I'm starting on my mast. It will be hollow, using a birdsmouth technique. I also am using a tabernacle, not a mast box so the mast will pivot on a bolt through both arms of the tabernacle. People have reinforced hollow masts at stress points (the pivotpoint, where eyebolts attach etc.). Anyone have a plan for how to do it right, and still get the mast to come out like it should? I assume the reinforcement is necessary.
Dave
Hollow masts have blocking inside to prevent point loading issues from attachments, thru bolts, etc. These blocks are what we call swallow tails because of their tapered ends. Inside a birdsmouth mast, they are more difficult to fit then a conventional box mast, but they all do the same thing, gradually increase the wall thickness until the mast is solid at the area where the load will be.
In a gaff rig, mast flexibility isn't a big issue so the slope of the "tails" doesn't have to be especially tall, say 8:1 or 10:1 the thickness of each separate piece, as they lie against each stave.
Most make their birdsmouth mast in two halves, by not gluing two of the seams during assembly. This lets you open the mast, insert hard points, halyard sheave boxes, exit boxes, wiring, maybe some conduit for all this stuff, etc. Dry fit the swallow tails and drill out the center for a drain hole (assuming there's nothing else inside the mast). A 1/4" hole will be fine, which lets any accumulated moisture drain to the bottom, where another hole will let it leave the mast. Do not seal it up tight.
Naturally, epoxy encapsulation is best. There is no need for 'glass fabric on the mast, though at the hounds, where the gaff will ride when underway, a wrap of fabric will protect the mast from the jaws. On the other hand, leather covered jaws will do the same thing.
Reinforcement for a thru bolt, like that used on a tabernacle, usually need a bushing too. The traditional way is to drill quite over size and insert (glue) a hardwood dowel, which is then drilled for the bolt. This serves as a compression post and a bearing. Now most people just fill the over size hole with thickened epoxy, then drill for the bolt. The epoxy is tougher and can't rot. If using a dowel, drill twice the diameter of the bolt and use that size dowel (3/8" bolt, use a 3/4" dowel). If using epoxy, drill a hole 30% larger then the diameter of the bolt (3/8" bolt, use a 1/2" hole).
Hollowing a Mast, just a question? On "A Weekender" after the 4"x4" Mast has been shaped from the outside would it be possible to "Rip it in Half" and hollow out the inside with several passes of the saw and a few hits with chisel and hammer leaving a minimun of 3/4" from inside to outside and then epoxy the two halfs back together?
    Alan & Fran
(04-29-2010, 02:59 PM)Alan Mallett link Wrote: [ -> ]Hollowing a Mast, just a question? On "A Weekender" after the 4"x4" Mast has been shaped from the outside would it be possible to "Rip it in Half" and hollow out the inside with several passes of the saw and a few hits with chisel and hammer leaving a minimun of 3/4" from inside to outside and then epoxy the two halfs back together?
    Alan & Fran
It's probably the hard way to get a hollow mast. It would be easier just to laminate it together with a hollow.Â
My mast is 2 2X4s laminated with a 3/4" spacer between them. I left enough room to run a plastic tube down the centre of the mast to use as a wiring chase for the mast head and steaming lights.
There were three mistakes I made (on the mast).
First - I made the top and bottom parts of the mast separately. They ended up being slightly different dimensions and making the joint in the tabernacle close up and be even was a significant pain. It would have been much easier to have put together the whole mast and then cut it.
Second - I gave up too soon during tapering. The resulting mast weighs a ton (so it seems) and the gaff jaws had to be adjusted so that the gaff would spin around the mast.
Third - One of the 2X4s I used for the upper part of the mast is some wood that continues to ooze resin.
Good luck with yours.
Actually, that's precisely how traditional hollow masts are done. An oval mast is featured in the latest WoodenBoat magazine, using just this method, so pick one up at the news stand. When doing it this way, it's important to "back out" the hollow so it's smooth and relatively uniform in wall thickness. Naturally, it means you need an especially straight grained 4x4, with limited if any run out, but it certainly can be done this way. Spar stock has to be selected carefully, especially if you make them light weight.
One of the reasons a birdsmouth mast is much preferred is the decreased amount of tools and skills sets necessary for it, compared to backing out a solid stick. A birdsmouth just needs a table saw (or router) and a plane/belt sander, orbital, etc. to knock it round. A traditional round, hollow mast requires scrapers, backing planes, gouges, chisels, drum sanders, etc. and more smoothing skills. I don't know a single novice builder with a backing plane, let alone curved scrapers, so . . .
Thanks for the response, Paul.
Dave