I’ve seen some canoes that are offered with the option of swapping out the thwart/carry bar in favor of a center seat. Does this make car-topping significantly more difficult? I have a small build, and therefore proportionally narrower shoulders.
Depends on a lot
How high are your bars off the roof ?
How much higher is the seat above the gunnels?
Do you load from the side or slide it on from the rear?
I just got back from a paddle with our tandem canoe.
I don’t have a center seat, but I have a portage yoke with high (4") cushey foam blocks that stick that far above the gunnels.
I load from the rear, and when I am sliding the canoe forward, I get the bow on the front bar, and then have to lift the boat as high as I can to get the portage blocks over the rear bar.
If I could not reach high enough, I would just load from the side.
With all that said. Measure the height of the bars above your roof. Then measure how far the seat is above the gunnels.
If the seat above the gunnel height is higher than the bar above the roof height, you are going to end up scratching the roof unless you can put some of those foam canoe blocks, or equal on your bars to raise the canoe seat up high enough to clear the roof
Where there is a will, there is a way!
hull over your head ? how heavy is the hull ? how long is the canoe, how wide is the…beam ?
Could you clarify the question?
I can't really tell what it is that you are concerned about. When I think of a canoe seat, I think of a bench seat or tractor seat. With either of those two, nothing changes when it comes to being able to attach the canoe to your roof rack. A center bench seat can be used as a carry yoke. I doubt that a tractor seat works so well.
The nature of your question could make a person assume that the center seat has a fixed backrest. I think that was Jack's assumption. If you use a seat with a backrest, follow Jack's advice, or perhaps you can remove the backrest.
If you need to remove the center seat for car-topping on account of it having a fixed backrest that gets in the way, you can solo carry a canoe by placing a boat cushion against the floor of the boat and using that in the manner of a carry yoke. The method leaves a bit to be desired (you end up with your head facing kind of downward), but it works, and you can put one end of the canoe on your roof rack or loading bar just the same as if you were using a carry yoke (that's how I carry and load a small aluminum Jon boat, and using the low seat of my guide-boat and smaller pack boat as a yoke is pretty much the same as using a cushion against the floor, since the seats are so much lower than canoe seats).