More Surprises and Problems
You will remember that we had removed a small piece of plank from the ships bottom for examination, particularly to look for the presence of Teredo Worm, luckily none was found. The type of timber was English Elm, well known for its durability in sea water and used extensively in the lower hulls of wooden ships prior to the 1850s. When the small piece was cut out, apart from some surface wear, the rest of the plank was in the same condition as it had been when it was assembled there in 1924. These bottom planks were substantial, each measured at least 28 cms wide, up to 20 cms thick and up to 10 metres long, fastened to the bottom Oak frames, themselves up to 30cms square section. What drew our attention most was how the plank seams had been sealed. The shipwrights of 1924 had inserted up to 12 layers of Oakum (caulking material made of tarred and spun hemp) into each seam (see picture), each individually hammered home tight into the seam.
A crude calculation tells us that the overall length of the Oakum strands in the seams of Discovery’s planks laid end to end would have stretched to over 11 kilometres. The seams were finally sealed with a white lead putty mix (illegal today because of it’s toxic nature).
The removal of this small piece of plank provided another, unfortunate problem. The steel fastenings had worn, producing a black liquid substance that oozed between the wood and the fastening. Immediately a further survey was made of the whole of the ship’s bottom, this time concentrating on the fastenings, looking out for evidence of the black liquid material we had found at our examination site. The outcome of that survey was not good news! Many of the fastenings, especially one third of the ships length from the stern, showed signs of distress. Every fastening that showed signs of distress had to be inspected further. The result was breathtaking! Over 100 fastenings were removed, some showed mild corrosion, others severe corrosion and at some of the sites the fastening had corroded to leave nothing but a black void in the plank. See pictures.
So what had caused this phenomenon? After much scratching of heads and examination of the evidence it was decided that there were two culprits, one visible and the other invisible. The visible one was the ingress of fresh water (rainwater) into the inner hull of the ship causing some areas, where the ships inner bottom was unprotected, to allow this fresh water to seep into the planks and then find a way down the sides of the fastenings. The invisible one, much more worrying, was the electrolytic currents within the dock itself slowly corroding the steel fastenings. How long this had been going on was difficult to tell. It could well have started when the ship first lay in London from 1932.
To explain the corrosive effect in layman’s terms is difficult. Let me try! First, we must have electricity. Second we must have cables that carry that electricity. Third we must have a static but floating object that is held together by metal fastenings. Water is a wonderful generator of
electricity. Put these all together and what do you get? Electrical charges within the water that desperately search for a way out and seek an earth connection. In the case of the Discovery her steel fastenings, metal plates and other metal fittings in or on the underwater hull are targeted. The fastenings and fittings become electrically charged setting up a chemical reaction in their immediate area that attacks the metal, corrodes the metal and ultimately if left unattended, destroys the metal.
In London live electrical cables would have been at or near the dock side or near the Embankment where she lay for so long. A constant supply of electricity would have been fed on board by a cable or cables. In Dundee electrical cables surround the ship on the quayside to the North, East and West of where she lies. A 240v constant electrical supply is fed on board by cable from the quayside. So all of the elements to cause electrolytic corrosion are or have been present around the Discovery for the last 80 years or so.
What can be done about it? Well, the secret is to provide the electric currents with what they want – an earth escape. Not an ordinary earth escape but one that provides the current with the least resistance, one that has been tried and proven beyond doubt. Zinc is the most commonly used metal to protect steel from the ravages of electrolytic corrosion. It comes in many forms but the usual is rectangular blocks in ingot form. Companies who specialise in cathodic protection provide both the amount required to suit a given case and advice on how to fit the protection. These Zinc Anodes have a finite life and cease to be effective when they reach a stage where they have almost completely oxidised, therefore, they should be renewed every 12 to 18 months in order to continue to protect from the corrosion. The picture shows the state of two anodes fitted on Discovery’s stern post near the propeller, they are long past their sell-by date!!
Next. The project continues with more surprises in store!
© Captain John J Watson OBE January 2012 Fethiye