and Harbour Defence Asdics (HDAs)
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Dr Richard Walding  
Research Fellow - School of Science
Griffith University
Brisbane, Australia

Harbour Defence Asdic HDA

The Harbour Defence Asdic is a special sea-bed mounted form of the ship-borne Asdic (now known as sonar). It  was developed by the Royal Navy towards the end of WW1 and first used in 1939. The US had access to this technology and called their HDA a "Herald" - harbor echo ranging and listening device. As with modern sonar, an oscillator (set in 'transmit' mode) sends out a short pulse of sound at about 15 kHz ( a 'ping') which would reflect off any large object in its beam. The resulting echo could be detected by the underwater oscillator (in 'receive' mode). The pattern of echoes could indicate position and speed of a ship. The name Asdic was first used by Churchill in the House of Commons. On 11 December 1939, the Admiralty were asked what it stood for and they said "Allied Submarine Detection Investigation Committee - a body formed during the war of 1914-1918 which organised much research and experimentation for the detection of submarines". However no such committee ever existed. The first reference to ASD was in the "Weekly Report of Experimental Work at Parkeston Quay - 6 July 1918" where ASD was used to stand for Anti-Submarine Detection. The suffix "ics" was added to make it a noun and to ensure the name of the device was unrelated to the operation of the device.  The term "Sonar" was invented in 1942 by F V (Ted) Hunt of Harvard Underwater Sound Laboratory. He wanted a phonetic analog to RADAR using sound (sonic), hence Sound, Navigation and Ranging (SONAR).


O/S Walter Flett (RN)
- later Captain

  Photo right: Type 135 HDA and tripod being lifted from the waters of Trincomalee Harbour, Ceylon, by the cable-layer HMS Bullfrog in 1945. This photo was supplied by Captain Walter J S Flett (RN), Findochty, Scotland who was an Ordinary Seaman aboard Bullfrog from late 1945 to early 1946. This is the only photo of the British HDA dome known. The total height of the HDA was about 12 foot with the dome being about 1'8" diameter and 3' long.

The USN Herald unit operates on the same principal:

Herald being lowered in NY Harbor. Photo supplied by Jim Carpenter, son of Lt Carpenter, Commanding Officer at Fishers Island, New York. The Herald was in the form of a triangular-based pyramid with a height of about 10-12 foot.
US Navy pontoon barges are usually used at advanced bases in transporting supplies from ship to shore. They are also very helpful when supplied with a crane, in planting Herald sea units (1944). Herald shore terminal unit being tuned by student watchstanders at the National Training School, Fishers Island, NY - 1944.

Asdic was a term used to describe echo location and ranging using high frequency sound waves.  It grew out of experiments undertaken by French scientists in the early 1900s on the propagation of sound in water.  Of particular note is the Fessenden electromagnetic oscillator which was a precursor to the modern transducers used in sonar, the term now applied to Asdic operations.  Other French scientists contributing greatly to the work were Langevin and Chilowsky who developed techniques for transmitting and detecting the sound waves.  In 1915, NZ scientist Ernest Rutherford and scientists from the British Bureau of Investigation and Research (BIR) developed the process further and the first Asdics were ordered by the Royal Navy in June 1918.  In the USA, experiments continued at Columbia University which lead to the formation of research teams combining the US Navy, the Submarine Signal Company, General Electric and Western Electric at Nahant, MA. Asdic research stepped up at the start of WW1 but it  was not ready by the end of the war, and didn't get to sea until 1920, when one was evaluated on the cruiser HMS ANTRIM. Four patrol vessels were also fitted with Asdic on an experimental basis, leading to adoption of a production Asdic system in July 1922. British submarines were fitted with Asdic beginning in 1926. It was in service in British destroyers from 1928.

The HDA was developed in the late 1920s at HMS Osprey (Portland Naval Base, England). It was given low priority and the first model was not ready until March 1932 when placed 1000 yards off the Portland breakwater. An improved HDA was laid 3 miles off the same breakwater in 1934 in conjunction with indicator loops. The valve transmitter of the early set was replaced with an high frequency motor alternator (HFMA) and a resonant circuit. After this, the first model with a number (131) was introduced.

By December 1941, the Admiralty was worried about midget submarines entering ports (against which the standard indicator loop was ineffective), particularly after Italian human torpedoes made daring attacks on the harbours at Alexandria and Gibralta.  The Type 131 HDA was not effective against small targets so an improved HDA (Type 135) was designed at the Underwater Experimental Establishment at Fairlie by the naval scientist H F Willis.

Whereas the Royal Navy used quartz oscillators, the US Navy based their oscillator on magnetostriction. Both the Admiralty and the National Research Laboratory (USA) worked on magnetostriction, their paths diverged before the war and each chose alternative methods.

Operation of the HDA can be seen in the following schematic (Figure 1):

Power was generated by a Mark 2VSO Ruston Vertical Oil (Diesel) engine producing 15 - 16 HP at 1000 RPM, connected by belt drives to a 220V, 7kW DC Generator for the HDA and a 110V 2kW DC Generator for the Indicator Loops. This was housed in a 18' x 10' concrete generator hut about 100 m from the control hut.

The Generator Hut at RAN 4, Bribie Island.  Artificer's Workshop at RAN

Board Charge-Discharge
The battery array consisted of two banks of high tension (H.T.) batteries each of 36 2-volt cells. These cells were quite small (about 10 cm high by 5 cm square) as they were only required to supply the 72V potential to the anodes of all four valves. Two lots of L.T. batteries each of 2V cells provided the 4V for the four valve filaments. The batteries and the charge-discharge board were located in the Artificer's Workshop in the Loop Control Hut. The HSD rating would change banks each 24 hours, test SG regularly, top up and clean terminals.

High Frequency Motor Alternator
The 220V DC was also connected to the High Frequency Motor Alternator (HFMA) which was also located in the workshop. The HFMA consisted of a shunt-wound DC motor driving an inductance-type alternator. The speed was controlled very accurately by a sensitive relay and governor. The frequency of the HFMA was between 10kHZ and 13kHz for HDAs. Although DC motors start readily, for safety for this high speed HFMA, a starting resistance was used as shown in the following straight line circuit (Figures 2 and 3):

Figure 2 - To start, close SW 1. Solenoid closes SW2. Motor starts with resistor in.

Figure 2 - At run, resistor is shorted out by SW3 which closes when back EMF from motor weakens holding off coil 4.

At the opposite end of the HFMA to the commutator was a simple governor (Figure 4):

Figure 4

The Disc and Ring are in the field circuit. If the motor speeds up, centrifugal force straightens the spring, breaking contact between disc and ring, opening the field circuit, slowing the motor. In operation, this is continuous, sparking is always visible, and the carbon ring must be frequently checked and replaced.

The HDA Room at RAN 4  Some 2-core cable in the HDA Room

Transmitting Panel
The HFMA output of 2000V AC was connected to a resonant circuit in the Tuning Panel. The Tuning and Transmitting Panels were housed in the one box (60cm high by 45cm wide) on the wall and had a removable cover. The resonant circuit consisted of a tuning coil toroidally wound (15" diameter) and at the centre of which was situated the condenser with suitably tapped condenser connections. This was located in the HDA room.

Constantly charged by the HFMA, this resonant circuit was discharged to the oscillator by the automated send/receive key (S/R Key) on the Transmitting Panel. The key itself was a simple rotating arm motor type with limited movement thus:

Figure 5

Perfect cleanliness was the answer here plus a smear of Pure Medicinal Vaseline. This substance was known in the service as "Starters" and the sight of it in a more senior officer's hand would terrify any midshipman.

The Amplifier
Four standard thermionic triodes were powered by the batteries and operated thus (Figure 6):

Figure 6

The output from the amplifier (tuner) was heterodyned by mixing the signal from the amplifier (e.g. 13 kHz) with a carrier signal generated by another oscillator within the amplifier that was 1 kHz higher or lower (e.g. 12 kHz). The resultant signal was the difference between the two (1kHz) which is audible and can be heard by the Asdic rating through headphones or a loudspeaker (a nice tone - just below the C above high C).

To adjust the frequency of the signal several changes had to be made to the equipment. For example, to choose a resonant frequency of 11 kHz (identification code "U"), the operator adds a set of copper discs stamped with the letter "U" to the governor of the HFMA which regulates its speed to produce an 11 kHz signal. He would also set the number of condensers on the transmitting Panel to "U", the Amplifier valve to "U" and the heterodyne also to "U".

Underwater Gear

Figure 7: Diagram of the quartz oscillator. It has a diameter of about 15".

A cut-away of a dome that would be lowered when a ship is using Asdic gear. The oscillator housing is in the centre.

The Quartz Oscillator consisted of three steel plates as shown in the figure above. Sandwiched between them were pieces of quartz of matched frequency. This circuit was connected via an armoured 2-core cable to a quartz oscillator housed in a pressure-tight bronze cylindrical dome about 2' long slung vertically (gimballed) under a 12' high tripod on the sea bed. An armoured 7-core cable delivered 220V DC to the Bearing Control in the Controlled Training Unit in the HDA Room and thence to the Training Motor in the dome out in the bay. This allowed control of the direction of the Asdic Beam in 5 steps (or 1 - 2 continuous). The dome was filled with water and the dome itself had a window (originally Dermatine but later stainless steel) through which sound could pass without diminution in volume or change in frequency.

The automated send-receive key dumped the condenser's charge every 3 seconds. The duration of the 'ping' was 3/100ths of a second. The speed of sound in water is 5000 ft/second (at 20C and 3.5% salinity) which enables the sound to travel 2500 yards and return in the 3 seconds before the next ping. This is the maximum range for detection any echo. The sound beam sent out by the oscillator was conical in shape (about 16 divergence). The cone pointed away from the transmitting ship, which meant that the area covered by the Asdic beams widened with the distance. Within the range of the Asdic, the father away a submarine was from the hunter ship, the more likely it was to be spotted.

Electrochemical Range Recorder
Any echo would be detected by the quartz oscillator (now acting as a detector) and the signal is sent back by the 2-core cable to the HDA Room. The send-receive key switched the transducer's terminals to the input of a A/S44 Amplifier where it is amplified and recorded on an A/S 3 Electrochemical Range Recorder (see Figure below). The receiver could also remain switched to the transducer, allowing it to be "passively" used as a hydrophone. As an hydrophone, the lower frequencies of the HDA (10-13 kHz) were better as they could detect a submarine's propeller noises and, if changing depths, compressed air passing into the submarine's ballast tanks.

First production model electrochemical range recorder A/S3, 1934. Instrument was slaved to the distance finder A/S5. Paper impregnated with potassium iodide starch solution contained in sealed tank shown in the right hand picture.  Each roll of paper was 30 yards long and a black warning mark would appear on the right hand edge signifying the approach of the end of the roll. After its appearance, there was still sufficient paper remaining to complete the attack. Source: ADM 186/526, Osprey (HY), No. 25, Fig 34 A and B.


1. Terminal to Amplifier
2. To Fuse Box
3. Perspex cover
4. Relative Speed Scale
5. Scale Change Knob 6. Range Scale with cursor: 0-2500 Scale 25, 0-1000 Scale 10.
7. Magnetic clutch revolved by motor through gear box aluminium pulley with steel centre and draws stylus carriage from left to right by cord.
8. Motor and Gear Box. Two scales: Scale 25 was normal; paper width equivalent to 2500 yards. Scale 10 was speed x 2.5 - paper width = 1000 yards. Switch to Scale 10 is ordered when range reduced to 1000 yards.  Length of transmission reduced by faster speed over S/R key contacts. It doesn't matter much (at 12 knots only 2 minutes to firing time). Transmission interval reduced to 1.2 seconds from 3 seconds.
9. Stylus assembly. Two plated rods with return springs. The stylus was dragged to the right by a motorised cord. When it struck the right-hand terminals, it broke the magnetic clutch circuit and the stylus flies back as it is pulled by the stretched spring. When it strikes the left-hand terminals, the S/R key is shorted and the oscillator pings.

HISTORICAL NOTE: precursor to the Asdic recorder

In the early days of the telegraph, a variety of methods were used for recording the signal transmitted over the wires. Bain's "chemical telegraph" used specially prepared paper: The battery current, decomposed the salts in the paper and united with the iron point of the pen-wire, left a light blue mark on white paper. Or, if the current were strong, a dark blue mark would be left on the paper. The color of the mark depended upon the quantity of the current upon the wire. During the magnetic storm of February 19, 1852, the current increased so much that a "flame of fire" followed the pen and set fire to the paper. The following is an actual account from the journal of an electric telegrapher:

Thursday, February 19, 1852
"Towards evening, a heavy blue line appeared upon the paper, which gradually increased in size for the space of half a minute, when a flame of fire succeeded to the blue line, of sufficient intensity to burn through a dozen thicknesses of the moistened paper. The current then subsided as gradually as it had come on, until it entirely ceased, and was then succeeded by a negative current (which bleaches, instead of coloring, the paper). This gradually increased, in the same manner as the positive current, until it also, in turn, produced its flame of fire, and burned through many thicknesses of the prepared paper; it then subsided, again to be followed by the positive current. This state of things continued during the entire evening, and effectually prevented any business being done over the wires."


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