When the sea is rough, and the screw leaves the water as a consequence
of the ship's motions, the rotary velocity of the screw and engine
increases to a dangerous degree, because the resistance that the screw
was meeting in the water suddenly disappears. When the screw enters the
water again, the resistance makes itself abruptly felt, and causes
powerful shocks, which put both the screw and engine in danger. Ordinary
regulators are powerless to overcome this trouble, since their
construction is such that they act upon the engine only when the excess
of velocity has already been reached.
Several remedies have been proposed for this danger. For example, use
has been made of a float placed in a channel at the side of the screw,
and which closes the moderator valve by mechanical means or by
electricity when the screw descends too low or rises too high.
[Illustration: BROWN'S ELECTRIC SPEED REGULATOR.]
Mr. Brown's system is based upon a new idea. The apparatus (see figure)
consists of two contacts connected by an electric circuit. One of them,
b, is fixed to the ship in such a way as to be constantly in the water,
while the other, a, corresponds to the position above which the screw
cannot rise without taking on a dangerous velocity. In the normal
situation of the ship, the electric circuit, c (in which circulates a
current produced by a dynamo, d), is closed through the intermedium of
the water, which establishes a connection between the two contacts. When
the contact, a, rises out of the water, the current is interrupted. The
electro, d, then frees its armature, f, and the latter is pulled back by
a spring--a motion that sets in action a small steam engine that closes
the moderator valve. When the contact, a, is again immersed, the
electro, e, attracts its armature, and thus brings the moderator valve
back to its normal position. It is clear that the contact, a, must be
insulated from the ship's side.
Several contacts, a, might be advantageously arranged one above another,
in order to close the moderator valve more or less, according to the
extent of the screw's rise or fall.
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