Environmental Engineering Reference
In-Depth Information
Animals were used in waterborne transport from antiq-
uity. As already noted, oxen hauled barges from Ostia to
Rome, and Horace in his Satires describes a journey on a
canal boat pulled by a mule. Their use increased with the
construction of major canals, first in China, where
the Grand Canal, begun during the Han dynasty, pro-
moted economic development in the country's core eco-
nomic areas (Chi 1936; Needham 1971), and much later
in Europe and North America. Chinese canal boats were
pulled by gangs of laborers or by oxen or buffalo. In Eu-
rope, where canals reached their greatest importance
during the eighteenth and nineteenth centuries, horses
were the main prime movers. They walked on tow paths,
pulling the barges at speeds of about 3 km/h when
loaded, and on a well-designed canal a single heavy horse
could pull a load of 30-50 t, 1 OM more than could be
managed on the best hard road. Steam engines gradually
replaced barge-towing animals, but many horses still
worked on smaller canals during the 1890s (Hadfield
1986).
All preindustrial seaborne transport of goods was
dominated by sailing ships, machines to convert the
wind's kinetic energy into forward motion through the
use fabric airfoils (sails) and stable and steerable hulls
(Van Loon 1935; Torr 1964; Chatterson 1977; Block
2003). The pressure differences generate lift and drag
forces acting on the sail (Marchaj 2000). With wind
astern, lift far surpasses drag and propels the ship; with
wind on the beam or slightly ahead of it, the force push-
ing the vessel sideways is stronger than the force propel-
ling it forward. If the ship were to steer even closer to the
wind, the drag would pushed it backward. The oldest
vessels, with square sails set at right angles across the
ship's long axis, proceeded fast only with wind directly
astern or less than 30
off course. The only way to cope
with this limitation was to keep changing the course by
resorting to wearing (making a complete downwind
turn). Consequently, Roman ships pushed by the north-
westerlies could sail from Messina to Alexandria in just
six days, but the return could take ten times as long.
Duncan-Jones (1990) argues that it is almost impossible
to say what speeds were typical.
Medieval square-rigged ships could proceed slowly
with the wind on their beam (90
), and their post-
Renaissance successors could move at an angle of about
80
into the wind. The canted square sail appeared in
the Indian Ocean during the third century
B
.
C
.
E
. This
was a clear precursor of triangular (lateen) sails of the
Arab world after the seventh century. In Europe only
the late medieval combination of square rigging and tri-
angular sails made sailing close to the wind possible.
Gradually, these ships were rigged with a larger number
of loftier and better adjustable sails. Ships with fore-and-
aft sails could tack, turning their bows into the wind and
catching it on the opposite side of the sail. Ships that
combined square sails with triangular mizzens could
manage 62
, and fore-and-aft rigs (including triangular,
lug, sprit, and gaff sails) could come as close as 45
to
the wind (modern yachts come close to the aerodynamic
limit of 30
). Capabilities for sailing into the wind have
thus advanced by more than 100
.
Better sails, deeper hulls, the stern-post rudder (a Chi-
nese invention), and the magnetic compass (in China
after 850, in Europe around 1200) combined to make
ships into more efficient wind converters. The simplest
vessels of this improved design began the great European
voyages. Portuguese sailors advanced first along the west-
ern coast of Africa (Senegal in 1444, the equator in
1472, to Angola in 1486). In 1492, Columbus crossed
the Atlantic; in 1497, Vasco da Gama rounded the Cape
