Tabernacle To Cabin Top Fastening

Discussion in 'Boat Design' started by viking north, Feb 8, 2012.

  1. viking north
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    viking north VINLAND

    Does one securily fasten the mast step(tabernacle) to the cabin top and risk it and part of the cabin top being torn out leaving a gaping hole due to a demasting or would it be a better design to fasten say with 1/4 full thread bolts thus engineering a sort of sheer pin break away resulting in less possible damage to the cabin top. This is a design feature I will have to consider in my present build which will be involved in blue water sailing. All input greatly appreciated, Tnx.

    A yacht is not defined by the vessel but by the care and love of her owner---
     
  2. philSweet
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    philSweet Senior Member

    One end of the mast flogging about the boat is bad enough. You certainly don't want BOTH ends flogging about the boat. The mast should break, buckle, fold, or what ever, but the base should stay put. A compression post and tension rods which run in the deck and vertically beside the post ought to keep the mast base in place. Generally, you try to engineer things so the mast goes forward or to one side, but not back into the cockpit.
    Ever see a mast potted with Spartite not want to come out. People pick their entire boat up trying to get their mast out. Deck stepped aluminum masts can get a pretty good hold of their bases if they haven't been pulled in twenty years. I have seen some who use lag screws to fasten the mastep down, but I would rather see a backing plate and throughbolts and rods. You should be able to tear the mast off the boat with a chain without harming the step. I run lines to the mast when towing, storm anchoring, or just when I need a few extra winches to solve a problem. I fully expect to be able to pick my boat up from the mast step and shake it.
     
  3. viking north
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    viking north VINLAND

    I agree Phil on a deck stepped mast where one can get into fastening to deck beams but I was thinking more on the lines of cabin roof beams. These being much dimensionally smaller and regardless of how much they are themselves reinforced it's highly debateable if that area can take the transverse stress of a mast going over the side. Hopefully the mast breaks at the top of the tabernacle before a section of the roof is torn out. Flange bolting the compression post to the keel and then thru bolting the tabernacle to a welded flange on top of the compression post might be one way. Or a headroom saving steel archway with the tabernacle thru bolted to the top of the arch could be another. It's not a question that i don't understand what is required it was a question does a breakaway system have any merits or has the idea ever been considered or used in a design. A regular deck mounted mast to mast step connection is usually a simple slide in and gravity lock setup with the jaws of the mast end locking into a transverse pin in the deck fitting. A tabernacle is a different beast usually involing two pins one at the base and one as the tilting axil. The upright legs of the unit itself certainly provides more leverage on the deck fasteners and the roof structure if the whole mast was to go over the side. Just a thought, but wouldn't a breakaway systen be of some benifit here. Food for debate ???? Geo.
     
    Last edited: Feb 8, 2012
  4. philSweet
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    philSweet Senior Member

    well, I wouldn't want it to break away, but a predictable failure mode is nice to have. There was a tabernacle thread a while back that talked about a way to convert an existing aluminum mast into a tabernacle by cutting it and inserting a core reinforcement and adding some cheeks. That allows a standard failure of the regular tube near deck level without putting a huge peel load from a higher up failure. You can reverse the cheeks by fixing them on the mast and running the compression post right out the deck if that helps you get around the peeling/prying loads. The cheeks probably shouldn't be separately mounted to the deck anyways. They should be attached to a bracket that takes all of the prying and the bracket gets mounted to the deck.

    found it: Columbia 50 in Santa Cruz mast system here: http://www.boatdesign.net/forums/boat-design/mast-raising-lowering-systems-38193-2.html
     
  5. viking north
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    viking north VINLAND

    Thanks Phil i'll check that out . It's just one of those nagging things thats always bugged me in that it seems damed if you do and damed if you don't. Of course thats typical of boat design in general. :)--Geo.
     
  6. viking north
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    viking north VINLAND

    Unique systems from which i have come up with an idea. I present it : Use a section of mast as the compression post. This section of the same profile but larger than the boats mast. The compression post/mast to extend above the deck say 24 to 30 to where the tilt axil is below the gooseneck fitting. Cut out two sections fore and aft say down to 2-3 in. above deck level. This should allow the boats mast to fit between the remaining legs on either side. Weld flat alum plate over the open inside face of the curved alum legs thus creating two inclosed U shaped tubes as the legs of the tabernacle. Weld in a plate to cover the top opening of the mast/compression post 2 -3 in. above the deck to keep it weather tight. Seal the thru deck opening around the whole tube with a good weather tight marine caulk and Bob's your uncle-- a strong weathertight tabernacle that places no vertical loads on the deck (cabin top)and greatly reduces the possibility of tearing out part of the cabin roof due to a total demasting. As an added feature comm. and electrical cables can be fed up thru the tube. ?????
     
  7. michael pierzga
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    michael pierzga Senior Member

    Your boat is small. Why not just sail with a deck stepped mast.

    Place two oposing metal wedges under the mast foot whack with a hammer to jack it up...pin them ... and go sailing. Then when folding time comes...place the tabernackle pin thru the mast...drive out the wedges...the mast is now on the pin...and fold ?
     
  8. michael pierzga
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    michael pierzga Senior Member

    Typical small craft tabernackle
     

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  9. viking north
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    viking north VINLAND

    Michael the boat is small less than 10m but it has to handle both the blue and the green. I.E. super strong-overbuilt. My opening post is my concern, a cabin top not a deck stepped affair. I am not impressed with the standard factory produced cabin top units and I add to the problem with the added leverage of a tabernacle. It has always been a pet peeve and concern of mine that I want to solve-reduce the possibility of a bad situation. Bad enough to lose a mast but a hole in the cabin top spells total disaster.
     
  10. michael pierzga
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    michael pierzga Senior Member

    I dont know...I would choose a simple system. Simple is always robust.

    By simple I mean no contraptions on deck and a system to hinge that may need to be assebled each time you fold. How many times will you fold..every day or every season ?
     
  11. viking north
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    viking north VINLAND

    Ya but robust is not always simple :) except myself as described by my wife :D. I'll have to lower the mast every time i melt a comm. antenna :p. It's not the number of times it's just the independance of being able to do it. Yacht clubs - cranes-boom trucks-- just a waste of hard earned cash if one can built it right in the first place. Besides how long more will the telephone company let me throw a vice grip weighted lowering line up over their pole cables ( my mast crane). :eek:
     
  12. michael pierzga
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    michael pierzga Senior Member

    And you might google the Netherlands for tabernackle insights...seems everyboat has a tabernacled mast...perhaps some good ideas. Ive never personally sailed with on.
     

  13. viking north
    Joined: Dec 2010
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    viking north VINLAND

    Good idea, the Dutch the old masters, bet they have a few versions. Something weird is happening -- two posts just disappeared -- ????

    Off to the shop --Cheers--Geo.
     
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