Why do some canoe paddles have a bent shaft?

As you reach your canoe paddle out at the beginning of your stroke, picture where your paddle goes into the water as the beginning of an arc.  The end of that arc is where you take your paddle out of the water.  At the beginning of that arc, the pressure you put on that paddle through your stroke actually pulls the canoe up and out of the water as you propel the canoe forward.  Toward the end of the stroke, as you are pulling and arcing your paddle behind you up and out of the water, the pressure you put on the paddle pulls the canoe down into the water.  Accordingly theory says and practice shows that the most efficient part of your stroke is the part that is in front of you at the beginning of the arc when you are pulling the canoe up and out of the water. 

The purpose of the bent shaft paddle is to create a little more distance at the beginning of the paddle stroke's arc by angling the blade out further .  Also, the bent shaft gives your stroke a longer period of time that your paddle is the most perpendicular to the water during each paddle stoke.  Both of these things increase the efficiency of the stroke through the use of a bent shaft paddle.