When to drag and when to lift

Not a lot of pretty pix this time, but a discursion on the Aleut blade and  the dynamics of GPs. OK, no pix at all.

When I bought the Novorca Greenland blades, I also bought a beautiful lapis-coloured Aleut. Like a Greenland, the Aleut blade has a very long, narrow power  face and a central loom that the blade diverges from, but unlike the GP it has an egg-shaped loom, the long flat axis of the blade is offset from the central axis of the loom, and the slightly (backward facing) concave power face has a long groove running down its centre.

The Aleut blade hails, remarkably enough, from the Aleutian Islands of the NW pacific. The Aleut were famed for their long, fast paddling journeys.  Ten to sixteen hours was apparently nothing unusual . It stood  to reason the Aleut blade would be ideal for longer runs such as the Hawkesbury.

So which side really is the power face? Wolfgang Brinck :

in the summer 1986 issue of sea kayaker magazine j. heath [4] explains that the most significant advantage of the greenland paddle is its effectiveness in coming out of the water. a kayaker can pull the blade out of the water at the end of a stroke more easily with a greenland paddle than when using a conventional blade. there are also apparent advantages in the unique stroke of the greenland paddle  which is called the canted stroke. in a canted stroke  the kayaker keeps the blade tilted slightly forward throughout the entire stroke. heath claims that the advantage of the canted stroke is the fact that vortex shedding only occurs on one edge. this allows the kayaker to have more control over his or her stroke

But the central ridge that runs down the length of the blade does more than give the paddle a combination of strength and light weight. The ridge also controls the flow of water over the face of the blade. When you use an Aleut paddle with the ridge facing backward, the ridge evenly divides the flow of water over the two halves of the blade. If you pull a flat faced paddle blade through the water, the blade alternately wants to slip right and then left and then back again, making the paddle flutter. The central ridge prevents this and gives you a solid steady stroke. While it is not impossible to paddle a flat faced paddle, you need to maintain a tight grip on it to prevent it from fluttering. Having the central ridge lets you keep a looser grip on the paddle and lets you put all your energy into forward propulsion

There’s another nice piece at http://www.kayakforum.com/cgi-bin/Building/Archive60.pl/noframes/read/62746.

As I discovered in my HCC trial runs though , as I used it it was inferior to the GP: the greater length meant slower cadence,  it doesn’t cant the way a GP does and initially had a tendency to develop flutter at the spear-shaped tip.  I gave up and went back to the GP.

I certainly noticed a tendency to suck air down the forward-facing  side of the blade  on acceleration. Brinck again:

The harder you pull your paddle through the water the greater the pressure in back of it and the lower the pressure in front. If the pressure in front of the paddle is low enough, air will get sucked down along the face of the blade and that will prevent you from getting any more thrust. The harder you pull, the more air you suck. This problem is worse with native paddles than with Euro paddles. You can plant a Euro paddle so the blade is completely in the water before applying any force on it. The deeper the blade is in the water before you start pulling, the less the chances of air getting pulled down the face of the paddle.

Since then I have gone out of my way to practice with the Aleut but I have found it a tricky beast. There is very little I can find on the web about technique for these blades, but I am sure the problem lies in the way I use it. It’s a beautiful piece of gear and I only wish I had the expertise  to unlock it.

I know the Aleut blade is used more vertically than a GP, and does not employ the cant so beloved of GP users,  no strangers to using cant of another kind!  Still I find it more strenuous and  just about the same speed as a GP.

Which brings me eventually but inevitably to a quick summary of what I have found about the hydrodynamics of “native” paddles.  My blunt understanding has been to date that while euro blades generate propulsive force overwhelmingly through drag, GP’s also generate a high degree of lift ( notoriously in the last third of the stroke). Ginni Callahan  taught me to really push down hard on the upside arm as the paddle entered the last part of the stroke to give an extra kick of lift at the exit . It noticeably increases speed, though I’m not disciplined enough to keep it up for more than ten minutes or so at a stretch ( tried hard in the HCC but, like the slide stroke, it was exhausting to do over a couple of km and quickly put me out into lactic acid boundary-riding ).

Chris Cunningham at Sea Kayaker http://www.seakayakermag.com/2011/Dec11/paddles.htm explains it like this:

Wing paddles may be among the latest major development in paddle design, but the principle behind it, using lateral movement of the paddle blade through the water to generate lift, is an old idea. It’s what gives Greenland paddles great power in spite of their narrow width. While a modern wing paddle develops lift by moving outward from the kayak, a Greenland paddle does so by slicing downward and using the opposite edge of its blade as the leading edge. That lateral movement also keeps the paddle moving into water that is as yet undisturbed and provides more resistance to slip. The Greenland paddle has an advantage over the modern wing paddle in its symmetrical design: It is equally effective moving either direction. The narrow Greenland blades also allow users to grip the paddle anywhere along its length for a variety of techniques.

While the blades are narrow, they are quite long and have an area the equivalent of many Euro blades. The long blade is like a glider wing; its high-aspect ratio is particularly efficient at generating lift. Euro paddles used for sculling braces function like planning watercraft. They provide a lot of support while skimming across the surface. Greenland blades generate more lift while submerged.

Mr Brink has a fine summary in his dynamics page at http://www.wolfgangbrinck.com/boats/paddles/dynamics.html:

 When you pull a paddle straight back through the water, the resistance you feel is called drag. The bigger the surface area of your paddle, the greater the drag. This is what we would expect based on experience. But another factor affecting drag is the shape of the blade.

A long skinny blade with the same surface area as a short wide one has greater drag. The difference between the two paddles is that the long skinny one has a longer blade edge than short wide one. For a given surface area, a circular blade would have the shortest edge. So what does edge have to do with drag? If you think about it, water has to move past the paddle when you pull it through the water, but it can't make a 90 degree turn when it comes to the edge of the paddle blade. Instead it has to take a wide turn around the edge. This wide turn in effect makes the paddle act as if it had more surface than it does. So more edge gives a paddle a greater effective surface area. I don't know that you would want to go into a lengthy discussion of this phenomenon with a casual critic at the beach, but now you know.

I was very conscious that the blade face areas of GP/Aleut  paddles  would be pretty close to those of the standard euro blades but had never heard that point about the edge.  I presume therefore that a sharper edge is better than a more rounded one, but the utility sticks I’ve made have pretty chunky edge profiles and they get along nicely.

Martin Nissan on Greenlandpaddle.com  has a good meditation on canted strokes at http://www.greenlandpaddle.com/index.php?option=com_content&task=view&id=42&Itemid=76 and cites some work in a back issue of Sea Kayaker by Al Bowers about the hydrodynamics of drag and lift:

‘In any surface that produces lift, two vortices are formed. In an aircraft wing, the two are opposing, and form the start-up vortex (where the lift originates) and the bound circulation vortex of the wing (which creates lift). The aircraft wing that is producing lift and flying has ‘shed the vortex off one side’. The reason flutter occurs in conventional paddles is because the vortex sheds alternately, first on one side, then on the other. The result of a fluttering paddle is an alternating stream of vortices behind the paddle; this stream of alternating vortices is called a Karman vortex stream (after the noted aerodynamicist Theodore von Karman).

However, if the vortex can be controlled to shed on one side only the other vortex is ‘trapped’ around the paddle and manifests itself as lift. The ‘tipped forward’ clue is also significant. There are two ways of inducing lift from a surface. If the surface is ‘cambered’, it can produce lift; this is the method used by modern wing paddles. The second way, if there is no chamber, is to set the blade at an angle of inclination to the flow. This is familiar to most kayakers with conventional paddles when doing sculling strokes. There are other similarities, such as the ease with which the blade leaves the water, i.e. ‘’the blade slices out of the water’. This attribute is often credited to wing paddles. Another is ‘grip of the paddle is noticeably firmer’. Again a typical comment associated with wing paddles.

June Heath http://www.seakayakermag.com/2000/june2000/JuneHeath2.htm

was very taken by the importance of the GP exit, and the vortex-shedding it does at that point of the canted stroke:

When a narrow paddle enters the water with the working faces at right angles, the water tends to flow equally around each edge. It makes a vortex that looks like a miniature tornado, along each edge. The axis of each vortex is parallel to the edge.

However, if the top edge of the paddle blade is tipped forward throughout the stroke, the vortex will shed toward one edge all of the time. It becomes predictable, instead of alternating, and the paddler can compensate for and control it. This might be an important factor in the effectiveness of this paddle stroke.

Chris Cunningham followed that up with a bit more explication:

At first, the stroke Maligiaq describes seems a bit unnatural—even precarious—because the angle of the blade draws the paddle downward as you pull on it at the catch (the insertion of the blade into the water). Since the angle of the blade causes it to slip downward as the blade slips into the water, it is very quickly buried in the water. You begin the stroke pulling on a well-buried blade; the solid connection to the water is evident.

The lateral movement of the blade, as John Heath suggests, sheds the vortex off to one side, preventing flutter. You can see the vortex, a little tornado-shaped cone of air twirling off the tip of the blade, its pointed end trailing away, pointing toward the bow. As the blade sheds the vortex, it moves into "solid" water, away from the air driven in at the catch. By contrast, in what Maligiaq refers to as the "beginner’s stroke," the blade does not slice into the water, but stabs into it end-on. Since the blade doesn’t enter the water quickly, it is only partially buried when you pull it back. Air driven into the water by the tip swirls in a pair of vortices as the water wraps around both edges of the blade, pulling more air from the surface as the paddle is pulled through the water. With so much air on the back side of the blade, the paddle does not have a good grip in the water.

At the end of Maligiaq’s stroke, the downward pressure created by the angle of the blade keeps the blade buried in the water to the very end of the stroke. At the release, the pulling hand does not lift up, as it does in the beginner’s stroke, but it pushes forward. The blade slices out of the water, moving up as it moves forward.

I lied about no pix. Cunningham's diagram of the angle on exit. 

Glad June is doing so well after the sex-change . 

And finally there’s a cracker of a an analysis at http://www.me.rochester.edu/courses/ME241/G11(Final).pdf, Jacob Farber’s thesis for a mech engineering degree looking at the GP hydrodynamics. Concluded the GP would be better for distance than sudden catch.  A clutch of equations, and a reference again to J Heath:

In the summer 1986 issue of Sea Kayaker magazine, J. Heath [4] explains  that the most significant advantage of the Greenland paddle is its effectiveness in coming out of the water. A kayaker can pull the blade out of the water at the end of a stroke more easily with a Greenland paddle than when using a conventional blade. There are also apparent advantages in the unique stroke of the Greenland paddle, which is called the canted stroke. In a canted stroke, the kayaker keeps the blade tilted slightly forward throughout the entire stroke. Heath claims that the advantage of the canted stroke is the fact that vortex shedding only occurs on one edge. This allows the kayaker to have more control over his or her stroke.

None of which really gets me any closer to working out why my Aleut stroke is such crap.