268

N1C1IOLLS S SEAMANSHIP AND NAUTICAL KNOWLEDGE

On board ship, however, it is the apparent wind we experience
and it is the true wind that should be recorded in the log book A
fairly good approximation of the direction of the true wind may be
obtained from the sea as the wind blows at right angles to the normal
wave crest, but our parallelogram may be helpful also.

Example.—The apparent wjnd is from East with a velocity estimated
at 20 m p.h.; the ship steaming North at 10 m p h. Find the direction

and velocity of the true wind.

Make AB 10 miles due North, and
AC 20 miles due East. Join CB and
complete the parallelogram if desired.
BC represents the direction and velocity
of the true wind S. 63° E, 22£ m.p.h.
This, of course, is the wind which
would be experienced if the ship were
stopped because it is the North-going

Flg 1T^eASnSt^nIEaSt' sPeed of the sbP Which makes the

apparent wind come from a direction

more forward than the true wind.

Boat Sailing.—The principle of sail propulsion may be illustrated
by .a parallelogram of forces, as in the figure, where XY represents the
plane surface of the sail and A the
centre of effort, the name given to
the point where the whole force of
the wind is concentrated. The wind
cannot go through the sail and the angle
of its incidence with che sail at A} angle
XAP is equal to its angle of deflection,
angle YAQ: the impelling force RAt&cts
along the line AB which is the bisector
of angle PAQ and is always at right
angles to the plane surface of the sail.

If this force were equal to a wind
velocity of 10 m.p.h., then make AB equal to 10 parts from a con-
venient scale, and make BC perpendicular to the ship's fore-and-aft
line drawn through A and we then have a triangle of forces The
parallelogram ADBC may be completed if desired.

AB represents the direction the boat would go if she were a free
agent; AD represents that part of the wind force which pushes the
boat to leeward and AC the part that pushes her ahead. The resistance

11 —Sail Propulsion