tabernacle question

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  • 29 Jun 2018 14:46
    Reply # 6349946 on 3251698

    I've had to cancel my offshore sailing plans again, I relaunched about a month ago and everything looked fine, but on my way to Nova Scotia stress cracks have appeared in the mainmast tabernacle and are spreading rapidly. The cause seems to be a tortional stress which has been a problem all along. The trouble is that the lateral stresses are not being taken up evenly fore and aft and are causing a twisting moment which I hadn't expected. I'll be hauling out again for repairs and hopefully get things right this time, I was planning to write an article about these tabernacles thinking they were a success, now I'm glad I didn't. When I do get things right I'll share what I've learned but more work and more testing are needed


  • 10 Apr 2015 05:20
    Reply # 3290900 on 3251698

    Or steel that could be left ashore or epoxy coated and stowed as ballast. New plan, I'll be at making the lower 1 /10th plus of the aluminum mast square by snugly encasing and epoxying it it in a fiberglass covered plywood box, ½ inch marine grade, and then using Pete Hill's proven design. The inside corners of the box can be filled, at least top and bottom, to provide an even bearing surface all the way around the mast. Got the plywood yesterday and am making progress elsewhere. Today I fit and attached battens to the sail and was relieved to find that they all fit in the pockets despite a couple dry spots that had me thinking I might have to pull some stitches. I'll get there yet.
  • 09 Apr 2015 15:25
    Reply # 3288989 on 3251698

    I have sailed a pointy sailed boat once with a wooden mast counter balanced with a length of concrete bolted to it , it came down and up no trouble , probably acted as ballast as well   .     alloy tubes don,t do  well in  compression perhaps a hard wooden or steel former of a fair length in side might do . 

  • 25 Mar 2015 13:42
    Reply # 3265947 on 3251698

    I think that's a good idea, it would help solve the problem I had with my foremast tabernacle, which is a lack of torsional stiffness. Not a problem when the boat is in calm water or on the hard, but could be if you wanted to raise/lower the mast in less than ideal conditions. The lower the axis, the stiffer it would be. If I had thought of that before, I might have done things differently. 

    I think I have a solution to the twisting problem, will post pictures when it's done.

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

    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 | Anonymous member
  • 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

    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
    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
    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


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