by 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."


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.



Service torpedoes in bold.
Design and Development
Service Dates/Total Production
Platform/ Target

New torp. Elect prop. and passive homing. Mk.13 envelope

Mk.13 with passive acoustic homing. Steam prop.

Active acoustic homing in azimuth only.


Passive acoustic homing.


CUTIE Anti-escort

Passive acoustic homing

Post WW II development, distinct from Mods. 0,1,2 and 3

Passive acoustic homing

Improved Mk.28

Sea water battery

Brush Dev. Co.
Backup for Mk.24

Passive homing.

Passive homing Mk.18

Active homing, FIDO envelope

Post WW II development, distinct from Mods. 0 and 1

BuOrd, GE, Exide
Passive homing

HUSL is Harvard Underwater Sound Laboratory; BTL is Bell Telephone Laboratories, ORL is Penn State University Ordnance Research Laboratory. NIS indicates "never in service".


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 follows.

Chronology Mine Mk.24 (Torpedo)

   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 1941-1950.

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 and space.

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 rudder.

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

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