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.

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.

To achieve this, HDMLs were fitted with two very large rudders and, to reduce resistance to turning, the keel ended 13 ft (4.0 m) before the stern.

Boats operating in tropical waters (including the Mediterranean Sea) were sheathed in copper below the waterline to prevent the attack of marine borers.

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.

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 was also a Gardner sourced engine of the type 1L2, and was a single cylinder hand start unit producing 7.5 horsepower (5.6 kW).

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

In Royal Australian Navy service they were used for coastal patrols around northern Australia, New Guinea and Timor, and for covert activities behind Japanese lines in Southeast Asia.

HDMLs were initially transported as deck cargo on larger ships for foreign service, which is why their length was restricted to 72 ft (22 m).

[clarification needed] 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.

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 Ocean in order to keep clear of the enemy occupied coast.

British HDMLs were normally manned by Royal Naval Volunteer Reserve (RNVR) officers with temporary commissions, and "hostilities only" ratings.

[citation needed] After the war many HDMLs were adapted for other purposes, such as survey vessels, search and rescue, dispatch boats and for fisheries patrols and training.

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.