What is a ton, aboard ships and maritime? A ton measures weight, right?

In this lesson we will explore the various definitions of tonnage and the differences between the definition on land and at sea. A short history lesson will reveal a critical international convention that assigned certain responsibilities for larger ships. Lastly, we will explain how and why this applies to yacht crew.

Index

• Tonnage Definition
• Historical Definition
• Tonnage Convention for Ships
• Yacht Tonnage
• Quick Definitions
• Summary
• Further Reading
• References

Tonnage Definition: American, Imperial, and Metric Unit of Mass

If you're an American, I'm assuming you were taught that this is equivalent to 2000 lbs (a short ton). Although this might seem like an arbitrary number it's actually quite negligible in comparison to 5,280 ft in a mile or any of the baffling volume calculations that litter our measurement system. On the other hand, if you're European then you know a tonne to be equal to 1000 kgs. Again, if you're American then this would be 2,204.62 lbs (a metric ton). To make matters worse, in the United Kingdom and Commonwealth you have what is known as an Imperial ton, which is 1016 kg/2,240 lbs (a long ton). So we can immediately set the standard that various countries have significant differences in the manner in which they calculate tonnage.^[1]

Historical Definition

Historically, the word ton originates from the word tun, which in the early 1800s served as a basis for a standard unit of measurement for wine, oil, or honey.^[2] One "tun" served as a volume of liquid equaling roughly 250 wine gallons, which usually equalled nearly 2000 pounds in weight. Interestingly enough, this "wine gallon" standard developed in the early 1700s and nicknamed the "Queen Anne wine gallon" still serves as the basis for the American gallon measurement today.^[3]

Hopefully I haven't lost anyone in the short history lesson. Without delving into too much unnecessary information, throughout the rest of the 1800s and early 1900s there were various standards of weight and volume set for ships, mostly all based on prior British models. Some were based on the carrying capacity/weight of cargo that a ship could hold in their stores while others were based on the actual weight of the ship. Some calculations incorporated length, hull shape, and other variables. Various countries used conflicting standards and there were attempts at uniformity in world gatherings such as the Olympics or competitive races.^[1]

Tonnage Convention for Ships

However, in 1969 the International Maritime Organization, IMO came together in London to standardize the calculation of tonnage on ships with the International Convention on Tonnage Measurement of Ships ^[4]. This outlined the new tonnage calculations requirements for new vessels built by 1982, a 12 year phase-in for existing vessels, vessels completing international voyages, and complete calculation instructions for the new tonnage standards. Similarly, it set the standard for various maritime terms and definitions still used today. Lastly, this Convention outlined that it did not include ships of war, vessels under 24 meters (79 feet), and most importantly for American yacht crew, vessels operating within certain geographic locations, such as the Great Lakes and St. Lawrence Rivers.^[5]

Fast forward to today and the most relevant information to you. It might seem counterintuitive, but whenever you hear anyone speaking regarding tonnage in the maritime industry they are actually referring to volume, not weight. This is the volume of space within the vessel as defined by a few conditions. If someone is referring to the actual physical weight of a ship then that is called displacement (or displacement tonnage). Volume is important because this is how many legal requirements are set for the vessel, such as crew requirements, safety, and other standards.

Yacht Tonnage

As such, for international yacht crew, you will find the sizes for all yachts listed in gross tons as per the convention requirements mentioned above. Most often, you'll hear of vessels above or below 500 GT as well as 3000 GT because as per the STCW code, these are the cutoffs for various important legislative distinctions. Just as with our original differences discussed in measuring tonnage, American (and Canadian) yacht crew will notice that their countries' licenses and documentation are listed in gross registered tons, which are now obsolete in most of the rest of the world (see CFRs 46 Subtitle II Part J Code 143, 145 & Chapter I Part G Code Part 69 Subpart A).^[6]

Because of this, the US was granted two equivalencies by the IMO in order to be in compliance with STCW codes set for the worldwide maritime community. These are for the 200 GRT, equivalent to 500 GT, and 1600 GRT, which is equivalent to 3000 GT. For example, an American vessel of 200 GRT crossing the international boundary lines established by the USCG on a seagoing or international voyage becomes subject to mandates applying to 500 GT vessels.^[7][8] Although this might bring headache to the actual registration of vessels in these situations, it will hopefully clear up what is covered by each license.

Quick Definitions

Gross Registered Tonnage (GRT) - A ship's total internal volume expressed in "register tons", each of which is equal to 100 cubic feet (2.83 m³). Gross register tonnage uses the total permanently enclosed capacity of the vessel as its basis for volume and was replaced by gross tonnage in 1982 under the Tonnage Measurement convention of 1969, with all ships measured in GRT either scrapped or re-measured in GT by 1994.

Gross Tonnage (GT) - Measure of a ship's total internal volume from keel to stacks. This is a dimensionless measurement. Most importantly, this is used for safety regulations, safe manning, and other legal requirements.

Net Registered Tonnage (NRT) - is the volume of cargo the vessel can carry calculated from the gross register tonnage less the volume of spaces that do not hold cargo (e.g., engine compartment, bridge, and crew spaces. It represents the volume of the ship available for transporting freight or passengers. Similar to GRT, It was replaced by net tonnage in 1994, under the Tonnage Measurement convention of 1969.

Net Tonnage (NT) - Measurement tied to the earning potential of a vessel based on a calculation of the volume of all cargo spaces of the ship. Net tonnage equates the gross tonnage minus all of the non-revenue earning aspect of a ship such as crew quarters, machinery spaces, etc.

Summary

• Tons in general are widely varying measurement across the world.
• In 1969, the IMO put forth the International Convention on Tonnage Measurement of Ships in order to standardize the tonnage definitions worldwide.
• This allows for STCW conventions applying to specific sized vessels to be easily understood and interpreted.
• The US and a few specific flag states were able to retain the prior tonnage classifications except for international or seagoing voyages (outside their pre-established Boundary Lines).
• Holders of USCG licenses should note that:
  • USCG 200 GRT = ITC 500 GT.
  • USCG 1600 GRT = ITC 3000 GT.

Further Reading

1. Builder's Old Measurement
2. Ship Displacement
3. Moorsom System
4. Guide to Tonnage
5. CFR 46.Chapter I.Subchapter G.Part 69.Subpart A § 69.20 
6. CFR 46.Chapter I.Subchapter G.Part 69
7. CFR 46.Subtitle II.Part J.Chapter-143
8. CFR 46.Subtitle II.Part J.Chapter-141 § 14101
9. CFR 46.Subtitle II.Part J.Chapter-145

References

1. Ton. Wikipedia. (n.d.). Retrieved from https://en.wikipedia.org/wiki/Ton

2. Zupko, Ronald E. (1985). A Dictionary of Weights and Measures for the British Isles: The Middle Ages to the Twentieth Century, Volume 168. Memoirs of the American Philosophical Society. American Philosophical Society. 168. ISBN 9780871691682. Quoting Gras (1918), p. 706

3. Cardarelli, F. (2003). Encyclopaedia of Scientific Units, Weights and Measures. Their SI Equivalences and Origins. London: Springer. pp. 49. ISBN 978-1-4471-1122-1.

4. International Convention on Tonnage Measurement of Ships, 1969. United Nations Treaty Collections. (n.d.). Retrieved from https://treaties.un.org/doc/Publication/UNTS/Volume%201291/volume-1291-I-21264-English.pdf

5. International Convention on Tonnage Measurement of Ships. International Maritime Organization. (n.d.). Retrieved from https://www.imo.org/en/About/Conventions/Pages/International-Convention-on-Tonnage-Measurement-of-Ships.aspx

6. Units and Systems of Measurement Their Origin, Development, and Present Status" (PDF). National Institute of Standards and Technology. Archived from the original (PDF) on 19 October 2011. Retrieved 17 October 2011.

7. How Much Does Your Boat Weigh? GRT VS. GT. Maritime TOAR Assessments. (n.d.). Retrieved from https://maritimetoarassessments.com/2018/10/26/much-boat-weigh-grt-vs-gt/

8. Boundary Lines. Code of Federal Regulations. (n.d.). Retrieved from https://www.ecfr.gov/current/title-46/chapter-I/subchapter-A/part-7

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