Harbour Defence Motor Launch

The Harbour Defence Motor Launch (HDML) was a British small motor vessel of the Second World War.

The HDML was designed by W J Holt at the Admiralty in early 1939. During the war, 464 HDMLs were constructed, mainly by yacht builders, in the United Kingdom and a number of other allied countries. In view of their later expanded combat roles in some Commonwealth navies some HDMLs were re-designated as "Seaward Defence Motor Launches" (SDML) or "Seaward Defence Boats" (SDB).[2]

StateLibQld 1 79147 Royal Australian Naval ship ML 1322 at Colmslie Naval Base, Brisbane, ca. 1944
ML 1322, an Australian HDML, at Brisbane in 1944
Class overview
Name: Harbour defence motor launch (HDML)
Completed: 486
Active: Until the early 1970s
General characteristics
Displacement: 54 tons (full load displacement)
Length: 72 ft (22 m)
Beam: 16 ft (4.9 m)
Draught: 5 ft (1.5 m)
Installed power: 152 bhp (113 kW) each engine[1]
Propulsion: Twin handed Gardner 8L3 marine engines[1]
Speed: 12.5 knots (23.2 km/h; 14.4 mph)
Range: 2,000 mi (1,700 nmi; 3,200 km) at 10 knots (19 km/h; 12 mph)(1,650 gallons)
Complement: 2 officers, 2 petty officers and 8 ratings
Armament: Typically twin 20mm Oerlikons, twin Vickers K machine guns and six depth charges

Design and construction

HDMLs had a round bilge heavy displacement hull 72 feet (22 m) long with a beam of 16 feet (4.9 m) and a loaded draught of 5 feet (1.5 m). Loaded displacement was 54 tons. The hull had a pronounced flare forward to throw the bow wave clear and provided considerable lift to prevent all but the heaviest seas from coming aboard. Although seaworthy, the boat had a considerable tendency to roll, especially when taking seas at anything other than directly ahead or astern. The cause was the round bilge midship section and a considerable reserve of stability, the effect of which was to impart a powerful righting moment if the boat was pushed over in a seaway. This, coupled with the round bilged hull and lack of bilge keels, would set up a rapid and violent rolling.

One of the design criteria was that the boat had to be capable of turning inside the turning circle of a submerged submarine. To achieve this, HDMLs were fitted with two very large rudders and, to reduce resistance to turning, the keel ended 13 ft (4.0 m) before the stern. A side effect of this was that the boat lacked directional stability and was extremely difficult to hold on a straight course.

The hull was of round bilge wooden construction, planked with two diagonally opposed skins with a layer of oiled calico between them – known as a "double-diagonal" technique. The hull was completed with frames or "timbers" riveted perpendicularly from the keel to the gunwale on the inside of the planking, forming a very strong hull. The hull was further strengthened by the addition of longitudinal stringers riveted inside the timbers together with further timbers, known as "web frames". They were fastened inside the stringers opposite every third main timber. HDMLs were fitted with a deeper section rubbing strake aft. Its purpose was to roll depth charges (kept in and delivered from racks on the side decks) clear of the hull and propellers.

Most HDML hulls were planked in mahogany, but as the war progressed this became scarce and larch was used, although this tended to lead to leaky hulls. The decks were also of double-diagonal construction and generally made of softwood. Boats operating in tropical waters (including the Mediterranean) were sheathed in copper below the waterline to prevent the attack of marine borers.

In order to lessen the chances of the boat sinking in the event of damage to the hull, they were divided into six watertight compartments. Provided that the bulkheads were not damaged, the boat could remain afloat with any one compartment flooded.

Q 1362
A Lend-Lease HDML, built in Rye, N.Y.

Some were constructed in the United States and nominally owned by the United States Navy, but delivered to the Royal Navy and other allies under Lend-Lease. Most were returned to the United States Navy at the end of the war before being sold to other countries, the majority to the Royal Netherlands Navy.


HDMLs were designed to accommodate a crew of ten. There were berths for six ratings in the fore cabin, which also contained a galley with a coal fired stove.[note 1] In the forepeak, there was a Baby Blake sea toilet and hand wash basin. The officers were berthed in the after end of the boat; the petty officers being in a cabin on the port side just aft of the engine room with their own separate toilet and hand wash basin. A small "Courtier" coal-fired stove provided heating.

The commissioned officers had comparatively roomy accommodation in the wardroom aft, although it suffered from being situated above the propeller shafts and therefore subject to noise and vibration. The wardroom also contained the ship’s safe, a dining table and seating, a wine and spirit locker, a small coal stove and a tiny footbath.

The boat's radio room was a small compartment situated aft on the starboard side, adjacent to the petty officers’ toilet.

The chartroom was located on the main deck. It contained the chart table, a casual berth and a second steering position. On the forward bulkhead a navigational switchboard was fitted, which included a duplicate set of engine revolution indicators, switches for the navigation lights, clear view screens and the "action-stations" alarm.

The main steering position was on the open bridge where the two engine room telegraphs were fitted. There were also voice pipes connected to the inside steering position, the engine room, the radio room and the wardroom.

Engine room

The HDMLs had a manned engine room which usually comprised two engine room staff when in Royal Naval service. There was no direct bridge control of the main engines or machinery. A small ship's telegraph system was used in conjunction with a buzzer system, with predetermined signals for the communication of orders between the engineer and master.

The engineer operated the machinery from a position between the main engine propulsion gearboxes on the centreline of the vessel. This was generally done in the sitting down position, using a removable seat which was hung from the engine room access ladder. Four levers were used to control the two Gardner 8L3s engine's RPM settings and the direction of drive to the propellers via reversing gearboxes. A governor (speed control) control lever was used to adjust the engine revolutions, and a gearbox lever was used with positions for ahead, neutral and reverse.

Settings for the engine governor controls were "slow" 250 RPM, "half" 600 RPM, "full" 800 RPM and "emergency full" 900 RPM, and those settings were possible with the gearboxes in ahead or astern. The vessel's telegraphs indicated the required settings for all levers at any one time.

Other operations included the monitoring of the water jacket temperatures of both prime movers. Gardner design engineers designed the early marine variants of the 8L3s to be direct salt-water–cooled, with an allowance for corrosion included in the water jacket wall thickness. To maintain the correct operating temperature of 62 degrees C, a bypass valve was incorporated in the cooling circuit. This allowed varying amounts of the coolant to be diverted back to the feed side of the pump, thus raising the water temperature before circulating it around the engine. This in turn resulted in a higher overall engine temperature.

A third engine acted as an auxiliary power unit. This was installed within the machinery space and provided motive power for electrical generation and to operate the fire and bilge pump set. This was also a Gardner sourced engine of the type 1L2, and was a single cylinder hand start unit producing 7.5 horsepower (5.6 kW).

Other features of the machinery space were five liquid storage tanks: two large fuel oil tanks on the centre of each wing, with two day service fuel oil tanks just forward of the former, which supplied fuel to all engines by gravity feed. The fifth tank was used to store lubrication oil, and this was generally sited on the port side aft area of the space. The adjacent space on the STB side provided space for the engineer's work bench.


The intended armament was a QF 3-pounder gun, an Oerlikon 20 mm cannon and two machine guns.[3] As constructed, HDMLs were commonly fitted with a QF 2-pounder gun on the foredeck, an Oerlikon 20 mm high angle/low angle cannon on the stern cabin which could be used against surface targets or anti-aircraft defence and a Vickers K gun or Lewis gun on each side of the bridge. They carried 6 to 8 depth charges on the aft decks. The 2-pounder guns were not particularly accurate, possibly because of the boats' tendency to roll, and many were replaced by another 20 mm Oerlikon HA/LA gun. Some Australian HDMLs also carried Browning .50 calibre machine guns.


HDML 1301 In harbour at Padstow in WWII
HDML 1301 in Padstow with extra fuel tanks and stores for a voyage to Malta

HDMLs were originally intended for the defence of estuarial and local waters, but they proved such a seaworthy and versatile design that they were used in every theatre of operations as the war progressed. They were to be found escorting convoys off the West Coast of Africa, carrying out covert activities in the Mediterranean and undertaking anti-submarine patrols off Iceland. They also played major roles in Operations Glimmer and Taxable, deception operations to draw German attention away from the Normandy landings.

In Royal Australian Navy service they were used for coastal patrols around northern Australia, New Guinea and Timor, and for covert activities behind Japanese lines in South East Asia.

HDMLs were initially transported as deck cargo on larger ships for foreign service, which is why their length was restricted to 72 feet. Later in the war, with many merchant ships being sunk, it was found to be much safer to move them abroad under their own power. Some HDMLs, undertook fairly substantial ocean voyages. Many belonging to the Mediterranean fleet sailed from the UK to Malta via Gibraltar in convoy, voyages which necessitated going well out into the Atlantic in order to keep clear of the enemy occupied coast. Three HDMLs were fitted with a second mast and sails with the intention of sailing to the Caribbean. In the event, they did not make this voyage, joining the Mediterranean fleet instead.

British HDMLs were normally manned by Royal Naval Volunteer Reserve (RNVR) officers with temporary commissions, and "hostilities only" ratings. The crews, however, gained an enviable reputation for their skill and expertise in the handling and fighting of their vessels.


After the war many HDMLs were adapted for other purposes, such as survey vessels, search and rescue, dispatch boats and for fisheries patrols and training. ML1387 (renamed HMS Medusa in 1962), was the last in Royal Naval service, operating as an hydrographic survey vessel until November 1965 and subsequently preserved by the Medusa Trust. The last Royal Australian Navy HDML. HMAS HDML 1321, was paid off in 1971.[4] Others were allocated to naval reserve units. Some were loaned and later sold to countries such as Greece, Cambodia,[5] and the Philippines. Some formerly with British colonial navies, such as the Royal Indian Navy, were transferred on independence to the new countries such as India, Pakistan and Burma. Some were retained by various governments for civilian use, such as police and customs. Those with the Free French Naval Forces during WW2 were incorporated into the French Navy, many based in overseas colonies such as Cameroon and French Indochina. Many were sold out of naval service to become private motor yachts or passenger boats, purposes for which they were ideally suited, with their diesel engines and roomy accommodation. Such was the superior design and build of these craft, that a number still survive today in their civilian role. Others continued in government service before finding their way onto the civilian market at the end of their working lives.

A Royal Hellenic Navy HDML was heavily modified to resemble a German E-boat for use in the 1961 film The Guns of Navarone.


The Medusa Trust maintains an extensive archive of documents, photographs and records of nearly all 480 HDMLs and their crews.[6]

The HDML ML1387 (renamed HMS Medusa in 1962), is a museum ship in Haslar Marina near Portsmouth and has recently undergone an extensive refit to keep her seagoing.

The HDML ML1301 is privately owned.[7] https://mecmuseum.nl/hdml-1301/

HDML 1321 survived as a dive/tour boat. In 2016 a trust was established to purchase and restore 1321 to her wartime configuration, as it is the first Australian-built HDML and took part in the Z Special Operation Copper raid on Muschu Island off Wewak in April 1945. On 19 October 2016, 1321 sank at her mooring in Darwin Harbour - funds are now being raised to salvage the vessel.1321 salvage fund


This is a partial list of known builders

United Kingdom


World War II


Other government departments

See also


  1. ^ Admiralty Pattern 3160
  1. ^ a b General directions for the operation of Gardner L3 Diesel engines, p. 10, The Cloister Press.
  2. ^ Sea Power Centre history page for SDB 1323, Australian Navy, archived from the original on 21 December 2013
  3. ^ Chris Bishop (2002). The Encyclopedia of Weapons of World War II. Sterling Publishing Company. p. 525. ISBN 1586637622.
  4. ^ John Bastock (1975). Australia's Ships of War. Angus and Robertson. p. 176. ISBN 0207129274.
  5. ^ Conboy, FANK: A History of the Cambodian Armed Forces, 1970-1975 (2011), p. 239.
  6. ^ Medusa Trust
  7. ^ "hdmlmeda". Archived from the original on 10 July 2012. Retrieved 1 May 2011.


External links

Amenities ship

An amenities ship is a ship outfitted with recreational facilities as part of a mobile naval base. Amenities ships included movie theaters and canteens staffed by mercantile crews of the Royal Fleet Auxiliary service. These ships were intended to provide a place where British Pacific Fleet personnel could relax between operations.

Coastal minesweeper

Coastal minesweeper is a term used by the United States Navy to indicate a minesweeper intended for coastal use as opposed to participating in fleet operations at sea.

Because of its small size—usually less than 100 feet in length—and construction—wood as opposed to steel—and slow speed—usually about 9 or 10 knots—the coastal minesweeper was considered too fragile and slow to operate on the high seas with the fleet.

Minesweeping, in conjunction with fleet activities, was usually relegated to the diesel-driven steel-hulled AM-type minesweepers, later to be replaced by the wood-hulled MSO-type minesweeper with aluminum engines.

Coastal submarine

A coastal submarine or littoral submarine is a small, maneuverable submarine with shallow draft well suited to navigation of coastal channels and harbors. Although size is not precisely defined, coastal submarines are larger than midget submarines, but smaller than sea-going submarines designed for longer patrols on the open ocean. Space limitations aboard coastal submarines restrict fuel availability for distant travel, food availability for extended patrol duration, and number of weapons carried. Within those limitations, however, coastal submarines may be able to reach areas inaccessible to larger submarines, and be more difficult to detect.

General stores issue ship

General stores issue ship is a type of ship used by the United States Navy during World War II and for some time afterwards.

The task of the general stores issue ship was to sail into non-combat, or rear, areas and disburse general stores, such as canned goods, toilet paper, office supplies, etc., to ships and stations.

Guard ship

A guard ship is a warship assigned as a stationary guard in a port or harbour, as opposed to a coastal patrol boat which serves its protective role at sea.


HDML may refer to:

Handheld Device Markup Language

Harbour Defence Motor Launch


HMAS HDML 1321, also known as Rushcutter was a 58-ton Harbour Defence Motor Launch of the Royal Australian Navy (RAN). Built by Purdon & Featherstone, Battery Point, Hobart, Tasmania and commissioned into the RAN on 11 November 1943, being the first Australian-built HDML to be commissioned and the last HDML in RAN service. She was assigned to Z Special Unit and delivered commandos for the 1945 ill-fated raid on Muschu Island. She was later reclassified as a Seaward Defence Boat and put into reserve after the war. She was recommissioned as HMAS Rushcutter (ML 1321) in 1953 and used as an unarmed training vessel for the Royal Australian Naval Reserve and Australian Navy Cadets until 1970. Paid off in August 1971, she was converted to pleasure craft MV Rushcutter and is now based in Darwin.

Rushcutter was moved from its mooring in Cossack Creek to the Small Boat Anchorage between Stokes Hill Wharf and the East Arm Wharf in 2016 while it was being offered for sale. It sank there on 19 October 2016. The hull, without any significant deck or hull structures, was raised and landed in July 2018; the wreck was bought by the conservation group from the owner, Ms Geddes, for a nominal AU$2. In mid July 2018, it was refloated by using buoyancy assistance and pumps, then removed from the water for further preservation.


HMAS HDML 1324, also known as Nepean was a 58-ton Harbour Defence Motor Launch of the Royal Australian Navy (RAN). Built by A.McFarlane and Sons, Birkenhead, South Australia and commissioned into the RAN on 12 June 1944. She was at the Timor surrender of the Japanese occupying forces in 1945. She was later reclassified as a Seaward Defence Boat. Parts from HDML 1324 were used to refit SDB 1325.

HMAS Rushcutter

Two units of the Royal Australian Navy (RAN) have been named HMAS Rushcutter, for Rushcutters Bay, New South Wales.

HMAS Rushcutter (naval base), a depot and training base originally operated as the headquarters of the New South Wales colonial navy, and later operating under this name from 1940 to 1979, when the land was granted to the public.

HMAS Rushcutter (ML 1321), a former Harbour Defence Motor Launch used as a training vessel.

HMAS Rushcutter (M 80), the lead ship of the Bay-class minehunting catamarans, which was launched in 1986 and decommissioned in 2001.

HMS Meda

Two vessels of the Royal Navy have borne the name HMS Meda:

HMS Meda (1880) was a survey schooner launched in 1880 at Barnstaple and purchased that year by the Navy. She was sold in 1887 to the Government of Western Australia.

HMS Meda (1943) was a Harbour Defence Motor Launch launched in 1943 at Bideford and previously known as SDML 3552, SDML 1301 and HDML 1301. The boat acted as a navigation marker during the invasion Elba in June 1944. After the Second World War she was used for survey work in the Mediterranean and English Channel, and renamed HMS Meda in 1949. The vessel was sold to a private individual in 1966.

HMS Medusa

Ten ships of the Royal Navy have borne the name HMS Medusa, after the ancient Greek mythological figure Medusa:

HMS Medusa (1785) was a 50-gun fourth rate launched in 1785 and wrecked in 1798.

HMS Medusa was 38-gun fifth-rate frigate launched in 1801. She was Nelson's flagship on his return to England at Harwich on 9 August, was present at the Action of 5 October 1804 and was broken up in 1816.

HMS Medusa was to have been a 46-gun fifth rate. She was ordered in 1816, reordered in 1830 and cancelled in 1831.

HMS Medusa (1838) was a wooden paddle packet launched in 1838 and sold in 1872.

HMS Medusa (1839) was an iron paddle gunboat launched in 1839 and wrecked in 1853.

HMS Medusa (1888) was a Marathon-class cruiser launched in 1888, on harbour service from 1910, sold in 1920 and resold in 1921.

HMS Medusa (1915) was a Medea-class destroyer, previously the Greek Lesvos. She was purchased in 1914, before being launched in 1915. She was sunk in a collision with HMS Laverock in 1916.

HMS Medusa was an M29-class monitor, previously named HMS M29. She was renamed HMS Medusa in 1925, converted to a depot ship and renamed HMS Talbot in 1941, HMS Medway II in 1943 and back to HMS Medusa in 1944. She was sold in 1946 and broken up in 1947.

HMS Medusa (1939) was an auxiliary minesweeper requisitioned in 1939 and transferred to the Royal Australian Navy as HMAS Mercedes in 1942.

HMS Medusa (A353) was a harbour defence motor launch, launched in 1943 as ML 1387. She served in D-Day, was renamed BDB 76 in 1946, SDML 3516 in 1949 and Medusa in 1961. She was paid off in 1963, and is now a museum ship.

Mine countermeasures vessel

A mine countermeasures vessel or MCMV is a type of naval ship designed for the location of and destruction of naval mines which combines the role of a minesweeper and minehunter in one hull. The term MCMV is also applied collectively to minehunters and minesweepers.


A minehunter is a naval vessel that seeks, detects, and destroys individual naval mines. Minesweepers, on the other hand, clear mined areas as a whole, without prior detection of mines. A vessel that combines both of these roles is known as a mine countermeasures vessel (MCMV).

Motor Gun Boat

Motor Gun Boat (MGB) was a Royal Navy term for a small military vessel of the Second World War. Such boats were physically similar to Motor Torpedo Boats (MTBs), equipped instead with a mix of guns instead of torpedoes. Their small size and high speed made them difficult targets for E-boats or torpedo bombers, but they were particularly vulnerable to mines and heavy weather. The large number of guns meant the crew was relatively large, numbering as high as thirty men.

Motor Launch

A Motor Launch (ML) is a small military vessel in Royal Navy service. It was designed for harbour defence and submarine chasing or for armed high-speed air-sea rescue. Some vessels for water police service are also known as motor launches.

Ocean boarding vessel

Ocean boarding vessels (OBVs) were merchant ships taken over by the Royal Navy for the purpose of enforcing wartime blockades by intercepting and boarding foreign vessels.

Repair ship

A repair ship is a naval auxiliary ship designed to provide maintenance support to warships. Repair ships provide similar services to destroyer, submarine and seaplane tenders or depot ships, but may offer a broader range of repair capability including equipment and personnel for repair of more significant machinery failures or battle damage.

Submarine chaser

A submarine chaser is a small and fast naval vessel that is specifically intended for anti-submarine warfare. Many of the American submarine chasers used in World War I found their way to Allied nations by way of Lend-Lease in World War II.

Submarine tender

A submarine tender is a type of depot ship that supplies and supports submarines.


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