Weighing up ships along the Dorset coast
Science Feature by Professor Glenn Patrick
LIKE many coastal towns, Lyme Regis was once home to a shipbuilding industry with shipyards flourishing around the Cobb.
According to the Lyme Regis Museum, the largest vessel recorded as being built in the town was a 475 ton, full-rigged ship called Salacia in 1853.
Shipbuilding inevitably declined, but the Boat Building Academy continues this tradition of building sea craft in the town. Formed in 1997, this ensures that the craftmanship and skills needed for boat building continue to flourish.
Living by Lyme Bay, we can still watch all sorts of boats and ships even if they are not built here. For the most part, these are small fishing vessels or pleasure craft, but occasionally something bigger arrives in the bay to provide a bit of excitement.
This happened recently when the cruise ships Marella Discovery and Marella Explorer II were anchored off West Bay during Storm Darcy.
Searching for some facts on the web, I established that the 866 feet long Discovery has a gross tonnage of 69,130 tons and a displacement of 34,837 tons, whilst the 814 feet long Explorer II has a gross tonnage of 72,458 tons and a summer deadweight of 7,260 tons.
Impressive numbers, but what do they all mean?
Confusingly, tonnage is sometimes used as a measure of volume not weight. This arises because the word “ton” originates from the old English word “tun”, which was the name for a large wine cask with a volume of 252 gallons.
Tonnage or “tunnage” was therefore introduced as crude measure of volume so that ships could be taxed according to the value of their cargo or passenger capacity.
As a full wine tun weighed 2,240 pounds or one ton, it is also the origin of our Imperial unit of weight.
Just to perplex us even more, it appears that there are at least eight different measures of a modern ship’s tonnage! Let us consider just four of these nautical terms.
The “lightweight tonnage” of a ship is basically the weight of a ship as it was built in the shipyard – the basic materials that go into making a ship’s hull, decks, and machinery – this is what would be used to measure the scrap value of a ship.
Then, we have the “deadweight tonnage”, which is a measure of the cargo carrying capacity and comprises the weight of all the cargo, fuel, provisions, supplies and crew on board the ship – an important measure for tankers and cargo ships.
For passenger ships though, it is another measure called “gross tonnage” that is of prime interest to customs and excise officers as this is a measure of the total internal volume of a ship and is calculated using a mathematical formula.
It has nothing to do with weight and represents the space in the ship that can be used for cargo and passengers – very useful for taxes, duties, port, and pilotage fees!
Finally, we have “displacement tonnage” which is the total weight of the volume of water that is “displaced” by a ship when it is sitting in the water at her designed waterline. This is the real weight of a ship and is equal to the deadweight plus the lightweight tonnage.
A good example is the Queen Mary II, which has also been anchored off the Dorset coast. Her displacement tonnage – what she would weigh if you could put her on a set of scales – is 76,000 tons. This varies according to her actual deadweight, which can reach about 19,000 tons.
Her gross tonnage, though, is listed at a mighty 149,000 tons, but remember that this is a measure of the volume of the ship not its weight.
We also have the added complication of the actual units that we use for tonnage – there are long tons, short tons and tonnes!
The British/Imperial ton or “long ton” is equal to 2,240 pounds whereas the US “short ton” – used by our friends across the pond – is equal to 2,000 pounds.
Since metrication in the UK and for the rest of Europe, we also have the “tonne”, also known as a metric ton, which is equal to 1,000 kg or 2204.6 pounds.
The difference between these units is only around 12 per cent, but such differences matter especially when you are dealing with large objects or large distances as encountered in space travel.
On reaching Mars in 1999 after 10 months and travelling 416 million miles, the $125 million Mars Climate Orbiter spacecraft was lost by NASA just as it was due to go into orbit around the red planet.
The reason? One piece of software built by a team performed its calculations in US units, but then passed its results to another piece of software written by another team that worked in metric units!
Returning to ships, I have been lucky enough to be invited to lecture aboard several of the cruise ships that have been anchored around the Dorset and Devon coast.
Like others, when I see the decks full of passengers and all the consumables needed for a voyage, I cannot help marvelling at how they keep afloat.
Occasionally, we also get to see impressive military ships along the south coast. The most imposing I have seen is the nuclear-powered aircraft carrier USS Theodore Roosevelt – also known as the ‘Big Stick’.
With over 5,000 crew, it was so large that it could not enter Portsmouth naval base in 2015 and had to anchor in the Solent. Fortunately, I was able to take a boat trip to view the carrier bristling with missiles and F/A-18F Super Hornet fighters.
Being a warship, the important measure is displacement tonnage running in at a muscular 104,600 long tons. At 1,092 feet long – almost the height of the Empire State Building – and constructed out of 60,000 tons of steel, again I could not help, but respect its ability to float.
There is a clue in the fact that the carrier has 4,000 compartments and spaces. This is all down to the Greek scientist Archimedes who according to legend leapt from his bath and ran naked down the street shouting “Eureka” after he discovered the principle of buoyancy.
When applied to ships, Archimedes Principle states that the buoyant force exerted on an object in water is equal to the weight of fluid displaced. If the downward force of gravity is less than the upward buoyant force, the object will float.
We must admire the ingenuity of the engineers who design massive ships weighing hundreds of thousands of tonnes such that they displace enough water that they float. It is all down to the shape of the ship and the air trapped inside those internal compartments.
Provided the total density of the ship is less than the same volume of water, the ship floats. If the density of the ship exceeds that of the water – for example if water enters the vessel – it sinks.
Ship building has come a long way since the days of Archimedes, but his principle still rules the waves!
Glenn Patrick is a particle physicist and science communicator who explores the quantum world of sub-atomic particles (including at the Large Hadron Collider) and now lives in Lyme Regis.