When he needed to lay out smooth arcs on the Adirondack chair on page 74 of issue 149, WOOD® magazine Master Craftsman Chuck Hedlund turned to his shop-made fairing stick. Chuck's version, shown above at left, features an adjustable cord with a sliding "toggle" that locks in the desired arc for hassle-free use.
To make your own, start with a 3/4"-wide piece of 1/8" tempered hardboard. The length is up to you; but at 24", this one handles most layout chores. Also cut a piece to size for the toggle. Now drill the four 1/8" holes, as dimensioned, through the ends of both pieces.
Next, thread a length of #18 nylon mason's cord (ours measured 38"), following the arrows in the drawing below. The cord gets tied to one end of the fairing stick, then goes through the holes in the toggle, loops through the other end of the fairing stick, and ties back to the toggle.
To use the fairing stick, start by figuring out the endpoints and midpoint of the arc you want to create. Here's where you'll appreciate Chuck's toggle device. Instead of using clamps or nails to hold the ends of the stick in place, just slide the toggle to flex the stick until it matches your desired arc. Friction locks the toggle in place, retaining the correct shape. Now align the stick on your workpiece and trace. If you have multiple pieces to mark, you can pick up the stick and move it without losing your setting.
When you're not using the fairing stick, slide the toggle to release tension on the stick. That minimizes any "memory" setting in. If this happens, just adjust the cord and flex the stick in the opposite direction.
Also, if you need a fairing stick greater than 3' long, increase the stick's width to about 1-1/2" to keep it from twisting sideways under tension. For a really long stick, switch to 1/4"-thick hardboard.