Page 170 - The Complete Rigger’s Apprentice

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section charts which take these factors into account What the heck is 12.71 in. ? It’s just 127,100
without further calculation. reduced to a more compact form by those “10,000”
Let’s go shopping for a mast section. Assume divisors in the formula. In either form, it’s our trans-
the stick on this boat is 42 feet above deck, and that verse moment of inertia.
the lower spreaders are 17 feet 7 inches above deck. The formula for the fore-and-aft or longitudi-
That’s about 42 percent of the total exposed length, nal moment of inertia is much the same, except for
a number that lends itself to wholesome propor- a different, smaller constant. It’s smaller because
tions, so that no part of the rig will take a dispro- the unsupported length is assumed to be the entire
portionate load. If the lower spreaders were higher exposed length of the mast. I is our symbol for
LL
up, the lowest section of the mast would have to be longitudinal inertia. So:
much stiffer—longer unsupported length. Since the
entire mast is scaled to this section, that would make
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the higher, shorter sections far too stiff and heavy. I (in. 4 ) = C f.a. 5 L (in. 2 ) 5 Load
LL
Conversely, if the lower spreaders were a lot lower, 10,000 10,000
we wouldn’t need as heavy a section down low, but Where C = fore-and-aft constant
the upper and now longer sections would not be I = length, deck to jibstay
LL
heavy enough to support their loads. So somewhere Load = RM 30 compression load.
around 40 percent of exposed length is a good loca-
tion for lower spreaders. For single spreaders, about Accordingly,
55 percent of the way up is a good location, for the I (in. ) =
4
same reasons. LL 5 254,016 5 25,272.7
In any event, 17 feet 7 inches is 211 inches. .54 = 34.67 in. 4
10,000 10,000
2
Square that, and we have an L of 44,521. Our
T
load from the rigging formula at RM 30 is:
As you’ll note in the constants table (Figure
RM 30 5 2.78 5-26), there are different figures for wood and alu-
1
⁄2 beam = 25,272.7 pounds minum masts, taking into account their different
Our constant, from the chart in Figure 5-26, is Figure 5-26. Values for the constant “C” in the formu-
1.13 for an aluminum mast, keel-stepped. las for moments of inertia about the transverse and
Plugging into our formula, then, longitudinal axis of a mast. These values assume a

I (in. ) = keel-stepped mast. Deck-stepped masts require values
4
tt
44,521 25,272.7 for the constant perhaps 50 percent greater, which
1.13 5 5 =12.71 in. 4 can be reduced to 20 to 30 percent if a big tabernacle
10,000 10,000
is present to provide partner-like support. (Adapted
from Skene’s Elements of Yacht Design, 8th Edition,
by Francis Kinney. Dodd, Mead, 1981)

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