There is an amazing lack of information on the Internet regarding tabernacle scantlings. I'm hoping that those with practical experience will weigh in here.
Note: (I had posted some really bad math regarding this on another thread, now deleted. Apologies to anyone who had tried to follow it. At least I'm not snapping out half-informed 3 a.m. tweets on complex foreign policy issues like some of my high-ranking countrymen.)
Pete Hill, in JRA's Issue 61, February 2013 wrote a helpful article on the tabernacles he's built over the years. He also gives his general guideline on scantlings:
"I make the cross sectional area of the tabernacle at least as large as the cross sectional area of the lower part of the mast, suing the same material as for the masts ((usually Douglas fir). For example, for a 7in x 7in mast base (49 square inches), the sides are 2 in x 9 in, and the back is 2 in x 7 in (50 square inches)... All my masts have been wooden, so I am not sure how these scantlings would translate to an alloy mast."
David Tyler recommended something similar.
Using Pete's general rule:
According to the PJR, my equivalent wood mast is 13 inches in diameter, which is 78.5 sq in. (The actual mast is 10" diameter Aluminum 6063-T6, 0.25 inch wall thickness.) I'd need tabernacle sides about 2 in x 12 in, and a back of 2 in x 14, equal to 76 sq in.
The assumption is that we're talking about Douglas fir here, too. That's about 70 board feet of Doug fir, weighing 183 lbs. That smells a bit high weight-wise, but visually seems to make sense.
For comparison, that same section of mast, if I don't use a tabernacle, is only 50 lbs.
A second approach:
The Yield Strength of Al 6063-T6 = 21,000 psi. The Modulus of Rupture for White Ash (of which I have plenty in my garage) is 15,000 psi. A quick use of the stress formula 3FL/bd^2 says that I'd need a 11.2 inch diameter white ash cylinder to get the equivalent strength of the mast. Which is a lot lower than Pete's method gives, and calls for only 0.6 inch thick sides and back if I snugged it up to the mast. That's just silly thin.
Then there's stiffness. White Ash is known for its ability to bend. I don't want a whole lot of bending going on, though. So thicker is better there.
My boards are about 6" wide. I intend to rip them flat sawn 4/4 white ash boards in 1.75" strips, turn every other one end for end, rotate each 90 degrees, and glue them up. The result is a quatersawn gluelam. I've done a few rudders and centerboards this way using just dimensional lumber, 2x6 or 2x8 from the local big box store, and I'm amazed at the stability and strength. The result will be 1.75" thick tabernacle sides and back.
Anybody want to stop me before I make a mistake?