What a great opportunity to use that high school calculus that 'no one uses in real life'. Trick would be coming up with a function that you could integrate in your head while driving cross-state.

: I always "ballpark" the displacement by assuming the boat is made
: in straight lines, diamond shaped, and symmetrical. Of course few boats
: are made this way, but I get a close enough estimate of the displacement
: with some simple math.

: For a diamond shaped boat that is 7 feet long and 28 inches wide (or 2 1/3
: feet wide) I calculate the area of the diamond by first dividing it into 4
: triangles, each with a base of 1 1/6 foot and a height of 3.5 feet. By
: reassembling pairs of these triangles ( in my head) I make 2 rectangles
: that are each 1 1/6 wide by 3.5 feet long. Putting those two rectangles
: side by side I form a bigger rectangle that is 2 1/3 feet wide and 3.5
: feet long. I figure the area of that rectangle as the footprint that the
: boat will make on the water. In this case it is about 8 square feet.

: It may help to draw a rough sketch to follow this, but gee whiz, the numbers
: I generally need for any kayak or canoe are only the beam multiplied by
: 1/2 the length of a boat.

: I figure a cubic foot of water at 60 pounds. It is a nice number that is
: evenly divisible by 12, and pretty close to the actual weight of fresh
: water. Salt water will have about 5% greater bouyancy, but I tend to
: ignore that. Anyhow, for 8 square feet, if the boat sinks an inch into the
: water ( 1/12th of a foot) then your displacement in cubic feet will be 8
: times 1/12, which is 2/3 of a cubic foot, or 40 pounds of water displaced.
: So, for each 80 pounds you put in the boat it goes down 2 inches, and so
: forth. If the boat itself weighs 30 pounds and your gear weighs 15, then
: you need to displace 175 (your weight) plus 45, or a total of about 220
: pounds. by my rough reckoning, therefore, this boat would sink into the
: water (or have a draft of) about 5 1/2 inches.

: For safety most canoes want 6 inches or more of hull sticking out above the
: water. This is called freeboard. Using this measurement you would want a
: canoe with sides that were 11.5 inches high at a minimum. With a kayak,
: though, a lot depends on what you want to do with the boat. You could
: build it with an inch or less of freeboard, and the covered deck will keep
: out waves and splashes. Sealing yourself in with a good sprayskirt would
: let you sit in the water with most of the boat underwater, and only the
: cockpit about it, sticking above the surface kinda like the conning tower
: on a submarine. In practical terms, though, kayaks tend to be high enough
: to allow the paddler to sit comfortably, and that means adequate room for
: the feet, so your minimum deck height in the center is going to be 1
: "foot" high at least. or maybe I shoudl say 1 "shoe"
: high.

: Curvature of the hull will reduce the volume of the boat, so it will sink
: lower in the water (greater draft) and a boat that has a lot of rocker
: will also go lower.

: Hope this helps

: Paul G. Jacobson
: Hope this helps.