The proprietor of Compass Marine Yacht Surveys, Andrew Edmonds, was given the task of almost entirely rebuilding a 35m long wooden motor yacht originally made in 1942.
Upon initially surveying the vessel, Andrew proposed that every keel bolt should be updated because the majority of them were very weak or invisible. Andrew recommended the use of Coldur-A® from Columbia Metals for this application.
Keel bolts join together the wooden keel and 'hog’ of the boat, also called the backbone of the vessel, and fasten this to the bulkheads and frames. The hog is partially wider than the keel. Both are created from Scottish larch wood and are in their original condition.
The scarph joints between the lengths of wood that create the keel and the hog needed to be secured by keel bolts, and also to fix bronze propeller shaft brackets to the hull. The reconstruction effort required 98 keel bolts of different sizes.
“The bolts will be immersed in the keel for years to come,” describes 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 fibers 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.”
Coldur-A®—The Preferred Material for Keel Bolts
Both of these factors meant that Andrew was directed towards Coldur-A® as the preferred 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,” Andrew explains.
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.”
This information has been sourced, reviewed and adapted from materials provided by Columbia Metals Ltd.
For more information on this source, please visit Columbia Metals Ltd.