The width of the bottom at the transom states 8.5 inches. This is incorrect.
It should be 18 1/2 inches.

When comparing the width's on drawing in JonBoat2.jpg and the width of the 
bottom on drawing in JonBoat4.jpg the plywood widths just do not add up. 

As to not waste sheet of plywood, you should loft this plan onto cardboard and 
test fit it together before cutting your wood.