Fitting a tabernacle

Discussion in 'Boat Design' started by Patmobile, Sep 7, 2016.

  1. Patmobile
    Joined: Apr 2016
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    Location: France

    Patmobile New Member

    My Tricorn cruising dinghy has a deck stepped mast which can be raised and lowered reasonably easily, by two people, when the boat is on its trailer. The operation cannot, however, be performed - at least, not safely or securely - when the boat is afloat, unless alongside a quay with the aid of a crane.

    I'm hoping, by fitting a tabernacle, to make it easier to raise or lower the rig both while afloat or onshore. Apart from being able to navigate under bridges, I believe there may be a couple of other advantages, such as spreading the rig load over a greater area of deck, and providing a couple more places for cleats and general halyard tidiness.

    This is the sort of thing I have in mind, though probably a little smaller:

    [​IMG]

    The mast is a Proctor alloy mast (F section, I think) dating from 1962. it's in good condition without any dents or repairs.

    My question is, does the mast need to be sleeved or reinforced in any way at the point where the pivot bolt will pass through? I'm worried that the bolt hole might become enlarged, and the metal around it cracked or torn.
     
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Yes, the pivot hole does need to be reinforced, if you make it fit a tabernacle. The usual method is to insert a well sized length of stainless tubing, to act as a compression tube, inside the mast. Ideally, you use a liner of HDPE inside the tubing, so the pivot bolt has something to ride on, instead of the edge of the mast extrusion.

    Tabernacle design is important too. The mast will be not as well supported in one, meaning loads are higher, not lower with it's use, so insure rigging attachments on the deck are well backed up. The bottom of the mast will need to "land" on the tabernacle, so consider more HDPE as a pad, for it to sit and slightly angled forward, so it can swing in and out of the tabernacle without binding. Lastly, loads on a tabernacle can be huge because of the cantilever involved, so make it stout. On your boat, I'd bend or weld up some heavy aluminum plate (6061 T-6 alloy) to keep the windage and weight down. It can be made from wood, but to have it stiff enough, it'll be bulky compairtivly.
     
  3. Patmobile
    Joined: Apr 2016
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    Location: France

    Patmobile New Member

    Thanks. Very grateful for your advice

    Not sure I have quite got it. Is the SS tube you prescribe to be stuffed tightly up longitudinally inside the mast, & drilled laterally to take the pivot bolt? Or could you mean a tube fixed laterally through the mast to act as a sort of combined bush and compression tube? My guess is the first, since it seems to provide strong engineering for the probable loads - but please let me know if I'm seeing this wrong.

    If I'm right about the longitudinal stub/tube/internal sleeve, how is it best fixed in place so that the pivot holes can not become displaced, failing to match those in the skin of the mast?
     

  4. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 472, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    The reinforcement tube is designed to serve as a compression post/tube for the pivot bolt and is sleeved to work as a bushing for this bolt/pin.

    Placing it (the tube) axially seems logical, but the mast is already stiff enough to work in compression, assuming it stays in column. The reinforcement tube is to locally reinforce the area, that will see compression from the pivot and help stiffen lateral loads, as the mast is raised and lowered.
     
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