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U.S. Boundary Line
(46 CFR part 7)
Overview
The Boundary Line marks the dividing point between internal and offshore waters for several legal purposes, including load line regulations.
The Boundary Line is sometime 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 general rule, the Boundary Line follows the trend of the seaward, highwater shorelines and across the entrances to small bays, inlets, and rivers. In most cases, this means that as soon as an outbound vessel crosses seaward of the entrance, it has crossed outside the Boundary Line (even if on a domestic voyage).
There are some significant exceptions, however. For example, the Boundary Line is 12 miles offshore along the Gulf of Mexico between the Marquesas Keys, FL and Rio Grande, TX. There may also be some local gerrymandering of the Boundary Line across the mouths of inlets or jetties.
Therefore, it is essential to consult 46 CFR Part 7 regarding the specific location of the Boundary Line for any particular waterway.
CFR regulations can be accessed through the GPO Access database. Scroll down the table to "Title 46" and select the current year.
Contact the Naval Architecture Division:
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Naval Architecture Division (CG-5212)
U.S. Coast Guard Headquarters
2100 Second Street S.W.
Washington, D.C. 20593-0001
(202) 372-1372
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