An excerpt from an article by Ruth Priest Dixon about Seaman's Protection Certificates in the National Archives Prologue Magazine:
Genealogical Fallout from the War of 1812
The impressment of American seamen by the British was one of the causes of the War of 1812. The practice also resulted in the creation of extensive records about merchant seamen that are of great value to genealogists and historians. These Seamen's Protection Certificate Applications for what might well be called a merchant seaman's passport have remained virtually untouched since they were originally filed. Now they are being organized and preserved, and those from the early years are already indexed. These records are in the Old Military and Civil Branch at the National Archives.
NARAtions also published an article about the Seamen's Protection Certificates.