Harbour defence motor launch

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ML 1322, a Royal Australian Navy HDML, in Brisbane in 1944
Class overview
Name	Harbour defence motor launch
Completed	486
Active	Until the early 1970s
General characteristics
Displacement	54 tons (full load displacement)
Length	72 ft (22 m)
Beam	16 ft (4.9 m)
Draught	5 ft (1.5 m)
Installed power	152 bhp (113 kW) each engine[1]
Propulsion	Twin handed Gardner 8L3 marine engines[1]
Speed	12.5 knots (23.2 km/h; 14.4 mph)
Range	2,000 mi (1,700 nmi; 3,200 km) at 10 knots (19 km/h; 12 mph)(1,650 gallons)
Complement	2 officers, 2 petty officers and 8 ratings
Armament	Typically twin 20mm Oerlikons, twin Vickers K machine guns and six depth charges

The harbour defence motor launch (HDML) was a 72 ft (22 m) long British-designed motor vessel used for harbour defence during World War II. Nearly 500 were built by numerous Allied countries during the war.

The HDML was designed by W J Holt at the Admiralty in early 1939. During the war 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).^[2]