Wednesday 29 October 2014

Prototyping a modified birdsmouth mast with an integrated sail track

Out here in the boondocks in Western Australia we don't have terribly much access to traditional boatbuilding timber. Douglas Fir? Not a chance. Spruce? You've gotta be joking.

What we do have in very plentiful supply are Jarrah and Tasmanian Oak. Jarrah is an incredibly dense, strong, durable timber. Great for railway sleepers (all the railway sleepers in the London underground were made from Western Australian Jarrah), but challenging to build a boat with. Tasmanian Oak is slightly less dense than Jarrah, but still isn't exactly a lightweight timber. So I'm wanting to build a wooden mast. You can see my dilemma. There's no way I'm going to build it out of Pine or Western Red Cedar, because both are much too soft. I'm interested in the birdsmouth construction, but I want a sailtrack, and I can't think of a straightforward way to do that with normal birdsmouth.

Well if a simple method won't suffice, surely a complicated method will. If I build a sailtrack piece out of Jarrah, then use that to replace the back three birdsmouth staves in an eight sided spar, then mirror everything so the left and right sides are the same, I end up with something that will do what I want.

So I designed my mast in sketchup. The birdsmouth bits are 30mm x 12mm Tassie oak. The sailtrack bit is made from two pieces of 30mm x 12mm Jarrah, cunningly joined after cutting the track with biscuits. Then a pair of 18mm x 30mm tassie oak bits get laminated on, and more cutting happens to make the back of the mast. After assembly the whole lot is planed to an oval shape 63mm wide x 76mm long.

Sounds easy! I wasn't so sure, so thought I'd prototype something. Importantly I want to use only methods of construction that will work with 6.1m long bits of timber. I'm thinking that means doing everything with my trim router.

I started by cutting the birdsmouths into my 30mm x 12mm tassie oak. I used a 12mm dia 90 degree V bit in my trim router, with some guides screwed into the base.

I repeated the exercise with a 12mm round bit in the side of some 30mm x 12mm Jarrah to make half the sailtrack.

Then I laminated two pieces of the sailtrack Jarrah, plus two lengths of 30mm x 18mm tassie oak. This is going to be the hardest part with the full length mast. Keeping squeezage out of the sail track is going to be hard. For the prototype I forced some wadding down the track once it was assembled to clear the squeezage. Doing that over 6.1m is going to be challenging. It might be easier to cut the slot part before gluing so that I can get access to the inside of the track.

Once my lamination had set up I cut rebates into the tassie oak part to locate the front five staves.

This is what my prototype looks like now. I still need to bevel the back sides and cut open the sailtrack slot, glue the whole shebang together, then plane it to an oval. I weighed this 300mm section this morning at 649g. That's with a fair bit of material still to be removed. That means a 6.1m mast built like this should weigh around 13kg, and hopefully be as strong as they come...

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