Saturday, June 18, 2011

Glued-Lapstrake (clinker) Plank Fairing

Glued-lapstrake is a deservedly popular method for building one-off boats in wood/epoxy - for a number of good reasons. In the case of the amateur and small-time professional builder it gives just about the fastest path to a handsome, round-bilged hull, with the added benefits of being light, strong, attractive given the plank lap lines. Very importantly, the building jig is simple and easy to build.

I'll write more about the particular advantages of the glued-lapstrake building method in another posting, and I've previously pointed out the way in which a fine entrance combined with the prominant plank laps keeps spray knocked down in a fashion similar to sprayrails.

In the above video, you can see how well the plank laps work at knocking down spray, although the boat (my Periwinkle design) is running in very light winds - but you can clearly see the principle.

To get back to the point, one particular advantage of glued-lapstrake construction is that the building jig need only consist of relatively few, widely-spaced molds or frames, with the plywood planks bridging the gaps between the molds in a fair curve.
Here is a typical glued-lapstrake jig, showing widely spaced molds made from disposable material. In this case the boat was a Francois Vivier-designed Aber which I built for a customer about five years ago.
Plywood is a wonderful material if you can locate good-quality sheets, but one of its virtues is also a liability. Plywood is of almost equal strength in all directions, and is frequently used in much reduced thickness compared with solid timber. The problem is that very occasionally the planks may tend to flatten out between the molds.

Plywood planks going on over widely-spaced frames and molds. This is Periwinkle

Some people put stringers into their molds, but this adds a lot of complexity to what should be a simple set-up, and adds considerably to the cost of the mold. In addition, it makes removing the boat from the mold difficult, especially if the boat has any tumble-home (that is where the sides of the boat curve in towards the centreline of the boat as they get towards the gunwale as frequently seen on "Canadian" canoes for example).

If the building jig is a simple one without stringers, it is easy to access the inside of the boat during construction, and when it comes time to remove the boat from the jig, all one has to do is knock the individual molds out one at a time.

However, there are occasions where the thin planking refuses to lie in a smooth, fair curve - usually just in a few odd places. When I am faced with that situation I use a technique where a stringer, or batten, is applied to the outside  of the hull. It need not even be a full-length batten - just enough length is required to span three or four molds. Temporary plank fasteners are driven through the batten to hold the plank while glue is curing, just as in the rest of the planking job.

Here you can see a temporary batten sitting over a plank lap on the original Periwinkle. There was a little flatness towards the bow (which has been fixed now with a minor alteration to the plans)
Close-up of the batten showing how it sits over the plank lap, and also showing where one of the temporary fasteners has been driven through.
For a job such as this you can usually get away with a single batten, re-used on subsequent planks if and where required. On Periwinkle the batten shown did the entire boat. But it needs to be empasised that the extra fairing is very rarely needed - but it is nice to have the technique available in situations where the ply is a bit too limber, or there is an inaccuracy in the jig.

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