Wednesday, April 13, 2011

Assembling a "Bird's Mouth" Hollow Mast

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!

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.
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. 
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.
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.
Another view of the insert
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.
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.
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.

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

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.
Start with a few, evenly-spaced cable ties, and then put on more in-between.
I use high-quality cable-ties and tighten them with the aid of pliers.
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.

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.
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!


  1. Hi,

    Great pics and nice boat. Can you talk about the sanding process? I am not able to find a video showing this step.