, the high AR paddles DO take more skill and experience to
use properly, and not a good choice for newbies, though I think they should
transition into them real soon if they intent to keep paddling over long
distances.
>
> My opinion (neither smart enough nor fast enough to make a conclusion):
> Racers use wing paddles because they are indeed faster and more efficient
at
> propelling a boat through the water.

How are you measuring efficiency?  If you mean least time over a given
distance, YES.  If you mean least expended energy over a given distance
(ignoring the amount of time it takes), NO.  This is like comparing a fuel
economy of a Honda Civic with an Unlimited drag racer.  Both do the job they
were designed for very well, but what they were optimizing is very
different!

On the other hand, it's not all about
> speed and efficiency. It's about fun. When I paddled with a Greenland
paddle
> I was pain free (although slower) for more miles. Paddling with a high
> aspect ratio paddle is arguably better for your body, especially when
> paddling long distances. When I'm older and wiser I'll stop paddling in
all
> situations with my WW paddle and start using something with a higher
aspect
> ratio.

I also enjoy the feel of a symmetrical high aspect ratio paddle.  But if
someone thinks it is more fun to paddle around using a tennis racket, who an
I to criticize.  There are some people that go jogging with weights around
the arms and legs to get a better workout, who am I to tell them they do not
know what they are doing?  It is not efficient, but that is clearly not
their goal.

Peter

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