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48
THE SUBMARINE TORPEDO BOAT
thrust of the water exerts an upward or downward pull
upon them, according to the direction of the inclination
and tends to move the boat bodily up or down with the
axis of the vessel remaining practically horizontal, or in
other words, to cause a vertical movement. The “even
keel” boat is also fitted with the usual diving rudders aft,
but here they are used not for the purpose of diving but
to counteract any tendency of the planes to throw the
vessel from an even keel condition. For this reason they
are called trimming rudders by the advocates of this
system.
However, with the development of dimensions it was
found to be impractical to submerge the vessels of the div-
ing type by means of the stern rudders alone, and these
boats were also fitted with forward diving rudders. The
distinction as a real difference of operation then no longer
exists to any marked degree between the two.
I say real difference of operation when to be more cor-
rect I should say difference of performance, as I mean be-
havior of the vessel itself while submerging rather than
mechanical operation. The actual operation is performed
on the one hand by setting the forward diving rudders to
a certain inclination and by constant operation of the stern
diving rudders, while on the other hand the “even keel”
boats are managed by constant operation of the hydro-
planes in addition to the stern diving rudders.
The contention between the advocates of the two types
seems to have been upon the subject of which method of
submerging was the most compatible with safety.
The handling of the early boats of the diving type seems
to have been attended with some fair degree of safety, but
this was because those boats were small, relatively quick