Thursday, December 8, 2011

Tapering Mast Staves - My Approach to the Problem

Increasingly, people are turning to the "Bird's Mouth" method for making hollow timber spars and even oars. I've written about various aspects of the system in previous posts - see here and here and here .

Over the last week I've been preparing material for two hollow masts and I thought I'd show you my tricks for getting accurate tapers on the individual staves quickly and easily.

Firstly, I machine the mast stave material down to the thickness required - on my plans, you will find that the thickness of the staves is annotated on the relevant plan sheet.

This is one of the sail plans sheets from the First Mate plans. Shown on the drawing is the length of each spar, and the diameters at marked intervals along the spar. Also included is the thichness of the "Bird's Mouth" staves and their width at marked intervals. This takes much of the painful work out of calculation the sizes of mast components.
After having machined the material down to the required thickness, I cutt eight staves slightly wider than the dimension at the widest point - I usually make tham about 3mm/1/8" or more wider than required. I then cut the "Bird's Mouth" cut-out as described in the earlier postings.

Cutting the "Bird's Mouth" on the table saw.
Once cut to a long, parallel-sided stave with a "bird's Mouth" it is time to cut the taper.

Showing the principle for cutting the taper on staves
My approach is as follows: -

Marking divisions along on single mast stave. This stave will become the "master"
At each division, draw a line across the stave, square to the long edge.
Here is a small section of one drawing, where you can see that this mast is 3816mm long, with divisions at 553mm intervals (these aren't alway even, but they are clearly marked on the drawings. You can specify metric or imperial. In the middle of the photo you can see that at that point, the mast is 73mm in diameter and the figure in brackets shows the if the mast staves are cut to 28.3mm at that point the mast will come out at the correct total diameter after rounding (or you could leave it octagonal for less labour).
More details taken from the drawing
At each division, I mark the width of the stave, working from the bird's mouth edge so that the taper is cut off the flat edge. I find vernier callipers to be the most convenient tool for this job (and for many others)
Once all of the offsets have been marked, I clamp the stave up against a straight-edge to keep it straight between divisions (this is very important) and I then draw a straight line from offset mark to offset.
The Bird's Mouth side is clamped lightly up against a straight edge, and in the forground you can see how I have marked the amount of material which needs to be removed
You can see that as you approach the tip, more and more material needs to be removed to achieve the required taper.
After marking the taper line, I cut the excess material off using a bandsaw, but you could laso do it with a hand plane. The bandsaw is good because it gives a square cut if properly adjusted. Cut just clear of the line and finish up with a sharp low-angle block plane.

Finishing off the cut with a low-angle block plane
Once the first "master" stave has been completed accurately, I then lay it on top of the succeeding staves, holding both against the straight edge fo accuracy. Then, trace around the master stave to mark the remaining seven in succession.

Marking the next stave by tracing around the master with a sharp pencil
Another shot showing the marking of the remaining staves by tracing around the master.

After this it is simply a matter of cutting and planing the remaining seven staves.

To assemble the eight staves can be very difficult if you are working single-handed, as the whole bundle tends to collapse like a house-of-cards. In this series of pictures I am only doing a dry assembly, but it shows the trick I use to make things fool-proof.

Staves are temporarily screwed to an octagonal disc of thick plywood of a generous diameter

Staves are temporarily screwed to an octagonal disc of thick plywood of a generous diameter
This procedure holds the eight stave in their correct angular relationship, greatly simplifying the assembly process. I then move to the far end and start clipping the eight staves together in a self-supporting bundle and put a temporary cable-tie on to keep things loosely in position.

The far end clipped together and held with a cable tie....
...then I move along progressively adding cable ties. At this dry-assembly stage, only a few cable ties are required....
...until I get to the base, remove the temporary screws and the plywood disc, and clip the final bit together.
After this I do things such as making octagonal mast plugs etc, and them pull the whole thing apart, apply epoxy, and glue it together for the last time. See the previous posts.

1 comment:

  1. Nicely done photos and commentary, makes me believe I could replicate the process. I am interested in trying some bird's mouth oars. Do you include such plans with your Flint design?