From Wikipedia, the free encyclopedia

The Harbour Defence Motor Launch (HDML) was designed by W J Holt at the Admiralty in early 1939. During World War II 486 HDMLs were built, mainly by yacht builders, in the United Kingdom and a number of other allied countries.

HDMLs were originally intended for the defence of estuarial and local waters, they proved such a sea-kindly and versatile design that they were used in every theatre of operation 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.

To begin with, HDMLs for foreign service were transported as deck cargo on larger ships, which is why the length was restricted to 72 feet. However, later in the war, with many merchant ships being sunk, it was found to be much safer to steam them abroad under their own power and some HDMLs (including 1301) undertook fairly substantial ocean voyages. Many of the Mediterranean fleet sailed from UK to Malta via Gibraltar in convoys, 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 Caribean. In the event, they did not make this voyage, and joined the Mediterranean fleet.

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

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 Burma, Malaya and the Philippines and became the backbone of their smaller navies. 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.

Design and Construction

HDMLs had a displacement hull of 72 feet (22metres) long with a beam of 16 feet (4.8 metres) and a loaded draft of 5 feet (1.5 metres). The loaded displacement was 54 tons. The hull had a pronounced flare forward to throw the bow wave clear, and providing considerable lift to prevent all but the heaviest seas from coming aboard. Although sea-kindly, there is a considerable tendency to roll, especially when taking seas at anything other than right angles. The cause, surprisingly, was their considerable reserve of stability, the effect of which was to impart a powerful righting moment if the ship was pushed over in a sea-way. 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 it had to be capable of turning within the turning circle of a submerged submarine. To achieve this, HDMLs were fitted with two very large rudders and, to reduce the resistance to turning, the keel ended 13 feet from the stern. A side effect of this was that the hull lacked directional stability, and it is extremely difficult to hold 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 “double-diagonal” construction. The hull is completed with frames or “timbers” riveted perpendicularly from the keel to the gunwhale on the inside of the planking, forming a very strong hull. The hull is further strengthened by the addition of longitudinal stringers riveted inside the timbers together with further timbers, known as “web frames” 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 on racks on the side decks) clear of the hull and propellers.

Most HDML hulls were planked in mahogany, but later in the war when 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. Ships operating in tropical waters (including the Mediterranean) were sheathed in copper below the waterline to prevent attack by marine borers.

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

HDMLs were designed to accommodate a crew of 10. There were berths for 6 ratings in the fore cabin, which also contained a galley coal fired stove – Admiralty Pattern 3160. In the forepeak, was a Baby Blake sea toilet and hand wash basin. The officers were berthed in the after end of the ship; 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, so that it was subject to a certain amount of 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 ship’s radio room was a small compartment situated aft on the starboard side, adjacent to the petty officers’ toilet.

On the main deck, was the chartroom, which contained the chart table, a casual berth and a second steering position. On the forward bulkhead was fitted the navigational switchboard, 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.

Armament

To begin with HDMLs were commonly fitted with a 2-pounder gun on the foredeck, an Oerlikon high angle/low angle machine gun on the stern cabin (this could also be used for anti-aircraft defence) and a Vickers 0.303" machine gun on each side of the bridge. They carried 10 depth charges, also on the stern cabin. The 2-pounder guns were not particularly accurate, possibly in view of the boats' tendency to roll, and many were replaced by another Oerlikon HA/LA gun.

• Adapted from “Medusa” by Mike Boyce

{{uncat}Cats: Royal Navy, Coastal Forces, World War2}