In article 0vtlqb.ijg.ln@Eskdale, Jim Wallis
wrote:
Interestingly if you compare Allan's post with mine you will find that
whatever may be implied in the first few paragraphs there is only one or
two areas in which our posts disagree.

There are. Whatever you may believe was implied :-)

Drag vs Lift:

I've never had the chance to study the path of a paddle through the
water, I assume probably incorrectly that most people messing around
with symmetric blades operate them in a mainly drag fashion

We will differ on that point, Jim. I don't believe that any paddler
*intends* to use drag or lift by choice, but from what I have observed the
major propulsive forces come from HDL. The same debate rages on a regular
basis in rowing where the lift forces are approx 9x greater than drag - in an
action where most people cannot conceptualise lift at all.

- it appears that Allan is in possession of data to the contrary which I
certainly can't argue with. Every bit of paddle motion that is not
perpendicular to the blade surface will produce lift (and drag), so a
curved blade will produce lift even if you do pull it in a straight line,
which when you really think about it - you can't.

....and in a normal paddle action, you don't.

I wouldn't have thought initially that the lift would be significant, but
then I was forgetting that the paddle is tilted in the Z direction no
matter how vertically you try and place it.

Asymmetry

As for asymmetric blades, I do feel they are more efficient, but maybe
not for the reasons you see quoted

Asymmetrics were introduced into racing (a bit before my time), because it
was believed the paddle would twist in the hands when one corner entered the
water first. They were therefore thought to be more efficient at the catch
phase and a benefit in reducing forearm fatigue and injuries.

There is no evidence that there is any benefit, either in improved
performance, comfort or anything else. They became a 'must have' as soon as
they were adopted by the best paddlers.

(which I often tell people for simplicity) - the shape of a wingtip can
have an incredible effect on the way the vortices are shed but also on the
stability of the wing, the asymmetry will effectively reduce the aspect
ratio which increases stability which will in turn affect the rate at
which you can apply power.

There night be something in that, especially in the first few strokes from
stationery - but once a lift-generating stroke is employed (ie most strokes),
those principles become less relevant.

Reducing aspect ratio for stability is a trade off for lift but I feel the
stability is more useful. It also seems to affect the feel of the bite and
if anything encourages the blade to sweep out for hydrodynamic lift when
compared to a squarer tipped paddle.

Again, we will differ on this point - the 'swing out' action is a consequence
of the blade )any blade) acting as a foil. It will adapt the angle of attack
and natural passage through the water according to the force applied - unless
the paddler attempts to control it for any reason.

It also exploits a more natural action, IMO - and, as I said - an action seen
in top paddlers before wing paddles were invented and HDL was considered.

The above does not constitute proof that asymmetry is better I do consider
it evidence though. It is a list of possible reasons why it feels better
to me - the chance to try 2 blades identical but for a cutaway is very
rare so I don't claim to have compared like for like.


....and a cut-off blade is not directly comparable, anyway, as it will have
reduces SA with the concomitant benefits that provides...

What does all this mean for Roo?

Get out there and try some new paddles! If your river blades have had as
heavy use as mine (they have probably had more) they are probably
impracticably small now. But shape is more important than size, 2
paddles the same size but different shapes will have different
characteristics, will absorb different amounts of power and produce
different amounts of propulsion for a given power.

One other point - whilst a lot of paddle manufacturers do just copy what
is trendy, most of the better ones do use trial and error to determine
what they consider to be good shapes.

sceptic mode

Yeah. Right.

\sceptic mode

I'm sure some are claiming to do CFD to determine the shapes (madyaker?)
but whether they have the correct models for the paddle path (and thus
water flow over the blade), I couldn't say!

Wing paddles were, we understand, designed at a UK university - as blades for
a water-pump (like a propeller)... which should give some clue as to how they
work and how they should be used.


Allan Bennett
Not a fan of trial and error - aka BCU Special Hearing Committee


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