Sunday, February 22, 2015

Birdsmouth mast glued up

I have had the parts for the mast cut out and ready to go for about six months (see <this post>), and this weekend seemed like the right time to assemble them.

I decided to construct a jig to facilitate holding the staves in position while getting them clamped up.  I gathered a couple sawhorses and several other stands and spaced them out along the length of the mast.  I then measured the mast rough diameter at each station and cut out matching profiles from some 1/4" plywood.

I then attached some short pieces of 3/4 plywood recycled from the building jig to each support so that I would have something to attach the profiles to.  Setting up a laser level made it fairly easy to align the supports.  On each one I had drawn a horizontal line and a vertical line, and layed out and cut my semicircles from the intersection of the two.  Having the lines left over on the supports was key to easy alignment.

Here's a shot showing the vertical alignment.

And here you can see the horizontal alignment.  I held the profiles in place with clamps until satisfied with the alignment, then drove a couple screws through to fix them in position.

This setup ensured the jig was straight in both directions, and aligning the center points automatically took into account the taper in the mast, which is not constant along the length.  Here's an overall view with the mast dry-fit in place.

With the jig set up, I next ensured I had the plugs for the ends of the mast ready to go.  The top end is plugged by a simple octagonal stick about 3/4" thick.  The plug at the base of the mast is also octagonal, but protrudes from the end with a square section that will fit into the mast step.  

I cut away the top of the plug so there would not be an abrupt transition from solid to hollow. An abrupt transition can cause failure at that point if the mast is under bending stress.  That won't be the case here, as this is a short plug about 1 foot long, and the mast is supported well above that height at the cabin roof, but I had seen it done elsewhere and thought I may as well do it.

With all the pieces ready, I lined up all the mast staves and clamped them together.  I thought it would be easier to apply the epoxy this way rather than handling each stave individually, and it worked out well.

I first wet them out with unthickened epoxy using a short 1" section of roller.  I did the V side and the back side, then the V side again.  The idea is to let the wood absorb what it can, so that it doesn't suck the epoxy out of the joint during glue-up, leaving a weak spot.

Once the staves and plugs were wet out, I mixed and spread thickened epoxy on the V side. I made a little custom spreading stick by sanding it to a point and filing notcches in the edge.

The notches leave some epoxy behind, so that there is an appropriate amount distributed along the joint.

Holding the stick at an angle allows it to fit the sides of the V.

No photos of the epoxy spreading, as things were just slightly frantic at this stage - I wanted to be sure the epoxy didn't start to cure before I could get it all clamped up.  I was glad to have my wife's help mixing up epoxy.

Once all the staves were spread with epoxy it was back into the jig with them, which made the alignment and fitting together of the joints a snap.

I used hose clamps to pull the staves together, and then applied zip ties to hold them in place, then moved the clamp down and repeated.  I had a number of hose clamps, so a left them in place periodically, rather than unscrewing them all the way to move them past the next support cradle.

The next day I removed the clamps and saw that things were looking pretty good.

I got out my block plane and planed down the projecting corners to make the mast 8-sided.

And then planed the corners to make it 16-sided

And then then 32-sided.  The mast is now a bit smaller than the support jig cutouts and tends to slide.  I moved the end support and raised it up to provide an end-stop.

And then planed the ridges again to be about 64-sided.  It's a bit hard to be sure where you have planed and where you have not at this point.

Lots of neat curls resulted.  Too bad I can't think of a use for these.

Next I'll switch to sandpaper to remove the remaining ridges, then round off the top and apply some finish.  Not sure yet whether I want to put a couple coats epoxy and then varnish, or maybe go with white paint instead.


  1. First saw your blog this morning by way of DuckWorks and spent a bit of time reading it. I'll be building birdsmouth spars for my Pathfinder this spring and learned some useful tips from you. Thanks.

    1. Thanks, Bob, I'm glad you picked up some tips. I recommend the laser level for getting things lined up easily. But really, I don't think a little bend in the mast would be a big deal. -- Dave

  2. Dave, love the pointy application stick, with notches no less! What a pro! NIce work. --Will

  3. Dave, excellent as always. Hey, please photograph how you are fixing/bulding the upper support block for the mast. I am going to place mine as high as possible (and still contact all 4 sides of the mast box). Also, how will you accomplish the mast rake angle? I'm still a little confused about how to build in the proper angle of the mast.

  4. Yes, will do, and I'll include a future post on that. The mast rake angle is set based on where the mast is supported at the base and at the upper end of the trunk. A bit of trigonometry is all that is needed to figure out the offset needed between the two support points to get the correct angle. -- Dave