To make sure that the boat keeps its shape you need to install to thwarts a little less than 1/3 of the distance from each end.
I measured along each side 32 inches from the stem and stern and made a tick mark (The tick mark is vissible in the first picture of step #15). Then measure across the boat at those points. Mine was about 24 inches. I then layed the thwart acorss the Beam (Top) so that they over hang just slightlyand traced the curve, at both ends of the thwart. Cut this on the Band saw. Next turn the thwart on its edge and mark a line that goes right down the center as in the second picture below (I made the line very dark so that it would show up in the picture). Being very careful resaw the stock on the Band saw, It will end up being about 5/8ths of an inch thick. It is much easier to sand this before you install them.