Tonnage Measurement History
When someone refers to the tonnage of a vessel they are usually not talking about weight. In some cases the actual weight is measured but that isn’t the information we are looking for today. Instead we will look at tonnage measurement systems that describe volume.
The oldest systems for measuring the volume, or capacity, of a ship appeared more than 800 years ago.
Wine was one of the most valuable commodities of the time and was carried in casks called tuns. If a ship could be loaded with twenty casks it was considered a twenty tun ship. Once the ship was designated with a certain capacity a fee could be calculated and charged each time the vessel utilized the port facilities.
Tuns became tons over time and by the middle of the 1700’s the concept of tonnage was the global standard for measuring the capacity of a ship.
Unfortunately this system suffered from a major flaw at this time. There was a common concept of potential value of cargo given the available capacity, but the means of measurement and even the unit of measurement were not standardized.
Some systems used a formula based on the exterior dimensions of a ship while others measured specific areas like cargo holds. Multiple units of measurement were also used which made the consistent measurement of volume difficult.
In 1849 England began the process of standardizing volumetric measurement. The Moorsom system was the product of the efforts and remains the basis for modern tonnage measurement systems. Moorsom’s system is based on the same concept introduced hundreds of years earlier in the Mediterranean and Aegean Seas. In order to assess a fee appropriate to the earning potential of a ship the cargo carrying areas were measured.
The Moorsom system was based on two measurements. The first, Gross Tonnage, was very similar to the most detailed and accurate way of expressing the total volume of the interior of a vessel. All interior spaces were measured and recorded with the exception of crew spaces.
Net Tonnage is a similar measurement but engineering spaces and crew quarters aren’t included. Net tonnage is meant to express the true cargo capacity and earning potential of a ship.
These terms might seem redundant in our modern understanding of maritime business but at the time technical innovations made the division necessary.
In the middle 1800’s when Moorsom’s committee was developing these rules the age of sail was still dominant but steam power was rising. The differences in cargo capacity between a sailing vessel and a steam ship were significant enough to warrant a separate system.
It was already accepted that crew spaces were exempt from inclusion and in the case of sail this was the limit for the exemptions. The vessel itself did not need a large support system, the supplies for ship and crew fit in a small area. Crews often found their sleeping area filled with supplies at times of high provisioning.
An engine driven ship requires a significant amount of space for equipment and fuel storage. Engine rooms and fuel bunkers take up space that would be used for cargo on a sailing vessel.
From the gross tonnage point of view a powered vessel and a sailing vessel of the same external dimensions have the same interior volume and cargo capacity.
Net Tonnage takes into account engineering spaces so when power and sail vessels of the same external dimensions are compared the powered vessel is measured closer to its true cargo capacity.
Modern Tonnage Measurement Systems
We still use Moorsom’s basic system today along with many additional rules added over the past century and a half. Each country or economic region has its own rules that spell out the specifics of tonnage measurement but they find many common roots in the rules of the International Maritime Organization.
Between 1959 and 1969 the IMO developed the Convention on Tonnage Measurement of Ships. The conventions came into force in July of 1982 for new builds.
Controversy abounds when it comes to tonnage measurement. The tonnage of a vessel not only dictates things like port fees and lock passages but in many cases it also dictates the minimum number of crew, safety equipment on board, and sanitation practices in certain waters.
Since so much potential cost could be incurred by a few extra cubic meters it is not surprising that vessels are built to maximum capacity while remaining just under the threshold for the next tonnage division.
Some ships need to have adjustments made if the flag of the vessel changes. The new country of registry might have a slightly different means of measuring so a small area will be blocked off and a hatch called a tonnage opening. These openings are strictly regulated since the purpose is to legally circumvent tonnage rules.
Classification societies have taken on the task of tonnage measurement in order to combine accurate consistent measurement with safe building practices. In an effort to reduce expenses some ships are designed with more consideration for capacity than seaworthiness.