My mast components have been languishing in the back yard for a year, covered in a length of heavy plastic to protect them from the elements. This has been mostly successful, but on pulling them out there's some evidence of water. I presume being in the bag in the sun there's been some condensation.
In any case, now I'm back to the mast proper. For those who don't remember (I barely do myself) I'm doing a modified birdsmouth construction, with a sail track, in Jarrah and Tasmanian Oak. Neither of these are traditional spar making timbers, on account of their density, but I figure I can get good performance by pushing the boundaries of the traditional birdsmouth technique, and thinning out the walls some. Plus, Jarrah and Tasmanian Oak are pretty-much indestructible.
Here's a photo of my prototype piece, that I did a while ago. It's mainly condtructed from 30mm x 12mm staves.
Luckily, the 6.1m long mast fits neatly across the back of the garage, on the bench I made specifically for this work. I confess this was something that I measured when we were looking at places in Perth, and I rejected a few on the basis that I couldn't do this work in the garage. The garage in the place we're in could be bigger (it'd be nice to fit Elena in lengthways with her bowsprit, but it'll do.
I've opted to fill in the base of the mast with Jarrah up to the boom fitting. If the mast is going to break anywhere, I figure it'll go at the boom, as that's where the main unsupported sideways forces are. There's a slot down the leading edge of the mast for drainage, and I've tapered the staves just above the boom, to minimise stress risers. You can also see here a length of coax that I've put in for a mast-mounted VHF antenna. Also two of the three staves that will fill in the front of the mast are mostly prepared - I just need to taper the tops and route the birdsmouth into the front. However I want to get all the inner details right before sealing up the mast, as I'll kick myself if I forget something.
Moving up the mast to the halyard exits. I'm using 6mm spunport line, which is a modern polyseter double braid, but in traditional colour and finish. I've elected not to use double-sheave exits, and have routed in a long slot for the halyards to come out at a shallow angle. If this proves troublesome, in use, I can always add exit blocks. Note also here that I've started finishing out the interior of the mast with filled epoxy. This is simply to ensure the halyards don't snag on anything.
So now I'm on to some design work. One of the reasons I put this aside previously was that I was unsure what to do with shrouds. The hounds really should be strong. Traditional masts tend to use a band encircling the mast, but that's not going to work well on this mast due to the sail track. I could split a mast band at the sail track, but then I'd be paranoid about the shrouds bending the band apart at the front and splitting the mast.
I think I've come up with a workable solution. Davey do chainplates with the eye at 90 degrees, intended for mounting on the stem at the bottom of the bobstay, or else on the gunnel for bobsprit shrouds, shown below:
Initially I was thinking of simply bolting these to the sides of the mast, with backing plates inside, but there's still the splitting problem as with the cut mast band. So then I thought I could join the chainplates to one another, clamping the mast. I have a pile of bronze rod that I've been turning all sorts of things from. All I need is a couple of lengths of rod with threads tapped in the ends, then stick these in holes in the mast, and flogg the chainplates up to either side. Something like:
The only danger is clogging up the space that's meant for running my halyards, but if I keep the diameter down to 12mm (oodles strong enough) there's heaps of space to run the halyards past.
Editing after some good advice from the woodenboat forum not to put the holes in line on the mast. I've tipped the fitting over a few degrees, and shrunk the holes in the timber to 4.8mm from 12.7mm. The bronze plugs now don't go through the timber section. To ensure that when the bolts (now 10g) are flogged up the mast walls don't get cut into by the plugs, I've added backing plates as well, which essentially act as large washers.
And another edit after a complete rethink. I've decided to fill in the mast in this section, bore a hole for the main halyard through my plug, and go with a more traditional hound. Here is is bolted to my prototype mast section. I used 2.4mm thick x 25mm wide bronze for the jib stay straps, and 3.2,, thick x 25mm bronze for the shroud straps. Bending the jib straps around the mast was a real pain.
The whole assembly bolts through the mast with a single 5/16 bolt (actually I'll use 5/16 bronze threaded rod with acorn nuts on either end.
I'll do a separate cutout for the jib halyard exit box immediately below the stay attachment. I can use the stay attachment for a roller furler, or use the halyard exit box for a normal jib.
My fingers are a mess after beating on those straps.
Oh dang! So excited to see you get started on the mast. Can't wait to see the process and how it turns out.
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