Lofting the lines
What a grandiose term for the process of transferring some 90 odd measurements from a book to a sheet of marine ply. It evokes the days of craftsmanship, wooden sailing ships and the shipwrights that built them. Back to reality, I am building a lapstrake canoe and using some pretty modern materials to do it.
Essential tools for lofting proved to be a good quality ruler, a sharp pencil and a large set square – oh and an eraser. My set square is metric but that does not matter – size is the key here with my 40 centimetre model proving only just big enough. Using a cheap “mechanical” pencil proved to be the best way of always having a sharp point – and you can buy them by the packet for very little.
“Measure twice and cut once” goes the refrain but in reality measuring more than twice looks like a good policy. I crosschecked the offsets by calculating and then measuring the plank widths at each station. I am not sure just how paranoid one should be about accuracy. The fact that you only have a set number of fixed points along the curve of a given plank (one for each ‘station’ set at twelve inch intervals) makes me think that minor inaccuracies can be resolved when drawing a fair curve between them. I also assume that any errors in the published table or (more likely) in my reading of that table should become reasonably obvious when drawing the curves. I would not expect to see anything other than a smooth line along the whole length. Humps or hollows would imply an error somewhere.
In truth, once into the swing of things, marking out the offsets does not take much more than an hour. Now the interesting bit – drawing in the curves.
Good news on clamps - you may remember that, in my first post, I wrote that I need at least sixteen clamps for this project. Paul suggested I look at 'F' clamps and I have found that the local “pile it high and sell it not quite as expensive as the others” builder's merchant has pairs of 'F' clamps at €1.30 the pair. They also had large spring clamps at around a Euro a piece.
Curve Drawing
My trusty guide and inspiration, Chris Kulczycki’s “The Canoe Shop” suggests that the best way to draw the curves that form the upper and lower edges of each plank is to hammer a panel pin into the lofted marks at each ‘station’ and to push a flexible wooden or metal lath against the pins – drawing a fair line against the curved lath. Our blog host Paul has posted pictures of the non-slip weights that he uses to hold a lath against the waypoints on a curve – these are the business as you need something to hold the lath in position – unless you are blessed with more than the usual complement or arms.
I ended up using a combination of pins, weights, spare hand and my knees to get my lines drawn in. Again, this is not a lengthy process. I took the opportunity to re-check every measurement but otherwise found that each plank took shape fairly quickly. I did have to check that my lath was square as it had a tendency to twist slightly so I needed to check that before running my pencil down it’s edge.
I am slightly embarrassed to have to admit that I have got to my current (great?) age without realising ‘till now what the wedge shaped back end of a cross-peen hammer was for. I now understand that it is to start panel pins held between finger and thumb – how brilliant! I have been using a French cross-peen hammer – a style that I have always regarded as looking somewhat crude. This one at least, is nicely balanced and a delight to use. Anyone with a twelve year old son will know why I had to buy a new one – my rather more stylish wooden handled one is “out there somewhere” along with quite a few other tools silently “borrowed”, never to be seen again.
The hardest curve to draw is the first twenty odd centimetres of the garboard plank (the one that forms the bottom of the canoe). This curve is quite tight and detailed offsets are not available to those of us too mean to purchase the full sized plans. This curve must smoothly run into the slightly convex flow of the next 90 centimetres or so of what will become the line of the keel. A ‘flexible curve’ – basically a length of lead between two steel ribbons with a moulded plastic outer - proved helpful in getting this to look right. This simple device retains it’s shape allowing you to make minor adjustments until the overall curve is fair.
No comments:
Post a Comment