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Thread: Mast Tabernacle Design

  1. #1
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    Default Mast Tabernacle Design

    Or perhaps hinged mast design. I've done some searching, but have not come up with much in the way of best practices or recommended techniques for designing and installing a tabernacle in a boat that doesn't have one. I have a 15' Joel White Marsh Cat catboat - it has a 20' mast with a max diameter of about 4". The mast isn't particularly heavy - I would guess around 40 lbs - but when stepping and unstepping through the deck it must be gripped ~3.5-4' from the heel, which places the CG up above your head. This can cause difficulties when trying to lower the mast to horizontal. Don't ask how I know.

    I know that other Marsh Cat builds have incorporated slots in the deck, hinged masts, etc. to cope with this problem - unfortunately I haven't been able to come up with enough detail on any of them to use them as a guide. What to do? Reinvent the wheel, of course!

    First off, here are the relevant details of the mast, partners, and step from the plans:

    MC_MAST_DETAIL_SNIPS.jpg

    Here's my first idea: Take a 4" square x 0.25" wall steel tube 24" long and cut pockets on the Fwd and Aft sides to allow the mast swing out, and cut slots in the LH and RH sides to accommodate a 1/2" bolt. The result would be something like this:

    MCT_SQUARE_03.PNG

    Then cut the mast about 12" off the deck and install the steel tube. In this way, the mast can be lifted and tilted aft to lower it, like so:

    MCT_SQUARE_01.jpg

    MCT_SQUARE_02.PNG

    The pivot would be above the gooseneck (actually a track, in my case) so that the boom and sail can remain furled in place - the gooseneck/track would need to be moved to the aft side of the tabernacle tube. This would require cutting flats on the 4" mast to bring it down to 3.5" so the tube can fit. Any thoughts as to whether this seems like the right way to go? I am loath to cut my mast, but I haven't yet come up with what seems like a reasonable way to controllably lift the mast vertically through the deck until the heel is free and controllably lower it. I have considered a big A-frame that would fit around the mast, but rigging and unrigging a derrick at the boat ramp seems like a suboptimal solution. I'd appreciate any opinions.

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    Default Re: Mast Tabernacle Design

    "The enemies of reason have a certain blind look."
    Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.

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    Default Re: Mast Tabernacle Design

    Personally I would put a slot in the deck with a mast box to guide the heel down into the step.

    It worked on an 80foot lugger with a massive mast, so will do for you.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Mast Tabernacle Design

    Your plan looks a lot like the Tabernacle designed for my Autumn Leaves from CLC.

    This is welded and powder coated aluminum. I wouldn't go with steel myself. It could also be done with stock aluminum pieces bolted together.

    This unit came in at 45 pounds. It's stepped on the keel.

    IMG_20190116_171708.jpg
    -Dave

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    Default Re: Mast Tabernacle Design

    Why reinvent the wheel? First thing you need to decide is whether you want a stayed or unstayed mast. On a boat as small as yours, unstayed is by far the most convenient and easiest to rig/unrig and there is less stay crap to deal with when its down. The pivot can be located high enough to avoid the crew's heads and cabin when trailing. Provided your mast is strong enough to support the sails in your expected wind, that is what I'd do. A simple hinge at the base is adequate for a stayed mast but an unstayed one needs to transmit all the loading to the hull and that calls for some engineering. Your example is good enough for a hinge but not for an unstayed mast.

    Lots more to consider as we don't know enough about what you want to do.
    Tom L

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    Default Re: Mast Tabernacle Design

    This is a very sensible, clear, article on small boat tabernacle design.

    http://www.jimsboats.com/1jan11.htm

    It's in wood, but making it out of 1/4" aluminum wouldn't be hard.

    Both masts on Drake are in tabernacles, to allow passage under low bridges. There are photos on my thread in Designs. But being a 40-footer they were welded up from 1/4" steel, hot-dipped galvanized, then painted.

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    Default Re: Mast Tabernacle Design

    Quote Originally Posted by Tom Lathrop View Post
    Why reinvent the wheel? First thing you need to decide is whether you want a stayed or unstayed mast. On a boat as small as yours, unstayed is by far the most convenient and easiest to rig/unrig and there is less stay crap to deal with when its down. The pivot can be located high enough to avoid the crew's heads and cabin when trailing. Provided your mast is strong enough to support the sails in your expected wind, that is what I'd do. A simple hinge at the base is adequate for a stayed mast but an unstayed one needs to transmit all the loading to the hull and that calls for some engineering. Your example is good enough for a hinge but not for an unstayed mast.

    Lots more to consider as we don't know enough about what you want to do.
    Marsh cat has a single fore stay as designed
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    Default Re: Mast Tabernacle Design

    Here are photos of the mainmast tabernacle on Drake.

    http://forum.woodenboat.com/showthre...ed-ketch/page6

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    Default Re: Mast Tabernacle Design

    Living where I do, a boat with a tabernacle is quite normal.I have a few comments about the design in post #1.It shouldn't need the slot for the pivot pin and the metalwork should not require the mast to be lifted into place.The heel of the mast should bear on the infill at the base of the tube and the cut should be as close to square as possible to transmit the thrust.You need to treat a tabernacle as simply a means to get the mast and when the mast is upright the forestay and the structure of the boat must provide all the support necessary.This is the tabernacle of a wherry which although four times as long as a Marsh Cat and much,much heavier has a mast with just a forestay.



    In this instance the mast has a counterweight of about a ton on the end of the mast.This view of a smaller boat shows the foredeck hatch that is near universal and which may be difficult to accommodate in a Marsh Cat.If the boat is used in anything other than restricted inland waters it is imperative that the hatch is secured.I have known a boat with an insecure hatch sink in a couple of minutes at sea when the hatch became dislodged.Fortunately the crew were soon picked up.You will probably notice the two long pieces of threaded rod protruding from the forward face of the mast.These are for holding the counterweights and they would almost certainly have been added when the mast was in the boat as transporting them in place would have been challenging.


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    Default Re: Mast Tabernacle Design

    Thanks for the replies. The boat and rig are already built per the design, more or less. I didn't build it, so I can't definitively say that nothing deviates, but as far as I can tell it's per the design. As specified in the design it has one port and one starboard stay and a forestay that runs to the end of a short bowsprit.

    That's all fine apart from the difficulty I've had with stepping and unstepping the mast - mostly unstepping. I'd like to install a tabernacle so that it's easier and safer to rig. I'm open to alternatives, so please let me know your thoughts.




    @Thorne: I have no aversion to a commercial product, but my mast isn't deck-stepped and I'd like it install a tabernacle or hinge above the boom so that the rig can stay more or less rigged. Similar to the Marshall Marine hinged mast in functionality, if only they sold a version that could be installed into a wooden mast.

    @Peerie Maa: I have no doubt that a slot in the deck would work well, but it would require some surgery to the boat. Maybe this is the best approach - modify the boat and leave the mast alone?

    @Woxbox: Is that a tabernacle that drops in to existing partners or is it permanently installed? I've been thinking that the simplest way to do this is to make all changes to the mast and leave the boat alone, but maybe I should install a more or less permanent bit with the tabernacle on it and then just shorten the mast?

    @Tom Lathrop: You make a good point about the need to transmit the mast loads to the boat. I think that a steel or aluminum tube similar to the proposed design will handle the shear and bending required, but I have not yet finalized my math. I'll post it when I'm done - in the meantime I'd like to hear any thoughts on load transmission that I should take into account.

    @Dave Hadfield: Thanks very much for the links - the article was interesting. I am thinking that this would be cold galvanized (zinc paint) and then painted - similar to what you have done. Has that held up well? The advantage to using a stock tube is availability and cost, but I'm thinking about something formed from .25" plate - more details to follow.

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    Default Re: Mast Tabernacle Design

    @John Meachen: Thanks for the suggestions. How close to balanced are the masts on the boats you reference? I notice that the wherry has an open bearing on which the mast pivots - I'm unclear how the heel of the mast sits against the base of the tabernacle if there's no vertical compliance. Similarly, for the Marsh Cat I don't quite follow how the heel of the mast will sit on the infill (mast stub) in the base of the tabernacle if there's no slot.

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    Default Re: Mast Tabernacle Design

    Quote Originally Posted by pandelume View Post

    @Peerie Maa: I have no doubt that a slot in the deck would work well, but it would require some surgery to the boat. Maybe this is the best approach - modify the boat and leave the mast alone?
    I think its the way to go. I think it better to keep the spar intact, especially about where it sees its maximum load. I also look to the solutions fitted to working boats, developed over the decades. They had to be robust and follow the KISS principle. So you have the Scottish luggers and Northumberland cobles slot to guide the mast as it is stepped and struck, held in place by a chock dropped in behind the mast. The only other solution, that does use a tabernacle is the Norfolk wherry, copied by the hire yachts, with a slot in front of the tabernacle allowing the heel of the mast to swing up forward.
    It really is quite difficult to build an ugly wooden boat.

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    Default Re: Mast Tabernacle Design

    Quote Originally Posted by pandelume View Post
    @John Meachen: Thanks for the suggestions. How close to balanced are the masts on the boats you reference? I notice that the wherry has an open bearing on which the mast pivots - I'm unclear how the heel of the mast sits against the base of the tabernacle if there's no vertical compliance. Similarly, for the Marsh Cat I don't quite follow how the heel of the mast will sit on the infill (mast stub) in the base of the tabernacle if there's no slot.
    They are balanced to the extent that the available force can raise or lower the mast.It isn't so hard to add or remove a balance weight and should the mast be broken and need replacing that is an entirely likely outcome if the new mast is heavier or lighter than the original.On the small cruiser mast it would be normal for the weight to be in slabs of half inch thick lead and the outer slab can be cut down or supplemented by another if necessary.

    As for the wherry mast being an open bearing;gravity ensures that the upper portion of the pivot doesn't see any load and it only needs to be round for the freedom to rotate.At the base of the tabernacle there is an infill block that the mast needs to bear on and as with the wherry mast breakage situation,the block can be removed and adjusted to suit a replacement mast.The old timers used to insist that the pin should spin under finger force with the mast up so it could clearly be seen to be supported and not just hanging on the pivot.The aft portion of the mast might have been eased just a touch to allow the mast to get fully raised and there should be no significant slope on the infill block.

    It might be a good plan to experiment a little with a couple of planks and a few spacers and a piece of fence post for a dummy mast.Mark the deck position on the sides and try a few different things.

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    Default Re: Mast Tabernacle Design

    Quote Originally Posted by Peerie Maa View Post
    I think its the way to go. I think it better to keep the spar intact, especially about where it sees its maximum load. I also look to the solutions fitted to working boats, developed over the decades. They had to be robust and follow the KISS principle. So you have the Scottish luggers and Northumberland cobles slot to guide the mast as it is stepped and struck, held in place by a chock dropped in behind the mast. The only other solution, that does use a tabernacle is the Norfolk wherry, copied by the hire yachts, with a slot in front of the tabernacle allowing the heel of the mast to swing up forward.

    Your remarks remind me of the Woods Hole Spritsail Boat, which has a mast that stepped in the eyes of the boat and is, I believe, lashed in position - in order to allow it to easily be lowered and raised singlehanded:

    https://smallboatsmonthly.com/articl...pritsail-boat/

    IMG_2765PSweb-800x533.jpg

    IMG_2746PSweb-800x533.jpg

    I'll have to take a closer look at how the Marsh Cat's deck is constructed to see how feasible a slot would be.

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    Default Re: Mast Tabernacle Design

    @Woxbox: Is that a tabernacle that drops in to existing partners or is it permanently installed? I've been thinking that the simplest way to do this is to make all changes to the mast and leave the boat alone, but maybe I should install a more or less permanent bit with the tabernacle on it and then just shorten the mast?
    The tabernacle drops into partners designed to accept it. It can be removed easily enough, but there's really no need to do that on a regular basis. I would agree that you want to shorten the existing mast and add a solid base to transfer the stresses down into the boat.

    Here's a Menger Cat, a heavy 19-foot boat that comes standard with a tabernacle. It's beefy but not tall. It's held up with a forestay but no shrouds needed. I think this demonstrates how compact a tabernacle can be if it's we'll engineered and well made.


    -Dave

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    Default Re: Mast Tabernacle Design

    I have to say that the tabernacle in post #15 looks precarious.It is the sort of thing I would be happy to use for a stayed rig as it would enable you to locate the mast heel with a good deal of control and when upright the standing rigging could deal with the loads imposed on the hull.To have a hefty mast and rig cantilevered from such a short structure makes me uneasy.The mass of spaghetti emerging from the base of the mast tells me that it is hung on two pins as the forestay meets the mast at a tiny angle and can't offer too much support.I have seen it stated quite a few times that shrouds should never meet a mast at less than thirteen degrees and can't think of any reason why the same logic wouldn't apply to a forestay.

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    Default Re: Mast Tabernacle Design

    Quote Originally Posted by John Meachen View Post
    I have to say that the tabernacle in post #15 looks precarious.It is the sort of thing I would be happy to use for a stayed rig as it would enable you to locate the mast heel with a good deal of control and when upright the standing rigging could deal with the loads imposed on the hull.To have a hefty mast and rig cantilevered from such a short structure makes me uneasy.The mass of spaghetti emerging from the base of the mast tells me that it is hung on two pins as the forestay meets the mast at a tiny angle and can't offer too much support.I have seen it stated quite a few times that shrouds should never meet a mast at less than thirteen degrees and can't think of any reason why the same logic wouldn't apply to a forestay.
    That's how it looks to me, too. But Menger has been making them like this for a long time, and they hold up. Engineering! (There is also a pin that goes through that lower hole, but I recall seeing one that didn't have this.)
    -Dave

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    Default Re: Mast Tabernacle Design

    Nice example in post 15. That must be well engineered given a cat boats stiffness. Do they use a tapered alloy spar or is that an alloy tube foot on a timber mast?

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    Default Re: Mast Tabernacle Design

    A little off-topic, but I ran across this for sale locally:

    00p0p_8i37QgsJewm_1200x900.jpg

    It's listed as 1/4" bronze, 10" square base, 12.5" tall, 4" wide center slot, and a 1/2" pivot hole. If I were converting to a deck-stepped mast it might be a good choice.

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    Default Re: Mast Tabernacle Design

    ^ That would be fine for a stayed mast, but not for a free standing. Its suggested a minimum of 10% of the spar length as bury, in your case, a tabernacle with 2ft sides. There was a simple article on a Jim Michalak page.

    http://www.jimsboats.com/1jan11.htm

    There was another catboat with a single forestay, but it had several spreaders down its fore side and so could be of quite light section. If i can recall where i saw it, i will post, i believe it was keel stepped.

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    Default Re: Mast Tabernacle Design

    You're quite right - I'm not sure how you'd have an unstayed deck-stepped mast. I do have stays, but I don't think they would be up to supporting a deck-stepped mast.

    I read that article but I must have missed the 10% rule. That's a good bit of information to have. Referring back to the OP and the proposed design, that would suggest that the tube hinge should be more like 48" rather than 24" long, with 24" of engagement on the fixed and hinged parts of the mast. The tube itself is stronger than the mast in both shear and bending - I think the concern is with transmitting the bending from the mast to the tube.

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    Default Re: Mast Tabernacle Design

    I cant recall what book, probably in a few, most likely "junk Rig", but 10% is an absolute minimum of bury. Using your tubed sketch as an example, the lower section only has to be strong enough to fasten securely, with the minimum 24in being the sides above the join, there is certain comfort in using a long lower section also to spread the load
    Last edited by skaraborgcraft; 06-06-2019 at 03:55 PM.

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