Andrew Edmonds, the proprietor of Compass Marine Yacht Surveys, was tasked with the almost complete rebuild of a 35m long wooden motor yacht dating back to 1942. When Andrew first surveyed the vessel he recommended that all the keel bolts be replaced as many were no longer visible or were very weak. For this application, Andrew specified Coldur-A® from Columbia Metals.
Keel bolts hold together the boat’s wooden keel and ‘hog’, or the backbone of the vessel, and fasten this to the frames and bulkheads. The hog is slightly wider than the keel. Both are original and made of Scottish larch wood. The keel bolts will also secure scarph joints between the lengths of wood that form the hog and the keel and to fasten bronze propeller shaft brackets to the hull. The reconstruction work needed 98 keel bolts of various sizes.
“The bolts will be immersed in the keel for years to come,” explains Andrew. “Oxygen will be absent and although modern sealants help considerably to exclude sea water, we cannot assume this will remain the case for all the bolts and so we needed a material that was highly corrosion resistant with a high tensile strength. The main alternative these days is stainless steel and in Greece, where the boat was located, little silicon bronze is used. However, stainless steel has two major drawbacks when it comes to this particular application. The first is that in the absence of air and the presence of water, especially chlorides, it is highly subject to pitting and crevice corrosion.
“The second drawback with stainless steel is related to electrochemical decay (ECD) in wooden vessels. When stainless steel is subjected to sea water in the absence of oxygen, it becomes ‘active’. Unlike passive stainless steel, active stainless steel is ignoble and close, on the galvanic series, to mild steel. With steel and bronze in close proximity, an electrical circuit can become established (through water and wet wood, for example). Like galvanic corrosion of metals, ECD involves mixed metals. However, unlike galvanic corrosion, ECD will damage the noble metal and, importantly, the wood around the noble metal. In the UK this is traditionally known as Nail Sickness and in the US as Angel Hair. Under these conditions, bronze can turn red and become brittle. The lignin in the wood around the noble fastening is destroyed and the wood fibres separate – a bit like the breakfast cereal Shredded Wheat. This means that, even if the fastening is still strong enough, the timber can be readily pulled off the fastening. Lives have been lost at sea as a result of ECD.”
These two factors led Andrew towards Coldur-A® as the choice material for the keel bolts.
“By using Coldur-A® keel bolts, along with silicon bronze fastenings in the planking and bronze fittings externally and internally, we were able to avoid mixed metals while also achieving high levels of corrosion resistance,” continues Andrew. “Using a steel/iron combination is an alternative to using mixed metals but this does not achieve high levels of corrosion resistance. Moreover, to replace all the screws would have been expensive because of crevice corrosion and the effect of ECD on the 200,000 or so plank fastenings. Replacing them all in, say, 15 years would have required the accommodation to be completely stripped out.
“For this project, we partnered with Columbia Metals as we experienced high quality customer service and a good price. The company had the various diameters of Coldur-A® we needed in stock, as well as 3mm thick sheet, and was able to deliver promptly to us in Greece.”
About Compass Marine Yacht Surveys
Compass Marine Yacht Surveys is a firm of marine surveyors specialising in sailing and motor yachts along the south coast of the UK and in Greece. The company produces detailed reports based on thorough vessel inspections and is run by Andrew Edmond, a diploma holder of the International Boatbuilding Training Centre.
Coldur-A® is a silicon bronze alloy renowned for its combination of strength, corrosion resistance and formability. Its attractive golden bronze colouration makes it a firm favourite with designers, sculptors, metalsmiths and fabricators alike. Columbia Metals holds stocks of Coldur-A® in the half hard condition in sheet form and from 1/4″ to 1.1/8” diameter.
Images copyright of Andrew Edmond