510 NICHOLAS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE
necessary for drawing such curves and it is the duty of the ship's
officer to be able to read them intelligently.
We shall illustrate five curves which are associated with the loading
of a ship, viz., curves of displacement, tons per inch immersion, stability,
buoyancy and metacentre.
Displacement and Tons per Inch Curves.—The scale of feet at the
side of Figure 31 is a scale of draughts and against each even foot is
the displacement (weight of ship+bunkers and cargo) given in tons, the
displacement in F.W. is given to the left of the draught scale and for
B.W. to the right.
The light load line is 8 feet and at this draught the displacement
of the ship is 3500 tons in F.W. and 3600 tons in S.W.
The load line draught is 20 feet in S.W. and the corresponding
displacement is 10,100 tons. The deadweight or carrying power of the
ship is (10,100—3600)=6500 tons when loading in S.W. The tonnage
at intermediate draughts can be found by simple proportion, but ship
draughtsmen dearly love a curve although the two shown here are
nearly straight.
The vertical lines (ordinates) represent draughts, the horizontal
lines (abscissae) indicate a scale of displacement at the bottom and a
scale of tons per inch immersion at the top, the two curves having been
drawn on the same sectional paper for convenience.
Example.—Required (a) the displacement and (6) the tons per inch
at 12 feet draught.
(a) Find 12 feet on the draught scale and trace along the horizontal
line until the displacement curve is reached, then move 'vertically
downwards to the bottom scale and read the displacement in tons.
It is a little less than 5600 tons, say about 5570 tons.
(&) Continue along the 12 feet line to the T.P.I, curve, then move
vertically upwards and read the T.P.I, whiqh is a little more than 42J,
say 43 tons per inch.
Example.—Required the displacement and tons per inch at a draught
of 17 ft. 6 ins.
Answer.—Displacement 8600 tons. T.PJ. 48 tons.
Curves seldom give results as accurate as the information they are
derived from, and usually on board ship any information desired ia
read from the scale, interpolating between the even feet when necessary.