 A total of 561 were built at various U.S. yards.Originally a class of "motor minesweepers" (MS), "Y" was added to distinguish them from other minesweeper types; sources disagree on whether "Y" stood for "yardcraft" – indicating a type of craft assigned for duty within a navy shipyard or naval base and not expected to go beyond waters adjacent to the base – or to indicate they were built by yacht-builders; YMSs were built at 35 yacht yards, rather than at larger shipyards, 12 on the United States East Coast, 19 on the United States West Coast, and four in yards on the Great Lakes.Records show that YMSs were used in the United States Navy to sweep mines laid by enemy submarines as early as 1942 off the ports of Jacksonville, Florida, and Charleston, South Carolina.