Defence of the realm
Of the 460 Harbour Defence Motor Launches that served the allied Navies from the 1930s through to the 1960s, just one survives intact. Mike Taylor reports
Of 460 Harbour Defence Motor Launches built from the 1930s-60s, just one, Medusa, survives intact.
Any mention of Coastal Forces craft usually elicits images of racy motor gun and torpedo boats making a splash at around 40 knots. In fact, these craft were just a part of a larger command, which included round-bilge Motor Launches (MLs) and Harbour Defence Motor Launches (HDMLs).
Of 464 HDMLs eventually built, only one, Medusa, remains fully operational, although there is a well restored variant of the design in New Zealand (see page 58) and numerous other hulls still afloat, mostly as houseboats. So how and why were they built and what are the obstacles to converting and maintaining them?
With the advent of hostilities in 1939 the UK was woefully ill-equipped. A major
marine development and building programme was quickly initiated involving many yards around the country and abroad. In addition, high performance engines were in short supply, trained crews did not exist and effective tactics had to be worked up. Urgency was essential and soon the various coastal forces craft were operational adding a vital component to the international war effort.
The HDML design was the work of WJ Holt, chief constructor and small craft expert at the Admiralty’s naval construction department. HDMLs had a round-bilge heavy-displacement hull with a pronounced flare forward to throw the bow wave clear of the forepeak and bridge, providing considerable lift and preventing all but the heaviest seas from coming aboard.
Their curved midships hull section gave marked reserves of stability but although extremely seaworthy they had a considerable tendency to roll, especially when taking seas at anything other than bow or stern on. This action could become quite violent in some conditions when maintaining a direct course could be impossible.
Planked construction
Construction was all wood comprising two diagonally opposed layers of planking, mostly mahogany, with a layer of oiled calico between. These were laid over Canadian rock elm frames riveted perpendicularly from the keel to the gunwale. The hull was further strengthened by longitudinal stringers fixed inside the frames and further timber
frames, known as web frames. Finally, HDMLs had a deep-section rubbing strake aft, providing a sturdy edge from which the crew could roll depth charges.
As the war progressed mahogany became scarce, so larch was used instead, though these later hulls had a tendency to leak. The decks were also of double-diagonal construction and generally made of mahogany or larch. Boats operating in tropical waters (including the Mediterranean) were sheathed in copper below the waterline to prevent the attack of marine borers.
The vessels’ hulls were divided into six watertight compartments to reduce the chances of the boat sinking in the event of damage. Provided the bulkheads were not damaged the boat could remain afloat with any one compartment flooded.
The forward area below decks comprised six berths for ratings, a galley with a coal fired stove and a heads plus washing facilities in the forepeak. Abaft the engine room, the petty officers had a cabin on the port side with a separate heads and wash basin. A tiny radio compartment was positioned on the starboard side. A small coal fired stove
provided heating. Abaft to starboard was the wardroom to accommodate the captain and first officer. Here was located a dining area, separate seating, a generous wooden storage ‘tall boy’, the ship’s safe, the spirits locker and a coal stove. The officers had their own heads right aft. Finally, on the main deck was a spacious chartroom accessed from the bridge. In addition to a subsidiary steering position with engine rev counters, it also housed the navigational switchboard and ASDIC (Sonar) equipment. For the navigator, there was a chart table with drawers below and an occasional bunk. Above was the open bridge with the main steering position, telegraphs and voice pipes to the engine room, radio room and secondary steering position.
long-range power
The engine room, manned by an engineer and a stoker, usually housed twin Gardner 8L3, 24-litre, eight cylinder inline diesel engines. A third unit, usually a Gardner 1L2 single cylinder engine, powered the dynamos for the ship’s 24V and 110V DC electrical system as well as bilge pumps and fire main. The main fuel tanks are forward of the engine room and fuel was hand pumped from here to the day running tanks in the engine room. Drive to the outward turning propellers was direct and with twin rudders gave HDMLs a commendably tight turning circle.
Armament varied, determined by availability. Today, Medusa mounts a 20mm Oerlikon high/low angle cannon on her foredeck with a second on the wardroom coachroof and twin 0.303 Vickers K machine guns on each side of the bridge as well as six depth charges suitably placed on the aft decks.
Originally, HDMLs were intended for the offshore anti-submarine defence of harbours. However, they quickly proved their capabilities, able to function in every theatre of wartime operations. This included escorting convoys off the west coast of Africa, carrying out covert
SOE-type activities in the Mediterranean, anti-submarine patrols off Iceland and landing coast watching teams behind enemy lines in the Far East. Initially transported as deck cargo on larger ships to foreign parts, as the war progressed they steamed under their own power in convoys from the UK to Gibraltar, which often made significant detours out into the Atlantic to pass well clear of enemy held coasts, their crews (often little more than youngsters) gaining a strong reputation as master seamen. From there they went to Cape Town and the Mediterranean via the East Coast of Africa and Suez as well as onwards to the Far East. Six were intended to go to the West Indies which was outside their fuel range and so were fitted with a secondary mast and sails.
Post war, HDMLs were adapted for surveying and as fast dispatch boats as well as being allocated to Royal Navy Volunteer Reserve units providing valuable seagoing experience. Others were sold to countries such as Sri Lanka, Burma, Malaya, Italy and Greece.
As their service careers came to an end most were sold off into private hands and eventually converted into houseboats and pleasure cruisers.