: I'm about to start building a kayak for a rather large guy. I figure around
: four hundred pounds. I was planning on increasing the sheer of a Mill
: Creek 13, and wanted to know if someone has a way to calculate how much I
: would need to add to give it another 100 pounds of capacity.

Typically if you enlarge a kayak in one dimension its capacity increases proportionally, in two dimensions capacity increases to the square of the increase and in three dimensions to the cube of the increase. But you cannot increase a S&G kayak in one (or two) dimensions without recalculating the panels. But you can increase S&G designs proportionately in three dimensions by increasing the panels to the same proportion in their two dimensions. So if you take a Mill Creek 13 and increase the paddler design weight from 250 to 400 you need to increase the volume by 1.6 times. Or the dimensions in all three directions (and the panels) by 1.169 times (cube root of 1.6). The Mill Creek 13 will have to be enlarged from 13 x 29.5 to 15.2 feet x 34.5 inches. Build a Mill Creek 16.5; it is designed for paddlers that weigh 230 – 400 pounds. The Mill Creek 16.5 is 16.5 x 33. I would go with it and avoid all of the math. And if you buy the kit you avoid all of the lofting and cutting. Stick with the proven design.