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).