Cat posted an interesting question to me this weekend. On the face of things, it seems like a very simple question. The question? "Where should I put my seat in the Wee Lassie II that I'm building."
First I need to mention a little about the design : the Wee Lassie and Wee Lassie II canoes that were an interpretation of the classic Rushton designs by Mac can be found in his book, Featherweight Boatbuilding which is available from numerous sources, including the publisher, WoodenBoat. I say these are an interpretation because Mac made some small changes to the boat's shape to carry a bit more volume aft of the paddler. This was because the original canoe design tended to "squat" in shallow water when paddled hard. I should also point out that there are some small errors in the mold data that have been corrected (see here) and in full size plans available from Feather Canoes and Newfound Woodworks
When you're locating seats in a canoe, you want to be balanced from side-to-side and fore and aft - this is so you're not riding bow high, or digging the bow in the water - you want the waterline of the canoe one the waterline... The reasons for this are good stability and good handling as you paddle as well as ease of paddling. If you're fighting to stay level or the boat's attitude in the water is poor, you will not have much fun paddling it. The hull shape might be fine, but poor weight distribution turns it into a pig. I'm sure you've seen the guy all by himself in a tandem canoe, sitting in the stern seat with the bow riding high and fighting to keep the thing going straight and trying to overcome every gust of wind...
But back to Cat's question - in Mac's plans, he shows the position of the thwart, but not the actual location of the seat - in scale, the seat is located about 2" forward of the thwart. First, I find this to be a bit close - with a life jacket on, this simply isn't enough room to lean back comfortably without having the thwart hitting me in the small of the back. Don't ask me how I know this...
Here's how we locate the canoe's seats and thwart when we build our canoes in class: First, we build the canoes up until the point that they're 'glassed, and have decks, grab-handles (if any) and gunwales installed. We then take a piece of pipe and put it underneath the canoe running athwart. (side-to-side) We then move the canoe back and forth and find the point where it balances on the pipe and put a piece of tape on the inside of the floor. Unless you have a significantly asymmetrical hull, unusually disproportionate decks, or a huge honkin' 10 pound piece of "ego bronze" for a fore-deck tie-down (i.e. : 1939 Packard hood ornament...) this is usually the center of the canoe's volume. This is where we locate the front rail of the seat - just behind this point. A paddler's body weight distribution is pretty evenly divided between your legs and the rest of the torso, so in a solo double-paddle canoe, your legs basically begin at the front rail of the seat and your torso is behind it. Voila - good balance.
We then locate the front edge of the thwart about 4" behind the rear rail of the seat. Unless you put in a swivel backrest like on Tom Hill's Charlotte, or a wide one the location of the thwart wants to be far enough back that you can lean back a bit as it isn't very comfortable to lean on. Keep in mind that personal geometry varies quite a bit, so you'll probably want to mock up the seat and thwart location to see how it will feel. Be sure to put on your life jacket when you do this as these can add a bit of thickness. Also, pay particular attention to the height difference between the seat surface and the thwart as well as the shape you've chosen for your thwart. A traditionally shaped thwart may want to be a bit further back than a gently curved one to cradle your back a bit.
Personally, I use a Crazy Creek canoe chair strapped down to my caned seat. It winds up looking like this:
One creative student came up with a removable swiveling backrest that was built into his thwart. This year, I have a student who wants a removable backrest that will "stake" into the rear rail of the seat frame, but be like a Howda seat. I'm sure I'll be updating this as we go along.
When considering seat location for a tandem canoe, the process is a bit different. Tandem canoes need to take into account the differing weight of paddlers. As I noted before it is a balancing act. In Ted Moores excellent book Canoecraft, he has a very clear and concise picture showing basically the following:
What this shows is the "ball" at the center of the canoe marks the balance point as shown above (with the previous caveats...) This would be the middle of a symmetrical canoe and is the point at which the portaging yoke should be placed. (usually a tad tail heavy to allow for easier portaging...) The "ball" on the edge of the stern seat (on the left) is the center of the stern paddler's center of gravity. The "ball" on the right is the center of the bow paddler's center of gravity. To figure where the seats go, you use the equation shown in the image - Sternman's weight times distance "A" is equal to the Bowman's weight times distance "B". Relatively simple math.
You paddle with people of significantly different weight - like either children or adults in the bow of your boat. - might be over 100 pounds difference in the paddler weight. How to deal with that conundrum? Sliding seats to move the paddler's center of gravity!
Beautiful sliding seat by Green Valley Boat Works
Some canoes have bow seats only that slide and some have both bow and stern seats that slide. It really depends on how much weight difference you have.
Another thing to keep in mind is that if you are carrying gear, you can re-position your cargo in the canoe to improve trim to account for different paddler weight.
Hope this helps!