Shipwreck

A shipwreck is the wreckage of a ship that is either beached on land or sunken to the bottom of a body of water. It results from the event of shipwrecking, which may be intentional or unintentional. There were approximately three million shipwrecks worldwide as of January 1999, according to Angela Croome, a science writer and author who specialized in the history of underwater archaeology .
When a ship's crew has died or abandoned the ship, and the ship has remained adrift but unsunk, they are instead referred to as ghost ships.

Types

Historic wrecks

Historic wrecks are attractive to maritime archaeologists because they preserve historical information: for example, studying the wreck of revealed information about seafaring, warfare, and life in the 16th century. Military wrecks, caused by a skirmish at sea, are studied to find details about the historic event; they reveal much about the battle that occurred. Discoveries of treasure ships, often from the period of European colonisation, which sank in remote locations leaving few living witnesses, such as, do occur as well.

Environmental hazards

Some contemporary wrecks, such as the oil tanker Prestige or, are of interest primarily because of their potential harm to the environment.

Hazards to navigation

Artificial reefs and dive sites

Other contemporary wrecks are scuttled in order to spur reef growth, such as and Ocean Freeze. Many contemporary and historic wrecks, such as, are of interest to recreational divers that dive to shipwrecks because they are interesting to explore, provide large habitats for many types of marine life, and have an interesting history.

Well known incidents and disasters

Well-known shipwrecks include the catastrophic Titanic, MV Doña Paz,,,,,,,, HMS Pandora and.

Abandoned or scuttled derelicts

There are also thousands of wrecks that were not lost at sea but have been abandoned or sunk. These abandoned, or derelict ships are typically smaller craft, such as fishing vessels. They may pose a hazard to navigation and may be removed by port authorities.

Causes

Poor design, improperly stowed cargo, navigation and other human errors leading to collisions, inadequate maintenance, bad weather, fire, and other causes can lead to accidental sinking. Intentional reasons for sinking a ship include: intending to form an artificial reef; destruction due to warfare, piracy, mutiny or sabotage; using the vessel for target practice; or removing a hazard to navigation. A scuttled ship can be also used as breakwater structure or to deny or restrict access to an area to other shipping.

Distribution

Shipwrecks are widely distributed, but are concentrated where there is more shipping and more navigational hazards.
Some shipwrecks have been recorded and the circumstances of loss may be known. other ships have disappeared without trace and are assumed to have been wrecked.

State of preservation

Many 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 life that consume the ship's fabric
temperature
the acidity, and other chemical characteristics of the water at the site

The above - especially the stratification and the damages caused by marine creatures - is better described as "stratification and contamination" of shipwrecks. The stratification not only creates another challenge for marine archaeology, but also a challenge to determine its primary state, i.e. the state that it was in when it sank.
Stratification includes several different types of sand and silt, as well as tumulus and encrustations. These "sediments" are tightly linked to the type of currents, depth, and the type of water, which implies any chemical reactions that would affect potential cargo.
Besides this geological phenomenon, wrecks also face the damage of marine creatures that create a home out of them, primarily octopuses and crustaceans. These creatures affect the primary state because they move, or break, any parts of the shipwreck that are in their way, thereby affecting the original condition of amphorae, for example, or any other hollow places. Finally, in addition to the slight or severe destruction marine animals can create, there are also "external" contaminants, such as the artifacts on and around the wreck at Pickles Reef and the over-lapping wrecks at the Molasses Reef Wreck, or contemporary pollution in bodies of water, that severely affect shipwrecks by changing the chemical structures, or further damaging what is left of a specific ship.
Despite these challenges, if the information retrieved does not appear to be sufficient, or a poor preservation is achieved, authors like J.A. Parker claim that it is the historical value of the shipwreck that counts as well as any slight piece of information or evidence that is acquired.

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 Mary Rose.
Steel and iron, depending on their thickness, may retain the ship's structure for decades. As corrosion takes place, sometimes accelerated by tides and weather, the structure collapses. Thicker ferrous objects such as cannons, boilers or the pressure vessel of a submarine generall survive longer underwater in spite of corrosion.
Propellers, condensers, hinges and portholes were often made from non-ferrous metals such as brass and bronze, which do not corrode easily.

Salinity of water

Fresh and low salinity water

Shipwrecks in some freshwater lakes, such as the Great Lakes of North America, have remained intact with little degradation. 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. However, bacteria found in fresh water can cause the wood of wrecked ships to rot more quickly than in seawater unless it is deprived of oxygen. Two shipwrecks, and, have been at the bottom of Lake Ontario since they sunk during a violent storm on August 8, 1813, during the War of 1812. They are in "remarkably good" condition.

Seawater

Wrecks typically decay rapidly when in seawater. There are several reasons for this:

Iron-based metals corrode much more quickly in seawater because of 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. Such cases are prominent on deep-water shipwrecks, such as RMS Titanic, , and . However, there are some exceptions; RMS Empress of Ireland lies in the saltwater portion of the St. Lawrence River, but is still in remarkably good condition.
Unprotected wood in seawater is rapidly consumed by shipworms and small wood-boring sea creatures. Shipworms found in higher salinity waters, such as the Caribbean, are notorious for boring into wooden structures that are immersed in sea water and can completely destroy the hull of a wooden shipwreck.
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, termed "grounding"
Collision with another ship
Catastrophic explosion, steamship boilers often explode when water covers them during the process of sinking
Fire that burns for a long time before the ship sinks
Foundering, i.e., taking in so much water that buoyancy is lost and the ship sinks ; some ships with a dense cargo may break up when sinking quickly and hitting a rocky seabed
Enemy action from aerial bombs or torpedoes that may cause destruction before sinking

After the loss, the vessel's owners may attempt to salvage valuable parts of the ship and its cargo. This operation can cause further damage.
Shipwrecks in shallow water near busy shipping lanes are often demolished or removed to reduce the danger to other vessels. On charts, some wreck symbols have a line under the symbol with a depth mark, which indicates the water depth above the wreck.

Depth, tide and weather

On the seabed, wrecks are slowly broken up by the forces of wave action caused by the weather and currents caused by tides. More highly oxygenated water promotes corrosion, which reduces the strength of ferrous structural materials of the ship. Deeper wrecks may be protected by less exposure to water movement and by lower levels of oxygen in water.

Temperature

Extreme cold slows the degradation of ship materials. Decay, corrosion and marine encrustation are inhibited in cold waters.

Marine pollution

Many modern shipwrecks contribute to marine pollution, mainly as sources of significant oil spills. A 2005 survey of shipwrecks since 1890 found that over 8,500 of the submerged wrecks may still contain oil. Oil spills can have devastating effects on marine and coastal environments as well as fisheries. In addition to being toxic to marine life, polycyclic aromatic hydrocarbons, found in crude oil, are very difficult to clean up, and last for years in the sediment and marine environment.
Shipwreck pollution may also originate with a ship's cargo or munitions, such as unexploded ordnance or chemical weapons canisters. German trawler V 1302 John Mahn, sunk in the North Sea in 1942, has multiple unexploded depth charges on board which render the wreck hazardous. Samples taken from the wreck and nearby sediment show the presence of heavy metals like nickel and copper, polycyclic aromatic hydrocarbons, arsenic and explosive compounds, which have changed the local microbial ecology.