How do I taper dowels for masts and spars?

First of all, try to avoid using dowels - they almost invariably contain cross-grained portions, making the result susceptible to warping. You're better off starting with straight-grained square stock of the proper size (and if you can split the stock to rough size, you can guarantee that the grain is straight; it's also something to use already-split wood for).

You can turn spars using a lathe, but I find that it's more trouble than its worth – long thin stock tends to 'whip' when being turned. Besides, most spars are not round – they often have square, hexagonal, or octagonal sections for at least some part of their length!

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You can easily shape spars using no tool other than a plane, but a simple fixture makes it easier to hold the stock on edge while removing the corners.

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To make the fixture, take a couple of boards (as long as your longest spar, a couple of inches wide, and 1/4 inch thick are convenient); they can be plywood or hardwood (but avoid balsa or other soft woods).

Saw out one as shown at B. – a short piece (B1) off the end (for use as a stop), and cut the remainder in half (B2 and B3).

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Chamfer one edge of each of the long pieces at a 45 degree angle.

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Glue the B pieces together on top of the A piece as shown; the resulting fixture can be placed on the workbench and used to steady the work.

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An additional piece of wood glued to the bottom front converts the jig into a "bench hook"; you can apply this to the edge of the workbench to help hold the fixture while in use.

To shape a spar, first reduce the square stock to the maximum size.

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Draw the taper required and remove that stock; taper both sides equally rather than just taking all the stock off one side.

Turn through 90 degrees and taper the remaining two sides.

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Next, we want to convert the square cross section to an octagon - place the tapered stock into the fixture and remove one corner. Don't remove stock if that part is supposed to be square (e.g., the inboard end of the bowsprit is often square in section).

Rotate through 90 degrees and remove a second corner. Continue until all 8 sides are equal in width. (The fixture may not be needed for the last two corners, but it makes the first two corners much easier to work).

Remove all of the octagon's corners to make the spar 16-sided (unless it's supposed to be octagonal; the center portion of yards often were.)

A light sanding should now result in a round cross-section.
{John O. Kopf}

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