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SHIP TONNAGE THE MYSTERIES OF BY CAPTAIN BARRY THOMPSON P ity the poor landlubber! We seafarers and our fellow conspirators, the naval architects, have really set the cat among the pigeons. What with gross tons, net tons, displacement tons, deadweight tons and freight tons (and a few more), how can the poor landlubber be expected to understand ship tonnage? With metric units arriving a few years ago, some of our own brethren have become confused, too. Other changes in recent years have altered the way we now consider the size of ships. The ton was originally a "tun" and referred to a barrel used in the wine trade as a measure of quantity. In 1842, a British Act of Parliament stated a tun should not be less than 252 gallons, the weight of which approximated the unit that became the ton. When New Zealand rejected the old Imperial system of weights and measures in 1976 and adopted the metric system it required a change for units of mass from tons to tonnes. Although many of us grew up with the ton, a unit of weight of 2240 pounds, we readily accepted the change to a metric tonne of 1000 kilograms, slightly less than the weight of the old ton. As long as we are talking about weight, there is not too much difficulty with some types of "ship tonnage", which probably came into use for levying dues on wine-carrying ships. When we now refer to the weight of a ship or her cargo (I emphasise "weight"), we now have to think in tonnes and not tons, and make a mental note that the unit is slightly lighter than the unit used previously. This means ships and cargoes now weigh slightly more tonnes than they used to weigh in tons. Otherwise there is generally not a problem. A warship's tonnage is usually stated in displacement tonnes, which is her weight in metric tonnes as if measured on a giant set of scales. It gets its name because it is actually the weight of water she displaces floating in seawater. This displacement tonnage can be either light displacement (the weight of the ship alone) or load displacement (complete with fuel, stores, ammunition and her ship's company), and it is necessary to state which it is. Unlike naval ships, the size of merchant ships is very rarely given as a displacement tonnage. Cargo ships, especially oil tankers, may have their size expressed in deadweight tonnes (DWT), a measure of the maximum weight of cargo the ship can carry without illegally submerging her load line when afloat in seawater. Today's seafarers remember with gratitude the work of Samuel Plimsoll, the British Member of Parliament who fought long and hard in the mid-19th century to prevent the overloading of ships, which were causing many seamen to lose their lives. The principal load line on a ship - a legal requirement - is often known as her Plimsoll line to this day. A master who overloads his ship breaks the law and strong safeguards are now in place to ensure ships are not overloaded. Dues used to be based upon the number of tuns of wine that could be fitted into the ship, a convenient, if not very accurate, measure of a ship's size. Today, much more accurate methods are used, with passenger and cargo ships actually being measured internally. Apart from its use until recently to express weight, the word ton had been used to express volume as well. The gross ton was a measure of a ship's enclosed volume, with a few exempted spaces allowed as a deduction under the tonnage measurement rules. Unlike displacement and deadweight tonnages, a gross ton had nothing whatever to do with a ship's weight. It was a measure of 100cu ft and was used to state the size of ships as being of " … so many gross tons". With the introduction of the 1969 Convention on Tonnage Measurement of Ships, which came into force from 1982, some ship tonnages underwent a significant change and the term gross tons (or tons gross) was dropped. Slightly different rules applied to its measurement and the term "gross tonnage" has taken its place. Gross tonnage is now simply shown as a number followed by GT (or gt), ie, 25,000gt, where previously it would have been 25,000 tons gross. Generally, the changes have tended to increase gross tonnages slightly, but remember, this is an indicator of the internal volume of the ship, not her weight. Gross tonnage, rather than displacement or deadweight tonnage, is almost always the method by which the size of passenger ships is given and is also common for cargo ships. When we read that the ocean liner Queen Victoria weighs 91,450 tonnes, this is absolute nonsense. It is probably meant to convey she is of 91,450gt, an indication of her size, not her weight. The weight (displacement tonnage) of two ships of the same gross tonnage can be significantly different if they are of different construction. Aluminium is often now used in place of steel for the superstructure and the two ships might be fitted with different engines and be differently equipped. Net tonnage is another measure of ship tonnage and is closely related to gross tonnage. Spaces such as the engineroom and the crew accommodation are deducted from gross tonnage to arrive at the net tonnage. This provides a more realistic indication of a ship's actual earning capacity. It is sometimes used for port dues and other charges but it is not of much interest to the layman. There was also a freight ton, now replaced by a revenue tonne. Used in New Zealand, albeit not universally, it provides the basis to calculate freight charges by applying the freight rate to either the weight or the volume of the cargo, depending upon whether its weight is concentrated (eg, iron ore or coal) or relatively light in relation to its volume (eg, bags of kapok). Finally, just to throw a few more terms into the ring, there are even more types of tonnage, especially Panama Canal Tonnage and Suez Canal Tonnage (the basis for charging canal dues). In the United Kingdom there is even a tonnage known as Thames Measurement for many small craft. I hope some of the fog surrounding ship tonnage has been dispersed, but it's likely to remain a complete mystery to many. March/April 2012 Professional Skipper 7