Page 146 - The Complete Rigger’s Apprentice

than the wire, is that there is now a compression sponding formulas, we can calculate moments with-
load on the vertical member (mast); the wire exerts out having to set up a real mast and attach tension
a downward pull of, in this case, 111.8 pounds. gauges.
The closer the lower end of the wire is to the mast, Figure 5-5 is a graph that represents the results
the greater the proportion of effort it expends in a of calculations to determine wire strain at a vari-
downward pull, and the more tension is exerted on ety of staying angles (the angle of the wire relative
it (Figure 5-5). In every instance, the wire must be to the mast). As you can see, the amount of ten-
strong enough to deal with the moment produced sion accelerates as the angle narrows, approaching
by a force exerted at the top of the mast, and the infinity as it approaches vertical. Since sailboats are
mast must be strong enough not to buckle under usually narrow things with tall masts, and since
the imposed compression load. In addition, the wire neither wires nor masts can take loads approaching
now exerts a horizontal compression at its base. On infinity, the angle-to-load relationship is central to
a boat, that base is the deck, so we need to be sure rig design. Notice that the greatest acceleration of
that the deck is strong enough to take that load. tension comes in the range of 0 to 12 degrees. If the
A very great thing is that all these forces can be shroud angle is 12 degrees or greater, it can more
resolved into components having defined direction easily resist the pressure of wind on sails without
and magnitude relative to one another—we can fig- generating undue strain. But can this be done? Rigs
ure out how much load is landing where. Using lines are tall and narrow to allow the greatest length of
of proportionate angles and lengths, or the corre- luff and closest headsail sheeting angles.

Figure 5-5. The curve shows tension on a shroud Figure 5-6. The closer the lower end of the wire is to
induced by a 50-pound lateral load at the masthead, the mast, the higher the compression load on the mast
given various shroud-to-mast angles and the tension on the wire.

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