How do wrecks decay?

​No ship yard ever built a ship with the intention that it should end up on the seabed. By design ships are intended to keep the water on the outside (or in controlled areas like ballast tanks). From the earliest hollowed logs and coracles to the ocean going supertankers, the aim is to find materials that create a good airspace and allow the vessel, cargo and crew to stay on the surface. As construction methods have evolved, so have the materials that are used. For any vessel owner, the necessary maintenance to keep the water out is an ongoing and relentless battle.

Once a ship sinks, the process of decay inevitably starts. There are a number of parameters that affect how quickly a wreck will break up. A shallow wreck is exposed to the mechanical shearing forces of wave action and the remarkably destructive scouring of sand. Many wrecks become wrecks because the end up punctured on shorelines. In the battle between the rock and the metal hull, rocks often come out the winner. However, deeper wrecks evade the action of the weather and therefore will remain intact for longer.

Any biological material on a wreck which will decay very quickly. Body flesh is quickly scavenged by crustacea and fish. The hard matrix of the bones that are left behind is mostly hydroxyapatite (a mixture of calcium and phosphate) which is soluble in the sea and becomes more soluble at depth. This is why the deep ocean isn’t several metres deep in fish and whale bones, they dissolve.
The next most fragile structure on any ship wreck is the wooden components. Modern ships with chipboard partitions fare particularly badly once submerged, and can fall apart within a very short space of years. Wood is made up of cellulose and lignin molecules. Cellulose is the main part of the cell wall from the tree that the wood came from. Cellulose isn’t water soluble but the chains of cellulose are held together by hydrogen bonds which water helps to promote. Surrounding the cellulose chains are lignins. These are complex polymers that give wood rigidity and resist rotting. The levels and type of lignin vary between different species of tree. Teak, the beloved material for decks of many vessels, has a high lignin content, which helps it to resist degradation.

For the metal components of a wreck to decay there are several factors that will affect the rate of decay. Salt minerals dissolved in the ocean, particularly sodium chloride (the same stuff you sprinkle on your chips) is a major player. In salt water metals will corrode about 5 times faster than in fresh water. Salts break into charged ions which allow the conduction of electricity and metal ions from the ship will enter the water, gradually thinning the metal plates of the hull. Salinity levels can vary massively depending on the location. A nearby source of freshwater can reduce decay. Salinity is maximum at the surface and decreases down to 500 metres, although it rises again around 2500 metres down.

Oxygen levels in the water will also affect the rate of decay.  Oxygen reacts with metals to produce metal oxides eg iron reacts to produce iron oxide ie rust. Metal oxides are weaker than the metal they derive from.  So gradually the layer of metal turns to rust, which will thin the metal hull even further. Oxygen levels are at a maximum near the surface and decrease down to about 1000m, and then they increase with depth.

Finally, let’s consider temperature. A higher water temperature means all the water molecule are moving faster and at a molecular level, all these reactions occur more quickly. Deeper wrecks in colder waters have a slower decay pathway.
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So for a shipwreck to survive, we require a well-constructed, high quality metal in thick sheets, sunk in fairly cold water, deep enough to avoid wave action and sand scouring, so somewhere sheltered would be ideal……welcome to the wrecks of Scapa Flow!