Friday, February 21, 2014

Sample Birdsmouth Spar Blanks

Yesterday I glued up two short spar blanks using epoxy. Here's one of them.

Today I cut the ends of one blank flush and trimmed the corners to get a nice octagonal shape. I used a drum sander to trim the corners - not the quickest way to do it but with my planes in the other shop and cabin fever setting in, I used what I had available. I'll take the second blank to my brother-in-law's shop for turning on a lathe.

I used pieces of a round dowel for plugs, which worked for this experiment but I think I'd want to cut octagonal plugs for the real spars.

Some thoughts and observations from this experiment:
  • Use a squeeze bag or caulking gun tubed epoxy to facilitate spreading the thickened epoxy.
  • Use enough epoxy for some (but not a lot) squeeze-out to ensure good glue joins. Use enough epoxy around the plugs to ensure filling of the spaces between the plugs and the wall of the spars.
  • Assemble the spar in two halves - glue up two sets of four staves and clamp together but do not glue the two halves together. This makes coating the inside surfaces of the spar with epoxy simpler and less messy (if that is possible with epoxy). It also reduces, by two, the sets of mating surfaces to be coated with neat and thickened epoxy.
  • Use wax paper or parchment paper underneath the hose clamps.
  • Have enough hose clamps on hand.
  • Try not to coat the handle of the screwdriver with epoxy (transferred from squeeze-out to glove to screwdriver)…
Using the Birdsmouth Calculators it occurred to me that by tapering the staves as shown in the illustration, the inside of the spar is tapered as well as the the outside. This creates - in my mind - an obstacle: how does one accurately cut tapered octagonal plugs? Or is this not really an obstacle/issue?

One other factor to take into account is John Welsford's note on the sail-plan drawing: "taper outside not the inside space." I am not visualizing a straight-forward way to taper and notch the staves prior to assembly. Any suggestions on how to do this?


  1. I didn't bother tapering my octagonal plugs. I cut mine to fit the smaller end of the taper and gooped them up with "peanut butter" when I glued the two mast halves together. Another idea you might find handy is to make several holding jigs for the mast. They are T-shaped stands, with a square base and half and octagon cut into the vertical. They will hold your mast together while you are gluing the staves together. You can see mine sitting on top of the sawhorses here:

    I think when John says "taper the outside not the inside" he means remove material from the outside of the mast, but do not try to remove any material from the inside. Removing the "yellow" area from the staves per your illustration accomplished John's goal. It will maintain a constant wall thickness and effectively add material to the inside by making the hollow smaller as you go up the mast. In other words do not try to keep the diameter of the hollow constant. To do that you would have to taper the inside of the mast which is John is saying not to do.

    1. Thanks, Joel. Your explanation of "taper the outside not the inside" makes sense. It is refreshing to hear other's perspectives/interpretations that cast a new light on what to me is a "problem." Also, thanks for the reassurance - based on experience - that there is no need to cut tapered plugs. That helps a lot. (sometimes I make things more difficult than they need to be)

    2. Bob,
      Looks very nice. Good thing you are showing your work because this birdmouth food is new to me. Looking up to it a little. I have no clue what you guys are tlking about but will reread a couple of times and study the technique before I venture into it after the painting is done.
      Good job!