Page 248 - NAVAL SCIENCE 3 TEXTBOOK

254 NAVAL SKIllS

A (ombillatiOi/ mine, detonated by the simultaneous actuation
of two or all three of the foregoing types of firing mechanisms, is
SHIP'S PRESSURE SIGNATURE A. more eA-ective beciluse it is less susceptible to activation by fitlse
,
H : targets ilnd harder to sweep. In order for stich a mine to detonate.
for example, the pressure influence of a ship mar have to close a
switch at the same time as its magnetic field influences a coil. The
U.S. arsenal contains mines detonated br simultaneolls actua-
tion of all three mechanisms. Such mines are almost impossible
to sweep.
In addition to the combination of various influence mecha-
nisms, "counters" have been installed in some mines. These are
designed to cause the mine to remain inert until the actuation pro-
cess has occurred a preset number of times. This is intended to
gi\'C the enemy a false sense of security. by setting up the mines for
activ<1tion after minesweeping operations have been "successfully
concluded" without mishap.
Perhaps the most unusual "mine" in the Navy arsenal is not
reall), a mine at all. Called the CAPTOR (encapsulated torpedo),
it consists of an acoustic homing torpedo moored to the bottom
with about 300 meters ( fcet) of cable. Primarily a US\-\' weapon,
A pressure mine is detonated by the change in water pressure caused the torpedo, upon identi~ling a submarine acoustic signature, is
by a ship passing over it. The movement of displaced water as the
ship passes is called the ship's pressure signature. This type of mine automatically released to home in on and destroy the submarine.
is the most difficult to sweep.
Mine Countermeasures
Mode of operation. Controlled mines are no longer lIsed by the
Mine countermeasures (MCM) include all actions t,lken to protect
United States Navy, but m.1I1Y varieties of them were llsed in riv-
friendly shipping against mines. The three major types of mine
ers by the Vietcong in Vietnam. The)' arc manually detonated by a
countermeasures arc ship treatment against magnetic and acollstic
person on shore when an enemy ship is ncar the mine. ~'fost mines
mines, minesweeping, and mine hunting.
today arc designed to detonate automatically when a firing mecha-
The two principal methods of treating a steel-hulled ship to
nism detects a ship nearby.
decrease the magnetic eA-ects that actuate magnetic influence mines
Method of actuation. Actual contact of a ship with it mine or its
arc riepel'lI/illg and riegn/lssillg. The hull of it steel ship norm.llly
antenna is required to detonate a COlllact mine. One common COIl-
acquires a significant permanent magnctic field during construc-
tact mine is equipped with lead horns encasing glass tubes con-
tion. This occurs bCC<luse the steel plates of the hull are constantly
taining an electrolyte. \Vhen a horn is struck and bent, the glass
being heated, riveted, and hammered during building, causing the
tube is broken and the electrolyte flows into a battery cell. generat-
iron molecules to align themselves with the Earth's magnetic field.
ing enough current to detonate the mine. Another type closes an
A ship also has an induced magnetic field caused b)· the interac-
"inertia switch," which completes an electric circllit.
tion of the moving ship with the E<trth's magnetic field.
There al'e three basic types of il/fllle/ice mines: magnetic, ,lCOllS-
The purpose of deperming is to reduce il ship's pernlilllent
tic. and pressure. The firing mechanisms of two or all three of these
magnetic field to a minimum. Done by mcans of wnipping the hull
maybe interlocked ill the case of it combination mine, making it
with electric coils after the completion of construction, this pro~
more difficult to sweep. Influence mines are normally of the bot-
cess is essentially a large-scale version of demagnetizing a magnet.
tom type. The //Iagnelic mine is triggered by the target ship's mag-
Degaussing was developed by the British during \\'orld \"ar II to
netic field. \"hen a ship p<1sses, the firing circuit is actuated and the
defeat the <1forementioned German influence mine. It neutralizes
mine detonates. The acol/stic mine is triggered by the noise pro-
the strength of both the induced ,md permanent magnetic fields of
duced by <l passing ship's propellers, machulerr. or hull vibrations.
a ship by means of an arrangement of electric coils installed within
The firi ng mechanism call be set to react only to specific sounds,
the hull of the ship. Basically, a direct current is sent through the
so it will not be actuated by any normal sea sounds. The presslIre
coils to produce a magnetic field in exact opposition to that gener-
mine is triggered by the change in water pressure callsed by a ship
ated by the ship itself, thus nulli~'ing nny magnetic eA-ects.
p;lssing OWl' the mine, It is the 1110st difficult of the three bilsic
influence types to sweep.