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Rules of the Road: Tricky tonnage measurement not about weight

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Rules of the Road: by Capt. Jake Desvergers

Tonnage is an important topic in the maritime and yachting industry. Tonnage forms the basis for numerous items, including safety regulations, manning scales, registration fees and port dues.

The term derives from the taxation paid on “tuns” of wine.  It was later used in reference to the weight of a ship’s cargo. However, in modern maritime usage, “tonnage” specifically refers to a calculation of the volume or cargo volume of a ship.  It is a measure of the size or cargo capacity of a vessel.  The term often confuses people and creates a common misconception. The tonnage measurement is not the vessel’s actual weight.

For centuries, each seafaring nation calculated a vessel’s tonnage by its own rules. Methods of calculating tonnage were not consistently applied and, because they were designed for sailing ships, could not be applied appropriately or fairly for the new steamships being launched in the middle of the 19th century. Substantial portions of a steamship were required for boilers, machinery, and coal, thus limiting the proportion of the ship’s space available for cargo.

In 1854, Admiral George Moorsom of the British Board of Trade was tasked with creating a system for measuring ships.  The British system concluded that harbor and other vessel fees should be proportional to the earning capacity of the ship, whether for cargo or passengers.  While the Moorsom System became the baseline for most tonnage measurement systems, there still was no standardized set of international guidelines. One country did it different from the next.

In 1969, the International Maritime Organization (IMO) approved the International Convention of the Tonnage Measurement of Ships. In its shortened form, one may hear it referenced as ITC69. The Convention harmonized the definitions for gross and net tonnages, plus the criteria for which measurements shall be made.

These tonnage rules applied to all ships above 24 meters in length built on or after July 18, 1982, the date of entry into force.  Ships built before that date could retain their existing tonnage for 12 years after entry into force, or until July 18, 1994. This extended phase-in period was intended to ensure that ships were given reasonable economic safeguards, since port and other dues are charged according to tonnage.  At the same time, and as far as possible, the Convention was drafted to ensure that gross and net tonnages calculated under the new system did not differ too greatly from those calculated under previous methods.

During a recent refit and major hull modification, I had the opportunity to mediate a “conflict” between the owner, captain and shipyard. The discussion revolved around two items in dispute: tonnage and length.

A key point of the discussion revolved around the yacht’s new tonnage. There was a disagreement between the calculated gross tonnage and the actual weight reading provided by the travel-lift. Clarification was needed between “tonnage” and the actual “weight” of the yacht.

There are multiple measurements used to reference the “weight” of a vessel, especially when dealing with cargo ships and oil tankers. In yachting, we focus primarily on displacement, lightship and deadweight.

  • Displacement is the actual total weight of the vessel. It is often expressed in long tons or in metric tons and is calculated simply by multiplying the volume of the hull below the waterline (i.e. the volume of water it is displacing) by the density of the water. One should note that the water density will depend on whether the vessel is in fresh or salt water, or located in the tropics, where water is warmer and hence less dense.  The word “displacement” arises from the basic physical law, discovered by Archimedes, stating that the weight of a floating object equates exactly to that of the water which would otherwise occupy the “hole in the water” displaced by the ship.
  • Lightship, or lightweight, measures the actual weight of the ship with no fuel, passengers, cargo, water, etc., on board.
  • Deadweight tonnage (often abbreviated as DWT for deadweight tonnes) is the displacement at any loaded condition minus the lightship weight. It includes the crew, passengers, cargo, fuel, water, and stores. Like displacement, it is often expressed in long tons or in metric tons.

A second point in the dispute was the assignment of the yacht’s new “Length.” The confusion circled around the differences between the term Length and Length Overall. It is very important to differentiate the two.

For Length, as defined in the Convention, it means 96 percent of the total length on a waterline at 85 percent of the least moulded depth measured from the top of the keel, or the length from the fore side of the stem to the axis of the rudder stock on that waterline, if that be greater. With some flag administrations, they may also refer to this measurement as the vessel’s “registered length.” This is the measurement noted on most statutory certificates.

In contrast, the Length Overall (LOA) is true maximum length from the fore side of the foremost fixed permanent structure to the aft side of the aftermost fixed permanent structure of the vessel. This is the measurement that defines how big the yacht is.

A ship or yacht on international voyages and greater than 24 meters exhibits its tonnage, length, breadth, and depth through an International Tonnage Certificate (ITC). This certificate is issued by the flag administration or a classification society on behalf of the administration. For vessels below 24 meters, it is the discretion of the flag administration whether to issue a comparable certificate or document, such as a Certificate of Tonnage or National Tonnage Certificate.

Capt. Jake DesVergers is chief surveyor for International Yacht Bureau (www.yachtbureau.org). Comments are welcome below.

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