Old Wood & Rusty Iron – by Mike Creasy
Before a recent meeting at the Naden museum, a few of us were standing in the parking lot above the Esquimault Graving Dock talking about the size of ships – trying to visualize the RMS Queen Elizabeth as she looked for two weeks in 1942.
Her size is given as 83,000 gross registered tons and if memory serves, she was 1030 feet long. So, how does that compare with the big battleships of the time? Bismarck was 820 feet long and about 55,000 tons displacement, while the American Iowa class were 890 feet and 52,000 tons displacement. Of course, what the Missouri carried in armour plate was likely matched by the Queen Elizabeth’s tonnage of silver plate, but still… if one of these monsters slipped off the keel blocks, which one would press you into a fossil??
Well, if you’re under the QE and you hear the blocks groaning, pull out your tape measure. The bathroom scales won’t do. You are about to be flattened by a VOLUME not a weight. That’s right – tonnage as it applies to civilian ships is usually a measure of internal volume.
Tonnage is derived from the word TUN, which was originally a type of barrel used to transport commodities. A tun was defined as 100 cubic feet. The number of tuns a ship could carry was important to shipowners, insurers, taxation authorities and others. Over time, tunnage became tonnage, and the rush to confusion with weight was on.
In the early days of ship-borne transport when all ship were sail powered, it was enough to know the internal volume of a ship’s hull as a means of deriving theoretical cargo capacity. In 1720, the Builder’s Measurement Rule provided a simple calculation for deadweight tonnage based on length and breadth of a hull. This was replaced in 1854 with the Moorsom System, which is the basis for modern tonnage calculations.
Keep in mind that until 1876, ship owners were free to load their vessels until they wouldn’t float no more. Samuel Plimsoll’s marking system changed that, and it became important to have a good paper comparison of the capacities of various ships.
Deadweight tonnage (DWT) is the lifting capacity of a ship, minus the actual weight of the ship. DWT is often applied to bulk carriers like the big coal ships or oil tankers. You’ve probably figured out by now that a volume-based measure is used for merchant ships because the internal volume never changes, while a ship’s carrying capacity in terms of weight will change with the density of the water and the quantity of fuel in the tanks, among other things. Naval ships don’t use these measures because they don’t carry commercial loads, using instead the displacement tonnage (DT or DisplT), which is similar (but not quite the same as) deadweight tonnage.
The Moorsom System had to be adapted in the late 1800s as sail ships began to give way to steam power, and the calculation of internal volumes had to be modified to account for engines and fuel.
Gross Register Tonnage (GRT) was the measure of total internal volume of a vessel (with some exceptions) while Net Register Tonnage (NRT) was the GRT minus all the non-cargo volumes. Both these terms are now obsolete, having been replaced in 1994 by the calculations for Gross Tonnage and Net Tonnage.
In all but DWT and DT, the ton is still the equivalent of 100 cubic feet – a measure of volume, not weight.
Now, for all you students of physics (or anyone else who has tried to bring a ship alongside). Ships have weight. Together with speed, weight produces momentum. Momentum is that which changes the shape of wharves and shell plates after the propeller has exhumed Jimmy Hoffa from the harbour muck.
Speed is fairly well understood; for docking manoeuvres there is dead slow, full astern and “oh my goodness”. The heavier the ship, the lower the speed at which “oh my goodness” occurs. Now, you might use a slightly different phrase aboard your admiral’s barge but the impact, so to speak, is the same.
As we’ve seen, weight is less well understood. Actual weight is what matters, not tonnage as a measure of volume.
A ship’s actual weight – if you could get it on the bathroom scale - is the equivalent to the amount of water it displaces with its hull. Ship designers generally calculate this volume for different draft depths and convert to weight – thus producing a table unique to each ship, for any given loading condition.
This actual weight is the one that contributes to momentum – and it’s what matters when you hear those keel blocks creaking……
Queen Mary 2
Janes Fighting Ships, 1946