-- Last Updated: Jun-30-13 9:47 PM EST --
Okay, it ended up being more than a "couple of thoughts", but I hope it helps.
First, you may have to lower the seat, but be aware that when kneeling, your butt won't be on the seat "as much" as when sitting. Further, until you get that seat tilted forward to match your kneeling posture, there's really no hope that you can kneel AND get much butt contact. The front edge of the seat needs to be lower than the back edge (personal preference varies, but I think mine are about 1.25 inches lower in front, maybe even 1.5 inches for my highest seat, since for any particular person, the higher the seat, the more slope it must have to get a decent fit for kneeling). You want the seat to be high enough that you can get your feet in and out from under it fairly easily, though the type of footwear you use will affect that too. But like Kayamedic already said, there's nothing to be gained by making the seat a whole lot higher than what's needed for foot convenience (and safety) either.
That 57-inch paddle surely must be too long for you. The "chair" method of choosing paddle length is a pretty good starting point, especially when you look at where the top of the blade and the shaft come together as a reference point, because as Kayamedic points out, only the shaft length matters. Total length of a paddle with the proper length shaft will depend on the style of blade. However, with "standard" blade size, and they are pretty similar across the board for the "basic" style that is most common, a chart that uses the "chair" method to figure out a total length is likely to be pretty close as well. Just be aware that beaver-tail and other traditional paddles will have a longer total length simply because the blade is longer. But finally, you'll need to tweak the shaft length to match seat height and personal preference. Personally, I find that I can use quite a range of paddle lengths with fair comfort, but even so, I usually take two lengths with me that are about two inches different, one for deep water and one for shallow water.
One other thing to think about regarding paddle length is that there's nothing to be gained by burying the blade of the paddle extra deep. Try that 57-inch paddle, and put the blade in the water alongside the boat just deep enough to fully submerge it. I bet that at that point, the grip end is quite a bit higher than your head, and if so, that's not good. I can't recall exactly how high the grip ends up while I paddle, probably about eye-level at it's highest point, and mostly a bit lower, but I'm not really sure. I do know that the "chair" method gives me a shaft length that I like, with no need for further adjustment.
Knuckle-bashing really shouldn't be affected by paddle length. If your grip hand is at the proper height while paddling, there is nothing to prevent you from placing your lower hand at the proper height as well. If you hold the shaft with proper hand spacing and the grip hand at the proper height, a paddle that's too long will have the blade too deep in the water, and a paddle that's too short will have the blade not fully submerged. If the paddle is too short, that will actually reduce the chances of knuckle-bashing, not encourage it, because with the blade not fully submerged, interference with the hull will guarantee that the shaft is farther away from the gunwale. At least, this will be true of you are holding the shaft quite near vertical, as viewed from the front or back. Bottom line, it's your technique, not the length of the paddle that's causing the knuckle-bashing.
For what it's worth, my personal style lets my lower hand glide along low enough that I often could bash my knuckles, but I never do. Haven't done so in many years. I even position my lower hand lower than the gunwale in certain special situations (like when reaching the blade beneath the hull). With my hand at its normal height though, I can slide or brace the heel of my hand on the top of the gunwale for a bit of levering action without actually prying the shaft against the gunwale itself (I avoid that because with square-edged vinyl gunwales, it puts big dents in the shaft). Again, the point is that it's all about technique, and I suspect if you get that shaft aligned more vertically, you'll stop banging your knuckles.
Also, in case you didn't notice, the "chair" method should provide one recommended length for straight-shaft paddles and a noticeably shorter length for bent-shaft paddles. For your starting length recommendation, make sure you are using the method for your kind of paddle (bent or straight).
Full Size Sail Rig
Wabakimi Canoe Pack
Bent Shaft Canoe Paddles
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