Boundary Lines

What is the Boundary Line? 

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


NOTE: 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.


What are the Boundary Line regulations? 

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


As a rule, on the Atlantic and Pacific 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.


The Boundary Line does not apply to the Great Lakes. Commercial vessels operating on the Great Lakes are required to have a load line when they operate offshore of harbor breakwaters, unless specifically exempted by statute or regulation.


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

  • In the Gulf of Mexico between the Marquesas Keys, FL, and the Rio Grande river mouth, TX, the Boundary Line is located 12 nautical miles offshore. This creates a 12-mile-wide coastwise marine corridor inside of which non-load line vessels may operate. For ease of navigation, this Boundary Line coincides with the "Territorial Sea" boundary marked on nautical charts of the Gulf;
  • Also in the Gulf of Mexico along the western coast of Florida, Congress widened the load line-exempted corridor to 15 NM offshore (i.e., 3 NM west of the Territorial Sea boundary shown on charts) between Crystal Bay and Hudson Creek;
  • In New England waters, the Boundary Line follows a series of lights and offshore buoys from West Quoddy Head Light (at the U.S./Canadian border) to Race Point Light (at the tip of Cape Cod). Like the Gulf of Mexico, this also creates a coastwise corridor inside of which all vessels may operate without a load line. However, unlike the GoM Boundary Line (which is a constant 12 NM from the coast), the New England Boundary Line does not follow the coastline and therefore is an irregular distance offshore.


For this reason, operators of non-load line vessels--including fishing vessels that are more than 79 feet in length and that are built on/after 1 July 2013--should especially familiarize themselves with the Boundary Line in New England waters (refer to 46 CFR Parts 7.10 and 7.15). Taking such a vessel outside the Boundary Line constitutes a load line violation;

  • And at other points along the U.S. coastlines, there may also be some local gerrymandering of the Boundary Line from the mouths of inlets or jetties out to the sea buoy and back.


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.