Page 168 - The Complete Rigger’s Apprentice

End Fixity But there are other ways to increase to the mast wall. With a round or oval mast this neu-
stiffness without adding mass. Try another noodle tral axis, around which all the forces are balanced, is
demo to demonstrate one of them: Take two long in the center of the mast. It can take careful calcula-
pieces of spaghetti. Break a couple of inches off tion to find this axis with exotic mast cross-sections,
one. Press down on this shorter one and notice how but the square-of-the-radius formula holds.
much pressure it takes to bend it. Notice also that This brings up the reason why most masts are
it bends in the middle. Now take the longer piece oval instead of round: Because shrouds attach at
and pinch it a couple of inches from the bottom. more points than stays, masts are generally sup-
Press. Even though the unsupported length above ported better laterally than they are fore-and-aft.
the pinch point is the same as the short piece, this You tend to get short, buckle-resistant lengths on
piece is stiffer. And instead of bending in the mid- the sides, and long, buckle-prone lengths on the
dle of its unsupported length, it bends closer to the front and back. Even on masts with multiple head-
upper hand, where spreaders would be on a mast. stays and running backs, induced mast bend can put
What you’ve just seen is a demonstration of the huge, let’s-see-if-we-can-fold-this-thing loads on a
effects of “End Fixity.” The short piece is said to stick. Therefore masts need to be stiffer fore-and-aft
have two “pin ends,” while the long piece has “one than they are laterally. The simplest way to accom-
pin end and one fixed end.” A boat’s partners do the plish this is to make a mast with the most heavily
pinching, so a keel-stepped mast will be stiffer than stressed sides farthest from the neutral axis. This
a deck-stepped mast of the same exposed length. A makes the mast stiffer in one plane than the other.
very stout tabernacle can add some degree of end A rectangular cross-section works well, but the mass
fixity, but if you’re interested in a light mast, keel- in the corners is pretty much wasted, adding mostly
stepped is the only way to go. lateral stiffness. An oval offers less wind-resistance
Why, then, are so many masts deck-stepped? A and less weight, so that is the most common mast
classic reason is that they’re easier to put up and shape. The main exception to this these days is the
down for traversing canals, passing under bridges, unstayed mast; it needs to be round, because there
or trailering. The convenience compensates for are no shrouds to stiffen it laterally. For more on
added weight. (See the discussion on Sojourner comparisons between stayed and unstayed masts,
Truth on page 303.) see the Mast Bend sidebar in Chapter 8.
But economics are also an issue, since it’s cheaper
to step a mast on deck than to cut a hole in the deck, Wall Thickness
and the sole, and install a mast step, mast collar, etc. Mast stiffness also varies directly with the thickness
And even the arrangement of the interior can of the mast walls. No free ride here; if you want to
be an issue. I once knew some owners who hired stiffen by thickening, you pay a price in weight.
an interior designer to redo their boat. The designer Some boats have masts that are “sleeved” with dou-
came below, took one look at the base of the mast ble wall sections over heavily stressed areas. This
that occupied a large portion of the saloon, and said, saves making the entire mast thicker-walled. But
“First of all, that has to go.” And go it did, to make more often, thick walls are resorted to when other
possible an arrangement of staterooms, sofas, and an stiffening methods have drawbacks.
entertainment center. For details of the consequences For instance, racing sailors will usually select a
of this simple act, see the description on page 169 very narrow mast for low wind resistance. They’ll
compensate for the loss of square-of-radius stiff-
Radius ness by making the mast very thick and by short-
There’s one other “weightless” means to mast stiff- ening the unsupported lengths with three, four, or
ness: making the mast fatter. Stiffness varies with the even five sets of spreaders. And they’ll accept a low
square of the distance from the neutral axis of a mast safety factor.

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