Journal of the Royal Naval Scientific Service. Volume 27, Number 2 ...

Torpedo History: Kirby S>
being used. i.e. whether a thermal engined
G7a or an electric G7e, but depended very
much on the speed of the weapon. Later work
showed that the noise originated at the propellers
and improved propeller design lowered
the self-noise, but initially it was calculated
that a torpedo would only be able to home
from a reasonable distance onto a ship if the
torpedo speed did not exceed 25 knots. Therefore,
a G7e was modified to run at this speed
and a simple homing device was fitted in the
nose. (Later in the war the Allies came to the
same conclusion regarding weapon speed but
had no weapon capable of such a low speed
nor a tactical requirement for slow torpedoes).
The first homing torpedo was issued to the
German fleet in January 1943 and was the T4,
also known as FALKE (= Falcon). About 100
T4 weapons were made and about 30 were
used. They were soon replaced by the T5
weapon which was given the code name during
development of ZAUNKONIG (= Wren). It
was also known to the Allies as GNAT
(German Naval Acoustic Torpedo), an altogether
much more appropriate name.
Gnats appeared in two basic forms, namely
the flat and the rounded nose types. Both
types used an amplitude comparison system
known as " amsel". Initial experiments,
carried out over many years beginning in
about 1936, eventually resulted in the T5 or
GNAT having a speed of 25 knots. This was
chosen as the upper speed because of the
deliterious effects of self-noise. This speed
gave the weapon a capability against ships
having speeds in the range 12 to 19 knots, the
lower speed being at the lower limit for noise
sufficient to activate the 75's passive homing
system.
The flat-nosed weapons, the nose of which
is illustrated in Fig. 32, carried four magnetostriction
hydrophones wired in alternating
pairs. A phase delay was introduced between
the sets of pairs so that the electrical output
%»**
FIG. 32. Nose and Homing Unit of German
"GNAT" Torpedo (1943).
was a maximum for sound arriving at ±25° to
the axis of the weapon in the horizontal plane.
The amplitude in each pair of hydrophones
was compared by switching to a comparator
at 100 cycles per second. The rudders were
then swung to steer the weapon towards the
side receiving the greatest noise. In the roundnose
weapon a bakelite cap was used to protect
the hydrophones. Only two of these were
used, each being placed behind a baffled
funnel to give maximum sensitivity at ±25°
as for the flat nose system. The funnels and
the cap were filled with glycerine and ethylene
glycol to give good acoustic transmission to
the hydrophones.
Due to an error it is widely believed that
over 6,000 T5 weapons were fired during the
war. In fact, careful research shows that 640
were fired with a hit rate of 6%. The Germans
claimed a rate of 53",,. Over 2,500 TS's were
test-fired in development.
Successes with ZAUNKONIG led to the
development of ZAUNKONIG 2. The
intended improvements were:
(a) Variable enabling ranges allowing escorts
to be passed by thus putting
merchant shipping at risk.
(b) Increased range. (The new batteries were
never actually brought into service and
the range remained at 6,230 yards.)
(c) Improved propeller design. This reduced
the self-noise and allowed ships with
speeds down to 9 knots to be detected.
(d) The ability to be fired at depths down
to 170 feet (compared with the maximum
depth of 491 ft. for the T5).
(e) Resistance to countermeasures. This
latter was included because of the
success of the FOXER, a towed noisemaker
deployed by Allied shipping,
which seduced the passive weapons awav
from the ship. The ZA UNKON1G 2, or
Til as it was also called, employed an
acoustic " dead spot " in the azimuth
sensitivity pattern at 20°. The weapons
were fired at about 20° on the target's
bow. The noise of the ship was not
picked up until very late in the attack
and then the weapon was programmed
to execute a sharp turn which, hopefully,
resulted in a hit on the ship.
Although the Til was issued to the fleet as
an operational weapon only one submarine
appears to have received these weapons and
none were fired.
An alternative to the "amsel " system was
the Pfau (— peacock) weapon homing system.