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U.S. Boundary Line

(46 CFR part 7)


The Boundary Line marks the dividing point between internal and offshore waters for several legal purposes, including load line regulations.

(The Boundary Line is also used in crediting inland/offshore sea service for mariner licensing; refer to the USCG National Maritime Center website for licensing issues.)

The Boundary Line is sometimes confused with the Demarcation Line, which is the dividing point between domestic rules-of-the-road (Inland Navigating Rules) and the international rules-of-the-road (Convention on the International Regulations for Preventing Collision at Sea, or COLREGS). The Boundary Line and Demarcation Line are different lines for different purposes, although they might coincidently overlap at places along the coastline.

Boundary Line Regulations

The U.S. Boundary Line is delineated in 46 CFR Part 7.

As a rule, on the Atlantic, Pacific, and Great Lakes coasts, the Boundary Line generally follows the high water shoreline, extended across the entrances to small bays, inlets, harbors, rivers, the ends of breakwaters or jetties, etc. In most cases, this means that as soon as an outbound vessel crosses seaward of the entrance, it has crossed outside the Boundary Line.

There are some significant exceptions to the above general rule, however:

Therefore, it is essential to consult 46 CFR Part 7, in conjunction with the appropriate nautical chart, to determine the specific location of the Boundary Line for any particular location.

Contact the Naval Architecture Division:

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Last Modified 5 Oct 2015