Last week at the Boot Bouw School in the Netherlands I spend several days learning how to make a hollow mast using the bird’s mouth technique. It is a technique that takes full advantage of modern woodworking techniques and glues and gives a wonderfully light mast. Or any spar: the gaff that I made is featherweight compared to the boom, which has the same length and diameter.
So how does it work? We start off by saying which diameter we want. For the sake of argument let’s take 100mm. A nice size for a larger dinghy mast, I had 80mm for the Bumst. Then we cut 8 pieces of wood 10mm by 20mm, precisely 10% and 20% of the desired diameter. Of course we use a wonderfully knot-free piece of wood, preferably Oregon Pine because it is meant to be the best. I am under the impression that the European wood that would be appropriate is Kiefer, also known as Scot’s Pine (Pinus sylvestris). And my greenie heart wants to use readily replaceable woods that are not transported over the planet. I am sure this is possible. This will be part of the next mast build.
These pieces are cut so that, as far as possible, the rings cross the shortest axis of the batten. This helps the wood work less as moisture comes into and leaves the wood. So the mast or gaff (the collective noun is “spar”) stays straigh(er).
We then use the table saw to cut two 45 degree slices into the wood from the corners so that they overlap and exactly meet at 90 degrees. The resulting shape looks somewhat like the open mouth of a bird: thus the name. The perspective in the photo looks more like some screaming chicks waiting for the parent to regurgitate down their throat, but that’s just a question of perspective.
The inner angle is then scratched out with a scraper using a good sharp right angled corner. In order to make the fit better, each batten loses one of the corners: on the lower right in the photo. The battens are also slightly tapered so as to bring a taper to the construction: the tip of the mast should be smaller than the base. This sounds simple but was a battle with the planing machine to make it work halfway okay.
So now we are ready for a test assembly. The diagram on the right shows how the pieces should fit together. It all looks very simple, but manipulating more than a handful of wobbly 4 meter long battens into a proper shape is a bit like herding kittens. Or getting media artists to go somewhere for lunch during the set-up phase for an exhibition.
But with 4 hands and some cable ties, a lot of experience (Bert) and luck (myself), we were able to get a test assemblage together and I finally saw that it actually seems to work.
Once the pieces are temporarily bound together, we can make the plugs that will fill out the hollow core of the spars. We used octagonal plugs for the base of the mast, the forward end of the gaff and the support block in the middle of the gaff (where the peak halyard attaches), and round plugs (made from some scrap mahogany) for the mast and gaff peaks. The octagonal plug shaping was quite easy for the gaff, as the battens were quite nicely and symmetrically built. The mast was a bit more messy; the diameter of the construction, i.e. the distance from one side to the other, varied across the 4 “diagonals” significantly. But we managed to get the pieces together, planed and fitting closely, but not so closely that there was no space left for the glue.
The Glue! It was time. We used epoxy and some filler to get it nicely thickened so it would not drip too much: the glue should remain in the gaps between the battens and not run out. A plastic strip was laid out to stop the workbench getting too gooped up. Then it got serious. Richard, a local fellow who volunteers at the Boot Bouw School, added an extra set of hands, all hands were clothed in latex gloves, and then we got going.
First we laid out the battens for the mast in the right order (because the pieces were not as symmetrical as hoped, we had to make sure the shape would match the shape of the plug once assembled) and held them together with clamps. Then the epoxy was spread out into the open bird mouths.
We opened the clamps and two of us assembled 4 of the battens into the bottom part of the mast form. Then the interior was painted with epoxy and the plugs laid into the hollow. Now it started getting innovative. Slowly we added the next battens, fitting them into the given shape, holding the form in one and a half hands while using half a hand to position the next batten. At some point it came together, the cable ties were pulled into action and the complete assembly was held in that shape.
We then made the assembly stiffer by wrapping it in a series of hose clamps and tightening the down onto the battens. Getting the mast as straight as possible was the next challenge: sighting down the lines of the battens held together, a short sharp “whack” rather than a push seemed to work better to coax the bends and wobbles out of the mast. Whack, rotate, check, rotate, whack. Eventually we decided we were “close enough” to straight, tightened the hose clamps again and moved on to the gaff, which was smaller and thus simultaneously more complex and yet simpler.
Then we went home. A folding bicycle and a Dutch train attendant who let me off paying because the machine would not accept my card. Back to the lovely people of FoAM Amsterdam who were putting me up. Another wonderful day!