Monday, 28 January 2013


Peggy's boat cellar is prone to tidal flooding. It also has a freshwater stream running through it. Since 2009 we have been recording the temperature and relative humidity (RH, in %) of the air in the cellar. There are plenty of resources on the www that neatly explain what RH is, but for the purposes of this blog its only necessary to know that it's RH that influences whether objects get damp or dry out.
Above 60% RH wooden objects are subject to increased risk of attack from fungi such as 'rot', and the risk grows as the RH rises. Wood-boring insects such as furniture beetle (woodworm) also like damp. In fact just about every natural decay process you can think of is helped along by high RH.
Iron, as we all know, rusts quickly when it is wet, and the wetter it gets, the faster it rusts. It'll keep on corroding until there is no metal left at all. If the iron is salty the corrosion will be more rapid and serious. The rate at which iron rusts is directly related to RH: the lower the RH the slower the corrosion. The only way to stop iron from rusting is to lower the RH of the surrounding atmosphere. In the case of iron contaminated with sea salt, one would have to lower the RH to levels similar to those you'd find in a desert to stop it rusting. This is quite a challenge! Very dry environments are not, unfortunately, very good for wooden objects.
The RH in the boat cellar is consistently 85-99%. This isn't good, obviously. Incidentally, we are interested to know why some of the grosser forms of fungal attack such as dry and wet rot are not more prevalent in the boat and building; we suspect the low temperature (8-12 degrees C throughout the year) may be a factor, but it's a question for future research. Aside from the kind of decay you can see and easily recognise however there remains the problem of more insidious decay we can't so easily see, and above all that of iron corrosion.
As the nails holding Peggy together rust they leach acid into the timber, weakening and destroying it. The evidence for this is all over the boat in the form of brown stains and circles. The boat would theoretically, eventually collapse, though this is not imminent. Some of the fixings are failing, nonetheless.

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