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 long with a beam of and a loaded draught of . 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 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 borer s . 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 rating s 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 "