Appendices -- Useful Supplemental Information

SPAR DIAMETERS AND TAPERS

Customers making spars for small craft rigs, if not following a sailplan that includes a spar plan, or if inventing their own rig, have occasionally queried me about a source for the proper dimensions and tapers of masts, booms, sprits, gaffs, and gunter and lug yards. I’ve always referred them to a table in Elements of Yacht Design, by Norman L. Skene, 1927 (and later editions revised by Francis Kinney). The table, left, and the numbers repeated below assume spruce. If using Douglas fir, the diameters can be reduced by 10%. The details in the scan of Skene’s page may not be legible, so here are the salient numbers for various small-craft spars, extrapolated in some instances from Skene’s definitions. For example, I suggest his “spinnaker boom,” a spar in compression only, as a surrogate for sprits and sprit booms, which are also in pure compression.

UNSTAYED MASTS FOR JIB-HEADED (TRIANGULAR) SAILS: Diameter (D) at partners to be .0165 (or .0155 for an unballasted boat) x length of mast above partners (L), in inches, tapering to .406 x D at the top. Diameters at four equally-spaced intermediate points to be .942D, .849D, .726D, and .575D. For example, a 120” spruce mast for an unballasted boat would be 1.86” at partners. The top at sheave would be .755”. The four intermediate taper points would be 1.75”, 1.58”, 1.35”, and 1.07”.

MASTS FOR GAFF, GUNTER, LUG OR SPRIT SAILS: Diameter at partners = .0204L, tapering to .99D about 2/3rds up, and then to .65D at top. For a 120” mast the result in spruce would be 2.45” at the partners, 2.42” 2/3rds up, and 1.6” at the top. (For GUNTER AND LUG YARDS, see Chapelle's recommendations below.)

GAFFS: Diameter at midpoint .020L, .93D at jaws, and .75 at tip.

SPRITS’L SPRITS OR SPRIT BOOMS (based on Skene’s spinnaker boom): D =.013L at midpoint, .75D at ends.

BOOMS: Diameter 2/3rds aft .015L, .79D at mast, .71D at boom end.

Howard Chapelle's Yacht Designing and Planning, 1936, gives additional specifications, under a section titled "Laying Out Spars, Diameters." His GUNTER AND LUG YARDS, for example, are to be greatest diameter .015 x length over all, .73D at the ends. Chapelle’s recommendations differ slightly from Skene’s at several points, and it might pay to check them as a cross reference.

Where these guidelines differ from a designer’s spar plan, you should of course ignore them. Results obtained using these guidelines ought to be checked by comparison with some similar craft or rig on the waterfront, or in a standard reference like Chapelle’s American Small Sailing Craft.



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