April 13, 2009

Build a Dinghy: Part 8

Whether its a go-fast, a sportfisherman or a little plywood boat like this Summerbreeze I'm building, when you build light, you have to build stiff. Stiffness is what maintains the hull's shape. Here, stiffness is acheived largely by the use of rails cut from dimensional lumber. I'll need to cut Gunwales, inwales, a keel ( shoe) and chine stringers.
I selected a 1" x 12" x 12-foot long piece of vertical grain Doug Fir for these parts. Douglas fir is strong for its weight and glues better than oak. The vertical grain insures better bendability and increased reliability. It has less chance of cracking or splitting in service because with such straight grain I can cut it so that the load paths all run along the grain. If the grain were wavy, the forces exerted on the parts in service would find a "way out" following the grain to the edge of the board and cracking it. Hey, buying a board like this is no guarantee, it just ups the odds. Nor is it cheap. I paid 60 bucks for it.
Of course I'll get my two chines, two rubrails, and my keel shoe from this one board. I'm going to use pine for the inwales, as I have a nice length of it. Plus, the contrast with the fir, which is a tad darker than pine, will look good when all is varnished.
Notice how I'm using one piece of plywood set up on horses as the bench for this boat. In the background is my bottom, glued up and waiting to be trimmed to shape. In the foreground is the board I'm cutting my rails from. I use a circular saw set to the design bevels and run it along a piece of aluminum channel as a fence to keep me cutting straight. The board is clamped, the channel is tacked to the work station and my cutting lines are heavily marked.

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