Heavy metal

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photo of 3 semi-circular structures on their sides in a row

As a child I had some difficulty grasping how the huge metal ships on the Mersey stayed afloat. I could understand wooden ships floating – after all twigs and sticks thrown in our local brook never sank – but steel and iron? This was the big question nobody seemed to be able to answer – just as, how do planes stay in the air?

The notion of building a ship entirely of iron challenged many owners and shipyards in the 19th century: “Who ever heard of iron floating?” was a familiar cry. But float it did and within a few decades ships made entirely of wood were the exception rather than the norm.

Prior to the 19th century, for many centuries ships were built of wood. As Britain became a naval world power, ancient forests were cleared to build warships.

The Napoleonic wars between 1803 and 1815 marked the high point of Britain’s naval sea power under sail. They were followed by 100 years of comparative peace when new sea technologies came to the fore. Before the Napoleonic wars, merchant ships were relatively small. Although much of the globe had been explored, maritime trade hadn’t developed enough to support an industrial society.

The Industrial Revolution, which began in Britain and saw the start of the factory system, created a demand for larger ships to carry more raw materials and manufactured goods. The material which made their construction possible was iron. At first the builders of iron ships tried to copy the trusted methods used in wooden ship production. Iron hulls were vulnerable to corrosion and marine growths, since no effective anti-fouling paints had yet been developed. As a result, ship builders compromised by constructing an iron framework covered with wooden planking.

At Merseyside Maritime Museum there are three models (shown here) showing cross-sections of hulls made from wood, iron and wood and iron alone. Copper sheathing remained the most effective barrier against barnacles and marine worms before the arrival of anti-fouling paints. From the 1870s steel, with its greater strength, began to offer overwhelming advantages over iron. Plate thickness and other metal parts of the hull and superstructure could be reduced by 25% with no loss of strength. This weight-saving resulted in greater speeds, fuel economy or cargo carrying capacity – whatever the needs of the owners.

A new Maritime Tale by Stephen Guy appears every Saturday in the Liverpool Echo. A paperback – Mersey Maritime Tales (£3.99) – is available from the museum, newsagents, bookshops or from www.merseyshop.com (£1.50 p&p UK).