The hardest class I
ever took was based on the book Stability and Trim for the Ship’s
Officer, written by U.S. Merchant Marine Academy instructors John
Ladage and Lee van Gemert. It was a deceptively trim little volume but
so dense and difficult that several of my fellow deck cadets stumbled
on one prolix, trigonometrical chapter or another and flunked out. Besides
being a stumbling block, the book was also an intensely practical exegesis
of seafaring stability, or “the tendency of a vessel to return to
its original position after it has been inclined due to external forces.”
Thanks be to merciful providence, I’ve had virtually nothing to do
with the terrible little tome for years, until just recently when the
subject of stability came up during a test of a salty, Australian-built
passagemaker called the Great Southern 76.
Great Southern? By naming his globe-trotting, ABS-certified, full-displacement
design for one of the roughest, most contentious oceans in the world,
Aussie naval architect Gavin Mair underscores his concern for one of the
most critical safety issues on the high seas: a vessel’s transverse
stability, specifically its GM or Metacentric Height. Unfamiliar term?
A boat with a large GM is typically “stiff,” with weight concentrated
low, and evinces a “snap roll” or excessive tendency to return
to her original position when inclined. A boat with a small GM is typically
“tender,” with weight concentrated high, and evinces a weak
tendency to return to her original position. The trick, of course, is
to create a vessel that comfortably splits the difference and gives you
an easy, safe roll, a task that entails way more weight analyses and computer
modeling than lots of naval architects are willing to perform.
But Mair says he’s done the deed with the Great Southern. In fact,
he claims that thanks to her GM of six feet (versus three feet for a top-heavy
fishing trawler with booms and A-frame or a whopping 15 feet for an exceptionally
stiff 50-foot powercat) the stout little ship is approximately twice as
stable as most commercial fishing vessels he’s designed for Australia’s
Great Southern Ocean. The point, he says, is to trade cargo-carrying capacity
and speed for improved roll characteristics and an increased capsize angle.
Families making passages across oceans don’t need to stow tons of
fish below decks, are seldom compelled to equate time with money, and
rarely have the heavy-weather expertise of their commercial counterparts.
So comfort, stability, and safety take precedence.
Mair’s basic strategy is simple. Borrowing from ship design, he puts
the Great Southern’s considerable fuel, water, and waste tankage
as low as possible, in double-bottom tanks. Additionally, by means of
the decreased shaft angle that a canoe counter-type stern engenders, he
can lower the engines as well. Moreover, the superstructure from the surface
of the weatherdeck up is constructed of lightweight, fiberglass-Kevlar
composite. All these things combine to lower the vertical center of gravity,
optimize roll, and produce a more stable ride, especially when teamed
up with other features like Wesmar fin-type stabilizers (with main and
back-up hydraulic power supplies); fixed, trapezoidal, and slightly up-angled
“antipitch” fins at the bow; and a carefully balanced, voluptuously curved,
slack-bilged steel hull.
Next page >
Great Southern 76 continued > Page 1, 2,
3, 4, 5