Shipwrecks

A shipwreck can refer to a wrecked ship or to the event that caused the wreck, such as the striking of something that causes the ship to sink, the stranding of the ship on rocks, land or shoal, or the destruction of the ship at sea by violent weather. A shipwreck is the remains of a ship that has wrecked, either in it having sunk or been beached. Whatever the cause, a sunken ship or a wrecked ship is a physical example of the event.[1] There are more than 3 million shipwrecks on the ocean floor, the United Nations estimates.[2]

Types of shipwrecks
Historic shipwrecks are attractive to maritime archaeologists because they preserve historical information; for example, studying the wreck of Mary Rose revealed information about seafaring, warfare and life in the 1500s. Military wrecks that were caused by a skirmish at sea are studied to find details about the historic event and reveal much about the battle that occurred. Discoveries of treasure ships, often from the period of European colonisation, which sunk in remote places, leaving few living witnesses, such as the Batavia, do occur but only very infrequently.

Some contemporary wrecks, such as the Prestige or Erika, are of interest primarily because of the potential harm to the environment. Other contemporary wrecks are scuttled in order to spur reef growth, such as Adolphus Busch and the Ocean Freeze. Wrecks like Adolphus Busch and many historic wrecks such as SS Thistlegorm are of interest to recreational divers who enjoy diving shipwrecks because they are often interesting to explore, provide large habitats for many types of marine life and have an interesting history.

Very few shipwrecks are famous catastrophes like the wrecks of the Titanic, Lusitania or Estonia. There are also thousands of wrecks that were not lost at sea but have been abandoned or sunk. These are typically smaller vessels such as fishing vessels. These vessels can provide an interesting recreational dive but are usually of little interest to historians. They may pose a hazard to navigation and may be removed by port authorities. These vessels are sometimes referred to as abandoned or derelicts

Shipwrecks and the law
Shipwreck law determines important legal questions regarding wrecks, perhaps the most important question being the question of ownership. The British Protection of Wrecks Act, enacted to protect historic wrecks, controls access to wrecks such as Cattewater Wreck which can only be visited or investigated under licence. The British Protection of Military Remains Act 1986 also restricts access to wrecks which are sensitive as war graves. The Protection of Military Remains Act in some cases creates a blanket ban on all diving, for others divers may visit provided they do not touch, interfere with or penetrate the wreck. In the United States, shipwrecks in state waters are regulated by the Abandoned Shipwreck Act of 1987. This act is much more lenient in allowing more open access to the shipwrecks.

Following the beaching of the MSC Napoli, as a result of severe damage incurred during European windstorm Kyrill, there was confusion in the press and by the authorities about whether people could be prevented from helping themselves to the flotsam which was washed up on the beaches at Branscombe. Many people took advantage of the confusion and helped themselves to the cargo. This included many BMW motorbikes [1] and empty wine casks as well as bags of disposable nappies (diapers). The legal position under the Merchant Shipping Act 1995 is that any such finds and recovery must be reported within 28 days to the Receiver of Wreck.[2] Failure to do so is an offence under the Merchant Shipping Act and can result in a criminal record for theft by finding.[3] After several days, the police and Receiver of Wreck, in conjunction with the landowner and the contracted salvors, established a cordon to prevent access to the beach.[4] A similar situation occurred after the wreck of the MV Cita in 1997.

An important international convention aiming at the protection of underwater cultural heritage (including shipwrecks) is the UNESCO 2001 Convention on the Protection of the Underwater Cultural Heritage. The 2001 UNESCO Convention on the Protection of the Underwater Cultural Heritage represents the international community’s response to the increasing looting and destruction of underwater cultural heritage. It forms part of a group of UNESCO standard setting instruments regarding the domain of cultural heritage, encompassing seven conventions adopted by UNESCO Member States, which constitute a coherent and complementary body guaranteeing a complete protection of all forms of cultural heritage.

The UNESCO 2001 Convention is an international treaty aimed exclusively at the protection of underwater cultural heritage and the facilitation of international cooperation in this regard. It does not change sovereignty rights of States or regulate the ownership of wrecks or submerged ruins.

Salvage of wrecks
Often, attempts are made to salvage recently wrecked ships to recover the whole or part of the ship, its cargo, or its equipment. A good example of this was the salvage of the Kaiserliche Marine High Seas Fleet at Scapa Flow in the 1920s. The unauthorized salvage of wrecks is called wrecking.

As a general rule, non-historic civilian shipwrecks are considered fair game for salvage; military wrecks, however, remain under the jurisdiction--and hence, protection--of the government that lost the ship, or that government's successor. Hence, a German U-boat from World War II still technically belongs to the German government, even though the Third Reich is long-defunct. Many military wrecks are also protected by virtue of their being war graves.

Historic wrecks (often but not always defined as being more than 50 years of age) are often protected from pillaging and looting through national laws protecting cultural heritage.[3] Internationally they may be protected by a State ratifying the Unesco Convention on the Protection of the Underwater Cultural Heritage. In this case pillaging is not allowed.

Causes

The sea is a dangerous place, and its hazards are legion. Human factors for the loss of a ship may include:

- Poor design or failure of the ship's equipment or pressure hull
- Instability, due to poor design, improperly stowed cargo, cargo that shifts its position or the free surface effect.
- Navigation errors and other human errors, leading to collisions (with another ship, rocks, an iceberg, etc.) or running aground
- Bad weather
- Warfare, piracy, mutiny, or sabotage including: guns, fire, torpedoes, depth charges, mines, bombs and missiles.
- Fire
- Overloaded with cargo, and exceeded the plimsoll line
- Intentional sinking (scuttling)
- To form an artificial reef
- Use as a target ship for training or testing weapons
- As a blockship to create an obstacle to close a harbour, river, etc. against enemy ships
- To prevent a ship from falling into an enemy's hands (e.g. Graf Spee)
- To destroy a derelict ship that poses a menace to navigation
- As part of an insurance scam

Design and equipment failure
Poor design allowed the ferry MS Herald of Free Enterprise to put to sea with open roll-on/roll-off bow doors, with tragic consequences.

Failure or leaking of the hull is a serious problem that can lead to the loss of buoyancy or the free surface effect and the subsequent sinking of the vessel. Even the hulls of large modern ships have cracked in heavy storms. Leaks between the hull planks of wooden vessels is a particular problem.

Failure of pumps can lead to the loss of a potentially salvageable ship with only a minor leak or fire.

Failure of the means of propulsion, such as engines, sails or rigging, can lead to the loss of a ship. When the ship's movement is determined only by currents or the wind and particularly by storms, a common result is that the ship is unable to avoid natural hazards like rocks, shallow water or tidal races.

Instability
Instability is caused by the centre of mass of the ship rising above the metacenter resulting in the ship tipping on its side or capsizing.

This can lead to a sinking if the openings on the upper side of the side are not watertight at the time of the capsise. To remain buoyant, the hull of a vessel must prevent water entering the large air spaces of the vessel (known as downflooding). Clearly for the ship to float, the submerged parts of the hull will be watertight, but the upper parts of the hull must have openings to allow ventilation to compartments, including the engine room, for crew access, and to load and unload cargo.

Bad weather
Poor weather can cause several problems:
- wind
- low visibility
- cold

Wind causes waves which result in other difficulties. Waves make navigation difficult and dangerous near shallow water. Also, waves create buoyancy stresses on the structure of a hull. The weight of breaking waves on the fabric of the ship force the crew to reduce speed or even travel in the same direction as the waves to prevent damage. Also, wind stresses the rigging of sailing ships.

The force of the wind pushes ships in the direction of the wind. Vessels with large windage suffer most. Although powered ships are able to resist the force of the wind, sailing vessels have few defences against strong wind. When strong winds are imminent, sailing vessels typically have several choices:
- try to position themselves so that they cannot be blown into danger
- shelter in a harbour
- anchor, preferably on the leeward side of a landform

Many losses of sailing ships were caused by sailing, with a following wind, so far into a bay that the ship became trapped upwind of a lee shore, being unable to sail into the wind to leave the bay.

Low visibility caused by fog, mist and heavy rain increase the navigator's problems.

Cold can cause metal to become brittle and fail more easily. A build-up of ice can cause instability by accumulating high on the ship, or in severe cases, crush the hull if the ship becomes trapped in a freezing sea.

Fire
Fire can cause the loss of ships in many ways. The most obvious way would be the loss of a wooden ship which is burned until watertight integrity is compromised (e.g. Cospatrick). The detonation of cargo or ammunition can cause the breach of a steel hull. Often a large fire causes a ship to be abandoned and left to drift. Should it run aground beyond economic salvage, it becomes a wreck.

Navigation errors
Many shipwrecks have occurred when the crew of the ship allowed the ship to collide with rocks, reefs, icebergs, or other ships. Collision has been one of the major causes of shipwreck. Accurate navigation is made more difficult by poor visibility in bad weather. Also, many losses happened before modern navigation aids such as GPS, radar and sonar were available. Until the twentieth century, the most sophisticated navigational tools and techniques available - dead reckoning using the magnetic compass, marine chronometer (to calculate longitude) and ships logbook (which recorded the vessels heading and the speed measured by log) or celestial navigation using marine chronometer and sextant - were sufficiently accurate for journeys across oceans, but these techniques (and in many cases also the charts) lacked the precision to avoid reefs close to shore. Marine chronometers were as revolutionary in the 19th century as GPS is today. However the cost of these instruments could be prohibitive, sometimes resulting in tragic consequences for ships that were unable to determine their longitude, as in the case of the Arniston.

Even today, when highly accurate navigational equipment is readily available and universally used, there is still scope for error. Using the incorrect horizontal datum for the chart of an area may mislead the navigator, especially as many charts have not been updated to use modern data. It is also important for the navigator to appreciate that charts may be significantly in error, especially on less frequented coasts. For example, a recent revision of the map of South Georgia in the South Atlantic showed that previous maps were in some places in error by several kilometres.

Over the centuries, many technological and organisational developments have been used to reduce accidents at sea including:
- International Regulations for Preventing Collisions at Sea
- Pilotage aids including lighthouses and sea marks
- Basic navigation tools such as the magnetic compass, nautical chart, marine chronometer, sextant, log and sounding line
- Advanced navigation tools such as the radio communication, radar navigation, gyrocompass, sonar, hyperbolic Radio navigation and satellite navigation
- Inspection of shipbuilding quality and maintenance of seaworthiness of the ship such as "A1 at Lloyd's"
- Intelligence and better defences to protect the ship from acts of violence, war and piracy
- Use of fireproof/unflamable materials to prevent fires from spreading rapidly, and modern fire-fighting agents such as gases and foams that do not compromise the buoyancy and stability of the vessel as quickly as water.
- Built-in devices to delay flooding long enough for rescue ships to retrieve survivors and/or tow the ship to the nearest shipyard for repairs, such as watertight compartments and pumps.

State of preservation
The Vasa is one of the oldest and most well-preserved ships salvaged in the world, due to the low salinity of the Baltic SeaMany factors determine the state of preservation of a wreck:
- the ship's construction materials
- the wreck becoming covered in sand or silt
- the salinity of the water the wreck is in
- the level of destruction involved in the ship's loss
- whether the components or cargo of the wreck were salvaged
- whether the wreck was demolished to clear a navigable channel
- the depth of water at the wreck site
- the strength of tidal currents or wave action at the wreck site
- the exposure to surface weather conditions at the wreck site
- the presence of marine animals that consume the ship's fabric
- temperature

Construction materials
Exposed wooden components decay quickly. Often the only wooden parts of ships that remain after a century are those that were buried in silt or sand soon after the sinking. An example of this is the Mary Rose.

Steel and iron, depending on their thickness, may retain the ship's structure for decades. As corrosion takes place, sometimes helped by tides and weather, the structure collapses. Thick ferrous objects like cannons, steam boilers or the pressure vessel of a submarine often survive well underwater in spite of corrosion.

Propellers, condensers, hinges and port holes were often made from non-ferrous metals such as brass and phosphor bronze, which do not corrode easily.

Salinity of water
Shipwrecks typically decay rapidly when in sea water; shipwrecks in some fresh water lakes, such as the Great Lakes of North America, have remained intact with little degradation. There are two reasons for this:
- Iron-based metals corrode much more quickly in sea water due to the dissolved salt present; the sodium and chloride ions chemically accelerate the process of metal oxidation which, in the case of ferrous metals, leads to rust.
- Bacteria found in sea water cause the wood on ships to rot more quickly than in fresh water.

In some sea areas, most notably in Gulf of Bothnia and Gulf of Finland, salinity is very low, and centuries-old wrecks have been preserved in reasonable condition.

Loss, salvage and demolition
An important factor in the condition of the wreck is the level of destruction at the time of the loss or shortly afterwards due to the nature of the loss, salvage or later demolition.

Examples of severe destruction at the time of loss are:
- being blown onto a beach, reef or rocks during a storm
- collision with another ship
- a catastrophic explosion (e.g. HMS Hood, HMS Sheffield (D80) )
- a fire that burns for a long time before the ships sinks

After the loss the owners of the ship may attempt to salvage valuable parts of the ship or its cargo - this operation can cause damage.
Shipwrecks in shallow water near busy shipping lanes are often demolished to reduce the danger to other vessels.

Depth, tide and weather
Wrecks are slowly broken up by exposure to wave action, the weather and the tides. Additionally, wrecks in deeper water suffer more degradation due to higher levels of water pressure.

Temperature
Extreme cold (such as in a glacial-fed lake) can slow degradation of organic ship materials.

References
- Scurvy, Death and Cannibalism [internet video]. Shipwreck Central.
- Curse of the $500 million sunken treasure
- BBC Radio World Service Broadcast, "What Lies Beneath" First broadcast Friday 22 August 2008
- Hans Blumenberg, Shipwreck with Spectator: Paradigm of a Metaphor for Existence (Cambridge, Mass.: MIT Press, 1997)