On May 6, 1861, Congress authorized presidential issue of letters of marque and reprisal against vessels of the United States. Ship owners were eligible for eighty-five percent of everything seized. Subsequent legislation provided for a bonus of $20 for each person aboard a captured or destroyed vessel, and twenty percent of the value of each enemy warship sunk or destroyed. The tightening Union blockade made the privateers less effective as they could not return prizes to Southern ports for disposition.