The Medusa Trust

HDML Design and Construction

The design of the HDML was by W J Holt, Chief Constructor of the Naval Construction Department, Admiralty. The text that follows is his description of the vessel, extracted from a paper he presented in 1947 dealing with the construction of a variety of Coastal Forces craft.

A design of a small motor launch, for protection of harbours and estuaries against submarines, was prepared in the Admiralty in late 1939. The staff requirements were for a craft not exceeding 72 ft in length and capable of being carried aboard the deck of a ship.

These craft had the following dimensions:

Length overall…...72 feet
Beam over rubbers….15 feet 10 inches
Draught forward and aft….4 feet 3 inches

The fig below shows the design outline:
The boat was round bilge form and was provided with two large rudders to ensure quick turning during an anti-submarine attack. The boats were fitted with two diesel engines, manufactured by either Henty and Gardner (150 HP each), Gleniffer (160 HP each) or Thornycroft (130 HP each). The engines gave the boats speeds of between 11 and 12 knots on 54 tons displacement.

The hull was of double diagonal planking with bent transverse timbers outside longitudinal stringers and with reinforcing bent timbers at intervals inside the stringers. This system of planking proved itself resilient and strong. Early boats were planked with mahogany. When mahogany became scarce, it was necessary to use larch for planking, and in some cases this lead to leaky boats. The use of larch was discontinued immediately the supply position for mahogany improved.

The craft were armed with one 3 pdr. Gun forward, one Oerlikon aft, two Lewis guns on pedestals, and eight depth charges. Asdic was fitted.

Fuel tanks were provided for 1250 gallons in the main tanks and 300 gallons additional in the engine room. The HDMLs had an endurance of about 2000 miles at 10 knots.

Accommodation was provided for two officers, two petty officers and eight ratings. The wheelhouse in early boats was fitted with protective plating, but in later boats the bridge was armoured instead of the wheelhouse.

HMS Medusa Medusa The crew