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Stroking...

Vortex shedding; If a flat blade is held and drawn through the water with the blade face at 90° to the direction of pull, water flow is induced across the blade face splitting in the middle and spilling over to create a vortex behind each edge. In practice these vortices are not balanced and one comes to build and dominate. This induces a lower pressure on that side and causes the blade to slip sideways towards the larger vortex. As soon as this happens the relative speed of the water across each half of the blade face, reverses, so that the larger induced vortex declines while the smaller vortex at the other side increases, causing the blade to reverse and slip in the opposite direction

In practice, the effect is that the paddle "wiggles", "zig-zags" or "flutters" in the water during the stroke. This is especially noticeable when accelerating. Standing on solid ground and passing the paddle through a water trough can simulate the equivalent experience of accelerating the kayak (Heath 1986).

Forward stroke; Zig-zagging of the paddle can be greatly reduced and the ‘grip’ of the blade in the water noticeably improved if the blade is presented at an angle at the start of the stroke (Heath 2000).

The angle causes the blade to dive under the water surface, a phenomenon known to canoeists as "slicing". Water is flowing in only one direction across the entire face of the blade. Used in this style the blade is drawn back past the paddler as far as is comfortable, at which point it is at the correct angle for a smooth exit. The blade angle enables the paddle to leave the water quickly and easily, an aspect Heath reported was important to modern Greenlanders to help free the blade for the return phase (Heath 2000).

When used in a ‘power’ mode of stroking, the style becomes more of a vertical stroke close to the kayak. Brian Day provides an excellent description of the versatility of Greenland paddling and the variety of bracing and rolling strokes which are possible (Day, 2000).

Conventional, contemporary thinking for the European paddle is that little propulsion results from that part of the stroke aft of the paddler and modern teaching encourages paddlers to end the stroke when the paddle is level with the canoeist’s hip (Train 1990).

There are a number of possible effects that might operate in the aft part of a stroke to aid propulsion. The first is that the blade angle is such that applied pressure results in a force with a downward component depressing the kayak in the water (Stamer 2000). Once the blade exits, the kayak rises, returning the energy to forward motion, possibly by helping the bow rise over the bow-wave created by the first part of the stroke. The second effect may be that the water is compressed between the blade and the side of the kayak at the end of the stroke and thus forced aft in a weak propulsive jet. Published accounts of Greenland strokes suggest that propulsion also results from the active lift of the paddle at the end of a stroke when the top edge of the blade is angled forward. Any or all of these effects may operate but no detailed analyses have been published to date.

Smoothness of thrust; Another factor is the degree to which the kayak is accelerated with each stroke. Every kayak has an optimum cruising speed above which the energy required for faster travel increases exponentially as the craft builds up a bow wave. The European paddle causes a rapid acceleration and the kayak decelerates before the next stroke. It may be that the narrow blade supplies a smoother delivery of thrust thus conserving momentum by reducing the acceleration and deceleration and this could represent a substantial energy saving. There is an analogy here with carrying of loads on land. In a study of African women carrying 60 lb loads on their heads (c27.3 Kg) it was found they used 20% less energy compared with figures for European soldiers carrying the same weight as a backpack. The most likely explanation is that the women use the flexibility of their backs and legs to smooth out the rise and fall of the loads with each step. Loads carried as backpacks, because they rest on the shoulders and hips, rise and fall with each step, requiring energy each time, which can be considerable by the end of a day’s walk.

- from: Sea Blades: Fashion or Function? (2001) © Peter Lamont, Isle of Luing, Scotland.

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