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how can i chanage to a tabernacle


DISTELTK

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Tabernacles on free standing rigs, need to be stout, but they're not that difficult to figure out. The usual approach, is to fix two solid lumber "cheek" pieces, through a hole in the deck. Of course they should be spaced apart the width of the mast, securely attached to the keel/bottom and at the deck. Above the deck, you'll have a pivot bolt that passes through the tabernacle cheeks and the mast, at the height you'd need. On a boat with a conventional boom, this pivot would be above the gooseneck fitting, with the gooseneck being attached to the back of the tabernacle itself. On a sprit boom rig, you can place the pivot any convenient place.

 

tabernacle1.jpg

 

This one is metal and the bulletproof way to make them. 

 

miketabb.jpg

 

This is a wooden one.

 

tabernacle.JPG

 

Another wooden one, though using plywood wouldn't be my recommendation. This image shows it going all the way down to the keel, which is important on a free standing rig.

 

If it's a stayed rig, all you need is a simple hinge at the bottom of the mast, on the deck and a compression post below it, to transfer loads to the boat's hull.

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As an alternative to installing a proper tabernacle some builders have placed a scoop on deck just forward of the mast  tube that helps capture the foot of the mast and makes stepping a little easier. Others have installed a temporary pivoting tabernacle that captures the base of the mast and helps guide it. I looked for the threads on this topic but could not find them.

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I once had an old cat boat that I wanted to make a tabernacle for, but couldn't cut into the deck to install one. Instead, I cut the mast above deck level and hinged it by bolting plates on either side of the lower mast section. The plates extended up to the hinge point in the upper section where it pivoted just like if it was in an actual tabernacle. Maybe a sketch will help.

 

post-1823-0-80501700-1446219665_thumb.jpg

 

 

 

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The thing you are all missing is that on a BRS15, the mast step is way forward.  The base of the mast very nearly meets the joint between the stem and keel batten.  (The BRS15 does have a stem, btw.)  I've attached a photo of someone else's boat, since mine isn't quite to this point.  I believe a scoop (second photo), or a temporary tabernacle are the only ways to help the situation.

post-3770-0-48675300-1446724033_thumb.jpg

post-3770-0-48999500-1446724107_thumb.jpg

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Both the slip in helper thingie and the pivoting ring deal still require the mast to be lifted out of the tube, when dousing the rig and placing it in the hole when setting up. On a cat rig, the "eyes" of the boat isn't a pleasant place to be fiddling with an unwieldy stick.

 

A tabernacle can eliminate the need to lift the mast and drop or pull it from the tube. In fact, a 25' mast I step regularly, uses a winch to haul it upright and to lower it. I use a regular trailer winch, mounted on the forward end of the centerboard case and it's lanyard is attached to a through eye, at the base of the mast. The pivot bolt is nearly 3' above this eye, so good leverage, but also a great deal of strain on the tabernacle, so I made it stout. I help the mast up with my shoulder as I start cranking the winch, simply to relieve some of this strain, but once it's started up, the pressure drops quickly and it's pretty easy to crank it the rest of the way. This isn't a B&B design and the rig is an aluminum stayed affair of conventional configuration. The mast is deck stepped, but the tabernacle is keel stepped. Simply put, you can skin this cat a few different ways, but ideally, you'd like to be able to step and unstep the stick quickly, preferably without having to dance on the scrawny foredeck. A previous tabernacle I used had temporary weights that where hung on the base of the mast, below the pivot, which counter balanced the mast's weight and it was nearly fingertip raising and lowering. Once the mast was up, the weights where removed from the hook on the front of the mast and placed in the bilge around the centerboard case. Do some interweb searching for tabernacles and you'll see several approaches. Most work, some are better than others, in terms of assembly and hoisting ease.

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It's not impossible to do.  But any solution that I come up with has serious structural issues.  There's not a lot of distance between the deck and the sail-- maybe 12", 14" at most.  The fact that the mast is un-stayed puts extra demands on the tabernacle and mast.  It'd have to absorb a lot of stress in a very short distance.  And the mast is so close to the stem, that it is not possible to do a conventional B&B Yacht Designs tabernacle, short of re-stepping the main aft about 15".  That's gonna start to wreak havoc with the mizzen as well as trim.  Here are some more shots of other boats, to illustrate my points.  To me, the lippy-mast-hole-funnel-thingy is the best possible solution.

post-3770-0-42570100-1446768449_thumb.jpg

post-3770-0-31221000-1446768504_thumb.jpg

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Most tabernacles don't end at the tack, but continue up a fair bit. On a sprit boom, the pivot could be as high as the snotter. I don't think this is a convenient height, but you don't need that much "purchase" on a rig like yours. If the tabernacle was from the keel batten to say a foot above the tack, you'd have more than sufficient purchase. Talk to Graham, as I'll bet he already has a solution for this.

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If you haven't covered the foredeck yet I think a combination of Chick's pictured tabernacle moved forward and PAR's idea of a taller tabernacle & higher pivot point so the bottom of the mast clears the foredeck should work.  I was conjuring up a similar system when I had that red BRS15 in your pics.  With the finished foredeck,  I thought of slotting the foredeck around the current mast step, sliding a tall 3-sided tabernacle down to the keel and anchoring it in to existing wood from the foredeck access.  The challenge is how the sails are mounted so they can be pushed/slid up (or removed if on track) to clear the higher tabernacle pivot.  Just thoughts, Rick

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always reserve the right to be wrong.  In this case, I'd love to be shown a way that one could do this.  My main intent has been to identify the structural challenges that DISTELTK will face in doing this.  I'm  a retired mechanical engineer.  My job (and current mindset) has been to identify the problems, and then set out to solve them.

 

Additional thoughts:  The pivot point will need to be quite high, because there is very little swing room below the plane of the deck.  The first question to Graham is how small a distance can be tolerated, between the pivot point and the bottom of the mast.  That's the key.  The rest is figuring out how to retrofit this on an existing boat.  

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The current mast purchase on your boat with the free standing stick is what 14". If you provide this plus a bit, you'll have the same or a little better leverage on the still free standing mast, assuming a stiff tabernacle. The tack doesn't have to be above the pivot, in fact I prefer it not to be on this rig, so it can be close to the deck. To lower the mast, you'd have to release the tack anyway, the same with a reef point. The sail, if on a slide would be flying free, below the pivot (maybe a foot of so), where the tack eye would live. You'd feed the slides into the track, just above the pivot, which is a convenient height BTW and the tack would simply "S" hook to an eye or possibly to a tackle is desired.

 

As far as the dynamics, if you provide a similar height of purchase for the now shorter, hence higher modulus mast, you'll be doing no disfavor to the rig. Again, this assumes a stiff, stout tabernacle. If it was me, retrofitting one, I'd simply glue a block on the keel batten or hull bottom, maybe tieing it to the stem too. I'd glue and screw cheek pieces to this, that extend up through the deck, with enough height to insure the pivot is the same if not taller than the original purchase (keel/partner) distance). I'd use an "I" beam shape on the tabernacle below decks and some simple blocking above to save weight, while keeping the cheeks apart. The only other consideration I might have is, athwart stiffness on the deck, which a beam would solve, if there isn't one there already.

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Don, 

 

There is no absolute minimum bury for a tabernacle, The smaller the mast diameter ratio to bury the higher the stresses go up. I try to get between 6 - 7 to 1 when I can.

 

The CS17 mk3 tabernacle has a 7.3 to 1 ratio. It has the same mast OD as the BRS but because the BRS does not have near as much stability, therefore much lower loads as the mk3 the mast has a thinner wall thickness. Without spending a lot of time on it I would guess that you could go down to 5 to 1 for the BRS if you had to.

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Thanks for all the info.  I think the best for me is to build a tabernacle with  slope from the cockpit. 

I will make a cutout in the deck and buldl a ramp to the mast can be slid in and pushed up from the

cockpit.  I will have an above deck portion the I can bolt the mast in a fixed postion.   The mizen mast

is not a problem.

Great site with a lot very useful information.

Thanx again  Tom D.  BRS 15 plan 127

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