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Scott Dufour

Tabernacle Scantlings for 43' unstayed mast

8 posts in this topic

Hey 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  

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Hi PAR  - you are exactly one of the brains I was hoping to pick.

I've considered it a metal tabernacle - but not seriously yet.  I'm certainly willing.

Are you suggesting it to save weight, or bulk, or for more certain strength?

I've been thinking wood because:

  • I have plenty and I know how to work with it and epoxy and fiberglass. 
  • I don't know how to weld. 
  • Wood in the interior is nicer than metal.
  • I like the sound dampening properties of wood - disconnecting the metal mast from the interior by way of softer material like ash just seems a good way to quiet some of the noiser elements of all those junk rig battens.  Not that they should bang all that much if I've designed the combination batten/downhaul parrels correctly.
  • I've just gotten rid of all chainplates and other major metal/deck penetrations.  That equals no leaks, and no sneaky corrosion hiding in the dark below deck and above headliner.  When I pulled those chainplates out, there was a fair amount of crevice corrosion, and I was hoping to avoid that potential.
  • The lowest batten (technically the boom) will be slightly below the top of the tabernacle.  It (and the reefed sail bundle) will be inconstant contact with it.  This may actually be an argument in favor of metal - a smaller interaction area due to reduced thickness.  I'm sure how that's going to work out yet.

But... if those concerns can be mitigated, or if I'm unduly concerned, or a hot dipped steel tabernacle has so many other advantages... well, that's why I'm here.

 

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Yeah, metal will be lighter and take up a smaller footprint too. It will also be stiffer and stronger. Given the size of your mast and the anticipated loading, metal has a strong favor for it.

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Are you thinking steel?  I'm pretty uninformed for this kind of structural analysis with metals. What's the grade of steel (I'm assuming not stainless) that's appropriate for this?

My quick calculations show that it could be 219 lbs of A131 ABS steel, or maybe as low as 100 lbs 6061-T6.  Would aluminum be recommended for this, given the machining nooks and crannies required and the potential for corrosion?

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I'd consider both stainless and aluminum, not mild steel. I'm not sure where you've come up with the steel weight, but you've likely made it way too stout. You can also use lightening holes in the sides and back of the tabernacle, to further reduce weight, maybe as much as 20% - 25%.

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That sounds like a job for a marine engineer, doesn't it? I think that I'm way out of my league there. 

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22 hours ago, Scott Dufour said:

That sounds like a job for a marine engineer, doesn't it? I think that I'm way out of my league there. 

I normally stay out of discussions where I don't have  expertise/experience, but this seems like a sound idea.

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