Shipwreck

A shipwreck is the remains of a ship after it has sunk or been beached as a result of a crisis at sea. A Shipwreck in the [[Douro River shore ]]

Causes

Ships are lost for many reasons, including:

• failure of the ship's equipment
• instability
• navigation errors
• bad weather
• warfare and piracy

Equipment failure

Failure or leaking of the hull is a serious problem that can lead to the loss of buoyancy and the sinking of the vessel. Even the hulls of large modern ships have cracked in heavy storms. Leaks between the hull planks of wooden vessels was a particular problem. Failure of the means of propulsion, such as engines, sails or rigging, can lead to the loss of a ship. A common problem is that the ship is unable to avoid natural hazards like rocks, shallow water or storms.

Instability

Instability is caused by the centre of mass or other forces acting on the ship rising above the centre of buoyancy 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 capsize. To remain buoyant, the hull of a vessel must prevent water entering the large air spaces of the vessel. Clearly for the ship to float, the submerged parts of the null will be watertight, but the upper parts of the hull must have openings to allow the crew to work and to load and unload cargo.

Navigation errors

Many shipwrecks have occurred when the crew of the ship have allowed the ship to collide with rocks, other ships or even icebergs. Often navigation was made more difficult by poor 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 techniques available were dead reckoning and the magnetic compass. These techniques lead to great scope for error and inaccuracy.

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. Vessel with large windage suffer most. Although powered ships are able to resist the force of the wind, sailing vessel 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 behind a sheltering 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 high on the ship can cause instability.

Warfare and piracy

Typical weapons used to sink ships have been: guns, fire, torpedoes, depth charges, mines and bombs.

State of preservation

Many factors determine the state of preservation of wreck:

• the ship's construction materials
• 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 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

Construction materials

Exposed wooden components decay quickly. Often the only wooden parts of wooden 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 its 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 phospor-bronze, which do not corrode easily in water.

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
• destruction in warfare

After the loss the owners of the ship may attempt to recover valuable parts of the ship or it cargo. This can cause damage. Ship wrecks 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 breaking waves, the weather and the tides.

Marine animals

Some species of marine worms eat wood.

See also

• List of shipwrecks

External links

*Database of 10,000 submerged wrecks and obstructions in the coastal waters of the United States

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