Kayak Paddle Blades - The Shape of Things...
The first paddle was probably created to move a raft across water so deep a push pole couldnt reach the bottom. As boats evolved into different craft so too did paddles. Many early Pacific Rim kayakers used a single bladed paddle - a more efficient design when kneeling in the cockpit straddling a bedroll as they did. Others, across the entire Arctic region all the way to Greenland also created double bladed paddles to propel their kayaks. Modern paddlers can still choose that traditional style as well as several other shapes that have evolved with technology and paddling styles. Each is designed to provide a more efficient and powerful means of propelling a kayak through the water.
What is the optimum shape for a kayak paddle?
Today's market offerings include everything from long flat panels to odd-shapes with flowing curves. Several factors affect the ultimate shape of a particular paddle offered by any of the dozens of manufacturers around the world. Opinions as to that ultimate shape tend to be based on a culmination of collective and interrelated aspects of paddle function and design.
Symmetric vs. Asymmetric
Envision an imaginary line down the center of the paddle shaft extending down through the middle of the blade, from heel to tip. If the blade sections on both sides of the line are an exact mirror image of the other, the blade is symmetric - two equal halves. Conversely, if one side of the line has a larger surface area than the other, it's an asymmetric blade.
Symmetric blades tend to be the choice of whitewater paddlers as well as those who prefer a high stroke while touring. Other design factors (keep on reading) will affect performance and power issues.
Those who tend to use a more leisurely low stroke tend to use the very common asymmetric kayak blade. In fact, it's because of that lower angle to the water that a blade has two different shapes but results in the same overall surface area. When the blade enters the water at the proper angle, the asymmetry enables both halves of the power face to have the same total area of contact with the water.
Width & Length
Basically a shorter, wider paddle blade is used for more forceful, more powerful strokes preferred for those who like speed or the critical bite of the paddle when needed quickly. The longer, narrower blade works for slower, easier-paced strokes. The "average" touring paddle blade might be at least 18-20" long and about 6" wide. Typically those who want speed and quick power might choose the same blade design, only significantly shorter and noticeably wider by several inches.
In both instances cited about, the focus has been solely on the relationship of the total surface area and how it is distributed relative to the shaft line along the length of the power face. But wait, there's still more to consider.
Flat vs. Dihedral
A flat paddle blade is exactly that flat across the surface. It may have a longitudinal curve or slight sweep to the blade, but the surface from edge to edge is flat usually. An exception is the addition of a rib down the center of an otherwise flat-paneled power face. The rib is there to guide the flow of water towards the outer edges of the blade while reducing flutter. Some paddle makers also use that rib to add strength to the blade.
If you look from the tip of a paddle back down lengthwise along its power face and notice that both sides angle slightly down and away from the shaft line out towards the outer edges you are looking at a dihedral blade.
Dihedral paddles have two plane/power faces. This lateral angle that can vary by manufacturer is designed to guide the water flow across the surface of the blade. By doing so it can reduce flutter in the paddle by directing that flow along the face to the outer edge. Like flat blades, a dihedral blade may also have a curve along its length.
Curved vs. Spooned
Adding a sweep to the paddle blade, either along the axis of the blade (curve) or through the cross-section (spoon) affects the bite a paddle has in the water. The curve is similar to the shape of a swimmer's hand during the power stroke. The curve is designed to provide an early catch during the beginning of the stroke. Because it's curved, the blade should be pulled from the water early to prevent the curve from lifting water instead of pushing it. The curve can continue throughout the entire blade evenly or can be applied more towards the tip.
A spooned blade is shaped like a spoon. If a spoon paddle were laid down flat with its power face upward, the lengthwise edge would actually enable you to add water without it flowing back off as it would on a "curved" blade.