Removable Tabernacle

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Re: Removable Tabernacle

Postby craig on Sun Feb 08, 2015 3:48 pm

Two of my friends and I set out yesterday for Titania's second sail. Temperatures were forecast to be 60 degrees and winds light at around 8 knots average, gusts to 15. The air was indeed quite warm, but the wind was WAY off - sustained 15-20 knots, gusts up to 30 or 35. Whitecaps were clearly evident in the middle of the lake and the waves were a good foot or more. We put one reef into the sail and set out but it was soon clear that we didn't even need that much sail to reach hull speed, and then put in the second reef.

Suffice it to say that the mast and removable tabernacle sustained a lot of pressure. About halfway through the day I thought I heard a "crack" and was understandably alarmed. Bulkhead 2 had separated from the cabin roof cleat right where the tabernacle is bolted in, formed a hairline crack. This crack extended along the cleat 2 inches along either side of the bolts. On the outside, I could see the bulkhead plywood bulging out slightly where the bolts go through, but the cabin roof/bulkhead joint was intact. Despite tightening the bolts with a ratchet wrench prior to setting out, fully 1/8" or more had opened up between the top of the tabernacle and the bulkhead. When the sail was dropped that space closed up. Confident that the bulkhead could handle the pressure, we hoisted sail again and didn't have any issues the rest of the day.

During construction I doubled-up the fiberglass cloth across the bulkhead 2/cabin roof joint. I don't believe the plywood would have separated from the roof. Rather, I think the flexing caused the cheap pine cleat to fail, causing the hairline crack.

The hairline crack is too small to fix. Rather, I will add a doubler of 9mm ply into the cabin, roughly a foot wide and extending from the cabin roof cleat to the lowest bolt. That should be sufficient to relieve some of the pressure from the 9mm ply bulkhead.

Summary for new builders:
I would recommend following Tattoo's advice from above: use high-quality wood for the cleat at that location. Double-up the fiberglass cloth across the bulkhead/cabin roof joint. Use a doubler inside the cabin, though I doubt it needs to be as large as his.
Titania, launched January 2015
craig
 
Posts: 284
Joined: Fri Feb 21, 2014 1:04 pm
Location: Chapel Hill, NC

Re: Removable Tabernacle

Postby tattoo on Sun Feb 08, 2015 7:11 pm

Hello Craig and others interested in the tabernacle bulkhead failures ...
I seem to remember that John Harris has recommended for new construction that a doubler plywood plate of certian deimentions be installed on the inside of the bulkhead. I would expand that modification so as to have the doubler extend from the port-side Dorade box to the starboard-side Dorade box and from the level of the anchor deck up to the cabin celing. But for repairs of a FAILED bulkhead, I'd go further by adding another doubler and increasing the bearing serlfaces of the nuts and bolt heads that hold the tabernacle fast to the bulkhead. It should be noted that although the failed bulkhead has been "well repaired," it will no longer provide [hardly] anyting towards the strength needed. That strength must come from the doublers. Within a few days, I'll post photos of the two failures that I experienced and the repairs made.

It may help to understand the mechanics and geometry of the failures. I've struggled with reaching an understanding of the forces involved. Notice that each of several failures occur in strong winds, caused a little noise (I thought it was something loose in the cabin), and was self-limiting. That is, the skippers continued to sail and the failure didn't worsen. The mast is subjected to four main forces: the boom pushing forward on the mast, the sail luff pulling the mast backward, the shrouds pulling down, the forestay also pulling downward, and the bolts holding the mast to the tabernacle/bulkhead. Another (and probably trivial) force is windage. I doubt the boom pushes hard enough to cause the failure. If it did, it probably wouldn't be self-limiting. The luff tension is in the wrong direction. That leaves the shrouds, forestay, and the bolts. The sideways forces of the shrouds and forestay basically cancel each other out. Now notice that the chain plates are placed only a few inches aft of the bulkhead. By design, far enough to be sure that the mast isn't blown down forward. It is certainly sufficient for that purpose -- if it weren't, then surely the mast would crash forward breaking off at some point or pulling the tabernacle with it. But that doesn't happen. That leaves the downward pull of the shrouds and forestay -- which puts the mast under tremendous compression. And while under compression, it is also pushed on by the boom. Although the boom's force is insufficient to directly cause the failure, it doesn't take much force to make it "unstable," such that it "bows" even slightly forward. Then, the geometry of the mast-shroud-forestay-boat tetrahedron shortens its height [of which the mast is its supporting column] -- resulting in a bowing out of the mast. It doesn't take much of a bowing angle to cause a little movement of the mast at the point of its attachment by the top bolt. But the small movement is irresistable because of the leverage that results from the geometry. If flexable, the mast must bow! Some mast are stiffer than others. I think John's prototype must have a fairly stiff mast. I think Tattoo's mast is quite flexable. It is made of Southern White Pine which is very light and flexable. And Tattoo's tabernacle/bulkhead has failed twice. I wonder what materils Craig's mast is made of. If builders are interested, I'd be willing to rig a defletion test to actually measure the mast's flexability. If setup conditions are well controlled, comparisons could be made. Comments, criticisms, welcom.
Pete McCrary, launched Tattoo Oct '10.
tattoo
 
Posts: 116
Joined: Wed Sep 16, 2009 7:09 pm
Location: Manassas, Virginia, USA

Re: Removable Tabernacle

Postby craig on Sun Feb 08, 2015 7:49 pm

Pete,

Thanks for your well-thought out reply. I have read through it twice and think I understand the physics you have laid out and it all seems to make sense. I'll outline my observations more completely:

My mast and tabernacle sides are douglas fir - the tabernacle rear is southern yellow pine. I installed a doubler of 6mm plywood around the mast bolt holes on top, because I saw both Tattoo and Pocketship #1 had metal plates at that location for support.

My tabernacle is supported by three bolts: one very close to the cabin roof cleat, one about 3 inches below that, and one at the bottom of the tabernacle. I had the two on top to strongly support where I thought the forces would be greatest. The bolts are obviously not expanding, and I have the nuts tightened down to the point that the washer is indenting the plywood. However, as I said the top of the tabernacle pushes away from the bulkhead visibly. This occurs when the wind is strongly blowing, and I can recreate it when I pull hard on the 2:1 jib halyard - the tabernacle pulls slightly away from the bulkhead. So I think the forward force is quite strong (maybe worse when the jib is used) and that in combination with any bending force causes the problems. I wish I had a better picture of how much forward force vs. sideways force is present when sailing: i.e. how much is the mast base taking vs. the shrouds. My shrouds are cinched quite tightly, but when sailing the leeward shroud is definitely loose. Pretty incredible amount of force.

I wonder if we could obtain better leverage by bolting directly through the cleat. That "should" stop flex at that position and provide the maximum strength for the tabernacle. Would look ugly though I guess. Based on your reply I will change my plans and install the larger doubler you recommended.
Titania, launched January 2015
craig
 
Posts: 284
Joined: Fri Feb 21, 2014 1:04 pm
Location: Chapel Hill, NC

Re: Removable Tabernacle

Postby tattoo on Sun Feb 08, 2015 8:03 pm

Craig,
A mast of Douglas Fir is definitely much stiffer than Tattoo's. So, I don't think the flexability is the cause. I think the side forces don't matter. I need to check the attachment point to see if the forestay pull creates any torque on the mast that would help or hinder any bowing action. Is its attachment point at the same level? It would be nice to understand the forces. On the other hand the design should take into account actual performance and make appropriate changes.
Pete McCrary, launched Tattoo Oct '10.
tattoo
 
Posts: 116
Joined: Wed Sep 16, 2009 7:09 pm
Location: Manassas, Virginia, USA

Re: Removable Tabernacle

Postby craig on Sun Feb 08, 2015 8:49 pm

Pete,

I'm not sure what you mean by "Is its attachment point at the same level?" I installed the shrouds, forestay, etc as per the plans. When I was figuring out the sails, I did a flexibility test of the mast by supporting it at the top and at the mast bolt location. I hung a weight of 50 pounds one-third from the top of the mast and tried to measure the deflection. I can't remember what it was - there was definitely flex, but I'm not sure what an average/normal amount of flex would be. The flex was small enough to where I had to be very careful in order to measure it.

- Craig

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Titania, launched January 2015
craig
 
Posts: 284
Joined: Fri Feb 21, 2014 1:04 pm
Location: Chapel Hill, NC

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