Admiralty law in the United States developed from the British admiralty courts present in most of the American colonies. These courts functioned separately from courts of law and equity. With the Judiciary Act, though, Congress placed admiralty under the jurisdiction of the federal district courts. Although admiralty shares much in common with the civil law, it is separate from it. Common law does not act as binding precedent on admiralty courts, but it and other law may be used when no law on point is available.
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Just as the Federal Rules of Civil Procedure placed law and equity under the same jurisdiction in 1938, the 1966 rules subsumed admiralty. Nonetheless, the Supplemental Admiralty Rules take precedence over the Federal Rules of Civil Procedure in the event of conflict between the two.
http://lii.law.cornell.edu/topics/admiralty.html