: I would like to build a 7 foot kayak, will this hold my weight i weight 178
: the beam will be 28 in. wide. will this hold me?

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.