Recently I got into a very short discussion of what constituted a canoe vs. what was a kayak. A very experienced friend of mine said that the blade is what makes the difference. So whenever I switch to a single I'm canoeing. And whenever I use a double I'm kayaking. Too bad there are no canoeing instructors around as I'd really like some tips on my forward single blade stroke. I bend forward and back with the single blade stroke doing little ab crunches as I go along. With the double blade the core movement is rotating side to side.
By the way I think the ideal kayak touring blade is a 46 to48 inch bent shaft single blade. I like to use that whenever I am not using my Lumpy Paddles Greenland Paddle. I prefer the double when it is really rough, but the rest of the time I keep up with groups just fine with a single.
The advantages of the single are: light weight, change of motion provides a rest break for some muscles, no useless second blade catching wind over your head, and DRY HANDS. Try one this winter!