Work continues on my spars. In my May 8 post, I left off with my mast staves being joined to reach their full 16ft length. Those joints came out good and I proceeded to laminate the halves of the yard and the boom to create the square section blanks that will then get shaped.
I wasn’t shy with the quantity of epoxy, but it was hard to get an even amount of squeeze out over the 12ft of length.
One of the main reasons for constructing the spars first was to determine the amount of bend they will exhibit and use that data to complete the sail’s final two edges. Other GIS builders have been pooling their data and the standard that has been settled on is to support the spar at its ends and hang 10kg (~22 lbs.) from the center.
Since the blanks have a seam up the center and the grain is oriented parallel with the seam, I was expecting the blanks to be stronger on that axis. Turns out, all four sides have different amounts of flex, and both blanks were different from each other.
So I made a chart to track all the measurements. As of this writing, I progressed with the yard blank from raw to tapered with the following results:
(Side A & C are the ones with the seams, B & D are perpendicular to the grain and the seam)
All four sides raw:
A: 35mm; B: 27mm; C: 32mm; D: 31mm
Then I read another method used by Dabbler Sails wherein one hangs 1/3 of the square foot area of the sail in pounds from the point where the halyard attaches. In my case that would be 35 lbs. The measurements are taken at the 1/4, 1/2, 3/4 marks.
All four sides raw:
A: 38-49-31; B: 30-42-27; C: 34-42-31; D: 31-43-29
Sides A & C tapered, (B & D raw):
A: 41-57-37; B: 32-45-30; C: 40-59-38; D: 35-49-32
All four sides tapered:
A: 44-64-42; B: 41-58-40; C: 44-64-43; D: 43-62-43
So I think I may have gone too far with this blank. I didn’t want flex in excess of 50-55mm. It’s certain that I will get even more flex after rounding off the corners into a fully circular cross-section. It’s interesting that each side flexes differently, even the opposing sides which are on the same axis. I guess when one side is in tension, the opposite side is in compression, so that variety of the grain will act differently... Meanwhile, the raw sides increased in flex when the perpendicular sides lost material. And when the final two side got tapered, the first two increased in flex again.
So after two pounds of shavings, I may have made a spar for a (smaller) future boat, or possibly parts for a Yawl rig for this one. But as of now, I am pulling the plug on this particular blank and will use the other heavier stiffer blank. Which means I'll have to start from scratch with my boom...
Next up: mast making.