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|Displacement:||270 long tons (274 t)|
|Length:||136 ft (41 m)|
|Beam:||24 ft 6 in (7.47 m)|
|Draught:||8 ft (2.4 m)|
|Propulsion:||General Motors 8-268A diesel engines, two shafts|
|Speed:||15 knots (17 mph; 28 km/h)|
The initial 80 ships were ordered by the U.S. Navy specifically for transfer to the United Kingdom under the Lend-Lease Programme. On transfer to Britain, BYMS-1 to BYMS-80 were assigned the British pennant numbers BYMS-2001 to BYMS-2080. Names were not assigned to the class members.
A further 53 BYMS bore hull numbers from 137 to 284. These were originally built for the U.S. Navy as part of the YMS class and transferred to Britain on, or shortly after, completion. On the Navy List they were designated BYMS, with their original U.S. Navy numbers. The remaining 17 BYMS vessels were delivered in a final batch.
Jacques Cousteau's well-known ship Calypso was originally built by the Ballard Marine Railway Company of Seattle, Washington, USA. She was a BYMS Mark 1 Class Motor Minesweeper, laid down on 12 August 1941 with the yard designation BYMS-26 and launched on 21 March 1942. She was commissioned into the Royal Navy in February 1943 as HMS J-826 and assigned to active service in the Mediterranean Sea, redesignated as BYMS-2026 in 1944, laid up at Malta and finally struck from the Naval Register in 1947.
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