During World War I, in 1916, the U.S. Navy began a registry of privately owned pleasure craft and yachts that were available for patrol service in the event the United States was drawn into the conflict.
"Section Patrol" ("SP") numbers were assigned in a series beginning with SP-1 and ultimately extending to well over 4,000.
As the registration process continued, other types of ships and craft were included for which the "Section Patrol" designation was clearly inappropriate, and these were generally given "Identification" ("ID") numbers in the same series as the "SP"s. In addition, some vessels that were numbered with an "SP" prefix before 1918 later had that prefix changed to "ID".
[1] This article incorporates public domain material from USN Ships -- by Hull Number: SP/ID -- World War I Era Patrol Vessels and other Acquired Ships and Craft.
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