Video: Selecting the Right Hullform
2018-Nov-05
Confusing Measurements of Ships
1.0 Measurements of Weight
Would you prefer long ton, short ton, metric ton, or tonnage? Ignore the tonnage, which is not a unit of mass. We’ll come back to that later. I’m no historian, but I think that the long ton came first, measuring 2240 pounds (lb). Then someone decided that they preferred round numbers and shortened that to the short ton, measuring 2000 lb.
Next the metric system tried to introduce order. It brought the metric tonne, which was only partially successful in the maritime industry. Table 1‑1 shows the common units of measurement. Pay careful attention to the abbreviations when distinguishing different measurements.
| Name | Abbreviation | Weight / Mass | Common Industries |
| Long ton | LT | 1 LT = 2250 lb 1 LT = 1016 kg | USA vessels |
| Short ton | ST tn | 1 ST = 2000 lb 1 ST = 907.2 kg | USA inland vessels, cranes |
| Metric ton | MT t | 1 MT = 2204 lb 1 MT = 1000 kg | Anywhere other than the USA. International corporations in the USA. |
| Kilopound | kip | 1 kip = 1000 lb 1 kip = 453.6 kg | USA civil engineering fields |
Side note: pounds and kilograms don’t measure the same thing. Kilograms measure the mass. Pounds measure weight: the force of Earth’s gravity as it pulls on the mass of an object. For some calculations, this is a critical difference in the two systems of measurement.
1.1 The Mistake of Tonnage
The most common mistake: tonnage does NOT measure weight or mass. I wish they picked a different word for this when they invented taxes. Tonnage bears more similarity to your taxable income. It has no correlations to physical mass. The first unit of tonnage was a tun, literally a large cask used to transport wine.
At first, tonnage was easy: count up the wine casks onboard the ship. But then the system expanded beyond just wine. Some people tried to cheat the system, and the codes changed. In the present day, tonnage admeasurement gets extremely complicated, requiring a specialist with experience working with your specific admeasurement system. (Oh yes, there are multiple systems for tonnage admeasurement.) Across the years of evolution and adaptation, we did agree on one thing: two important numbers: gross tonnage, and net tonnage, and plenty of confusion about their significance. (Table 1‑2)
| Name | Abbreviation | Weight / Mass | Simple Explanation |
| Gross tonnage | GRT | Nothing to do with mass or weight. | Vessel’s gross income per cargo load. |
| Net tonnage | NRT | Nothing to do with mass or weight. | Vessel’s net income per cargo load. |
2.0 Measuring Distance
When measuring length onboard a ship, we use reasonable units like meters or feet. But to measure the distance of travel, the maritime industry uses a nautical mile. Intended for navigators looking at charts, a nautical mile was historically one minute of latitude (1/60 degree). Table 2‑1 gives the appropriate conversions. And just to catch the land lovers, when mariners speak of distance, they normally just say “mile”. Usually, this implies nautical miles.
| Name | Abbreviation | Distance | Common Industries |
| Nautical mile | nm | 1 nm = 6076 ft 1 nm = 1852 m 1 nm = 1.151 mile | Maritime and aviation industry |
3.0 Measuring Speed
Speed derived from the measurement of nautical miles. The knot was originally measured with a knotted rope. But today, it’s formally defined as the distance of nautical miles traveled in one hour. (Table 3‑1)
| Name | Abbreviation | Speed | Common Industries |
| Knot | kn | 1 knot = 1 nm / hr 1 knot = 0.5144 m/s 1 knot = 1.6878 ft/s | Maritime and aviation industry |
4.0 Conclusion
The maritime industry has its own way of doing things, including its own measurements. Don’t ignore this. When ships weigh thousands of tons, you really need to know which type of ton. Keep these conversions handy, and you too can impress everyone with your esoteric knowledge.
