US NAVY TORPEDOESby Frederick J Milford
Part Four: WW II development of homing torpedoes 1940-1946
Reproduced with permission from the April 1997 issue of THE SUBMARINE REVIEW,
a quarterly publication of the Naval Submarine League, P.O. Box 1146, Annandale, VA 2200
Important as the WW II improvements in conventional
torpedoes were, the real revolution was in the development of
homing torpedoes, i.e., torpedoes which autonomously seek their
targets at least during the final portions of their trajectories.
The exact date when the homing concept first occurred to torpedo
developers is lost, but the general idea was discussed early in
the twentieth century1 when torpedo ranges got long enough that
very accurate aiming was required and relatively small angular
dispersion could cause misses. Not, however, until the mid-thirties,
when electronic technology provided the means for implementing
the concept, was it possible to begin serious development of
homing torpedoes. Programs were initiated by the German navy
in the mid-1930s and by the Royal Navy in the late 1930s. The
German program suffered a hiatus from 1939 to 1942 because the
expectation of a short war lowered its priority, but two torpedo
types for U-boat use against surface vessels were produced during
1943. Royal Navy results, mainly dealing with acoustics, were
not pursued, but were made available to the US Navy. US programs,
as we shall relate, began in December 1941 and produced an air
launched anti-submarine torpedo that entered service and sank
submarines seventeen months later, in May 1943. Several other
important homing torpedoes were developed for the US Navy before
the end of the war and two of these were used against enemy targets.
Homing torpedoes are dramatically different from
the gyro-controlled, set depth torpedoes used against surface
ships in that once they acquire their target, they home on it
autonomously using on-board controls. In addition to the obvious
advantage of homing in the horizontal plane in attacking surface
targets, homing can operate in the vertical plane thus providing
an important capability against submerged submarines or shallow
draft escorts. The homing concept is obviously very attractive,
so attractive in fact that only one new non-homing torpedo has
entered service with the US Navy since 1944 and that was the wire
guided Mk.45 to which special constraints applied.
A successful homing torpedo must:
Rational analyses of target signatures and probes
that might provide information about target location for use in
homing torpedoes have been made many times. The result, even
today, is invariably that the best, and possibly the only practical,
possibilities are acoustic. Target detection and tracking using
underwater sound had, of course, been developed during the inter-war
years for surface vessel anti-submarine purposes and for defensive
and offensive use by submarines. These sonar systems were of
two types, passive, which simply listened for noise generated
by the target, and active, which detected the reflection or echo
of a probing sound pulse emitted by the system. Such shipboard
systems provided starting points for torpedo homing systems, but
their size and weight were both much too large for torpedoes.
Developing equipment that satisfied the size, weight and performance
constraints associated with installation in a torpedo body was
a challenging task. The first US homing torpedoes used passive
systems that detected ship noise, primarily cavitation noise
from the screws. The directivity needed to generate homing rudder
orders was provided either by mounting the hydrophones around
the circumference of the torpedo and using body shadow and hydrophone
directivity to provide directivity or by mounting an array of
hydrophones in the nose of the torpedo and relying primarily on
hydrophone directivity. Soon after development of passive homing
began, US work was started on active homing based on a miniature
active sonar. The problems associated with fitting an entire
sonar system, using vacuum tube technology, into a torpedo body
while leaving room for the propulsion system and a meaningful
warhead were very severe. It was, in fact, not until early 1944
that the first active homing torpedo made a three dimensional
acoustically controlled run. Ultimately, however, acoustic torpedoes
incorporated passive homing for target acquisition and active
homing for the attack phase.
Detecting a target and indicating its direction are
not enough. This information must be converted to rudder motions
that will direct the torpedo to the target. Conceptually this
is relatively simple. In the case of passive homing, amplified
signals from say the left and right hydrophones can be compared
and the control circuits arranged to move the rudders to steer
in the direction of the stronger signal. A similar, but slightly
more complicated, system can be used for control in the vertical
plane. This approach was used in the Mk.24 torpedo, also known
as FIDO, discussed below. Simple as the process sounds, there
were many problems that were important in these early days of
electronics. For example balancing the left and right amplifiers
was enough of a problem that the early systems used a single amplifier,
which was switched back and forth between the left and right
channels. Stability of the control system also required study.
In 1942 these were problems at the cutting edge of engineering
technology. That they were solved expeditiously in the face of
similar demands for communications, radar, sonar, fire control
and nuclear weapons, to mention some of the competitors for electronic
development, was a tremendous triumph.
An acoustic homing system can work only if the torpedo
is quiet enough that its self noise does not mask the noise or
echo that is the target signal. This means minimizing both the
hydrodynamic noise, especially that originating in cavitation,
and the propulsion machinery noise. These issues and the constraints
of electrical propulsion, which was used with most WW II homing
torpedoes, led to rather slow, short range torpedoes, in many
cases so slow that they were effective only against submerged
submarines or slow moving actively searching escorts.
As with conventional torpedoes, there were, during
WW II, three launch platforms for acoustic torpedoes, aircraft,
submarines and surface vessels, and two classes of targets, surface
vessels and submarines. These platform-target combinations impose
constraints or design requirements on homing torpedoes that are
not operative, or at least much less important, in the case of
conventional torpedoes. The major new safety requirement was
that the torpedo should not home on the launching platform2 or
other friendly vessel. This requirement was satisfied in a variety
of ways. To protect surface vessels, ceiling switches disabled
the homing system of air launched weapons when the depth was less
than a preset value, say forty feet. Floor switches similarly
protected submerged submarines from their own anti-escort torpedoes.
Straight enabling runs to the vicinity of the target; anti-circular
run devices and other safety features were also added to some
of these new torpedoes. Further, during WW II Allied aircraft
did not drop homing torpedoes when operating in conjunction with
surface ASW forces. Incidents did, however, occur. HMS Biter
was chased by a homing torpedo giving rise to the doggerel "Biter
bitten by FIDO."
US NAVY HOMING TORPEDO DEVELOPMENT DURING WW II--AN
The development of homing torpedoes during WW II
was done almost entirely under the auspices of the Office of Scientific
Research and Development (OSRD) and its subsidiary the National
Defense Research Committee (NDRC). Wartime production of homing
torpedoes was accomplished by standard BuOrd procurement contracts
with industrial firms, primarily Western Electric, Westinghouse
and General Electric. Major research and development contracts
were issued under the authority of the Office of Emergency Management
(OEM) to Harvard University, Western Electric Co. (Bell Telephone
Laboratories), General Electric Co. and Westinghouse Electric
Corporation with smaller contracts to other universities and commercial
firms. Many subcontractors worked for the major contractors on
special aspects of torpedoes. Each of the major contractors and
Brush Development Co.3 developed one or more homing torpedoes through
the prototype stage. In several cases two contractors developed
competing models designated by the same Mark, for example, the
Bell Telephone Laboratories (BTL) and the Harvard Underwater Sound
Laboratory (HUSL) developed competing versions of the Mk.24 and
HUSL and GE developed competing versions of the Mk.32. In other
cases competing torpedoes had different Marks (The Brush Mk.30,
for example, was developed, as a backup, in parallel with the
Mk.24. Thus, there was significant competition, but also a great
deal of cooperation. This combination helped to produce the first
operational US homing torpedo in the remarkably short time of
seventeen months from initial concept to first combat success.
One estimate suggests that the competition saved a full year
in the development cycle.
Homing torpedoes developed along two lines: torpedoes
based on straight runners (primarily Mk.13, Mk.18 and Mk.19) with
standard 21" x 246" or 22.5" x 161" envelopes
and smaller torpedoes with 10" or 19" diameter envelopes
seven to eight feet in length. The principal technologies that
were newly incorporated to make homing torpedoes were underwater
acoustics (hydrophones); hydrodynamic and mechanical quieting;
electronic controls and servomechanisms. Though such items are
commonplace today, in the early forties they were revolutionary.
The number of torpedoes under development was large
as indicated by Table 1, but only three, Mk.24, Mk.27 and Mk.28,
saw service during WW II. All but one, Mk.21 Mod.2 (a homing
version of Mk.13), used electric propulsion and this was the
dominant mode of propulsion for new US Navy homing torpedoes until
high submerged speed nuclear submarines forced a return to thermal,
albeit advanced thermal, propulsion in the cold war era.
HUSL is Harvard Underwater Sound Laboratory; BTL is Bell Telephone Laboratories, ORL is Penn State University Ordnance Research Laboratory. NIS indicates "never in service".
SELECTED USN HOMING TORPEDOES--WW II ERA
Among the acoustic torpedoes developed during WW II there were two that represented critical milestones. The Mk.24 was the first passive homing torpedo developed for the US Navy and the Mk.32 was the first active homing torpedo. The Mk.35 was the first active-passive homing torpedo and it was based on research and development started during WW II. The actual Mk.35 torpedo development program seems to have begun quite late in the war and more properly belongs to the post-WW II era. We will focus here on the Mk.24 and Mk.32 torpedoes and comment briefly on some of the others.
Passive Homing and the Mine Mk.24 (Torpedo)4
The first of the new homing torpedoes was a response to the damage
being done to Allied shipping by German U-boats. From the beginning
of WW II through 1941 Allied shipping losses to submarines averaged
over 170,000 tons/month and aircraft were proving to be remarkably
ineffective in destroying submarines5. One consequence was that
even before the US entered World War Two, parts of the Navy were
reconsidering homing torpedoes as air launched ASW weapons. In
"the fall of 1941" (probably late November or early
December), the Navy asked NDRC to consider the feasibility of
a small, relatively slow-speed, acoustically controlled, air launched,
anti-submarine torpedo6. Submarines were thus specifically added
to the torpedo target list rather than being incidentally included
when surfaced or at periscope depth as surface vessels7.
In response to the Navy request NDRC convened a meeting at Harvard
on 10 December 1941. Two weeks later at a second meeting the
following requirements were outlined:
size to fit 1000 lb bomb rack, i.e.,smaller than 19" x 90" droppable from 200 to 300 ft at about 120 k. electric propulsion using lead acid storage battery 12 knots for 5 to 15 minutes 100 lb high explosive charge acoustic homing with greatest possible range
The participants in the meeting responded as follows: General
Electric agreed to design and fabricate the propulsion and steering
motors. David Taylor Model Basin would assist in any way possible,
primarily hydrodynamics and propulsion. DTMB actually supplied
the propeller and shell designs and the first few actual shells
used in the Mk.24 program. The Harvard University Underwater Sound
Laboratory (HUSL) and Bell Laboratories (BTL) each undertook the
independent, but cooperative and information sharing, development
of experimental torpedoes with their main contributions being
acoustic control systems and integration. The entire project proceeded
very rapidly. Some of the key events in the development of Mine
Mk.248 (FIDO), are shown in the almost unbelievable schedule which
First Meeting: Dec 1941 HUSL Proposal Dec 1941 BTL Proposal: Jan 1942 +1 month Design Freeze: Oct 1942 +10 months First Production Unit: Mar 1943 +15 months 500 units by: May 1943 +17 months First kill May 1943 +17 months
The entire development from conception to first kill was accomplished
during the general time period during which the previously described
Mk.14 problems were solved. The contrast in the rate of progress
on the two problems is striking. Mk.24 also established the four
hydrophone acoustic sensor arrangements that were the dominant
passive homing system for US acoustic torpedoes in the period
The Mk.24 that emerged was 84" long, 19" in diameter
and had a total weight of 680 pounds. It was propelled by a General
Electric five horsepower, 48 volt electric motor using an Exide
lead acid storage battery for power. The warhead containing 92
lbs of high explosive which occupied the forward 14 1/2 inches
of the weapon. These features were substantially different from
those of earlier torpedoes, but more significant differences were
to be found in the control system.
Target detection was accomplished by four hydrophones symmetrically
arranged around the circumference of the torpedo mid-section in
the left, right, up and down positions. Such an array is useful
for target acquisition because the four hydrophones together cover
essentially all directions from the torpedo and for homing because
"body shadow", meaning that the hydrophone on the right
side, for example, being in the acoustic shadow of the torpedo
body could not hear a target on the left side, provides directionality.
The basic idea is to compare the signals from the left and right
hydrophones and move the rudder in such a way as to steer towards
the stronger signal. In the BTL implementation of this scheme,
the hydrophone signals were amplified, rectified and subtracted.
This net signal was combined with the voltage from a potentiometer
which was coupled to the rudder. The combined signal drove a
DC amplifier which, in turn, controlled a differential relay that
caused the rudder motor to move in the appropriate direction to
reduce the input voltage (hydrophone derived voltage plus rudder
potentiometer voltage) to zero. The vertical control circuit
was identical except for including inputs from a hydrostat that
measured depth and a pitch pendulum, which were also voltages
derived from potentiometers. These signals caused the torpedo
to operate at a fixed depth until a sufficiently strong acoustic
signal was received. When such a signal was detected, the hydrostat/pendulum
control was disabled and acoustic depth control prevailed. As
a safety feature, acoustic depth control was disabled and hydrostat/pendulum
control re-established if the torpedo rose above a ceiling set
at about forty feet. This prevented the torpedo from attacking
surface vessels including surfaced submarines. These control
systems produced rudder angles that were proportional to the
difference in strength between the signals from the right and
left (or up and down) hydrophones. Such proportional control
was distinctly different from the "bang-bang" (rudder
hard left or hard right) controls that had been used ever since
the Obry gyro was introduced, but detailed analysis and experimental
work at HUSL showed that the "bang-bang" (no rudder
position feedback) controls would perform equally well.
The Mk.24 development program was notable not only because of
the speed with which it was completed, but also because of the
thorough development testing and subsequent quality control.
During subsystem development there was a continuing series of
tests to measure and verify essential performance characteristics.
Testing included drop tests, checking fitting to aircraft and
occasional drops from aircraft in addition to the usual laboratory
testing of the mechanical, electrical and electronic designs.
BTL alone conducted 192 in water test runs with their experimental
models between 16 April and 20 October 1942 and a comparable number
of tests was conducted by HUSL. Later, HUSL conducted an extensive
series of tests on Western Electric production torpedoes dropped
from PBY aircraft.
Both the HUSL and the BTL programs produced successful prototypes.
The BTL Mk.24 production design, which started from the BTL experimental
model used important features from the HUSL model and incorporating
a number of improvements suggested by development testing. The
design was frozen in October 1942. At that time Western Electric
was given a sole source contract for production of the torpedoes.
Subcontractors included General Electric, Electric Storage Battery
Co., and interestingly enough, a bathtub manufacturer for the
shells. The first production model was delivered in March 1943
and 500 had been delivered by May 1943. The first U-boat using
the Mk.24 was U-640 which was attacked and sunk on 14 May 1943
by a PBY from US Navy VP 849. The Mk.24 was eventually responsible
for sinking 37 enemy submarines10, about fifteen per cent of the
submarines sunk by air escort or air ASW operations between May
1943 and the end of the war. This torpedo was a major success
whose achievements have long gone unheralded.
Reflecting the perceived urgency of the requirement for an air
dropped, homing, ASW weapon, another passive homing torpedo, Mk.30,
was developed by Brush Development Co. under a BuOrd contract
as a backup for the Mk.24. This 10" diameter torpedo progressed
through the successful prototype stage, but because of the success
of the Mk.24 it was never put in service. It was, however, a
precursor to the active homing Mk.43 Mods.1 and 3 which were in
service from 1951 to 1957.
Two other passive homing torpedoes saw service in WW II. The
Mk.27 torpedo was a submarine launched anti-escort weapon based
on the Mk.24. The original Mk.27 Mod 0 was a minimally modified
Mk.24 with wooden rails to fit 21" torpedo tubes, a floor
switch (instead of a ceiling switch) so it would not attack the
launching submarine, and various arming, warm-up and starting
controls to suit a torpedo tube, swim-out launch mode. Eleven
hundred Mk.27 Mod.0 torpedoes, known as CUTIE, were built by Western
Electric and delivered between June 1944 and April 1945. Production
on a subsequent order for 2300 torpedoes continued until the end
of the war. One hundred and six were fired against enemy escorts.
Thirty-three hits sank 24 ships and damaged nine others. Later
versions of the Mk.27 were longer and heavier. Mod.3 which was
slightly over ten feet long and faster; it had a 200 lb warhead
and a gyro for straight runout before beginning to search for
its quarry, Only six were completed before the project terminated
at the end of the war. The post-war Mk.27 Mod.4 was different
from the wartime versions, especially in that it could attack
submerged submarines, and is discussed in the next part of this
series. The Mk.28 was a 21" x 246", 20 knot, submarine
launched anti-surface vessel torpedo with a 585 lb warhead.
It was equipped with passive homing and gyroscopic control which
competed for rudder control. About 1750 of these torpedoes were
produced by Westinghouse and Western Electric. Only fourteen
were fired with four hits during WW II, but the torpedo remained
in service until 1960.
The remaining passive homing torpedoes developed during WW II were generally and perhaps surprisingly successful, but were overshadowed by earlier successes or reached production readiness too late in the war to be used. Some of these programs did, however, influence post war torpedoes. The Mk.29, in particular, was the first torpedo designed to use a sea water battery11 for propulsion and offered other improvements that were used in later torpedoes. The Mk.33 appears to have been the first submarine launched antisubmarine torpedo developed by the US Navy, but only thirty of them were built for test and evaluation.
Active Homing and the Mk.32 Torpedo
Active homing, the second milestone, is significantly more complex
than passive homing and only two torpedoes of this kind, Mk.22
and Mk.32, were developed during WW II. Mk.22 began as an effort
to add active homing to the Mk.14 torpedo but ended up as a standard
Mk.18 electric torpedo design modified by Westinghouse and Bell
Telephone Laboratories to include active homing in azimuth only.
The homing system transmitted a pulse of 28 KHz sound using both
halves of a left-right split transducer. Echoes received by the
two halves were processed separately and their relative phase
was used to determine the direction of the target. From the relative
phase a course correction signal was generated and this signal
controlled a change in the gyro angle. The gyro maintained course
control between pings of the sonar. The implementation of this
scheme with minimal modification of the basic Mk.18 torpedo required
a great deal of ingenuity including, in particular, a complex
mechanical device called the "translator" which took
signals from the servo amplifiers and power from the propeller
shaft to drive the course input for the gyro. One of the problems
that is encountered in active acoustic homing systems, but not
in passive systems, is reverberation, i.e., reflections of the
transmitted sound pulse from random features in the surface, body
and bottom of the ocean. Reverberations are effectively false
targets and without special features an active acoustic torpedo
would often home on them. Fortunately, reverberations die out
quickly. In the Mk.22 system, the receiver was blanked for 40
milliseconds after the transmitted pulse and the amplifier gains
programmed to increase with time (time variation of gain, TVG)
in order to avoid the reverberation problem. The guidance system
was successful, but by 1944 azimuth only homing, even for 21"
torpedoes, was less attractive than the combination of vertical
and horizontal homing offered by competing systems. Work on the
Mk.22 was terminated before production designs were completed.
Two competing designs were developed for the other WW II active
homing torpedo, Mk.32. One design was developed by HUSL and the
other by General Electric both beginning in 1942. The Mk.24 body
was used, in fact Mk.32 was designed as a conversion of that weapon12
with the passive homing system replaced by a small active sonar.
Size and weight constraints were severe. The total available
volume was less than two cubic feet in the mid-section of the
torpedo, space for the transducers in the nose and the space occupied
by the Mk.24 depth control in the tail section. Weight was limited
to less than fifty pounds. These space and weight constraints
meant that the best options could not be used if there were a
lighter or smaller option that could do the job satisfactorily.
The second problem was to devise a control system that functioned
on the basis of short, 30 millisecond, widely spaced, 0.7 second
separation, inputs rather than continuous inputs characteristic
of passive homing systems.
The GE system that emerged used a magnetostrictive transducer,
four elements wide and eight elements high, that was split into
an upper half and a lower half. This configuration made it possible
to use phase comparison and proportional control in the vertical
plane where it was necessary to home on a submarine hull that
measured around seven meters from keel to deck. In the horizontal
plane, where the target was about 70 meters wide, a simpler on-off
control was used. In the absence of an echo the rudders were
hard over to port and the torpedo circled in that direction.
When an echo was received the rudder was shifted to hard starboard
and remained in that position until about one second after the
last echo was received. At this point the rudder was reversed
and the process repeated. The torpedo thus apparently homed on
either the bow or stern of the target, but the dynamics of the
torpedo and the electronic time constants shifted the actual homing
point toward the center of the target. The main virtue of this
homing system was that it used the same amplifiers as the vertical
control system without adding complex circuitry and so saved weight
Homing signals in the vertical plane were derived by comparing
the phase of the signals from the two halves of the transducer.
The up or down signals were used to drive a pendulum frame in
which the pendulum was suspended. Electrical contacts connected
the horizontal (diving) rudder motor to its power source in such
a way as to keep the pendulum centered in the frame. The system
thus controlled the pitch angle, and consequently the rate of
climb, directly. A hydrostat was installed, but it was used only
to control the mode of operation, e.g., set the depth ceiling,
and did not provide servo inputs that affected the horizontal
Reverberation and other false target problems were dealt with
by a combination of time variation of gain and blanking. It is
interesting that this system also switched between a search mode
and a pursuit mode presaging the on board logic of modern torpedoes.
An experimental Mk.32 produced by General Electric made a successful
sound controlled three dimensional run in February 1944, 22 months
after the concept was first presented to NDRC. Tests against
target submarines began in July 1944 and were successful. Leeds
Northrup was selected to produce the GE version of Mk.32 and ten
pre-production units were completed and tested before the project
was canceled at the end of WW II. Later, with deliveries beginning
in 1950, Philco produced a substantial number (about 3300) of
the somewhat different Mk.32-2 torpedoes for fleet use by destroyer
type vessels. This torpedo is discussed in a subsequent part
of "US Navy Torpedoes".
The HUSL system was different. The transducer was symmetrically
divided into four quadrants. The echo signals in these four quadrants
were processed in an ingenious electronic system to obtain rudder
orders. The system also contained a Doppler enabling system that
prevented homing on reverberation and other false targets including
wakes. While the HUSL system was not selected for the Mk.32 torpedo,
many of its features were incorporated into the Penn State Ordnance
Research Laboratory Project 4 system which was the basis for the
very successful Mk.37 torpedo.
Homing torpedoes ascended to paramount importance during WW II and the principal practical techniques, active and passive acoustic homing, were well established by the end of the war. The stage for subsequent US Navy torpedo development was thus, as we shall see in the next part, set during WW II.
1J.Küsters "Das U-Boot als Kriegs- und Handleschiff" Berlin, 1917 quoted in Eberhard Rössler "Die Torpedos der Deutschen U-Boote" Herford: Koehlers, 1984, p.136. Küsters mentions Swedish Captain Karl O. Leon’s idea of adding "ears" and mechanisms to control the rudders of long range torpedoes in such a way as to home on the target’s propeller noise.
2With non-homing torpedoes the main threats are prematures and circular running torpedoes, which have caused a number of tragic submarine losses, damage to firing submarines and near misses.
3Brush developed the Mk.30 outside of the NDRC framework under a direct contract with BuOrd..
4The Mk.24 homing torpedo has not, in my opinion, received the attention it deserves. The most comprehensive published document is Mark B. Gardner "Mine Mk.24: World War II Acoustic Torpedo", Journal of the Audio Engineering Society, Vol.22, No.8, October 1974, pp.614-626. "A History of Engineering and Science in the Bell System: National Service in War and Peace (1925-1975)", Murray Hill: Bell Telephone Laboratories, 1978 contains some information that is not included in Gardner's paper. These publications focus on the BTL/Western Electric projects, but clearly indicate that important contributions were made by other organizations. More recent is Tom Pelick "FIDO - The First U.S. Homing Torpedo", Submarine Review, January 1996 and correspondence by Milford and Polmar in the April 1996 issue of SR. Robert Gannon "Hellions of the Deep" University Park PA: Penn State University Press, 1996 tells more of the Harvard story. The primary documentation is contained in reports submitted to NDRC by HUSL and BTL/WE.
5This oversimplifies a complex situation. Between September 1939 and December 1941 aircraft were credited with sinking four U-boats and shared credit for four other kills. The major problems were inadequate aircraft and ineffective weapons. Improvement in both and revised attack tactics resulted in more successes and for the entire war more U-boats were sunk by aircraft than by surface vessels.
6Summary Technical Report of Division 6 NDRC, Vol.1 "A Survey of Subsurface Warfare in World War II", Washington: NDRC, 1946, p.209. The request probably evolved from a memorandum by Capt. Louis McKeehan USNR dated 24 November 1941 in which he asked "Is it feasible to devise acoustic equipment for homing control of a self-propelled, torpedo-like body?" McKeehan was a mine expert and had been Desk "N" Mines and Nets at BuOrd. The reorganization of BuOrd in February 1941 put R&D for all underwater weapons in Section Re-6 of the Research Division (Re). McKeehan headed Re-6 for part of the war.
7Conventional torpedoes had been fired at submarines, mainly surfaced, during WW I and the practice continued during WW II. The US submarine patrols from East Coast bases and Panama during 1942 were essentially anti-submarine patrols. WW II, however, saw the first development of specific ASW torpedoes capable of attacking submerged submarines efficiently and effectively. We view this as a significant augmentation of the torpedo target list.
8Several reasons for calling the Mk.24 torpedo a mine have been advanced. Security was certainly one reason. The other is given variously as recognizing the role of the mine warfare establishment or keeping the torpedo establishment and its baggage out of the project.
9The often reported sinking of U-266 by an RAF Coastal Command Liberator has been re-evaluated and is no longer attributed to FIDO. U-640 and U-657 were interchanged in early post war reports. The statement in the text reflects the most current evaluation available to me.
10Various numbers of kills are reported. In my opinion, the most probably correct numbers are 340 torpedoes dropped in 264 attacks of which 204 were against submarines. In 142 attacks US aircraft sank 31 submarines and damaged 15; in 62 attacks against submarines other Allies, mainly British, sank six and damaged three. Most of these submarine sinkings were German U-boats in the Atlantic but five Japanese submarines were sunk by Fidos, one, I-52, in the Atlantic and four in the Pacific. OEG Study No. 289 , 12 August 1946, is the main source for this conclusion.
11The first torpedo to use a sea water battery was a Mk.27, but this was purely experimental.
12"Acoustic Torpedoes" Vol.22 of the Summary Report of Division 6, NDRC. Washington: OSRD, 1946, p.76