I'm about to rehab an old Bell Morningstar canoe. I'm planning on getting Northstar contour seats and would like to make my own drop arches, just to keep the cost down. But how far down should the seat be mounted?
Also, I thought I might install a kneeling drop for paddling solo. How far down should that be?

I would look at link below. Scroll down to what Charlie Wilson says in the post. He was involved in the design of some of the Bell Canoes and later was co-owner of Placid Canoes. This will give you a good idea of where to start even though the post is talking about a different Bell canoe model. You might also search seat height too for more posts but Charlie Wilson would be the expert.

Seat height

Perfect seat height depends on your habits and comfort zone. If you kneel a minimum seat height will also depend on size of your feet and kind of shoes you wear: for instance, big-soled hiking shoes might require almost an inch higher seat than running shoes. If you have other boats you use - that would be a good place to start.

The seat drops on my Northstar tandem are 4 inches and so are all the standard ones sold by edscanoe...so I recommend 4 inches. It will be very stable (the contoured seats drop your butt another half inch or so). If both bow and stern paddlers will be kneeling 100% of the time or if you have trouble getting big feet under the seat when kneeling you knight make them a little shorter; I'd go 1/2" at a time. The kneeling thwart typically wants to be higher and in your case I recommend that you start on the long side (say a 4" drop in front and 3-3.5" drop in back and see how it feels. You're trying to balance the weight between your butt and knees so that the kneeling thwart takes some weight off of your knees but you don't want all your weight on your butt because you're sitting on a board. You also want the angle to be comfy so the front edge of the thwart doesn't dig into your legs/butt. So I'd say make it a touch long and plan to trim it (tune it) for max comfort for you.

Why not just make them as long a practical? Then you can shorten them up to a comfortable position after trying out a few times. They are much easier to shorten and adjusted later.

butthead

In general I'd agree with butthead's advice but in this case I know that with 4" drops and Northstar countoured seats I can barely get my feet under the seat in my Polaris to kneel even with no shoes...the seats are on the low side and the boat is incredibly stable...and Morningstar is more stable than Polaris. Plus the 6 inch screws are perfect with 4 inch drops so if you plan to start with even longer drops I suggest that you find long enough screws first.

Understand about practical limits. In my own case on my first solo canoe I dropped the seat low as I could. The more I got used to paddling solo I kept raising the seat in steps, to the point it was as high as I could get them over a period of about 2 years. Lowering serves a point, just be ready to go the opposite direction as you get familiar with paddling the canoe.

butthead