-- Last Updated: Nov-04-12 7:47 AM EST --
Although downriver racers and a very few whitewater paddlers use bent shaft paddles, the vast majority of whitewater boaters use straight shaft paddles with a T grip. Bow draws, cross draws, and J strokes can all be done fairly well with a bent shaft, but stern pries and stern draws are awkward unless you palm roll the paddle. And you don't want to be palm rolling the paddle in whitewater of any significance.
The low brace works with a bent shaft paddle as long as one is bracing on the power face, but you can't smoothly transition from a forward stroke, stern pry, or J stroke to a low brace or reverse sweeping low brace without palm rolling the grip (awkward to do with a T grip) and palm rolling requires you to relax your grip on the paddle with both hands, increasing the likelihood of having it knocked out of your hands when you need it most.
Straight shaft paddles are nearly always at least a couple of inches longer (in both overall length and shaft length) than bent shaft paddles used by the same person.
Furthermore, straight shaft paddles used on whitewater are often a little bit longer that straight shaft paddles used on flat water. Part of this is due to the fact that whitewater boats are nearly always deeper than flat water boats. Furthermore, the kneeling position used by whitewater paddlers might put their torso a touch higher than a sitting flat water paddler. In paddling whitewater I often have my shaft hand a bit higher relative to the water than I do paddling flat water as this makes it less likely to jam your hand between the side of the hull and a rock. To do this and avoid choking up on the paddle (which would reduce the power of the stroke) requires a little longer paddle shaft.
As an example I am about 5' 11" and use bent shaft paddles with an overall length anywhere from 48" to 53" in overall length. For straight shaft paddles in flat water I might use a paddle 54-56" in length but my whitewater paddles are all 56-59" in length.
A common recommendation for sizing flat water paddles is to use a paddle that places the grip hand at about shoulder height when taking a forward stroke with a good vertical shaft angle and the blade just fully immersed. A common recommendation for whitewater paddle sizing is to use a shaft length that puts the top of the T grip at eye or forehead level.