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.

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 - 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. 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 (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. 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. 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.

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. 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!

JIM