Sunday 18 October 2015

Delicate work on the mast

I figure one of the reasons I'm doing a hollow mast is to hide things in it. Specifically hide the sail track, halyards, and VHF radio antenna. I've been spending a little time building a prototype antenna, and working on the details of halyard management and sail track.

First the antenna. I based the design off this five element collinear, using 086 semi-rigid coax for the 1/2 wave sections and a 1/16" brass rod for the 1/4 wave section on top. By using 086, which is about 2.2mm diameter, I'm able to insert the whole antenna in a 6mm diameter polyethylene sleeve to seal it up nicely, and then put the whole shebang in the 1/4" slot that I've run up the antenna.

Here's detail of one of the joins between 1/2 wave sections, along with the polyethylene sleeve (actually air line):

I'm having all sorts of difficulty getting a match out of this arrangement. I was expecting to have to trim the radiators to account for the dielectric constant of the sleeve and wood, but it looks like the whole thing is just miles from 50 ohms. A network analyser plot is shown below:

Not so good. The marker is in the middle of the marine VHF band, at 158 MHz. The only promising resonance appears to be due to the feed line, at 90 odd MHz. I'm debating whether to design a stub match to brute-force match the antenna to 50 ohms, or else to abandon the idea of hiding an antenna in the mast and instead run RG-223 up the slot I've routed and bring it out the front of the mast above the jib halyard.

So while vaccilating I did some other work on the mast. First I cut some exit slots for the jib halyard and main halyard:

Next I cut a 12mm diameter dowel from Jarrah, and used it to plug the top and bottom of the sail track. By using Jarrah here, I get the best possible strength for screwing the halyard block and (possibly) the vang eye. I might yet run the vang off the tabernacle.

With the top and bottom of the sail track plugged, I then had to machine an escape for the slugs, just above where the boom will attach:

Finally I've started making reinforcing blocks for where the hardware attaches. The one shown is where the spreaders attach:

This one has both halyards run past it, so I've got a large ramped slot so the 8mm halyards can run past freely. The next one up will be where the shrouds and jib halyard attach, so it'll have to be made just so to guide the jib halyard to its sheave. Similarly the block for the top, which will (sort of) seal up the top of the mast, will have to include a path for the main halyard to exit via a sheave.

I'll also fill in the bottom of the mast, with Jarrah for strength, from the foot to about 200mm above the boom. I'll put a very gradual taper in this block to ensure I'm not creating a stress riser, plus a slot so that I can bring the antenna lead out the bottom, plus allow water to drain.

Saturday 3 October 2015

Getting back to work on the mast

So, with the bench knocked over, it's back to the mast. The plan is to build a modified birds mount mast, from Jarrah and Tassie Oak, with a sail track cut into the back. I previously knocked out a prototype, so I could figure out how I was going to do this The prototype is just 30cm long. The real thing is 6.1m long.

Unlike a regular birds mouth mast, which has a structure that's self-supporting and self-cantering, and thus keeps itself straight during assembly, my modified birds mouth doesn't. I've already constructed the sail track section from two pieces of Jarrah. This exercise in wrestling a pair of floppy staves on the floor of the hallway was what convinced me that I needed to build a decent bench for the purpose.

Here's the sail track piece sitting on the bench, with a couple of 3m planks clamped to the face of the bench as a straight-edge. There's quite a bit of wobble in the sail track.

Normally I would scarp planks to make a long plank, then join it to the structure. I don't have enough clamps to do the whole 6.1m length, so instead I've cut the scarfs, and am gluing on a pair of 2.4m long staves at a time, working my way up the mast and gluing the scarfs as I go. I quite like this technique - it makes for much more manageable quantities of goop.

At the top of the mast I'm tapering staves, so in order to keep everything lined up I add cardboard spacers to account for the taper.

Once this pair of staves set up, I cut the detail into the back of the sail-track-containing laminated plank using my trim router, so that I can join the next couple of staves. I've gone off the prototype a tad - rather than a pair of v-shaped cuts in the back to reduce weight, I've just done one cut, 6mm wide by 6mm deep, with a round nose cutter. This groove will be a nice snug fit for some RG-58 coax, which I'll use to build a VHF collinear antenna, embedded in the mast.

Here's a pic of some side staves being test-fit, looking into the inside of the mast. The sail track is down, against the bench. I've made the mast about 1mm wider than the prototype, so it fits my boom fitting better. Doing things a couple of bits at a time gives me much better control of the glue lines, and allows me to keep things significantly cleaner.