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, 486 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).

Design and construction
HDMLs had a round bilge heavy displacement hull 72 ft long with a beam of 16 ft and a loaded draught of 5 ft. 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 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 are 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, 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.

Accommodation
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 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, they had 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, “Kent” 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 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 (HDMLs fitted with Gardner 8L3s) 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 was installed within the machinery space to provide 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 hp.

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.

Armament
Initially 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 0.303 Vickers machine 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.

Service


HDMLs were originally intended for the defence of estuarial and local waters, 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.

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 (including 1301), 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.

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.

Post-war
After the war HDMLs were adapted for other purposes, such as surveying, or were allocated to RNVR units to provide valuable seagoing experience for this important part-time navy. Some were sold to countries such as France, Cambodia, Burma, Malaya, Sri Lanka and the Philippines and became the backbone of those countries naval forces. 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 as Cutters for HM Customs, before finding their way onto the civilian market at the end of their working lives.

The last known active HDML is ML1387 (Medusa, one of a handful which were the last in Royal Naval service. ML1387 has recently undergone an extensive refit to keep her seagoing.

HDML survivors
The Medusa Trust maintains an extensive archive of documents, photographs and records of nearly all 480 HDMLs and their crews. The HDML 1301 is privately owned.

Builders
This is a partial list of known builders


 * Australia
 * A.McFarlane and Sons, Birkenhead, South Australia
 * E.A.Jack, Trevallyn, Launceston, Tasmania
 * Purdon & Featherstone, Battery Point, Tasmania


 * United Kingdom
 * Berthon Boat Co Ltd, Lymington
 * M W Blackmore & Sons Ltd, Bideford
 * G Bunn, Wroxham
 * A H Moody & Sons, Swanick Shore
 * R A Newman & Sons, Poole
 * Leo Robinson, Oulton Broad, Lowestoft
 * Anderson, Rigden & Perkins, Whitstable.
 * Bolson, Poole, Dorset.
 * Bute Slip, Clyde, Scotland.
 * Elkins, Christchurch, Dorset.
 * Harland & Wolff, Belfast, Northern Ireland.
 * Hillyard, Littlehampton, West Sussex.
 * Lady Bee, Isleworth.
 * McGruer, Clynder, Scotland.
 * McClean, Renfrew, Scotland.
 * Morgan Giles, Teignmouth Devon.
 * Ranalagh Yacht Yard, Wooton, Isle of Wight.
 * Robinson, Tewkesbury, Gloucestershire.
 * Sittingbourne Shipbuilding Co, Kent.
 * Sussex Shipbuilding Co, Shoreham-by-Sea.
 * Thornycroft, Hampton, London.
 * Herbert Woods, Potter Heigham, Norfolk.


 * USA
 * Ackerman Boat Works, Azusa, California.
 * Dodge, Newport News, Virginia.
 * Edgar, Rye, New York.


 * Elscot, City Island, New York.
 * Everett Marine Ways Inc, Everett, Washington (near Seattle)
 * Freeport Shipyard, Long Island.
 * Grays Harbour Shipbuilding Co, Aberdeen, Washington.
 * Harris & Parsons, Greenwich, Rhode Island
 * Hiltebrant, Kingston, New York
 * Huskins Yacht Corp.
 * Chas. P. Leek, Lower Bank, New Jersey.
 * Madden & Lewis, Sausalito, Marin Peninsula, California.
 * Perkins & Vaughn, Wickford, Rhode Island
 * Quincy Adams.
 * Thorsen, Ellsworth, Maine.
 * Truscott, St. Joseph, Michigan.


 * others
 * Africa Marine, Mombasa.
 * Alcock Ashdown, Karachi.
 * Armedi Shipyard, Bombay.
 * Bombay Dockyard.
 * Bombay Steam Navigation.
 * Garden Reach, Calcutta.
 * Hooghly Dock & Eng. Calcutta.
 * India General Navigation, Calcutta.
 * Irrawadi Flotilla Co., Rangoon.
 * Mohatta, Karachi.
 * Nichol, Durban, South Africa.
 * Pehara, Alexandria, Egypt.
 * Rangoon Dockyard, Rangoon.
 * Scindia, Bombay.
 * Spadbrow, Durban, South Africa.
 * Thornycroft, Singapore.
 * Walker, Colombo, Ceylon.