Thursday, April 14, 2011

More about "Bird's Mouth" Spars

Last night I wrote a short piece about the process of assembling a "Bird's Mouth" mast. Since then, I've dug out some photos from my achives to show some of the steps in the production of the "Bird's Mouth" cut-outs.

The equipment required is basic, with the primary component being a table saw with a tilting arbor. This does not need to be expensive - mine is a second-hand Makita which cost me $120. I do use an old thicknesser and a cheap (and old) bandsaw - but these are luxury items, and the job can be done without them - it just takes a little longer.

This is the blade of my table saw tilted at 45 degrees, and set to cut to the required depth. On the right you can see the fence of the saw, and on the left is my home-made feather-board which will hold the sections of timber firmly against the fence while I make the cuts.

This shows one of the mast staves being pushed through the saw, with the home-made finger board holding the piece firmly against the fence. Accurate cutting is essential if you hope to get a mast with the correct diameter at each location.
Note the use of a 'push-stick' at the end of the run - be careful with power tools, as they can be very unforgiving and dangerous. If in doubt, get an experienced person to do the job for you.
First cut completed on one stave. After this, turn the stave end-for-end and run it through again.
This is how it should look after the second pass - now just do the same thing seven times more!
After all of the staves have had the 'Bird's Mouth' cut-out completed, I mark the pre-calculated taper on the side opposite to the bird's mouth and cut to the line with a bandsaw. This is a fairly tedious job, as there are eight very long cuts to be made, but it can be done with a plane and/or a jigsaw if you don't have access to a bandsaw.

A rough drawing to show the material to be cut away from the side opposite the 'Bird's Mouth'. This produces the taper in the finished, assembled spar. I usually give all the required dimensions in my more recently-completed plans.
Two masts (a main mast and a mizzen mast for a cat-ketch) given a trial (or "dry") assembly
Here they are, loosely assembled.
These are the solid inserts I put inside the bases of my masts. The 'V' cut-out is so there is no abrupt transition in stiffness between the solid and the hollow sections of the mast.
The section cut out of the solid plug. If you look carefully at the apex of the cut-out on the right of the picture you will see that I have finished the cut-out with a round hole. This is to prevent splitting of the solid plug.
Testing the fit prior to gluing.
A nice fit!
A glued-up mast.
Have a look at the previous post to see some pictures of the assembly process.


  1. Nice pics but I am not sure they make the process any less daunting - An open mind and a sense of wonder it might seem.

  2. Graham,

    I can make anything look more complicated than it is!

    Actually, as with most boatbuilding operations, the job provides answer to most questions as you go along. The main thing is to be organised, and aware of a few of the pit-falls.

  3. I built Ross' Flint and decided to have a go at the Bird's Mouth technique - it certainly isn't as difficult as it would first appear. I used a metre length of oregon veranda post (120mm x 120mm) to produce a 3.5m x 50mm mast - lots of ripping and scarfing. One tip I read online (after I had finished my mast) was to use tape on two (opposite) 'mouth' faces. This meant the mast could be split in two after the epoxy had cured, internal faces coated with epoxy, and the mast then re-assembled. Trying to coat the internal faces as well as the 'mouth' face at the same time can get very messy - especially as it helps to roll the staves around something during assembly. Just my 2c worth.

  4. Ross,

    What timber are you using for birdmouth spars? it looks like you have two contrasting timbers in the photo of the mizzen.