tabernacle question

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  • 22 Mar 2015 15:55
    Reply # 3261315 on 3251698
    Deleted user

    I have an idea for a tabernacle to share. I was playing with tabernacle designs for my aluminum tube mast last night and woke up with a very different idea. All of the pivoting tabernacles I've seen pivot at or near the top. Because of the stress on the mast there, hinges are usually used on hollow masts as opposed to drilling and bolting. Drilling holes in freestanding aluminum masts is discouraged for good reason. One may be able to do that with a solid wood mast or one with very thick walls especially if there is some reinforcement around the hole. Phil Bolger specified reinforcement with a long 1/16 inch stainless steel plate held by a number of very small screws on the wood mast, along with the tabernacle, he designed and I successfully built for my O'day many years ago. My second attempt to build wooden mast was a disaster but that's another story. Tightening the large bolt through the two thick wood sides of the tabernacle locked the mast in. It was a strong simple arrangement that served well.

    I have a scheme to be able to bolt through an aluminum tube mast to allow for a simple two sided tabernacle while keeping ample strength. First, the pivot point would be about midpoint vertically on the tabernacle. I think there there is no great lifting advantage to having the mast pivot at about 10% of length unless it is counter weighted, a great option with a boat with a bow well or if you pull downward from the mast base as Robert has. A wood plug would fill the tube to eight inches or so above where the hole would be drilled for the pivot bolt. The mast would be locked in the tabernacle by the pivot bolt in at this point providing good reinforcement side to side. The top and base of the mast would be clamped in as well. When you consider that this part of the mast does not need the same strength as at or above the partners, as seen in the tapered bury of stepped masts, I think this arrangement would stand up well.

    Having the pivot below the top of the tabernacle has the advantage of locking the mast between the uprights to limit side to side sway when raised, more so as more of the mast goes higher and the bury moves more between the uprights. Pivoting at the tabernacle midpoint would, I think, give it a longer arm of side to side support going up and down. On my boat the mast would still clear the cabin top when horizontal. I made and used a gin pole and a short temporary stay bridles to single hand for my stayed mast last year. A gin pole alone attached at the pivot bolt would allow the same.

    I have some good ideas and some wild ones as well. I'm pretty certain this one is sound but would appreciate any feedback.

    Phil

    P.S. I think I need to learn to shorten my posts.

    Last modified: 22 Mar 2015 15:56 | Deleted user
  • 19 Mar 2015 21:40
    Reply # 3259018 on 3251698

    Using some sort of pole arrangement like the 2 X 6 might work, if it were stayed at the right angle to take those torsional stresses. Also, I still haven't dome up with a man overboard recovery system, so something like that might do double duty. If I could use a spare batten, even better, triple duty.

     I do plan to add tubular braces aft of the fore tabernacle which hopefully will reduce the twisting. I'm reluctant to weld gussets onto the sides because of the distortion problems, the hardest thing about making these tabernacles was keeping them straight while welding. The uprights are 3/8 inch steel and I'm sure the lateral strength will be adequate.

    I made steel frames below decks which curve downwards and attach to the sawn frames, but I'm still not sure if the stresses might be a little too high when the boat is pitching so I decided that a couple of tubes angled aft will help that and reduce twisting as well. I hate to add more weight but would rather be on the safe side.

  • 18 Mar 2015 18:06
    Reply # 3257347 on 3251698
    Deleted user

    Robert,

    You still may be able to use temporary stays despite the narrow bow. For those kind of stays to work without needing to adjust them during raising or lowering they need to be horizontally in line with the mast's pivot point. What if you welded a small drilled angle iron ear at the back of each upright just below the pivot rod. Before raising or lowering the mast you could then then could the bolt something like a 7 or 8 foot 2”X6” piece of wood or a piece of aluminum channel to those ears giving you a wide base for the stays that, with the added height of the securing eye bolts at the ends, could be at the same level as the pivot.

    I can see why you don't want to add much more weight up in the bow but have you thought about adding some triangular gussets or braces from the tabernacle base to somewhere near the top. It would add a bit more weight and windage but would give more support to the top. Dave Z's aluminum tabernacle has two really neat bars that give both lateral and fore aft support to the top of the tabernacle. Something like that may do the same on yours. I think he also anchors it to the bulkhead as well. A piece across the back, allowing room for your lifting rig, would also add strength and stiffness but I'm sure you thought of that as well.

    How have you anchored the tabernacle bellow deck? The one tabernacle that I built was a very beefy simple two sided keel stepped one with a bolt through the middle of the square mast which was close to solid at that point. Working with hollow a round mast and trying to keep weight and windage down is very different.

    You are right on the cable comealong. I've used one too many times but fortunately I am still in one piece and still have all my fingers. I admire your fabrication work.

    Phil

  • 17 Mar 2015 14:24
    Reply # 3255240 on 3251698

    The chain come-along idea turned out to be one of my better ones. I've used those at work for years and have never seen one fail. I think it's the safest way you could do mast raising and lowering because they have a built in brake and there's no chance of anything slipping or fouling. The one I'm using at present is a cheap one from a local discount tool store, I plan to invest a bit more in an industrial grade one when I get underway. Even if something did fail I can work from a safe place where I'm out of the line of fire or the fall line of the mast. They're sold as "lever chain hoists" but commonly known as "come-alongs". I would definitely not reccommend using the cheaper cable come-alongs. Those are very unreliable and prone to jamming.

    Temporary shrouds would work for the main mast because there is adequate beam at that point, but the fore mast is too close to the bow to get enough lateral spread. This is a work in progress and it's clear the fore mast tabernacle needs a little more work. But I am very pleased with the raising/lowering system, it works even better than I had hoped it would.

  • 17 Mar 2015 09:24
    Reply # 3254985 on 3253581
    Deleted user
    Robert Leask wrote:

    I am a little concerned about the torsional stresses when the mast is near horizontal, I think these tabernacles will be strong enough when the masts are raised but the leverage is very high when they're on the way up or down, and I'm worried that if the boat were rolling a lot it could permanenly deform them. I'm trying to think of a way to stiffen them without adding too much weight or clutter. Suggestions welcome.

    Hi Robert,

    That's been a concern for us, too. Last time, we decided to leave the sail bundle attached, knowing we had plenty of power to lift. And we did... with me pulling on the masthead from beach ahead of our dried out boat (no rocking).

    Problem was, with the mast over halfway up, that bundle lifted from a little off-center, so swung across. The mast followed. I (over) corrected. It swung back. The mast followed. I (over) corrected. Repeat.

    I eventually got it under control before we levered our tabernacle apart. Or ripped the hinge off. Or something I yet fail to imagine. But WHOA! I was sweating bullets! 

    Got me thinking about this problem, and we thought this up:

    Since, with your set-up, you can take it slow and hold position, how about a pair of temporary, running shrouds, tended every few feet of lift/drop (stretchy line will dampen travel but allow some progress between resets)?

    To save a trip up the mast, you can tie them to a loop around the mast, and haul it up with the halyard (don't forget a downhaul line to bring it back to earth!). Maybe not all the way up, but to wherever the angle looks right (high and wide are both good, but work against each other).

    We use this method to lift one mast to 45+deg with the other's halyard. Then pull it home from the beach. But love your come-along arrangement!

    Good luck and work safe... that's a mighty big nutcracker!  8/

    Dave Z


  • 17 Mar 2015 08:46
    Reply # 3254949 on 3251698
    Deleted user
    Phil Brown wrote:

    Slacktide does well with a round solid wood mast clamped square in a square tabernacle.

    Hi Phil,

    We've had best results wrapping the mast with line or old firehose to fill out the gap between round mast and square tabernacle. This allows fine tuning with no carpentry, and no shifting or clunking between tacks.

    I've got some posts with pics, if you're interested:

    http://triloboats.blogspot.com/2012/03/masts-as-if-they-grew-on-trees.html

    http://triloboats.blogspot.com/2012/02/free-standing-masts-and-tabernacles-who.html

    First one has the pics of the cushions.

    Dave Z


  • 16 Mar 2015 12:51
    Reply # 3253581 on 3251698

    I'm just finishing tabernacles for my new 3 masted rig and hoping this link will work: 

    https://app.box.com/s/mggikxpabes6w22h6gsse51k8mrw907p

    The relevant pictures are titled MastDown 1 and 2. I made mine out of steel and will get them galvanized when I'm satisfied they are completely done. My idea for raising and lowering with a common chain come-along worked out very well, it can be easily and safely done with little effort, a one hand operation for the foremast at least. Will be trying the main mast shortly.

    I am a little concerned about the torsional stresses when the mast is near horizontal, I think these tabernacles will be strong enough when the masts are raised but the leverage is very high when they're on the way up or down, and I'm worried that if the boat were rolling a lot it could permanenly deform them. I'm trying to think of a way to stiffen them without adding too much weight or clutter. Suggestions welcome.

  • 13 Mar 2015 21:41
    Message # 3251698
    Deleted user

    I'm planning a tabernacle for the Brenda B, my Com-pac 19. The sail is in two pieces, roped with webbing and waiting for some heavier material to finish the short batten pockets by the mast. The 4 ½ inch x .125 inch 6061 t6 aluminum mast tube is on order and the snow is melting off the boat cover. My recent sailing has been from a dock on a lake a several miles from home which means I can be sailing in minutes, and with even less hassle with the new junk sail. While this increases my sailing time and allows things like for spur of the moment supper and evening sails, I miss time sailing out among the islands of Penobscot Bay. The tabernacle will make getting to the coast for a cruise or a temporary mooring much simpler than a keel stepped mast.

    Pictures of Daze Z's Slacktide tabernacles and Pete Hill's tabernacle write up in JRA mag #61 have given me some new ideas. I'd been thinking that to hold a round aluminum mast in a square tabernacle I would need half round shapes at the tabernacle's top and base to hold and clamp the mast. Slack tide does well with a round solid wood mast clamped square in a square tabernacle. It seems that that this would would work with hollow aluminum. Is this sound thinking? After seeing how the two tabernacles pivot I decided to use a wide heavy duty s.s. strap clamp designed for pipes to clamp the mast to a rugged hinge with the other flap of the hinge lagged to the back of the tabernacle. The intended pipe size is 4 3/8 inches and the space could be shimmed tight with leather providing a little cushioning.

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