Why it Works!...
For a given area and all other factors being equal, a flat shape with the highest coefficient of drag gives the best grip on the water. When various flat surfaces of equal area are pulled through the water, a round shape gives the least drag and a long rectangular shape, such as a Greenland style, gives the highest drag or best ‘bite (which is what is desired)’. A wide blade paddle falls somewhere in between these shapes. When a paddle is pulled through the water, the water on the working blade moves outward to the edges of the paddle and curls around forming an eddy or vortex on the back side These are shed alternately and is the reason for the zigzag motion felt at certain speeds. It is these eddies that create the ‘push’ on the paddle. Only with a narrow paddle is this vortex shedding noticeable because the vortex is large in relation to the blade.
The opposite is true for the non working blade. In hydrodynamics, the rectangular shape has the highest coefficient of drag but aerodynamically it has one of the lowest. This coupled with the lower Greenland stroke makes the effect of wind on the blade minimal. With feathered paddles, as the working blade is pulling through the water, the other is edge on to the wind. This may be a slight advantage for wide paddles when heading into the wind but what happens when paddling with a beam wind. A sudden, powerful gust may catch the unprepared paddler and capsize them. The working blade is edge on to the water and will offer no support to counteract the effect of the wind.
Where a wide bladed paddle is useful for quick, powerful strokes such as would be needed on the river or in racing, it can be very fatiguing for touring. Similar to a mountain bike in low gear for going through dirt and sand versus a touring bike in high gear. Initially, a narrow paddle will offer less power but after a kayak is at cruising speed it makes no difference.
Another consideration is that a narrow paddle gives less muscle shock since it develops resistance more slowly. Wooden paddles flex more, acting as shock absorbers in the beginning of the stroke and giving back the snap at the end.
It has been said that the Greenlanders did not have the technology to make wide bladed paddles or feather them. Nonsense. The paddle could have been made as a frame and covered with skin or thin pieces of wood or they could have built up the width the same way they attached the bone or ivory tips and edging to the narrow bladed paddles. As far as feathering goes, there is no secret to making a feathered Greenland paddle.They have been made quite easily and there are a few feathered Greenland paddles in the museums so the validity of that statement disintegrates . The paddle evolved as a narrow and unfeathered blade because, quite simply, it works.