Open canoe paddles 2: The shaft

The main purpose of the shaft in a paddle is to act as leaverage to transfer the force from the upper hand to the blade. The lower hand should mainly act as the pivot. Therefore the length of the shaft has a very big effect on the paddling. The other things which should be considered is the form (round or oval, or a combination of both) and thickness.
I've split this article into 3 sections; Length, Thickness and Form.

Length

'Shaft length' extends from the top of the grip to the throat of the blade, just where it starts widening.
The most common way to decide if a paddle is the right length is to put it on the ground and see if it reaches somewhere around your mouth. This way of measuring the paddle isn't really correct. Since what matters is the shaft length, not the paddles total length. You want a shaft length where, when you sit in the canoe and the whole blade is submerged, your upper hand is at about the height of your eyes. If the shaft is longer you have to stretch out your upper arm which forces you to use your arms instead of your torso. The right shaft length makes it much easier to use the torso while paddling. And with a long blade youre forced to make longer strokes. If you start the stroke far towards the bow you're just pressing water downwards, moving the canoe up. And similarly if you end the stroke far toward the stern you push water up, pulling the canoe down which just raises resistance.
A too short paddle on the other hand will lessen the leaverage which makes it harder to paddle. And the blade will also be partly above the water which creates "ventilation". The blade pulls down air behind the paddle which makes the water you press away in front of the blade fall in behind the paddle. So you lose power in the stroke.

So summarizing it, the better you are at using the torso (efficient paddling) the shorter shaft you'll want. And the shorter shaft you have the easier it will be to paddle efficiently. But if you paddle incorrectly with a too short shaft you'll get tired really fast.

Now that's the basics. But unfortunately, this isn't all there is to it. Which canoe you paddle matters, since the seat height and gunwhale height over water varies in different canoes. And if you tilt the canoe while paddling or sit on your knees that also changes your distance from the water. Your technique also affects the length. If you do short quick strokes rather than long ones you need a shorter shaft. And if you're stern paddler you might want a little longer shaft for more effective steering strokes.
The method I use to find out correct length is to try a paddle and analyze my stroke. At the point in the stroke where the paddle is at the deepest I want the blade to be just below surface and the upper hand in height with my eyes.
The easiest way to find this is with a paddle with adjustable shaft length. If you don't have one you can try with a slightly too long paddle and see how far under the surface the throat of the blade is (when at the deepest part of a stroke which has the grip at eye height) and remove that length from the shaft when you make or buy a new one.
But generally people use too long paddles. So my personal recommendation for you is to get a paddle just a little shorter than you think you need. The latest solo paddle I made has a shaft length of just 56 cm (22") and I feel I could cut it down by an inch or two still.

Thickness

Thickness is mainly about strength. A thicker shaft is obviously stronger. But it's also about balance, weight and comfort. A thinner shaft is lighter and easier to grab since you hold it in a loose grip. While a thicker shaft aids in balancing the paddle as it puts more weight on the grip side.

Form

An oval shaft is perpendicular to the blade for strength and comfort.

The form of the shaft is either round or oval. A round shaft is easier to flip when for example doing j-strokes. While an oval shaft gives a better grip and feel of orientation. An oval shaft is perpendicular to the blade to maintain the strength of a round shaft and to fit better in the hand. This means that by using an oval shaft you save some weight without losing strength. But for a paddle maker this makes it harder to balance the paddle (see previous post on balance) since you remove weight on the shaft side, which is already too light. A solution for this problem is to have the shaft round near the grip and then bevel it to an oval shape towards the blade. The thinner shaft near the throat also lessens the ventilation (see above).