Thursday 30 October 2014

Finishing the birdsmouth mast prototype

Continuing the exercise, I opened the slot up with a 6mm router bit, cut a couple of V cuts into the back of the sailtrack section to lighten it, and bevelled the sides of the sailtrack section with a plane. Then I glued the rest of the staves together, forming a rough birdsmouth section.

Once this had set up I planed off the sharp edges, then planed it into a sixteen sided figure.

Then finally I planed it further to a 32 sided figure, and sanded it reasonably smooth. I think the results speak for themselves. If nothing else, it's rather pretty.

And now for the moment of truth. 489g for 30cm. That's 1.6kg per metre, or right on 10kg for a 6.1m length. I reckon that's reasonably light.

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...

Saturday 25 October 2014

Inner deck supports, two goes at a mast tabernacle, and battery tray

I haven't been able to spend terribly much time on the boat of late, on account of being away for work rather more than usual. Much of the time I have spent was wasted, building a tabernacle that I ended up chucking out. Here's the first go. I had it in mind that I'd run it down either side of the spine through the bridge between the centerboard case and front thwart.

The result looked really awful. The tabernacle looked very heavy and the spine rather light. It looked kinda perched there.

After spending some time in the moaning chair, I decided to beef up the spine, and mount a smaller tabernacle to the top of a wider, thicker bridge piece rather than the sides of the spine. So I got the saw and chisel out and removed the reinforcing either side of the spine, then replaced it with a larger piece. I also put a second reinforcing piece under a wider bridge, horizontally. This makes the bridge about 25mm thick rather than 6mm, and mostly solid Tassie oak, sitting on a thick reinforced spine.

The tabernacle is made from Jarrah, with 16mm thick side plates and a solid wedge shaped piece between them. I bored an 18mm dia hole for the mast pivot bushings. I'll turn up some bushings to go in there and use a 12mm bronze shaft for the mast pivot. I'm waiting on some 12ga bronze screws to screw the base of the tabernacle down to the bridge piece. I'll attach the centerboard lifting block to the back of the tabernacle once it's installed.

I'm done with steaming for now, having steamed the inner deck supports. I also steamed a thinner piece of Tassie oak with which to form the back of the fo'c's'le. Nice gradual curves. When you sit on the forward thwart facing aft, the curved piece should make a nice comfy back rest.

To further procrastinate before starting planking, I knocked together a battery tray, which goes in the front thwart. This accepts a 20AH 12V sealed lead acid battery, which should be plenty to power a fixed VHF radio plus AIS transponder. I've put a slot in the tray for a tie-down strap, which will affix to the bulkhead above the battery. That way the battery will stay put even if the boat is capsized.