Copper sheathing



Copper sheathing is the practice of protecting the under-water hull of a ship or boat from the corrosive effects of salt water and biofouling through the use of copper plates affixed to the outside of the hull. It was pioneered and developed by the Royal Navy during the 18th century.

Development
Deterioration of the hull of a wooden ship was a significant problem during the Age of Sail. Ships' hulls were under continuous attack by shipworm and various marine weeds, all of which had some adverse effect on the ship, be it structurally, in the case of the worm, or affecting speed and handling in the case of the weeds. The most common methods of dealing with these problems were through the use of wood, and sometimes lead, sheathing. Expendable wood sheathing effectively provided a non-structural skin to the hull for the worm to attack, and could be easily replaced in dry dock at regular intervals. Weed, however, grew rapidly and slowed ships. Lead sheathing, while more effective than wood in mitigating these problems, reacted badly with the iron bolts of the ships causing sometimes severe damage.

Even older than the sheathing methods were the various graving and paying techniques. There were three main substances used: White stuff, which was a mixture of whale oil, rosin and brimstone; Black stuff, a mixture of tar and pitch; and Brown stuff, which was simply brimstone added to Black stuff. It was common practice to first apply wood sheathing and then pay it with white stuff, although black stuff was occasionally used in this way.

The use of copper sheathing was first suggested in 1708 by Charles Perry, though it was rejected by the Navy Board on grounds of high cost and perceived maintenance difficulties. The first experiments with copper sheathing were made in the late 1750s: the bottoms and sides of several ships' keels and false keels were sheathed with copper plates.

In 1761 the experiment was expanded, and the 32-gun frigate HMS Alarm was ordered to have her entire bottom coppered, in response to the terrible condition in which she returned from service in the West Indies. HMS Alarm was chosen because in 1761 a letter had been sent regarding the ship's condition, saying that the worms from the waters had taken a significant toll on the ship’s wooden hull. Before the copper plates were applied the hull was covered with Soft stuff, which was simply hair, yarn and brown paper. The copper performed very well both in protecting the hull from invasion by worm and in preventing the growth of weed for, when in contact with water, the copper produced a poisonous film, composed mainly of oxychloride, that deterred these marine organisms. Furthermore, as this film was slightly soluble it gradually washed away, leaving no way in which marine life could attach itself to the ship. However, it was soon discovered by the Admiralty that the copper bolts used to hold the plates to the hull had reacted with the iron bolts used in the construction of the ship, rendering many bolts nearly useless. In 1766, because of the poor condition of the iron bolts, Alarm's copper was removed.

After this experiment, and deterred by the unanticipated and not understood electrolytic reaction between the copper and iron, lead sheathing was tried again, though it was found to be unsuitable to the task, as the plates tended to fall from the hull alarmingly quickly. In 1768 a ship named the Dolphin was sheathed in the same way and sailed for a few years around the world. It came back with corrosion on the iron components of the hull; they were basically irreparable and had to be replaced. In 1769 another attempt was made at coppering a ship's hull, this time on a new ship that had been constructed using bolts made from a copper alloy. The results were far more favourable this time, but the onset and intensification of the war with America prevented the re-bolting of the Royal Navy's ships necessary to allow a full-scale coppering programme.

Humphry Davy's experiments with copper sheathing
In the late 18th to early 19th century, Sir Humphry Davy performed many experiments where he had various thicknesses of copper submerged on the shore and then measured how much the sea water had degraded each one. Sheets of different metals remained in the seawater for four months and then were examined. Two harbour ships were also used in this test, one with a zinc and the other with an iron band. Both the zinc and iron became covered in a carbonate which allowed weeds, plant life and insects to attach themselves to the metal. The sheets that had cast iron or zinc were free of any attached life forms or discoloration. The main purpose of these experiments was to determine how to lessen the corrosion that the seawater caused on unprotected copper sheathing. Unprotected copper, such as that which was not covered in another metal such as iron, would quickly go from a reddish color to a greenish color of corrosion. When the other metal was mixed in copper in ratios from 1/40 to 1/150, there was no visible sign of corrosion and minimal weight loss. When the ratio was changed to 1/200 and 1/400, there was significant corrosion and weight loss. Davy concluded that cast iron, which was the cheapest to manufacture, was the best for protection of the copper since malleable iron and zinc wore down faster.

Widespread implementation
With the American war in full swing, the Royal Navy set about coppering the bottoms of the entire fleet. This would not have happened but for the declarations of war from France (1778), Spain (1779) and the Netherlands (1780): Britain had to face her three greatest rivals, and coppering allowed the navy to stay at sea for much longer without the need for cleaning and repairs to the underwater hull, making it a very attractive, if expensive, proposition.

Fortunately the Parys Mountain copper mine on Anglesey, Wales had recently begun large-scale production that had glutted the British market with cheap copper; however the 14 tons of metal required to copper a 74-gun third-rate ship of the line still cost £1500, compared to £262 for wood. The benefits of increased speed and time at sea were deemed by the Admiralty to justify the costs involved, and in May 1779 all ships up to and including 32 guns were ordered to be coppered when next they entered dry dock. In July this order was expanded to include ships of 44 guns and fewer.

It was then decided that the entire fleet should be coppered, due to the difficulties in maintaining a mixed fleet of coppered and non-coppered ships. 82 ships of the line had been coppered by 1781, along with 14 50-gun ships, 115 frigates, and 182 unrated vessels. By the time the war ended in 1783 problems with the hull bolts were once more becoming apparent.

Finally a suitable alloy for the hull bolts was found, that of copper and zinc. At great cost, the Admiralty decided in 1786 to go ahead with the re-bolting of every ship in the navy, thus finally eliminating the bolt corrosion problem. This process lasted several years, after which no significant changes to the coppering system were required, and copper plating remained the standard method of protecting a ship's underwater hull until the advent of modern anti-fouling paint.

Civilian use
With its widespread adoption by the Royal Navy, some shipping owners employed the method on their merchant vessels. A single coppered vessel was recorded on the register of Lloyd's of London in 1777. By 1786, 275 vessels (around three percent of the merchant fleet) were coppered. By 1816, this had risen to 18 percent of British merchant ships. Copper sheets were exported to India for use on ships built there. In the late 18th and early 19th century, around 30 percent of Indian ships were coppered.

Merchant ship owners were attracted by the savings made possible by copper sheathing, despite the initial outlay. As the coppering was expensive, only the better owners tended to invest in the method, and as a result the use of copper sheathing tended to indicate a well-found and maintained ship, which led to Lloyd's of London charging lower insurance premiums, as the vessels were better risks.

Coppering was more commonly used on merchant ships sailing in warm waters. Ships sailing in colder, northern waters often continued to use replaceable, wooden sheathing planks. Wood-boring organsms were less of a problem for these vessels and they were often routinely beached - an operation that could cause considerable damage to expensive coppering. Coppering was widely used on Slave ships. After the Abolition of the Slave Trade Act became British law in 1807, the slave trade became illegal and slavers valued fast ships that were more likely to evade patrolling Royal Navy ships intent on capturing them.

The term copper-bottomed continues to be used to describe a venture, plan or investment that is safe and is certain to be successful. The related copper-fastened (and verb form copperfasten) is used similarly, though with the nuance of “secured, unambiguous”, rather than “trustworthy, reliable”.