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!
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).
To shape a spar, first reduce the square stock to the maximum size.
Turn through 90 degrees and taper the remaining two sides.
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.
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