Tabernacle / bulkhead failures & fixes . .

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Re: Tabernacle / bulkhead failures & fixes . .

Postby Pascal on Wed Feb 18, 2015 7:05 pm

I did not have the experience with Naoned with heavy wind . The most I got was about 19 mph . I really should have reefed but was having too much fun ( wife not so much ) this to say that nothing broke. If all adjusted right it should work fine and safe. :) Naoned tabernacle is 3/4" solid maple laminated with leftover 6mm marine plywood.
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Re: Tabernacle / bulkhead failures & fixes . .

Postby craig on Sun Feb 22, 2015 10:18 pm

I appreciate your posts, Pete, and everyone else who weighed in. I have made some slight modifications to my build to make myself feel more comfortable (although I'm confident things were A-OK before).

The bulkhead 2/cabin roof joint is encased with a full sheathing of fiberglass running over the joint and overlapping the anchor well floor fiberglass. A second layer extends over the joint in the central area where the tabernacle is. This sheet extends down the entire face of bulkhead 2, and midway up the cabin roof.

I have installed two plywood backers into the cabin and upgraded my tabernacle bolts to a larger diameter and larger washers (I can't remember what they were called, but these things are enormous). Because of the abundant fiberglassing, the bulkhead was in no real danger of separating from the roof; however, the loads were causing it to flex quite a bit. The plywood backers now prevent that from occurring to such an extent.

Most importantly, I have tightened down the shrouds as much as possible. Thanks to John for finally making me realize that the shrouds prevent FORWARD motion just as much as side-to-side motion. Of course that makes sense in retrospect. Most things do I suppose...

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Titania, launched January 2015
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Re: Tabernacle / bulkhead failures & fixes . .

Postby DanaDCole on Wed Mar 18, 2015 8:45 am

After carefully reading this thread I gleaned three points:

1. Make sure those shrouds are tight!

2. Forget any ideas I was harboring about a removable or folding tabernacle.

3. The backer is still a good idea, and much easier to do before cabin roof is installed. In case of a mishap (e.g., twist during raising/lowering or shroud break) I think I would much rather deal with a broken tabernacle or even mast than a ripped bulkhead.
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Re: Tabernacle / bulkhead failures & fixes . .

Postby Bflat on Thu Mar 26, 2015 5:06 pm

Would moving the chainplates aft one or two inches be worth considering as a partial solution?
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Re: Tabernacle / bulkhead failures & fixes . .

Postby John C. Harris on Fri Mar 27, 2015 9:44 am

>>>>>3. The backer is still a good idea>>>

No. No it is not. I do not endorse this. I've got six hard years of heavy air sailing, mast dropping, and general abuse of the tabernacle on PocketShip #1, which has what might be the most lightly-built tabernacle of any PocketShip. Just 3/4" cypress, a softwood. About the worst thing that happened was that some dummies working for me moved the boat on the trailer with the mast not lashed to the gallows, and it swung a foot-and-half to one side, seriously torquing the tabernacle. Nothing much happened. The only thing I've done over the years is add some light aluminum plates in way of the pivot bolt.

If the dingdang shrouds are set up so that the mast can't rotate forward past vertical, you're never going to have a problem with the structure as designed. Period. Full stop.

The backing plates I'm seeing are just plain ugly, represent undesirable weight high up in the boat, and STILL won't be sufficient to support an "unstayed mast," ie, one that is allowed to cantilever forward without the support of the shrouds.
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Re: Tabernacle / bulkhead failures & fixes . .

Postby DanaDCole on Fri Mar 27, 2015 5:32 pm

Good points all. My idea that the tabernacle might give before the bulkhead pulls away from the deck if doublers are added probably would not work (my only reason for this was in case of a shroud failure of some kind).

I'm also waiting to hear John's thoughts on moving the chainplates back an inch or two. I don't think that would be a good idea though--the design has proven itself as-is as John point out, and my guess is the shrouds are as far back as they can be without interfering with the boom.

The best course by far is making sure the shrouds are tight and checking their attachments and chainplates etc. along with all the other checks that are necessary before embarking. Every pilot goes though a long checklist before every flight. No need to be extreme, but a checklist of the most important areas to check before going out would be a good idea. I often think I ought to have one even for the little Skerry--I'm always forgetting something!
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Re: Tabernacle / bulkhead failures & fixes . .

Postby craig on Fri Mar 27, 2015 7:23 pm

Off-hand, I would think that moving the shrouds back would make them less effective and introduce more force on the forestay/bowsprit than before. I will happily admit that my setup was not properly done at first, since my stays were not as tight as I (now) understand that they need to be. With that corrected I have sailed with no issue whatsoever with fore/aft torque on the tabernacle or bulkhead 2. I also put nearly my entire weight on the forestay to test its strength, and had no issues (which would be the equivalent of sailing in a force 4 wind unreefed).
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Re: Tabernacle / bulkhead failures & fixes . .

Postby Bflat on Tue Apr 21, 2015 8:43 am

I'm cool with simply making sure the shrouds are tight enough to keep the mast leverage in check but, I find myself contemplating other measures as well (it entertains me).

1. Rake the mast. With the mast raked aft a bit the base of the mast couldn't push against the base of the tabernacle, thus robbing it of its leverage even if the mast bows. The manual states that the prototype was found to sail better with some rake anyway.

2. A more radical idea: After raising the mast, insert a pin near the base of the mast/tabernacle and remove the pivot pin. This would relieve much of the leverage on the tabernacle when sailing while still providing the convenience of a tabernacle. You would never sail with both pins in. Reverse the process to lower the mast. A caveat is that the rake couldn't be changed without drilling a another hole in the base, but even this could be alleviated by simply finding the proper rake before committing to drilling the hole for the base pin.

3. I'm still wondering about the effects of moving the chainplates aft an inch or two (thus increasing their "backstay" effect). With the asymmetric spinnaker, sailing dead down wind isn't much "in the cards" anyway so the boom travel range could be compromised a bit with little bad effect it might seem. The boom travel restriction would seem to be pretty negligible.

Just some wild thoughts, and I might be way off.
Bob
Last edited by Bflat on Tue Apr 21, 2015 7:55 pm, edited 4 times in total.
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Re: Tabernacle / bulkhead failures & fixes . .

Postby DanaDCole on Tue Apr 21, 2015 9:03 am

Your idea #2, second pin at the base of the mast, sounds interesting, but as you point out, pretty radical. Can hardly wait to hear John Harris's reaction to that one! If he doesn't come up with any serious objections though, I might consider it. Most important thing is to keep the shrouds tight of course, but things can and do go wrong, especially on a sailboat it seems. :)
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Re: Tabernacle / bulkhead failures & fixes . .

Postby John in CC on Tue Apr 21, 2015 7:57 pm

2. A more radical idea (kind of a thought experiment): After raising the mast, insert a pin near the base of the mast/tabernacle and remove the pivot pin. This would relieve much of the leverage on the tabernacle when sailing while still providing the convenience of a tabernacle. You would never sail with both pins in. Reverse the process to lower the mast. A caveat is that the rake couldn't be changed without drilling a another hole in the base, but even this could be alleviated by simply finding the proper rake before committing to drilling the hole for the base pin.


Problem I see is if the halyard fails while sailing upwind, the mast will not fall harmlessly onto the boom gallows. It could and the manual says, "tear the tabernacle right off the boat in a cloud of splitters"...
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