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.
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!
One of the hardest skills for some divers as their diving career progresses is learning to use a compass. Once you’ve mastered the technical aspects of making sure it’s moving freely and not locked off inside the casing, the biggest hurdle is trust. You need to gain a Jedi like perspective as you accept Obi-Wan’s guidance to “Use the Force”. And generally, that’s fine, until there’s a nearby wreck and your compass stops being attracted to the earth’s magnetic field and starts interacting with the ship’s magnetic field instead.
When iron hulled ships were introduced, the effect of the metal hull on steering compasses was first observed. During construction the metal in the ship adopts the magnetic field of the dockyard used for construction. In modern construction methods, the high currents used for welding the steel plates together create magnetic dipoles in the steel, thus magnetising the ship…
During the American Civil War, mines were developed that were activated by contact. The target ship hits the horn on the mine. The soft metal of the horn buckled under impact, smashing a glass ampoule with battery acid inside. The acid electrolyte dropped into the waiting battery, energizing it and heating a platinum wire inside the mercury fulminate detonator. Boom.
At the start of WW2, the Germans developed a new magnetic trigger for mines. For a while the British were stumped as to how these mines worked. But in November 1939 a German mine was dropped from an aircraft and landed on mudflats in the Thames estuary at low tide. The mine was disarmed and taken to Portsmouth, where the mechanism was examined. A magnetic needle which was pulled by the target ship’s magnetic field completed the circuit and fired the mine. Later sophisticated versions would use a counter that didn’t fire for the first few ships to pass.
Establishing how the mine worked held the secret to protecting vessels, you just need to wipe out the magnetic signal. Magnetic field strength is measured in units named after Carl Guass, so the process of removing the magnetic signature is known as degaussing. Remove the magnetic field from the ship and it can safely pass over the mines without triggering an explosion.
There were two ways of cancelling out the ship’s magnetic field. The permanent one was to put thick bands of electrical wire around the length of the vessel, known as coiling. Passing an electrical current through these cables generated an electromagnetic field that cancelled out the ship’s own field, thus rendering the ship invisible to the mine mechanism. Royal Navy Commander Charles Goodeve oversaw this system, and it even allowed for the polarity to be reversed when ships were in the southern hemisphere so that the ship appeared to have the same magnetic field as the natural background. But this equipment was expensive and difficult to install.
Measuring a ships natural magnetic field was a complex business. A series of magnetometers are anchored to the seabed about 5 metres apart for a 150 metre run. Each magnetometer wa connected to a fluxmeter on the shore. The ship passed over the magnetometers and the readings from the fluxmeter were used to create the ship’s signature. From this starting point the number of turns of the degaussing cable could be increased or decreased, or the current altered until the signature was minimised.
A second quicker method was to wipe the hull of the ship, with a current carrying cable running a pulse at about 2000 amps. The large cable was dragged down the sides of the ship in a process known as deperming. This wasn’t a permanent solution though, as the ship travelled through the Earth’s magnetic field it slowly became magnetised again. This started in late 1939 and helped protect many of the vessels that carried out the evacuation from Dunkirk. In a 4 day marathon session prior to the evacuation over 400 ships were ‘wiped’ in this way, though there are concerns that some of the ‘wiping’ may have not been as effective as hoped. The Isle of Man vessel Mona’s Queen was lost to a magnetic mine on 29th May 1940 just outside Dunkirk harbour, and stories persist to this day among the relatives of the crew that the ship wasn’t properly protected.
Any diver who has been to Scapa Flow to visit the remains of the World War I German naval fleet will know the story of the 21st June 1919 when 74 ships were scuttled. Due to some heroic efforts only 52 ships actually hit the seabed. Initially, the British Admiralty were determined to leave the German fleet on the seabed and let them rust. But the wrecks were a considerable hazard to local vessels, with several being grounded on up-turned hulls. By 1922 the demand for scrap metal had increased and the Admiralty started selling off the wrecks for salvage, £250 for a destroyer and a mere £1000 for a whole battleship (of course 100 years ago that was the equivalent of £54,000 but that still seems cheap).
Over the next 8 years Ernest Cox developed some incredible techniques to lift huge battleships from the seabed within Scapa Flow. The wrecks were salvaged for fixtures and fittings with artefacts being recovered in good condition including bottles of wine, musical instruments, and the metal ships were broken up for scrap sales. By 1930, the price of scrap metal had crashed leaving the whole operation in danger of financial ruin and by 1933 Cox sold out to Metal Industries Group and they lifted the last of the battleships in 1939 as World War II loomed. Just 3 battleships and 4 cruisers remained. These are the wrecks that divers now visit, but they all bear the scars of the salvage work carried out by Nundy Ltd and then from 1970 by Dougall Campbell as Scapa Flow Salvage Company. Explosives were used to blow holes into engine rooms to get the non-ferrous metals and to open up the hulls to recover the valuable torpedo tubes. Campbell also realised that the armour belts of 14 inch thick steel with a high content of nickel and chrome were easy to recover and valuable enough to be worth the effort. [Dougall freely shared his amazing knowledge to a number of Scapa Flow projects in advance of the centenary, and sadly passed away on July 26th 2018].
Sensibilities have changed regarding wrecks and their salvage. HMS Vanguard is now considered a war grave after she was lost with 845 men following an explosion in her magazine in July 1917, but by the 1950s and then again in 1970s, she was salvaged for her propellers, condensers, torpedo tubes, armour plating and Weir pumps. It wasn’t until 2002 that the wreck became a Controlled Site and diving was restricted, but for 85 years she was stripped of any useful metals ie the ones that would command a good price on the scrap market.
Early salvage operations were driven by the demand for scrap metal. In post WWI industrial development was being held back by lack of metal. But post WWII there was a new driver for recovering steel produced before WWI, lack of background radiation in the metal itself. As the nuclear and space races took hold on the 1950s and medical advancements in 1960s, there was a growing demand for steel that had very low levels of background radiation. Atomic testing in the 1950s released Cobalt-60 into the atmosphere. Steel production involves pumping air (or oxygen) through molten pig iron, which reacts with impurities creating oxides that can be removed as slag. Atmospheric atomic tests released atmospheric radiation which peaked in 1963, so any air or oxygen used to produce steel since that time introduced low levels of background radiation into the metal.
Ordinarily, this isn’t an issue. You wouldn’t notice or be affected by a trace in your cutlery or the spice rack in your kitchen or the panels in your car. But it matters if you are trying to build sensitive instruments to detect radiation. Geiger counters, body scanners and space equipment can all be affected by the low level of radiation introduced during the manufacturing process. That makes the steel armour plating from the pre-atomic era significantly more valuable than would otherwise be the case. Our latter found sensibilities to protect the wrecks as war graves wasn’t apparent decades ago. In Scapa Flow the salvage has stopped, but internationally wrecks are disappearing in their entirety. Happily, the half-life of Co-60 is short, only 5.26 years. Since the moratorium on atmospheric testing in 1963, levels of atmospheric radiation have been rapidly dropping, with only occasional contributions from Chernobyl, Windscale, Three Mile Island. This means that production of low background radiation steel is now possible, combined with computerised correction for background levels. However a drop in the market value of pre-atomic steel is unlikely to save wrecks around the world, particularly in Asia where even the former pride of the British Navy HMS Prince of Wales and HMS Repulse have been targeted by metal scavengers.
Anyone who runs a retail business will tell you that they are regularly approached for donations to a variety of charities. It’s a common thing for businesses to choose charities that reflect their employees’ interests or even to have an annual pitch from the employees to pick a charity of the year. In our dive centre we have collecting boxes for the local Hyperbaric Chamber and the RNLI, both close to divers’ hearts and interests.
The reality is that I’ve never been to the chamber for anything more than a planned visit and I’ve never called out the lifeboat. In fact we’ve responded to several emergencies at sea and reduced the times that Port St Mary lifeboat has been scrambled. So although we actively fundraise for both these charities, we definitely count in the low/no user group. But it’s reassuring to know that they are both there if we ever need them.
I have come to the conclusion that the RNLI isn’t always the diver’s best friend. With improved navigation aids and communications, excellent training and really powerful pumps our local lifeboat has been involved in many incidents. We know many of the crew, in fact we taught several of them to dive. The Isle of Man is blessed with numerous wrecks, many still unidentified. These are the product of two World Wars and, in the past, considerable navigation errors. Stick a lump of rock in the middle of the Irish Sea and it seems like a considerable number of vessels will manage to run into it! But all these wrecks are gradually deteriorating, leaving just the boilers and maybe the prop shaft behind amidst a collapsed mess of plates and ribs. As time marches inexorably onwards, the decay takes its toll and within the next few years several of these sites will all but disappear. So my big dilemma is this, where will the next generation of wrecks come from?
I can’t have been the only diver who watched the Riverdance drama unfold in 2008. Once the crew and passengers had been safely lifted off and she was adrift towards the Blackpool coast, I admit I was willing her, telepathically transmitting a “Sink. Sink. Sink” message. The seabed between the Isle of Man and the North West of England is rarely more than 40m. Just imagine a wreck of that size as a dive site. The Zenobia of the North West. But oh no! A conspiracy between the RAF, coastguard, ship owners and RNLI meant that they had manoeuvred her towards the shore until she was abandoned and by then she was so far inshore that she ran aground. She rapidly became a big tourist attraction, but after several failed attempts to refloat her, the owners sliced Riverdance into scrap metal and carted her away to an ignominious end. What an utter waste of a brilliant dive site!
So much as I love the RNLI I have come to the conclusion that they are not really a diver’s best friend. All this pumping out boats and towing them back to harbour malarkey is not good for maintaining a decent number of wrecks to dive. A recent faulty fire alarm panel in a neighbouring building saw the fire brigade arrive at 7am outside the dive centre. The building was locked up, but having established that no-one was in the building and there was no fire, they all accepted a cup of tea (in our DDRC mugs of course) and left with the alarm still ringing. I want the RNLI to take the same approach – if there’s no one in danger then just leave the ship to become a wreck. Don’t worry about where it’s going to end up, we have sounders and towable cameras to find it. A last known position will do and we’ll take it from there.
Sir William Hillary lobbied for the inception of the RNLI after witnessing the destructive power of the Irish Sea around the Isle of Man. He took part in commanding a volunteer crew in the heroic rescue of all the passengers of the St George as she struck Connister Rock outside Douglas harbour in 1830. The Isle of Man is proud of our longstanding association with the RNLI and Douglas boasts the first Life boat station. But in the early days the rescues were aimed at saving lives, the saving vessels part came later. All I’m asking for is the crews to be a little more pragmatic…..the next generation of wrecks lies in their hands!
Michelle has been scuba diving for nearly 30 years. Drawing on her science background she tackles some bits of marine science. and sometimes has a sideways glance at the people and events that she encounters in the diving world.