Boat Planform Shape

Before you get your tools and lumber out you should get a general idea what your boat's shape will be. This is the first step of rack of eye. This doesn't all come at once. I got my look from Google Map and looking down on some boat images from Smith Island. If you study these, you'll see that most of the boats follow the "Law of Thirds" meaning they are generally 1/3 as wide as they are as long ... and they the widest part of the boat comes 1/3 forward of the transom. Again, this is a generality and you can adjust it to fit your need. But this is a good starting point.

Setback: is the width at the transom. This can range from 0 (square stern) to several inches less than the Widest Beam. Only a few inches of setback will give the boat a graceful line. Too much setback will likely make the boat squat when engine power is applied.


But at this early point I'm sitting with some important information. I know how long and wide my boat will be, and I also know how wide my transom will be.

For you: If you want to build a 15-ft LOA boat, your widest beam will be 15*(1/3) or 5-ft.

My example: My boat needed to be 8-ft wide to get out of the garage, so that calculates out to 8*3 = 24 ft. But I wanted a little more beamy boat because the duck hunter need stability. so I made the LOA 22-ft. Again, this was just my preference.