Wing blade shape (vs. "thick" foil)

-- Last Updated: Apr-11-12 10:57 AM EST --

Lots of topics lately on paddle efficiency and shape, so here's another one for you fluid dynamics folks (you know who you are-;)

Wing paddles are shaped as concave surfaces formed from a pretty much uniformly thin sheet of material (with the exception of the Lendal Kinetic Wing, which has a thick foam core and the concave side is partially filled with it).

So, why is that? For maximum lift at slow speeds, if I understand correctly, the cross-section of the foil shape is supposed to be quite thick. But the cross section of wing paddles is just a curved line with no volume b/w the front and back of the paddle.

Remember the old airplaines? Their wings were shaped like today's wing paddles. More modern aircraft have thick wings, supposedly because that thicker cross-section creates more lift and less turbulence than the thin wings? Or is there another reason? I can think of a few engineering reasons to use a thicker profile over a thin one, but I don't understand fluid dynamics enough to figure out how the "lift" and "turbulence" forces for either shape differ ...

Why is this shape advantageous in paddles? Why not use a thicker leading edge as opposed to the spoon shaped one?

I've used the Lendal paddle only once a long time ago when I was just beginning to learn the wing stroke, so my impressions might be middied by my poor technique. But from the short time I used that "thick" wing, I thought it was actually very nice to use, except it was rather heavier than the alternatives... I've been toying with the idea to build-up the inside of my Epic paddle with construction foam to create a thick leading edge and see how it behaves (a temporary fix without glassing anything in, just temp-glue some pink foam insulation and shape roughly to see what happens)...

Weight matters a lot

– Last Updated: Apr-11-12 12:03 PM EST –

Repetitive motion where each gram adds up quickly.
Why put "extra" material for minimal added gain.

A blade spends time in Air and in Water which are
2 very highly different densities .
Water is about 800 times more dense than Air.

Enjoy the math

This analysis comes to the conclusion that
a wing paddle merely gives a 4% increase in speed
- and that's theoretical percentage not reality.
Other factors quickly degrade the advantage.

I suspect
lack of understanding provides a lot of inspiration in kayaking.

Ideally one would run a whole lot of CFD sims and pick the ideal shape, I am quite sure it would be different from the current state of art. They would probably get closer to the most efficient impeller shapes.

As to the weight comment - for a sprint kayaker a few grams here or there don’t make any difference.

Dynamic not static

– Last Updated: Apr-11-12 12:23 PM EST –

Simulating the hydrodynamics of a moving paddle would be unbelievably tedious and time consuming. How does one generate the shapes to test? There are no fixed criteria, so it would generally be through intuition and experience. What path should the paddle take through the water? What should the applied force exerted on the paddle as a function of time look like? Again, there is no single correct answer.

If one did get some real data to use to model the paddle path and the force-time curve, the results would always be subject to interpretation, as the assumed superiority of one shape over another will depend on the specific testing method.

Simply testing foil shapes statically is a huge ordeal - cf. the standard reference: Abbott & von Doenhoff, Theory of Wing Sections, a 700 page 'synopsis' of real NACA airfoil data. However, static tests are only partially useful for a dynamically unsteady situation like flow over a paddle.

To answer the OP, at low speeds, the hollow within a standard wing paddle may contain a zone of recirculating fluid. When there is a contained eddy like this, the fluid flowing past the paddle essentially 'sees' a solid foil shape, at least to some extent. An actual solid surface would be beneficial when the speed become greater, as that eddy can be periodically swept out of its hollow and show up as a shed vortex. My feeling is that the weight increase of a large airfoil section would counter any lift benefit. It would be interesting to see what's actually happening on a real wing paddle.

PS Willi - I just saw your link to the paddle propulsion paper - thanks for that reference. If you have any more like it, I would be grateful to see them.

In a ridiculous coincidence, my first published paper was at the same conference in Tasmania, years before my interest in kayaking began!

I sat through a couple of presentations on the advanced airfoil designs

All of them were computer designed, and then verified. Design involved either supercomputers, or cloud computing on big servers.

None of them even came close to resembling NACA shapes.

Not being able to achieve a solution does not imply absence of solution, probably something you tell your students each day? :wink:

Weight, yes, but I’m more interested in
the in-water process. From Carldello’s explanation, that there might be an area of water behind the edge that behaves as a virtual power face wall of a fat foil makes sense, at least intuitively. Although, also intuitively, I envision losses there, which would be minimized by an actual fat foil.

Propellers move a lot faster through water than the paddle (and there is also less of a circular motion in a paddle blade than in a propeller blare). So while general principles might be similar, the optimal shapes I doubt would be that similar.

A key problem with a paddle blade is that it’s angle to the water varies through the stroke. A propeller’s blade follows the same endless circular path over and over, so it can be optimized for certain speed and boat drag. A paddle blade can never be optimized for the entire stroke (even if we assume constant boat speed and drag) - if it works best at the beginning, it will likely not work optimally in the middle or the end of a stroke. So, that’s another challenge without a clear winner-solution…

Interesting reading that paper …

Anyway, let me get out for a couple of hours and see how the wet kind of water (as opposed to the virtual one) beaves today at the local playspot -:wink:

modern airplane wings
Do not want to sully anyones memory of their science lessons in school, but I do not think modern airplanes get most of their lift from the shape of their wings, but rather from the angle of attack on the air. Modern wings are very small relative to the weight of the airplanes.


Well, talking about slow-flying planes
What I had in mind was the slow-flying traditional stuff that probably relates more closely to our subkect than the small-winged computer-controlled supersonic stuff out there that probably can’t even be flown manually…

lack of understanding
I suppose it’s true for everything - the less you understand “why?” (on a fundamental level, not because you are just dumb) the more inclined you are to experiment, the more you experiment, the more chances you are going to come up with something really interesting, and that will get the ball rolling.

So, I guess we just fire up the supercomputers and solve the problem once and for all, eh? I wonder how did the supercomputer jockeys verify their designs? I think there won’t be a single best paddle, as we all know they are used in very different situations and for a multitude of purposes. Very different from designing an optimal foil to operate at a specified design condition.

BTW, what I tell my students is: “stop texting, watching TV and playing video games - hit the books, read the damn assignment, do the homework and don’t count on a computer to make your life easier.”

Lift or Drag? Fling-ring or Axial-flow?

– Last Updated: Apr-11-12 6:12 PM EST –

Which is it that actually contributes the most to propulsion when using a wing paddle? So even if you were to increase the efficiency of lift by 100 or more per cent in your wing paddle, it would not make a whole lot of difference, unless it was the major contributor of propulsion. Better to increase the efficiency of the force that actually contributes the most propulsion. Unfortunately, there are no studies that have determined this, and I'm still waiting for the latest Ross Sanders and colleagues study on this, if any?

Linked above
I recommend you read the article linked above by willi-h20. It has some info on what you ask. From looking at Figure 1, it appears that the lift and drag forces on a wing paddle both contribute to propulsion, the extent depends on orientation and paddle path. Also, if you could point me to some info on the researchers you mention, I would appreciate the info.

Air - Density - Engines - Water - Hands

– Last Updated: Apr-11-12 9:00 PM EST –

Water is very, very dense compared to Air.
Airplanes use engines, lots of power/thrust/force
working in an extremely un-dense medium of air.

Comparing apples to oranges in my opinion.

Getting a human pedal powered airplane to fly
took a massive amount of physical training and
a huge amount of technical design to make it happen.

Modifying a winged paddle for human powered paddling
won't pay any large dividends, maybe 1% .
Remember that article by Epic Kayaks involving rudders
- ""A rudder will add less than 2% total drag.""

How much of a percentage do you expect (in theory)
might be advantageously created via paddle blade shape

Cost/Benefit ratio doesn't give me bang for the buck

A couple of links

But thinking is free : )

Yes, plus if we should not get stuck

– Last Updated: Apr-12-12 11:16 AM EST –

on the air vs. water density issue. It was purely an example. Both thin air wings and fat foils operate in air. Do the planes with thin wings drop down because air is thin??? The simple point is that I want to understand what's going on. Plus, when you adjust for the speed at which airplanes move in the say 200-300 mph range (as I said, looking at older but not aincient planes) against the density of air and water and consider the speeds at which the paddle moves through the water, I have the feeling things will translate b/w air and water rather nicely.

I have actually done very simple airflow tests for designing my own bullet for the scupper on my surf ski. The results from my test in air carried very well in my actual trials on the water. I had a failed design that I never tested in air, that did not really do me any good in the water. After trying several designs in air and selecting the one that provided the most results in the air tests, I put that on my boat and indeed it worked better than the original Epic-designed scupper without the bullet. In that case, not only there was good correlation b/w air and water (with very different speeds of the flows to compensate for the different density of the fluids) but I also got a design that worked better than what the "pros" had built into my boat. Not only was the scupper more effective in draining water it was also with less resistance in the water while doing so (that was part of my air test - by looking at the turbulence patterns). I got the video to prove it -;) The improvement in suction by the way was about 30% in that particular case, which translated in almost 1 mph lower speed at which I got complete drainage and faster overall drain speeds in my surf ski...

Video: air test that translates to water

Interesting note in the first article:

“efficiency (L/Dmax) is probably a more important driver to maximum speed than maximum lift coefficient (CLmax)”

That jives with my thinking - if we can minimize turbulence/drag things will be better (and the payoff higher relative to simply increasing lift)

Okay dokey - Viscosity it is then

– Last Updated: Apr-12-12 12:27 PM EST –

Air will never equal water when it comes to
viscosity for a fluid's internal resistance
to flow i.e measure of fluid friction.

A gas has loose molecules and little "friction"
A liquid has many molecules; a lot of "friction"

The dynamic viscosities of liquids are
typically several orders of magnitude higher
than dynamic viscosities of gases.

The improvement possibly gained by manipulating
an edge on a paddle blade is quite small indeed
compared with 175 lbs of person/boat moving
relatively slow in the water (assume under 8mph)
The power output of normal humans just won't
benefit a great deal from a few millimeters of "edge"

LIft is the goal?

– Last Updated: Apr-12-12 12:24 PM EST –

I'm not very knowledgeable about HOW the wing paddle helps. But clearly the goal of a good paddle is to generate maximum forward movement, not maximum lift?

So a little bit of lift helps forward movement doesn't mean more lift will translate into more forward movement.