First of all, Tonnage is not the ships displacement. Displacement is the buoyant force that floats the vessel. It must equal the vessel’s weight. The Moorsman System is not used to calculate displacement.
Tonnage is a legal measurement that attempts to quantify tlhe vessel’s cargo carrying capacity. Unlike displacement, it is a measurement of volume. It is a legal calculation used to assess taxes and fees. The term tonnage is derived from the Old English word Tun. In the 1500’s wine was imported to England in barrels called Tuns. By counting the number of Tuns aboard an arriving vessel, the import duty could be easily calculated.
The next evolution was to rate each ship by the number of Tuns that it could supposedly carry. This could be published in a register for use by those with a cargo looking to charter a vessel to carry it. Many shipping fees were also and still are based on this published tonnage. Originally tonnage was calculated using simple rules (Length x Beam x depth)/100; The volume of a Tun being about 100 cubic foot. During some periods, the depth term was replaced by Beam/2. This resulted in dangerous, narrow but deep ships. Historically, at one time or another, the divisor has also been 94 instead of 100.
Since the mid 1800’s there has been an effort to more accurately determine tonnage by measuring the ship’s actual
cargo carrying volume. This has resulted in more complicated rules such as the Moorsman System that you mention.
Roger
