HMAS Curlew, M1121, Minesweeper to Minehunter
The Ton Class minesweeper/hunter, pennant number M1121 was built by Montrose Shipyard, Montrose, UK, completed on 20th. February 1953 and named HMS Chediston, later renamed HMS Montrose, renamed HMS Chediston, then sold to the Royal Australian Navy in 1961 and renamed HMAS Curlew.
Australian involvement in maritime mine warfare began during the colonial period in the 19th century with the employment of controlled minefields, maintained by specialised launches, as part of the defenses of colonial capital cities. The RAN began combating sea mines following the sinking of the SS Cumberland off Gabo Island, Victoria on 6th. July 1917, after she struck a mine laid by the German raider Wolf. Three auxiliary minesweepers (converted steam trawlers) were requisitioned and joined the RAN Brigade, to sweep the Victorian coast near Gabo Island. Thirteen German mines laid by Wolf were swept and destroyed. Since then Australia has maintained a mine warfare capability, conducting minesweeping operations in Australian and New Guinea waters during and after WW11, as well as in China and Malaysia.
There was a gap in minesweeping activities after WW11 until the RAN purchased six Ton Class Minesweepers from Britain in 1961; two of the minesweepers were built as minehunters, but functioned as minesweepers until the late 1960s when they were converted to minehunters, ushering in a new era in the RAN's mine warfare operations. Sister ship to Curlew was HMAS Snipe (M1102) built as HMS Alcaston by John I. Thornycroft and Company at Southampton and launched on 4 September 1952, bought by the RAN in 1961 and commissioned in 1962. Converted to minehunter 1968 to 1969 and paid off on 3rd. June 1983.
HMAS Curlew decommissioned on 9th. December 1966 after returning from the Indonesian Confrontation and was converted to a minehunter at Garden Island Dockyard, Sydney. The minesweeper HMAS Snipe, then under the command of Bill W decommissioned on the 12th. December 1968 and Bill re commissioned the Curlew the next day, the 13th. of December 1968 as the first minehunter in the Royal Australian Navy.
The conversion entailed fitting a high definition, short range sonar, the A/S 193 into the dome space, fitting up the ops room, rudders were replaced with active rudders powered by small diesel engines in the tiller flat and the ship was steered with a joystick via a computer instead of the traditional wheel. The crew was made up of 3 officers and 28 other ranks with 5 being Clearance Divers. Tons converted for minehunting had their influence minesweeping systems removed but retained their Oropesa mechanical sweeps, a precaution against deep moored mines. The hunters were instantly recognised by the pair of davits just aft of the funnel.
A Note on Mines
The naval mine is an efficient force multiplier and is one of the most cost effective weapons in the naval arsenal. Mines are small, easy to conceal, cheap to acquire, require virtually no maintenance, and can be easily and simply laid from almost any type of platform. Naval mines can be used to deny hostile forces access to the coastal zone and to defend important targets, such as ports, anchorages, and offshore structures, from amphibious or seaborne attack. Mines can quickly wipe out, or seriously impair, the effectiveness of surface and submarine forces. Emplaced mines are also difficult to counter and neutralize, especially in the presence of hostile forces. Because of these factors, mines are one of the most effective and deadly weapons that a naval force can employ. Left is an early Spanish Contact Mine similar to those used in the Phillipines against the Americans in the 1800s. Right is a WW 1 Mine cutout showing the explosive space (white) with primer pocket. The space above is battery and circuits plus a lot of "nothing" which is of course for buoyancy. This mine is fixed to a sinker, which would separate when the mine was layed. The two "bottle screws" are removed befor the mine is layed.
The Navy must be prepared to counter technologically advanced mines, as well as their low-tech predecessors. The success of simple World War 1 and World War 11 vintage mines means that these weapons will undoubtedly continue to threaten Naval forces. Since the mid 1980s, the number of naval mines and mining capable countries has increased dramatically. At present (2006), more than 50 countries, including those in politically sensitive areas, have mining capabilities with more than 300 mine types available.
Early mines are familiar as horned contact mines seen in many movies waiting for an unsuspecting ship to collide with it, causing a huge explosion damaging or sinking the ship. Mines are cheap to produce when compared to other weapons of war and cause a huge effort to clear. Moored mines of WW1 were simply cleared using a mechanical sweep consisting of wires towed by a minesweeper or between two minesweepers. Partly to combat the minesweeper and to cause chaos if a shallow draft vessel was to explode, a floating snare line could be attached to the horn of a contact mine.
Following on from moored contact mines came moored influence mines which were first used by the Germans in WW11 from 1939. The original types used a passing ships magnetic signature or sound signature to detonate the mine. Further development of mines resulted in ground or bottom mines, however they are designed to be used in shallow water to be close to a ship to use one of the ships signatures or a combination of two or more to detonate and to avoid problems from water pressure crushing the casing. Ground mines are generally shaped like a torpedo so they can be laid using submarine torpedo tubes, dropped from aircraft, fired off small fast boats or simply dropped over the side. Modern day container ships with "roll on, roll off" via a stern door could be converted to minelayers in a very short time and with their fast speed and advanced navigation systems be a major threat to any opposing sea forces. Bottom mines are not usually laid in waters deeper than 50 metres.
Sowing a "mixed bag" of mines was usually the strategy. To further confuse the enemy, mines can be and mostly are fitted with devices to prevent them being swept by conventional means.
Day clocks in the British Navy were able to be set from 1 to 44 days and a laid mine did not become active until the required number of days passed. Mines were laid using a mixed bag of settings so as to make minesweeping as difficult as possible.
Ship counters were also placed in the electrical circuit after the day clock and could be set to activate the mine after a set number of ships had passed over it, from 1 to 14 activations. This further frustrated minesweeping efforts as just one mine exploding meant that the whole area had to be swept again. The enemy had no way of knowing if another attack had occurred by minelayers or not.
With the development of the pressure mine which detonated using the pressure wave generated by the hull of a ship passing through the water, an operation had to be found to neutralize this type of mine. There are two areas of increased pressure (beneath the bow and the propeller) and one of reduced pressure (beneath the hull) of a ship underway which are used in combination to detonate a pressure mine. Sweeping this type of mine is almost impossible although a pressure body (sled) towed from a helicopter is used by the US Navy. This method of minesweeping is fast but very expensive and not always successful. An exploding mine can also bring down or damage a helo. The two objects which appear to be suspended under the aircraft are actually rear vision mirrors. Minesweeping helos were first used during the invasion of Vietnam.
"Stonefish" is an example of a British modern ground or bottom mine. It uses a combination acoustic/magnetic/pressure trigger and replaces earlier British mines. It can be launched by plane, ship or submarine for use in depths of 100-660 feet (30-200 metres). The mine weighs 2,180 lbs. (990 kg) with a 1,320 lbs. (600 kg) Torpex (a high explosive originally developed for torpedo warheads by Britain) warhead. Shelf life is 20 years with a 700 day operational lifetime. The Mark 2 is shorter and lighter with a 1,100 lbs. (500 kg) charge of PBX (a very high explosive developed in the US for torpedo warheads) and a much simpler firing circuit (over 30 components were eliminated). The stonefish has been produced for export to friendly countries around the world: Friendly one day potential enemies the next!
To answer the sea mine "Threat"? ... Minehunting!
The operation of clearing mines entailed locating the mine using the A/S 193 sonar, maneuvering the ship with the active rudders to keep it stationary while divers were sent in an inflatable boat using radar ranging to a position directly over the mine where they either placed an underwater charge by diving down or winched an explosive device to a position alongside the mine. The principal advantage of minehunting over minesweeping is that minehunters use forward-looking sonar, which enables the vessel to avoid passing over a mine while searching. It is currently the only practical MCM technique to counter the pressure mine. The key to successful minehunting in shallow water is high definition, short range sonar.
The Early History of High Frequency, Short Range, High Resolution, Active Sonar
During the period between WWI and WWII, the British were more active in the development of mine hunting sonar than were other navies. In general, most of these were anti-submarine warfare (ASW) sonars that were modified to operate with a shorter pulse, although some also operated at a slightly higher frequency. Beginning with the German use of influence mines in 1939, the emphasis was on being able to detect mines resting on the ocean floor. In brief, the British concluded that none of these sonars was adequate for the task. It is interesting to trace the threads of development of one of three types of sonar that are historically important: pulse-type ahead searching sonars, continuous transmission frequency modulation (CTFM) sonar, and very high resolution side scan sonar.
Pulse Type Ahead Searching Sonars
The first major U.S. effort to develop a minehunting sonar was the Underwater Object Locator (UOL) program at General Electric Co., initiated in 1942. The idea was to use a narrow beam (1.5 degrees), operating with continuous transmission at 1.0 MHz to mechanically scan a field in elevation and azimuth to paint an acoustic picture of the target (an acoustic camera). The program moved from the UOL MK-1 thru the UOL MK-IV, with a steady reduction in frequency (1.0 MHz to 100 kHz). In 1951, in reaction to the disastrous encounter with mines in Korea, the U.S. Navy greatly expanded its effort in mine countermeasures and procured the AN/UQS-1 in quantity (160). The AN/UQS-1, a 100 kHz sonar with 1.0 ms pulse, and 2.0 degree horizontal resolution, was subjected to extensive operational evaluation in the early fifties. It was determined that it could detect conventional bottom mines at ranges up to a few hundred yards, but it suffered from a critical weakness. In most shallow water areas with high traffic the ocean floor is cluttered with lots of junk, many pieces of which are comparable in size and target strength to mines. To investigate every target detected reduced the search and clearance rate to unacceptable low levels.
The inability of the AN/UQS-1 and other sonars developed before about 1950 to discriminate between mines and other targets at acceptable ranges led several navies to develop sonars that could classify targets detected as being mine like or nonmine like. While there are several target classification clues, by far, the most successful approaches were to image the size and shape of the target and its acoustic shadow which required better range and cross-range resolution. It was straight forward to obtain higher range resolution by using shorter pulses (0.1 ms), but better cross-range resolution was more difficult to achieve. Higher frequencies were employed, but at the cost of shorter ranges. In brief, several successful sonars were developed that did provide crude but useful images with the range resolution being much better than the cross-range resolution.
In the post WWII period, the British conducted an excellent basic research program on minehunting sonar that resulted in the development (ca. 1960) of the Type 193, a dual frequency sonar (100 and 300 kHz) with a 100 microsecond pulse and horizontal beamwidths of 1.0 and 0.3 degrees, which was the first successful operational mine detection-classification sonar: On a slightly later time schedule (ca. 1965), the U.S. Navy (USN) developed the AN/SQQ14 mine detection classification sonar with parameters similar to those of the Type 193. This was the primary USN surface ship minehunting sonar until the early nineties, and is still in service in several navies. By 1970 the French also had developed a two frequency system (Type DUBM 20/21) that was similar to the Type 193 and SQQ14.
These sonars were used on most of the ships (from eight countries) that detected and destroyed almost all of the 1200 mines planted by Iraq in the 1991 Persian Gulf War.
Early attempts at minehunting were made by the Royal Navy in the 1950s and 1960s. One system trialed was the Magnetic Towed Gradiometer (MTG). This employed a string of magnetic sensors towed across the seabed. When a metallic object was detected, a trace was made by the appropriate sensor pen on a paper roll onboard the ship and a marker buoy could be released. It soon became obvious that this system had severe limitations and more efforts were placed on the use of sonar for detecting mines.
In 1964 HMS Kirkliston became the first Ton Class (MCMV) to be converted into a Coastal Minehunter (MHC). She was fitted with the Minehunting System (Acoustic) Mk 1 (MHSA 1). This comprised the Plessey hull-mounted minehunting Sonar 193 and the Interim Navigation System Mk 1 (INS Mk 1). The INS Mk 1 included a Mk 20 plotting table on which a projected circular graticule was driven by an operator who manually "strobed" the radar echo of a short scope buoy using a cursor on his radar set. The geographical position of the short scope buoy had been established as accurately as possible and sonar contacts were plotted on a tracing paper "roller plot attachment" according to their range and bearing from the ship as represented by the centre of the moving graticule.
The identification and disposal of mines was achieved by a clearance diving team. The divers used an inflatable boat and a transit mounted to the top of the bridge and were "conned" on to the sonar target by matching the echo of the sonar reflector suspended beneath their dinghy with the echo of the "mine like" contact on the sonar display. Tons converted for minehunting had their influence minesweeping systems removed but retained their Oropesa mechanical sweeps.
HMAS Curlew, the first "Minehunter" in the RAN
Following a conversion that took almost 2 years, Lieutenant Commander B. A. Wilcox RAN, commissioned HMAS Curlew on 13th December 1968 as a minehunter. The first order given by the skipper to the wheelhouse was "port 60, slow ahead port rudder"! To the untrained this order was complete "double dutch"!! Curlew underwent an extensive workup, with the new A/S 193, active rudders, transit indicator, divers and "rubber duckies" all having to learn to work together. She then deployed to exercises with the Royal Malaysian Navy in waters off the East Coast of Malaya in the FESR, March 1969. Curlew succeeded in finding all her allocated mines and so was recognised as an efficient minehunter.
Minehunter Curlew's first Commanding Officer, My old skipper from Confrontation, Bill W, is respected today as the "Father of Minehunting in the Royal Australian Navy"
Typical use of the Ton Class Minehunters in the RAN
Syd B was midshipman in Curlew in 1965 in the FESR before leaving us in Tawau to fly to Singapore to join HMAS Duchess, which must have been a shock to him after our relaxed lifestyle. Against the advice of Admiral Twiss, CNS Far East Station, he became a Clearance Diver and was First Lieutenant and later Commanding Officer of Curlew. Syd has compiled from memory, the typical operations that Curlew (M1121) and Snipe (M1102) performed.
"The hunters had only the type 193 sonar and carried a team of five clearance divers. The diving equipment used was the Drager FGT 1A, a German set which was very quiet and with a low magnetic signature. A one man recompression chamber was located in the starboard waist. The diving capability was to 180 feet. Typical exercises were:
* Workups were usually conducted off Broken Bay and Jervis Bay particularly for both minehunting and minessweeping exercises.
* Mine Counter Measure exercises involving Squadron only took place off Port Lincoln in South Australia.
* Major exercises were conducted in Shoalwater Bay, North Queensland, the Kangaroo Series of Exercises involving Australian and US forces an example.
* Combined exercises with the Royal New Zealand Navy were held in the Hauraki Gulf, NZ.
* Combined MCM Exercises with the RN in the FESR. Minehunters Curlew and Snipe rafted up with other Tons in Singapore
* Bootless Inlet Mine Clearance: Both Curlew and Snipe were involved in the location and disposal of WW2 mines from Bootless Inlet PNG , just to the east of Port Moresby.
* Mine warfare Pilot Surveys (MWPS) were conducted on the approaches to and in major ports of Australia and the Great Barrier Reef recommended tracks throughout the seventies. These operations involved plotting seabed objects, identifying bottom (seabed) types and using divers to obtain bottom samples and conditions.
* Minehunting for lost or sunken objects, vessels and equipment.
* WW2 Mine Location and Disposal in Australian waters.
* Location and recovery of an RAAF F111 Aircraft from the seabed of Hauraki Gulf New Zealand. The lift of the largest section of fuselage was undertaken by HMAS Kimbla.
* Escorting and standing by USN nuclear powered submarines during port visits to Australian ports (Melbourne).
* Searching for Foreign hydrophone systems in West Timor Sea.
* Acting as sounding vessels with HMAS Moresby conducting hydrographic surveys in the Joseph Bonaparte Gulf.
* Apprehension and preparation of the subsequent prosecution of a Japanese Squid fishing vessel detained off the East coast of Tasmania.
* Provision of emergency recompression facility to civilian divers.
* Acting as the starting vessel for the Sydney to Hobart Yacht Race.
* Patrol Duties.
* Showing the Flag particularly at smaller ports.
In May and June 1975 the minehunters, HMAS Curlew and HMAS Snipe, using their A/S 193 sonar, together with the minesweeper HMAS Ibis surveyed the approaches to Darwin and the harbour itself, locating trawlers sunk during Cyclone Tracy, and other navigational hazards. Both hunters surveyed the approaches to all major ports in Australia, surveyed the recommended passages through the Great Barrier Reef and helped with the location and recovery of an RAAF F111 which crashed into the sea in New Zealand.
HMAS Curlew Decommissioned
HMAS Curlew decommissioned on 30th. April 1990, after a service with the RAN of 28 years and 38 years after being laid down. The tradional ship's paying off pennant was so long it was hard for the many balloons to keep it aloft!