2017.04.07 - Plugs and Tapers

As is normal at the moment I spent some time before breakfast applying varnish. I'm nearly finish with the items left in the paint shop but the spars and mast will soon need their five coats of varnish as well.

Lunchtime saw me start the reinforcing plugs for the mast. Although a hollow mast is strong there are certain places where it is advisable to reinforce the mast with a plug on the inside that makes that part of the mast solid. The two places I will be carrying out this reinforcement is the bottom of the mast from the foot to just above where the gooseneck will be mounted. This not only strengthens the mast in the tabernacle but also the mast where the pressure on the sail is transmitted to the mast along the boom.

The second point is higher up the mast where the force from the sail is transmitted to the mast from the gaff.

The hollow inside the mast is octagonal and should be just over 62mm wide. I used some of the 32mm square timber that I have, planed all four sides to make it 31mm and then epoxied the four pieces together to give me a 62mm square piece that is 4' long. The plugs will both be 2' long, the lower one will be tapered at the top and the upper one will be tapered at both ends.

This tapering is to avoid a sudden change in the strength of the mast as would occur if the plug were square ended. The tapering for the plug is inside out and the photo below shows how this looks.

This photo is taken from the website of Ross Lillistone of Ross Lillistone Wooden Boats in Australia. He has an informative page about birds mouth spars which you can see here.

Here is the plug timber glued and clamped up.

The task for the afternoon was to prepare the mast for gluing up tomorrow. To that end I needed to taper the strakes, build an assembly jig and dry fit the mast.

This is probably the biggest jig I have ever made. It needs to be this big as the strakes are long and need to be supported for most of their length whilst the tapering is done. the business end of the jig is about 5' long.

Two scaffold planks have been clamped together along with some other timber to form a channel into which the strakes will be placed.

At the business end the white piece of wood is the guide for the circular saw and has been screwed to the scaffold plank at the very shallow angle required.

The distance from the guide to the edge of the strake is 58mm as you can see here.

At the other end the guide is 10mm further away.

I had to use a piece of plywood to raise the level of the scaffold board a bit.

The strake is slid into the channel up as far as the end stop and then the circular saw run up the guide cutting the taper.

Difficult to see but this end is now 10mm narrower than before.

The depth of the cut was set just a fraction too shallow on the first cut and you can see the thin piece of wood left after the cut piece has been removed. Subsequent cuts didn't have this problem as I changed the depth.

And here are the tapered strakes. The one on the right has been turned around so that you can see the before and after widths.

Once that had been done I took the jig apart and put the two scaffold planks end to end and clamped then together. Four U-shaped supports were constructed from the gash pile and set out at 5' intervals.

And the mast strakes were assembled without glue, just a dry fit. The big jubilee clips that I bought were too small so I had to use some cable ties. I'll see if I can buy some bigger clips tomorrow before the glueing.

Not a bad fit. The mast is bigger than necessary but not hugely.

The 'thin' end.

So a very good afternoon's work, very satisfying. I stood the mast up to see what it looked like. I was surprised at how light it was and the taper looks to be about right. It will look different once it has been rounded.

I left the mast assembled since the next job is to construct the plugs and that requires the mast to be assembled to check the fit of the plugs. I did take it inside the workshop, however. The scaffold planks stayed outside.