Wednesday, February 19, 2014

Tapered Spars?

Good question… and another post prompted by a comment. Thanks, Ed.

The Pathfinder plans specify extruded aluminum for spars so there is no taper. But John Welsford states in the plans that wooden spars will work/are acceptable if the wooden spars are 5% larger in diameter than the aluminum spars specified in the plans.

When John drew up a balance lug yawl sail plan for me, he specified a 19'10" wooden main mast with a diameter of 80mm. The mast tapers from 80mm at 5'10" from the foot of the mast to 58mm at the top. He recommended using staves of at least 16mm if using the birdsmouth construction method.

This link provides a lot of information and calculators re birdsmouth construction. About half way down the page is a paragraph and illustration dealing with tapering.

If I am reading that information/illustration correctly, the taper can be cut after the V-notch and before assembly. My thought is to set up a jig and use a flush cut trim bit in the router to cut the taper. I suspect I will experiment with a test sample before dealing with 20+ feet of CVG Douglas fir…

The mizzen mast for the balance lug yawl plan is "as per (original) yawl rig" - in other words, extruded aluminum. I plan to build a wooden mizzen mast using the birdsmouth method but I haven't figured out the taper and I haven't asked John for his recommendation either (I may do that soon). The main boom and yard offer opportunities to test the milling and assembly sequences for longer spars. I already have a main boom built so I may just experiment - including tapers and, possibly an oval cross-section - with the yard.


  1. To cut my tapers, I used a router with a flush trim bit and a straight edge (the edge of a sheet of plywood makes a nice 8' long straight edge).

  2. Joel, Thanks for the tip and the link. I'll share your "Redneck lathe" with my BIL, who just be game to try this set-up. Worth a shot, anyway.