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Tabernacle Scantlings for 43' unstayed mast
Scott Dufour posted a topic in Design ForumHey guys, it's been a long while since I posted on here, but I'm looking for some of the smart people's brain power. Life changes made me abandon the PS26 build, but I'm not through with boats. I've got another large project. I have a gutted Pearson 10M, 1979 GRP, 12k lbs displacement. I'm converting it to a junk rig. Not one of those old flat paneled junk sails that won't go to windward, but a Split Rig junk, developed by Slieve McGalliard about ten years ago. My intent is to contribute as much as possible to the testing of this relatively new rig. She will be a serious offshore cruiser, and my home in a few years. I'm also designing to minimize as many maintenance headaches as possible, and enable as much self-suffiency as is practical. Plus, I'm a lazy (as in CS17 ease of sailing lazy) sailor. That's part of what the unstayed Junk rig offers. The venerable Atomic 4 is gone, replaced with an electric motor and a big ol' battery bank. No flushing head - it's a composter. That kind of thing. The junk rig enthusiasts over at the Junk Rig Association are a small but energetic bunch, and they remind me a bit of this crowd here: not overly enamoured with orthodoxy, but sticklers for good design. They've been excellent and experienced regarding all of the rig design elements except maybe one. I've got all the heavy lifting done for the sailplan (mast location, bury, deck reinforcement, et cetera) , and was ready to order the mast when the idea of tabernacle surfaced and I realized that I want one. The weight is a concern, to be sure, but the ability to lower my mast without a crane is just so tempting. But because weight on the deck is never a good thing, I don't want to overbuild it, as is my tendency. And unfortunately, there is an amazing lack of tabernacle design and build information on the internet. Some details: The mast is 6063-T6 10" diameter at base, 4" or 6" diameter at top, wall thickness of 0.25", and 43 feet above deck. That says that I need 4' 3" of tabernacle above deck. It's six feet from the underside of the deck to the mast step. I'd like to taper the tabernacle from under the deck to the step. Some guidance for wooden masts suggest that from the partners to the step, a mast can taper to 1/2 the max diameter. I'd assume that a tabernacle could do likewise? I'm making it from white ash. My back of the envelope calculations say that the sides need to be 1.75" thick (gluelam), making an open square box of 13.5" sides. That's a lot of weight. something line 180 to 200 lbs. And that doesn't include the tabernacle hardware, the epoxy and fiberglass, or other finishwork. The amount of mast that I'm not using (from partners to step) is about 50 lbs. Have I overdesigned? If not, I'm not sure that the extra weight it worth it. I've tried to paste in a table below of the rig's forces a the partners. Not sure if it will show correctly. Disp (lbs) 12273 Disp (kg) 5567 Heel (deg) RT ARM (ft) RT MOM (ft-lbs) RT ARM (m) RT MOM (Nm) 25 1.293 15,869 0.39 21,523 30 1.45 17,796 0.44 24,136 60 2.018 24,767 0.62 33,591 90 1.435 17,612 0.44 23,887 120 0.124 1,522 0.04 2,064 150 -0.984 (12,077) -0.30 (16,379) 165 -1.008 (12,371) -0.31 (16,779) ID (in) OD (in) Section Modulus Stress @ partner 30 deg Heel 10 9.5 0.00049 36,318,061