The Law of Gross Tonnage is an accepted nautical convention that when a sea-going vessel has the right-of-way as established by the International Regulations for Preventing Collisions at Sea, 1972 (72 COLREGS), it should nonetheless give way when faced with a larger vessel. This law is regularly invoked in non-maritime situations, such as when a bicyclist with the right-of-way invokes the Law of Gross Tonnage to avoid the Sport Utility Vehicle (SUV) that is about to cut them off.

The heavier vessel always has the right-of-way. There is no explicit directive in maritime regulations or law for the the Law of Gross Tonnage other than it is common sense that giving way and being alive is usually better than forcing one's right-of-way and being dead.

Example:

Two ships sailing in a battle group are operating off the coast of Southern California near San Clemente Island in the Southern California Operations Area. First, Oliver Hazard Perry class Guided Missile Frigate USS RENTZ (FFG-46) weighing a svelte 4,100 tons is going 20 knots at heading 090 (due east). Second, conventionally powered Forrestal class Aircraft Carrier USS RANGER (CV-61; aka "Danger Ranger") weighing in at a pudgy 81,000 tons is sailing at 22 knots, heading 180 (due south). The two vessels are approaching on collision headings.
                      -------
                     |       |   22 knots
                     |       |
                     | CV-61 |     |
                     |       |     |
                     | USS   |     \/
                     |RANGER |    180 (S)
                     |       |
                     |       |
                     |       |
                     |       |
                      -------




USS RENTZ    __
FFG-46      |__> 


  ---> 090 (E)
   20 knots 

COLREGS Rule 15 states that "When two power-driven vessels are crossing so as to involve risk of collision, the vessel which has the other on her own starboard side shall keep out of the way and shall, if the circumstances of the case admit, avoid crossing ahead of the other vessel."

This would indicate the RANGER should take action to avoid RENTZ. In fact, COLREGS Rule 17 requires that RENTZ maintain current course and speed so as to allow RANGER to maneuver appropriately out of danger of collision.

However, the Officer of the Deck (OOD - i.e., the guy in charge of directing the ship's maneuvers) on RENTZ has had many previous encounters with RANGER and invokes the Law of Gross Tonnage to immediately relinquish right-of-way and turns starboard to course 190. The COLREGS allow this when it is apparent that the "give-way" vessel is not maneuvering as required.

True Story: One day on the RENTZ we observed the crew of the RANGER practicing small arms fire (shotguns, .45 caliber handguns, etc.) off of the fantail, or back end, of the RANGER. The RENTZ OOD placed a call to the RANGER OOD resulting in this conversation:

RENTZ: Uh, RANGER, is your crew practicing small arms fire off of your fantail?
RANGER: Affirmative, RENTZ.
RENTZ: Uh, are you aware that we are 1000 yards astern of your fantail?
RANGER: That's an affirmative, RENTZ.
RENTZ: Could you stop shooting at us, please?


Sources:

Surface Ship Operations (NAVEDTRA 12973, SuDoc D 207.208/2:OP2/994) pp. 7-8 to 7-9.
USS RENTZ web site http://www.rentz.navy.mil
USS RANGER web site http://www.uss-ranger.org
Personal Experiences

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