d upon that the boat would be still harder to
upset and it is by such methods that the natives of the South Seas sail
their catamarans and proas at terrific speed and with huge sails out of
all proportion to the hulls. Sometimes one may see a “flying proa”
tearing along in a perfect gale with half a dozen persons hanging on to
the slender _outrigger_ extending from the weather side, and by their
weight alone preventing the queer craft from turning turtle.

All the above remarks refer to stability, but there is another factor
which must be considered and which is known as _lateral resistance_, or
in other words, the resistance offered to the water when moving
sideways. A boat might be very stable and yet it might be worthless if
it did not possess lateral resistance, for in that case it would merely
slide sideways instead of going ahead and a properly designed boat must
combine both stability and lateral resistance to the highest possible
degree.

When sailing in any direction, save directly before the wind, there is a
strong sideways pressure against the sails and unless the boat is
provided with some means of overcoming this she will slip sideways or
diagonally or will make “leeway,” as a sailor would say. Deep, narrow
boats have great lateral resistance but their resistance to the water
when moving forward is also great and hence the lateral resistance is
usually obtained by means of deep, narrow keels, centerboard or
leeboards. The knife-like keel offers little resistance to the water
when moving forward but great resistance when moving sideways, while the
centerboard may be pulled up entirely when moving forward with a wind
from the rear, thus still further reducing the friction against the
water.

If the boat possesses stability and lateral resistance and is properly
rigged the wind blowing against the sails will have a tendency to force
the stern of the craft away from the wind and the bow towards it. To
overcome this the rudder must be turned until the pressure of the water
against it has enough fo