The easiest tonnage to understand is displacement tonnage. This is the actual weight of the ship, called displacement because the weight of the ship is equal to the amount of water it displaces, per Archimedes’ principle. Of course, the displacement of a ship can change greatly depending on what’s aboard, so several different displacements are often used. A good reference will give three values, light, normal, and full. Light is basically an empty ship. There are some technical details, but in essence it’s what the ship weighs if we take all the people, fuel, cargo, ammunition, and supplies out of it. Full displacement is exactly what you’d expect from the name. The ship is as heavy as it’s ever going to be, with full fuel, ammo, and cargo. Normal is essentially a ship with full cargo and crew, but two-thirds fuel and ammunition. On warships, it’s common for light displacement to be 75% or so of full displacement, so it’s obvious that a poor choice of reference numbers can cause confusion about the actual sizes of the ships in question. Displacement tonnage in non-metric references is always given in long tons of 2,240 lb each. This is to allow easy conversion, as a long ton of salt water has a volume of almost exactly 35 cubic feet.
Displacement tonnage is the only tonnage value commonly used for warships, while merchant ships use a completely different set of tonnages, usually deadweight tons, gross tons, and net tons. Deadweight tonnage is at least an actual measure of weight, and specifies how much cargo the ship is able to carry, excluding the weight of the ship itself. Gross tonnage and net tonnage are both basically volumetric measures, specified using complicated formulas. Gross tonnage is based on the total volume of all enclosed spaces of the ship, while net tonnage is based only on the volume of the spaces that carry cargo or passengers. These values are used to calculate things like port duties and what regulations apply to the ship. They replaced gross and net register tons, which were also volumetric, of 100 cubic feet to the ton. These were based on a phenomenally complicated set of rules to determine what spaces were and weren’t counted, with shipowners trying to game the system and drive down their tonnage. Worse, different countries had different regulations on what counted under each value, so in 1969, register tonnages were abolished and replaced with the existing system.
While distance, speed, and “tonnage” measurements are the most prominent nautical idiosyncracies, there are a few other values that bear mentioning. The maximum width of a ship is the beam, apparently from the beams that used to run across ships during the days of sail. Draft (or draught) is how deep the hull is below the waterline. This is a very important number to make sure the ship doesn’t run aground, but it varies continually with the amount of stuff on the ship. Air draft is sometimes used for height above the water, important for clearing bridges and the like. Length, much like tonnage, comes in several values. The obvious one is length over all, LOA, which is exactly what it sounds like, measured from the furthest forward to the furthest aft point on the ship. This is obviously useful, but for hydrodynamic purposes another value, waterline length (LWL), is what the designer cares about. The last value is length between perpendiculars (LBP), measured between the vertical stem (bow) post and the vertical stern post. This is another value that was developed for civilian use. Some ships of a given LOA have a great deal of overhang at bow and stern, making them much smaller than other ships with the same nominal LOA. Using LWL is not a good solution, as it varies with draft. LBP solves both of these problems, but isn’t of great importance to warships.
Bean, “Nautical Measurements”, Naval Gazing, 2018-08-10.
November 25, 2020
QotD: Tonnage measurement(s)
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