Wednesday, May 11, 2011

Hollow Masts Built Using a Traditional Approach

A while back I put up two posts describing (briefly) the process of building a hollow mast using the "Bird's Mouth" method. The "Bird's Mouth" system is just a variation on a traditional method where a number of staves are edge-bevelled and glued together in something like an elongated barrel.

Another traditional approach is to make a spar in two halves just as you may build a solid spar, but to hollow out the internal parts of the halves before assembly. I do not like this method, because the skill required to ensure that the wall thickness is even on both sides is substantial. In addition, and more importantly, the mast is likely to bend in varying conditions of temperature and moisture content - somewhat like a bi-metallic strip.

Some time back, I was asked to make a mast this way for a customer, and here are some pictures in case you are interested in the alternative method.

One of the test pieces I used for calculation, along with my rough drawings
In the photo above you can see a dummy section of the stock from which I intended making the mast. I had drawn two cross-sections of the mast so that I could calculate how deep to run a circular saw blade at various distances from the centreline to approximate a cylindrical cut-out. I think I was limiting the wall thickness to 20% (the mast was for a Penobscott 14) and I divided the cutting process longitudinally into two main sections of the mast so as to build in an approximation of internal taper.

Two halves with saw kerfs finished
Above you can see the two halves of the mast with the saw kerfs finished. This was a job which demanded patience and attention to detail, as any slip would ruin the valuable mast halves. In addition to keeping the wall-thickness even, I had to take into account the places in the mast where solid sections were required.

Adjusting the cut for breadth and depth to account for taper

Look carefully in the photo above, and you will see how the width and depth of the cuts has been changed to account for taper.


Solid portion at the tip where halyard sheaves will be placed
I terminated the cuts against a circular line so that the hollowed out end would finish in a hemispherical shape
Using a chisel to remove the kerfed sections. Demanding work, as a slip or a bit of chop-out due grain reversal would be a disaster.
Close-up of chiselling work - lots of adrenalin!
Finishing the interior with a convex plane.
Mast halves being glued together
In the photo above you can see the finished hollow halves being glued together on a strongback to ensure straightness. The yellow colour of the epoxy squeeze-out is because I was using WEST System Brand 207 Special Coating Hardener. I did this because the mast would end up being varnished, and I didn't want the thin glue line to turn opaque and yellow over the years of exposure to UV. A small point where there is only a glue line, but very important where large exposed surface areas of epoxy are clear coated.

All spars were marked for eight-siding using this homemade device which automatically adjusts to the taper of the spar
Initial cuts were made using a drawknife, but a plane will do it with less risk, even though it is slower.

Finishing off drawknife cuts with a plane
After planing to an octagonal section, I mark with pencil to more easily guage the cut when planing to a sixteen-sided shape
Planed to a sixteen-sided shape. See how the pencil marks help with visualisation. The process then continues to a thirty-two side shape, and then the spar is sanded to a final round
Halyard sheaves and shoud and stay chocks at mast head - dry-fitted at this stage

The design called for three cleats and a boom shelf. I do not like screwing fittings to a spar as it can cause stress concentration at the screw holes, but these wooden fittings were glued as well, so they added structurally to the mast.

Close-up of the cleats I made prior to gluing. These types of fittings can be quickly and easily made from timber found around the workshop, so you don't need to buy plastic ones - wood looks better, is cheaper, and works better.

Gaff jaws and boom jaws before being glued to the spars.
I show these photos as a matter of interest, but my recommendation for hollow spars is to use the "Bird's Mouth" system. It results in a spar which will stay straight, the work is less nerve-racking, and the resulting component is stronger.

2 comments:

  1. You have a great blog, keep up the good work.
    C.O.
    http://whatsintheshop.blogspot.com/

    ReplyDelete