View Full Version : Cores for birdsmouth masts
08-02-2003, 11:06 AM
Recent threads have shown considerable success with birdsmouth construction for masts and spars. But what about compression that comes from fastening fittings such as mast collars or tangs? It seems like it might not be easy to shape short compression "cores" to handle fasteners in birdsmouth construction. Has anyone found an easy way to shape these hexagonal(?) cores?
08-02-2003, 12:21 PM
Good question, Ron. I have asked the same thing myself at last years' Port Townsend WBS and nobody had built anything that required cores however a couple of ideas were suggested. Best off was to make a reasonably accurate estimate of the location of the proposed fitting and mark on one stave. Cut a core that would fill the hollow space without hampering the tight fit of the staves. Glue the core to one stave and when set combine all the rest of the staves. If your mast is to have some taper, the core will have to be shaped accordingly.
Another suggestion was to build without the core and drill an oversized hole to take a metal sleeve with thin nuts on each side. Then you could pass the mounting bolt thru the sleeve to which you could hang mounting tangs.
I'll pass on here something of interest to me that could be inspiration to anyone working with epoxy and woodstrip constrution. Get yourself a copy of Jane's book of Fighting Aircraft of 1919. In the section dealing with German aircraft,(because they were defeated, we will tell you about their secrets and not our own), alot of space is given to construction methods and details. Its simply amazing what was being done back then with lo-tech materials. Things like composite structures, hot molded plywood, box spars, etc.
08-02-2003, 01:06 PM
I've done it several ways depending on mood and size of the spar. On a small spar such as Prairie Islander's gaff I glued the staves together. After cure I ran a drill into the two ends the distance required and glued in a dowel to fit the hole.
I don't like the idea of glueing the plug to one stave because I wouldn't trust myself to get it ezakly where it should be. Cut the plug to the size measured when you dry assemble the spar, slather it with goop, and roll the assembly around it. It'll position itself and help keep the assembly from collapsing until all the birds are fed so to speak.
On medium sized spars I've just inserted a dowel about the right size and let the epoxy fill the corners. And, on occasion I've cut an octaginal plug to lay in a solid section where a crane bolt would need it.
What ever floats your boat.
08-02-2003, 03:13 PM
I did mine this way. I epoxied the staves with wax paper seperating the halves. Once dry the spar is split in two pieces. I used 2 x 12 stock for my plugs. I ripped a square to the largest dimention of the inside hexagon. I ran the square through the table saw at 45 degrees and a hexagonal plug came out.
My description is not good but maybe the picture helps.
08-03-2003, 10:20 AM
Cool, Chris! Looks like that's the way to go. Pretty straightforward. I'll bet there's a way to taper the inside of the core so that there's not a "hard spot" at the end. Easy if I had a wood lathe, but... But this is a great approach. Thanks again for the description and picture.
08-03-2003, 10:43 AM
Havn't done it yet, but.
If you made eight staves with an isoc-- (two equal sides) triangular shape, with a 45 degree apex, and the base equal to the exposed inside width of the bird's mouth staves, it would all glue up at once.
You would get the proper grain orientation for each sector of the core. For fastening and moisture movement, etc.
08-03-2003, 12:47 PM
Here's a plug detail I drew up following Phillip Rhode's detail for rectangular masts and plugs . Eight deep cuts with a band saw produces eight tapered legs in the octagonal plug . The point of this is to avoid stress concentrations that would result from a sudden change of section , though it also makes for a fairer bend . If the plug is slipped in from the end I imagine this detail also gives excess epoxy somewhere to go rather than forming a rigid lump on top ( just what's not wanted ).Tines would be sawn in both ends of intermediate blocking .
[ 08-04-2003, 01:59 PM: Message edited by: Bill Perkins ]
08-04-2003, 12:06 PM
Dont' forget that the blocking should be both hollow and tapered— a hollow (but very nearly solid) octagon. The taper should be on the inside of the octagonal plug.
You want the taper so as to not induce a hard spot in the finished spar; you want the inside hollow so as to allow the spar to drain. Without the hollow, any water that gets into the spar via holes for wire, fittings, halyards, fasteners, etc., ...or just getting in from the top of the spar, will wind up sitting on top of the blocking.
It will eventually rot.
You can imagine what happens next.
Boys, ya know... [this just occurred to me]
You could, if you planned it properly, engineer the blocking into the birdsmouth staves themselves— a gradual 'swelling' of the wall thickness where the blocking is required.
Or make the staves for the blocking and glue them into place on each birdsmouth staves prior to gluing up the birdsmouth.
[ 08-04-2003, 01:10 PM: Message edited by: Nicholas Carey ]
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