Most
of the masts that I make are built hollow using the so called
"Bird's Mouth" technique. This is just a development of a vast range of
similar methods used over many hundreds of years, but it is particularly
suited to this day and age - mainly because of tilting-arbor table saws
and power routers being readily available.
In this series of photos, I'll just show some basic steps in the
assembly process. The marking and cutting of the staves which make up
the spar need an article of their own.
Because of the large surface area which needs to be covered with epoxy, time is critical, and you need to be organised!
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First four staves
laid up dry to test fit. I am using 'U'-shaped plywood forms to hold
the staves loosely together and in alignment. The forms are clamped to a
long piece of timber acting as a strongback. |
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There are only
about four forms for this entire mast, which is 23 feet long. They are
simply there to prevent the eight staves getting out of
control during the assembly. Once together, the structure is
self-supporting, but it does need to be straightend very carefully
before the glue goes off. |
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I always make
solid inserts for the bottom of the mast. It usually extends up to above
the level of the mast partners. The internal 'V' cut-out is there so
that there is no abrupt transition in stiffness between the solid and
hollow sections. |
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I carefully shape
the solid inserts so that they fit the internal shape of the mast
without holding the staves apart. A slightly loose fit is good, as it
allows a rich glue-line between the insert and the inner walls of the
mast. |
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Another view of the insert |
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All eight staves
are lined up in order, and at a convenient working height. Here I have a
helper, as we need to be able to get epoxy out fast. We are applying
the sealing coats to what will eventually be the inside of the mast, as
it will not be accessible after assembly. |
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I use foam
rollers to apply the epoxy, but you need to be careful to avoid pressing
too hard and causing the wet epoxy to foam like whipped cream. After
each coat is applied, I run over it lightly with a paint brush to smooth
the surface and break any bubbles. |
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These are the
staves at the masthead end. As you can see, these are almost at the
minimum size limit for this method - I had done careful
calculations ahead of time to make sure it would be feasible. You can
see here that the epoxy coatings are building up. |
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This shows me
working on another mast. We had already applied the glue to the gluing
surfaces using various techniques, and at this point I was rapidly
adding extra where required, and was smoothing the glue on all
mating surfaces |
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The assembly just
after putting together and placing a few cable ties around it to keep
everything in place. Note the squeeze-out beginning to occur. It is
important to have the glue fairly runny, as excess viscosity makes
it very hard to clamp the total assembly tightly enough so that the
mast assumes the correct diameter. |
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Start with a few, evenly-spaced cable ties, and then put on more in-between. |
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I use high-quality cable-ties and tighten them with the aid of pliers. |
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As you can see,
I'm serious about getting a good and even clamping! Although you can't
see it well in the photo, I use multiple windings of packaging tape to
apply extra pressure where needed. There are many different methods of
clamping, but this system works for me. |
Once the clamping has been completed, I then spend time
making sure that the mast is straight. While the glue is still wet, you
can fairly easily push, pull, and hammer the thing until it is straight.
Put some effort into this, as when the glue goes off, the shape is
fixed forever! You will find that the human eye is a very effective tool
for determining straightness.
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Slices sawn off
the top and bottom ends of the mast shown in the construction
photos. You will note that when I cut the bird's mouth cutouts, I run
the saw blade just a little deeper than required, so that I get a little
resevoir for epoxy, and that the staves fit in without hang-ups. |
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Here is a finished mast on another boat I built a couple of years ago. |
Be open-minded about this process - it may look intimidating, but in fact it is quite easy - as long as you are organised!
Hi,
ReplyDeleteGreat pics and nice boat. Can you talk about the sanding process? I am not able to find a video showing this step.
Great work
ReplyDelete