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6o
THE SUBMARINE TORPEDO BOAT
is essentially the form for under-water navigation, having
greater stability in that condition and offering less resist-
ance to propulsion than the submersible.
Summing up, the question of type then resolves itself
into the question of whether a maximum surface or sub-
merged speed is sought.
At present it has become the tendency to adopt the lines
of the torpedo boat to make for less surface resistance and
increased speed. This has necessitated a double hull
construction, the outer hull having the torpedo boat lines
and the inner hull being of circular cross section to resist
the pressure of submersion. The speed gained in this con-
struction has however been shown in practice to be very
inconsiderable and it is questionable whether it justifies
the necessary extra expense of construction. As subma-
rines can never attain the speed of which a torpedo boat
is capable is it probable that better results may be gained
by a compromise of the single hull form by effecting a de-
sign having a full entrance and a long fine run. This
would afford the least possible resistance and retain at
the same time all the advantages of the single hull con-
struction.
As the displacement calculations are similar to those of
ordinary ships and are familiar to all engaged in the prac-
tice of naval architecture or marine engineering they will
not be gone into here.
Stability
It is important with respect to the stability of the vessel
that the center of gravity of the hull weights be kept as
low as possible. The significance of this may be seen when
it is considered that this factor is about thirty-five per cent