Page 171 - The Complete Rigger’s Apprentice

properties. It should not be thought that because
the constants for wood are so much higher the masts
will be that much heavier; the constants are scaled
to stiffness in a given material, which is not neces-
sarily the same thing as weight or even size. Alu-
minum masts should work out a little lighter none-
theless, but it is not unusual to find them at least
as heavy as wooden ones, perhaps as an anti-law-
suit safety factor, perhaps because someone ran the
numbers wrong, or perhaps because that was the
section that the spar builder had lying around. Also
notice that the constants assume a keel-stepped
mast and two lower shrouds per side; if a mast
you deal with is deck-stepped or single-shrouded,
adjust accordingly.

Mast Charts Figure 5-27. A mast chart for oval sections. (From
Figure 5-27 shows the final step in mast design: Understanding Rigs and Rigging, by Richard Hen-
choosing a mast cross-section of sufficient stiffness derson, International Marine, 1991)
to stand up to our calculated moments of inertia.
The cross-hatched graph is a “mast chart,” which In practice, these two decisions are set-
precalculates for us the effects of radius and wall tled according to the nature of sailing the boat is
thickness in determining the desired section. We intended for. Cruisers will naturally be inclined to
need only read up from the bottom, which has the go a size up when in doubt, and to opt for a moder-
transverse scale, to intersect a line drawn across from ately large section, if only because it is less expensive
the side, which has the longitudinal scale. Where the than a very thick one. And likewise, racers are likely
lines meet, we can read off our dimensions—roughly to shave away scantlings in the interest of lightness,
5 inches on the transverse axis and 8 inches on the even as they minimize diameter to minimize wind-
longitudinal axis in our example. age, and hang the expense.
A leafing-through-a-catalog-ish alternative is In the case of our cruising cutter, the catalog
to consult a list of the dimensions and moments of mast section that most closely matches our require-
available extrusions, and pick out the one that comes ments has dimensions of 8.06 inches (205 mm) by
closest to our requirements (Figure 5-28). Either 4.88 inches (124 mm) by .180 inch (4.6 mm) thick.
way, we’ll usually be faced with two variables:

1. Since calculated moments rarely coincide WIRE/ROD OPTIONS
precisely with those of available mast sections,
a choice must often be made between sections Design is a search for the appropriate. The above
that are somewhat stiffer and somewhat more bulletproof cruising cutter rig is not the only bullet-
supple than our ideal.
proof cruising cutter rig; it and any number of other
2. Since masts come in varying wall thickness, configurations fit the job description. We could have
we have a range of choices, with a very small- made a single-spreader, deck-stepped, single-shroud
diameter, thick-walled mast at one extreme, design as well. Or we could have made the mast out
and a very fat, thin-walled mast at the other. of wood, which can be custom-made and tapered to

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