Tuesday, 5 February 2013

Hogging and Sagging

As I mentioned in my first post, Peggy has been propped upright on frames since 1950. We don't know exactly what she weighs, though we estimate about 2.5 tonnes.
Last week I introduced some of the ways in which the boat has decayed since she was made, and showed how her fixings (nails) are failing and damaging her. We wanted to be absolutely clear about this so, in 2010 I removed five timber core samples from the hull, each containing an iron nail.

Me removing core samples from Peggy's hull
I sent the samples to Dr David Watkinson of Cardiff University for analysis. David is one of the world's leading experts on marine archaeological iron. His tests showed that in every case there was practically no metallic iron left where the nails should have been, only rust. They also showed high amounts of sea salt (= bad) and advanced mineralisation (acid attack) of the surrounding timber. The failure of her nails puts Peggy's timbers under strain.
Wooden ships and boats often bend and warp if they develop structural faults or if they are otherwise put under unnatural strain. The commonest of these are termed hogging and sagging.
You might expect Peggy to have sagged because she is not evenly supported all along her hull as she would have been when she was afloat. In fact you can see that she has if you look at the distorted shape of the timbers adjacent to the 1950 props.
Hogging refers to bending of the keel. Its not easy to assess this in the tight space of the boat cellar. However in 2012 we commissioned a 3D laser survey of Peggy's hull, with the generous assistance of National Historic Ships UK (see links to the right of this post). You can clearly see the keel is slightly bent, a little like a 'hog's back'.

A preliminary scan image of Peggy showing hogging. The colours represent separate patches of data.

Next time I'll discuss how we came to decide what needed to be done to preserve Peggy.

No comments:

Post a comment