For a vessel with a given displacement, the metacenter and center of
gravity being known, it is easy to lay off in the form of a diagram
its stability or power of righting for any given angle of heel. Such a
diagram is shown in Fig. 3, in which the abscissæ are the angles of the
heel, and the ordinates the various lengths of the levers, at the end
of which the whole weight of the vessel is acting to right itself.
The curve may be constructed in the following manner: Having found by
calculation the position of the transverse metacenter, M, for a given
displacement--Figs. 1 and 2--the metacentric height, G M, is then
determined either by calculations, or more correctly by experiment, by
varying the position of weights of known magnitude, or by the stability
indicator itself. Suppose, now, the vessel to be listed over to various
angles of heel--say 20 deg., 40 deg., 60 deg., and 80 deg.--the water
lines will then be A C, D E, F K, and H J respectively, and the centers
of buoyancy, which must be found by calculation, will be B1, B2, B3, and
B4. If lines are drawn from these points at right angles to the water
levels at the respective heels, the righting power of the vessel in each
position is found by taking the perpendicular distances between these
lines and the center of gravity, G. This method of construction is shown
to an enlarged scale in Fig. 2, where G is the center of gravity, B1
Z1, B2 Z2, B3 Z3, and B4 Z4 the lines from centers of buoyancy to water
levels; and G N, G O, and G P the distances showing the righting power
at the angles of 20 deg., 40 deg., and 60 deg. respectively, and which
to any convenient scale are set off as the ordinates in the stability
curve shown in Fig 3.
[Illustration: STABILITY INDICATOR FOR SHIPS. Fig. 1.]
Having obtained the curve, A, in this manner for a given metacentric
height, we will suppose that on the next voyage, with the same
displacement, it is found that, owing to some difference in stowage,
the center of gravity is 6 in. higher than before. The ordinates of the
curve will then be G¹ N¹ and G¹ O¹--Fig.2--and the stability curve will
be as at C--Fig. 3--showing that at about 47 deg. all righting power
ceases. Similarly, if the center of gravity is lowered 6 in. on the
same displacement, the curve, B, will be found, and in this manner
comparative diagrams can be constructed giving at a glance the stability
of a vessel for any given draught of water and metacentric height.
[Illustration: STABILITY INDICATOR FOR SHIPS. Fig. 2.]
[Illustration: STABILITY INDICATOR FOR SHIPS. Fig. 3.]
The object of Mr. Alexander Taylor's indicator is to measure and show
by simple inspection the metacentric height under every condition of
loading, and therefore to make known the stability of the vessel. It
consists of a small reservoir, A, Fig. 4, placed at one side of the
ship, in the cabin, or other convenient locality, communicating by a
tube with the glass gauge, B, secured at the opposite side, the whole
being half filled with glycerine, which is the fluid recommended by Mr.
Wm. Denny, though water or any other liquid will answer the purpose.
At one side of the gauge is the circular scale, C, capable of being
revolved round its vertical axis, as well as adjusted up and down, so
as to bring the zero pointer exactly to the top of the fluid when the
vessel is without list. Round the top of the scale, at D, are engraved
four different draughts, and under these are the metacentric heights.
Test tanks of known capacity are placed at each side of the vessel, but
in no way connected with the reservoir or gauge. The metacentric height
is found as follows: The ship being freed from bilge water, the roller
scale is turned round to bring to the front the mark corresponding with
the mean draught of the vessel at the time, and the zero pointer is
placed opposite the surface of the liquid in the gauge. One of the test
tanks being filled with a known weight of water, the vessel is caused
to list, and in consequence the liquid in the tube takes a new position
corresponding with the degree of heel, the disturbance being greater
according as the vessel has been more or less overbalanced. The scale
having previously been properly graduated, the metacentric height for
the draught and state of loading can be at once read off in inches,
while as a check the water can be transferred from the one test tank to
the other, and the metacentric height read off as before, but on the
opposite side of the zero pointer. At the same time the angle of heel is
shown on a second graduated scale, E. Having obtained the metacentric
height, reference to a diagram will at once show the whole range of
stability; and this being ascertained at each loading, the stowage of
the cargo can be so adjusted as to avoid excessive stiffness in the one
hand and dangerous tenderness on the other. It will thus be seen that
Mr. Taylor's invention promises to be of great practical value both in
the hands of the ship-builder and ship-owner, who have now an instrument
placed before them, by the proper use of which all danger from
unskillful loading can be entirely avoided.--_The Engineer_.
[Illustration: STABILITY INDICATOR FOR SHIPS. Fig. 4.]
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