You are right that there should be a simple method for starting the design process -- and there is -- but it has nothing to do with how big of a thumb you have, nor how long your arms are.

Your prime determination is whether the thing will float,. so you have to ensure that the displacement of the hull is greater than the weight it will carry.

So, the main anthropometric measurement you need is one the primitive societies had a hard time measuring: your weight

Beyond that point, with a canoe you want to have the sides of the boat high enough tkeep out waves, so you have to find the average wave height around your vicinity. With a kayak you have a deck covering the boat, so waves can go right over the top of the boat without filling it and swamping it (assuming a tight fitting sprayskirt secured to a suitable cockpit coaming) In this case you want a boat that is high enough to permit your feet to fit in comforably. In this case you need your shoe size, or at least a measurement of the bottom of your shoe. Shoe size doesn't matter for canoes.

Your hip width is a minimum for the width of your seating area. A boat narrower than this will be uncomfortable.

Beyond that all the other numbers only affect performance issues: speed, stability, tracking, weathercocking, draft, etc. These are not trivial items, and great debates ahve been held over minute aspects of these matters -- but if you have the necessary figures for displacement and minimum width you can tailor any boat design into any direction you wish.

Using weight to determin your displacement requires a little math, and some simple physics which we all probably learned in grammar school. First, I make the assumption that my boat is in the shape of a long, skinny diamond. I can split that diamond in half at the middle and have two long skinny triangles. I cna figure the area of the triangles -- and then the volume of them -- and use those numbers to determine my displacement.

So, for instance, if I am thinking of an 8 foot long kayak which is 18 inches wide, I picture this as a diamond that is 8 feet tall and 18 inches wide. I cna split this into two triangles which are 4 feet tall and have an 18 inch wide base. The area of either of these triangles is 1/2 of the base times the height. I have two triangles, so I would later double that number. To make it simple for myself, I just figure the width of the boat (thats the base of the triangles) times half the length of the boat (4 feet for an 8 foot boat) gives me the area. In this case it is 6 square feet. I know that a cubic foot of water weighs a bit over 60 pounds. If I want a cubic foot I need something that is a foot on each side. If i have a shape that is 6 square feet, then it needs only be 1/6th of a foot thick to have a volume of 1 cubic foot. (A sixth of a foot is two inches) So, this prototypical boat would sink 2 inches in the water if it was loaded with 60 pounds. That also means it sinks an inch for every 30 pounds. That assumes the water is perfectly flat.

A 300 pound paddler could use a canoe like this if the sides were a minimum of 16 inches high. 10 inches of the boat would be submerged to displace the weight, and 6 inches of the sides would be above the water ("freeboard" is the term) just to keep out the typical waves in my area.

Now a boat that sinks 10 inches would get hungup on the rocks in the shallow river by me. I want something that will only have 5 inches of draft. So, I want something with a 'diamond" area of 12 square feet. I can make it longer, or wider, or both. A simple answer is to go with a boat that is 2 feet wide and 12 feet long. If I wanted more stability I could go with one that was 3 feet wide and 8 feet long ( short rowboat?!) but the added width would make it slower to propel, and a bear to steer. If I wanted more speed I could make the boat narrower, say keep the 18 inches, and double the length to 16 feet.

Whatever I do to change the width and length will affect stability and speed, but I am keeping my displacement the same, so it WILL float me.

If you have a boat with straight sides and a flatt bottom then it will sink into the water evenly and proportionately as you add weight, following the guidelines above. If the bottom is rounded, then when the boat is set on the water and lightly loaded, if it were clear, you could see the outline of the water on the botttom of the hull, and it would be a much smaller "diamond" shape. Adding more weight would make that diamond larger until the "diamond" represented the outlines of the sides of the boat. This makes estimating displacement a bit more difficult, but the guidelines given above will get you in the ballpark. And of course a computer can do the calculus needed to figure the volume enclosed by these curved surfaces with no problem. Here is where you need the design programs.

General guidelines seem to call for open canoes to be designed to have about 4 inches of draft and 12 inches of freeboard, with minimum load. When they are at "maximum" load there is still 6 inches of freeboard. The difference between those amounts determines how many paddlers, passengers and their gear can be carried.

Building without plans? Make a rectangular frame 12 inches high, and as wide as your hips. Get 4 boards that are 16 feet long, and about the size of a firring strip (nominal 1x2 or 1x3). Gather them in pairs, and secure the ends of each pair. Spread open one pair of these strips and set the frame in the center so the widest part of the frame holds the boards apart at the bottom of the frame. Spread open the other pair of boards and set them at the top of the frame.
You now have the outline of a boat. You can add a piece of wood for a bow and another for a stern, and secure it to the paired strips at the respective ends. You can add crosspieces, or other frames to provide support along the lenght of the "boat", and then you can cover the outside with canvas and paint it for waterproofing. Or, just wrap it with a blue poly tarp. If you want more stability, make the center frame wider at the top than at the bottom. Or , make the center frame wider overall. If you want a round bottom, make the frames with curved pieces instead of straight ones, and lay thin wood strips over the frames. Then when you cover the thing with cloth, the cloth will follow those defined curves. Need more displacement, use longer strips or wider center frame, or both.

Now you have a mental picture of how to design a basic boat. You can modify this concept endlessly, but it still revolves around displacement.

Hope this helps

PGJ