Many times in my consulting business I need to know what the “Gross Tonnage” of a particular vessel is. This will be the driving force of MANY regulations and how they affect a mariner’s licensing progression as well as the myriad of regulations that may affect the vessel’s operation and regulatory process. Many mariners and vessel operators are still unsure of what “tonnage” is and how it is determined.

Often the answer I receive is “my boat weighs 40,000 lbs”. They may be confusing this with “displacement” which is a measurement of how much water a vessel displaces, based on the volume of water being pushed aside by the submerged portion of the hull. This has absolutely nothing to do with gross tonnage.

There are two measurement systems used to determine a value for gross tonnage of a particular vessel. There is the “Regulatory” tonnage system, which is our U.S. Domestic tonnage measurement system. The initials for this are “GRT”, which is found on a vessel’s certificate of documentation (COD), a certificate of inspection (COI), or in some cases a “tonnage certificate”. Under the Regulatory System there are various formulas that allow spaces to be deducted. GRT is an INTERNAL VOLUME measurement of your vessel based on the dimensions of your vessel, such as Length, Breadth, and “depth” (not draft). The Coast Guard will accept formal admeasurement calculations from classification societies, such as ABS and once it’s officially documented with a GRT, that IS the tonnage that will be used for regulatory purposes.

The second measurement system is called the “Convention” tonnage system. This is based off of the International Convention of Tonnage Measurement of Ships, 1969. Yes, that’s correct, 1969! This system uses the same basic principle of measurement of the internal volume (carrying capacity) of a vessel, but is much less forgiving with respect to “exempted” spaces and formulas that allow this. The initials for this system are “GT” and you may also see this tonnage listed on a vessels COD or COI if it is measured under this system. In some cases a vessel may be measured under BOTH systems. This system is required for vessels built after a certain date and for those that are international trade.

The STCW code uses the Convention tonnage system as it’s basis for the convention’s rules and regulations pertaining to the certification of mariners. Within the code, the United States was able to have the International Maritime Organization (IMO) place “equivalencies” within the scheme for two of our U.S. tonnage endorsements. These are 200 GRT and 1600 GRT. Under this equivalency clause, 200 GRT is equal to 500 GT and 1600 GRT is equal to 3000 GT. This also applies to vessel’s measured under our GRT (Regulatory) system and when STCW applies. For example, a commercial vessel that is over 200 GRT, sailing beyond the Boundary Line (46 CFR Part 7), which is considered “seagoing”, the STCW code applies to that vessel and the mariners employed on board. This is because the vessel falls under the category of vessels of 500 GT or more, under the STCW code.

This can be very confusing on both a licensing and vessel operational scheme. Please contact us today if you have any questions!