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44
THE SUBMARINE TORPEDO BOAT
gravity lowered, one by the increase of displacement due
to the emerged volume, and the other by the added weight
of ballast taken into the tanks in the lower portion of the
hull. Therefore, B.G. becomes greater and the stability
is consequently increased.
Now in the double hull ship form of construction, it is
evident that in trimming, the position of the center of
buoyancy must be raised and the center of gravity lowered,
because the stability of the vessel under water cannot de-
pend upon form or inertia, but must rather depend upon
the principle analogous to that of the suspension of a
weight from a supporting element, therefore C.G. must
pass and take a position below C.B. The double hull or
wide superstructure owing to its shape cannot be con-
structed sufficiently strong to withstand much pressure,
so must be filled with water when submerged as is the
open superstructure of the submarine. This fact brings
the C.B. of the submersible back to practically the same
position of C.B. in the single hull type, but the position of
C.G. is much higher in the double hull boat owing to the
relatively high position of the center of gravity of its hull
weights due to the large superstructure. G.B. then, and
consequently the stability, is less in this type when sub-
merged than in the single hull or submarine type of
construction.
To illustrate the foregoing a set of stability curves has
been prepared, Figure i, for both types of boats, each
reduced to the same total displacement submerged for
fair comparison.
In summing up it may be well to point out here the rela-
tive tactical values effected by these rival types.
The ship form boat is evidently better suited to high