tabernacle material

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tabernacle material

Postby captain charlie on Sun Oct 14, 2012 6:47 pm

I will be building the tabernacle in a few weeks and have a question about the wood to use. I have some 1" thick macore, a South American mahogany, and was wondering if this will be strong enough, or do I need to get some oak , ash, or similar wood? I will be making the tabernacle removable to get in and out of my garage. Any thoughts from you veterans? Thanks, Captain Charlie
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Re: tabernacle material

Postby JonLee on Mon Oct 15, 2012 9:25 am

Mine is 1" sapele. It definitely seems strong enough.

I made my tabernacle removable too. I installed strips of 1"x3/4" timber on the hull, giving the tabernacle a nice place to nestle in to. The idea is that these guys will act structurally a little like the fillets that you'd have if you installed it permanently. In theory, they provide a more direct load path into the hull for some of the twisting loads than the default "tabernacle sides->tabernacle back->bolts->bulkhead" route. http://leeboatworkspocketship.blogspot.com/2012/09/countdown-5-days.html
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Re: tabernacle material

Postby decurtis on Mon Oct 15, 2012 11:21 am

My tabernacle is the same tiama plywood I used for the transom. From my research the 3/4" plywood is probably stronger than solid timber because the layers rae at right angles to each other and not running in one direction.
As experienced by myself and others the weakness will not be the tabernacle but the cabin wall the bolts go through. The inside of my cabin wall is now reinforced for the bolts and the tabernacle has fillets securing it to the outer cabin wall.
If your tabernacle is removable please be sure of the strength of your design as I am certain John C. harris is no longer recommending a removable tabernacle on Pocketship.

P.S. to JonLee

Sorry, I just saw your message now about my trailer and from your blog it looks like you have already answered your own trailer questions. There were no secrets to the placement of the side supports, it was just trial and error getting the length and angles correct. remember the boat is supported by the keel trough and the side supports are just for balance. One improvement I did make a year later was to increase the depth of the trough as much as possible so that the keel didn't as easily float or slip out of the keel trough when retrailering. Nothing worse than pulling the boat up the boat launch only to find the keel is not in the trough.
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Re: tabernacle material

Postby John C. Harris on Wed Oct 17, 2012 10:30 am

The best tabernacle I've seen yet was fabricated by Bob in Sydney, Australia. It's aluminum and beautifully executed. He writes: "It's 5mm 5083 marine alloy, laser cut, welded, with all the fittings tapped and screwed on. I'm using a 10mm alloy backing plate behind #2 bulkhead in the cabin, so as I said in the forum, I think I'll lose the stick before this pulls out."

A few photos:

Tabernacle 1.JPG
Tabernacle 1.JPG (41.54 KiB) Viewed 7890 times


Tabernacle 2.JPG
Tabernacle 2.JPG (70.25 KiB) Viewed 7901 times


All that aside, it definitely doesn't need to be aluminum to work! PocketShip #1 has probably been sailed harder than any PocketShip on the planet, including in truly abusive conditions, and it's persevering with an 18mm cypress tabernacle.

I made up this drawing about 18 months ago as a suggestion for reinforcement in a setting with a removable tabernacle. One of the most important things in the drawing is acknowledging that the cabin roof MUST be fastened securely to Bulkhead 2 to help with the loads. The one failure of a tabernacle I know of involved the bulkhead getting loose from the cabin roof. What's required is strong blocking in the corner between Bulkhead 2 and roof, as well as plenty of fiberglass turning the corner as shown in the drawing. And, ultimately, making sure that the side shrouds are taking up the offwind loads well before the mast starts trying to lever the tabernacle out of the boat.

PocketShip Removable Tabernacle.jpg
PocketShip Removable Tabernacle.jpg (136.46 KiB) Viewed 7924 times
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Re: tabernacle material

Postby decurtis on Thu Oct 18, 2012 9:45 am

I just thought I should clarify that my occurrence of my non-removable tabernacle pulling away and cracking the cabin wall was NOT sailing related. It was caused by an "oopsie" when the mast snagged the the top of the portable garage door opening when backing up the trailer and the tabernacle was pushed towards the bow. Definitely not a stress that should occur during normal sailing.
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Re: tabernacle material

Postby mark48 on Fri Mar 17, 2017 5:50 pm

Are those of you PS builders with removable tabernacles gluing the backing plate to BH #2 (through which the tabernacle bolts protrude)? Or are you just bolting your tabernacle through BH #2 and the backing plate (in addition to adding the reinforcing FG layers and taking care with the blocking at the BH #2 / roof corner as recommended by John Harris in his post above)?

Thanks,
Mark Nunlist
Lebanon, NH
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Re: tabernacle material

Postby Wayne G on Sun Mar 19, 2017 7:38 am

If using a backer I would imagine that gluing would distribute the loads more effectively.

On my removable tabernacle setup I do not have a wooden backing plate but use two through-bolted brass plates (not glued) with 4x 3/8 bolts; one on the inside of the tabernacle and one directly against the bulkhead in the cabin. These prevent the tightened fasteners from digging in to and marring the tabernacle and bulkhead.

Keeping the mast shrouds tight seems to be the most critical aspect of minimizing the tabernacle loads.
Recently I had a very careless accident where I trailered the boat with the mast up and made contact with an overhead lighting cable (slow learner) and even though traveling very slowly the boat was almost pivoted out of the trailer. Although the keel suffered a couple of very deep gouges there was no cracking or other damage to the tabernacle, bulkhead or bow.
Wayne Gray
Orlando Florida
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Re: tabernacle material

Postby craig on Mon Mar 27, 2017 2:19 pm

RE: gluing. Mine is not glued in because I had already painted the cabin. Maybe I would use epoxy if building from scratch.

Completely agree about keeping the shrouds tight. The mast/tabernacle will pivot forward until stopped by the shrouds. The top of the tabernacle will pull away from the bulkhead, and the bottom will push into the bulkhead. The majority of the force seems to be around the top bolt, which is why I used a 1/2" bolt there and use a doubler at that position. Before my shrouds were tight enough, I could actually see the (unreinforced, no doubler) bulkhead plywood bulging outward from the force of the mast on the stainless fender washers. With the doubler in place, that force is distributed across a much wider area, and the bulkhead can't fail as easily. Make sure you tighten the shrouds though!
Titania, launched January 2015
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Re: tabernacle material

Postby Bflat on Thu Mar 30, 2017 8:24 am

craig wrote:RE: gluing. Mine is not glued in because I had already painted the cabin. Maybe I would use epoxy if building from scratch.

Completely agree about keeping the shrouds tight. The mast/tabernacle will pivot forward until stopped by the shrouds. The top of the tabernacle will pull away from the bulkhead, and the bottom will push into the bulkhead. The majority of the force seems to be around the top bolt, which is why I used a 1/2" bolt there and use a doubler at that position. Before my shrouds were tight enough, I could actually see the (unreinforced, no doubler) bulkhead plywood bulging outward from the force of the mast on the stainless fender washers. With the doubler in place, that force is distributed across a much wider area, and the bulkhead can't fail as easily. Make sure you tighten the shrouds though!


That's a fascinating and informative description. I'm curious if you've noticed that the stresses on the tabernacle and bulkhead appear to be more on certain points of sail vs. others.
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Re: tabernacle material

Postby craig on Thu Mar 30, 2017 9:32 am

I wish I could tell you, but I didnt pay attention at the time, and once I properly tightened the shrouds, there is no appreciable movement on any point of sail.
Titania, launched January 2015
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