Teardrop shape (low aspect) paddle blades are made to be used at a slower cadence (or for only a few quick strokes) and the tip of the blade carries most of the surface area, so even if the entire blade is not in the water it still has a good hold. Rectangular blades are made for faster paddling cadences and for more efficient paddling over long distances. Adjusting the depth of the paddle blade in the water has a greater impact on the amount of force the paddle produces.