"Easy" mast stepping - alternatives for a tabernacle?

  • 27 Feb 2018 10:54
    Reply # 5879533 on 4611808

    Thank you Arne for your (as usual) positive and helpful reply.

    Since the last post I have found that the cost of cutting and folding 6mm sheet is a bit more expensive than originally quoted, but still – together with the cost of the aluminium sheet – cheaper than making the same thing in douglas fir here in New Zealand – if I could get the wood.

    6mm is the heaviest the local engineer can handle, but maybe this is enough to enclose and support a round section mast of 5mm wall thickness. The aluminium company will sell me half of a 1200 x 2400 sheet of aluminium, cut lengthwise, enough to fold into a 2.4m length of channel.

    I presume your suggestion to increase the thickness by 50% was to allow for loss of strength due to welding? I can not go thicker than 6mm. The engineer reckons there will be no significant loss of strength caused by welding, but my intuition tells me you are right and welding not a good idea here. Besides the cost of welding. Maybe I can do the whole job myself with no welding.

    The engineer can cut and fold a 154 x 154 channel – in fact he can do better than that, he can make a “top hat” section by folding out a 25mm flange on each of the open sides of the channel. These can be drilled, and the piece of plate which makes the 4th side of the box just drilled and bolted directly to this face. Any other fittings (such as turning blocks for halyards etc) can be bolted on here too.

    I can use a disk cutter to remove this fold-out flange from the below-deck part of the channel, which will remain open except for two or three wooden diaphragms down its length.

    I think I can make the tabernacle waterproof (and make it part of the structure) where it goes through the reinforced deck. Same as would be done with a wooden tabernacle. And I think a heavy wood block diaphragm can fill the tabernacle at deck level, to close off and separate the above deck and below deck parts of the tabernacle – and for the mast to stand on. If necessary the entire tabernacle below deck could be filled with wood – but maybe that would be over-kill - maybe a depth of 200 - 300mm would give enough of a gluing surface... and or maybe some fastenings could be tolerable here...?

    Is my reasoning sound? There is a reliance on the ability to glue wood to aluminium at various points – I would prefer to avoid fastenings – but with large gluing surfaces, care taken, and no glue joints put into tension I am hopeful epoxy will be OK. Any comments, criticisms or better suggestions will be welcomed.

    (The hinge at the top of the tabernacle may involve a small weld there, but no welding or drilling of the mast. I have that part sorted I think, and it will be very simple.)


    Last modified: 27 Feb 2018 11:05 | Anonymous member
  • 26 Feb 2018 13:11
    Reply # 5877704 on 4614280
    Jami Jokinen wrote:

    Economically there seems to be one problem at least for me: the aluminium tubes sold here seem to be 6000mm long. That would mean buying a more expensive tube than the mast itself, and only use maybe 1500 mm of it. Ouch.

    Jami, it might be worth asking the supplier, last summer I had to make a new mast step for stalemate and needed a 300mm length. I contacted market metals expecting a hefty bill.  The lovely people gave me a 300mm length for the cost of the postage. your supplier may just charge you for the 1500mm length?

    Good luck

    Mark

  • 24 Feb 2018 09:36
    Reply # 5875269 on 4611808
    Anonymous member (Administrator)

    Graeme,

    it seems that you are on the right track here. Since the tabernacle sits outside the mast, it will tend to be stronger: A square section which sits outside a round section, will be quite a bit stronger than the round one, if the same material and wall thickness is used. Bending strength varies with the cube of the diameter, you know.

    I have never made a tabernacle myself, but I have made batten hinges with the load-bearing hinge outside the battens, and these never failed (unlike some ‘inside hinges’ I have read about).

    Avoid making a weak (and leaky) spot where the tabernacle goes through the deck, and make sure that the lower end of the tab. never ever come loose from its step.. Welding on a squarish U-shaped flange or two, one at the top, and one where you want the leading blocks (in addition to the deck flange), cannot hurt. However, welding will take out any tempering of the aluminium, so I would anyway increase the plate thickness in the tabernacle with at least 50%, compared to the mast.

    Cheers, Arne


    Last modified: 24 Feb 2018 09:37 | Anonymous member (Administrator)
  • 23 Feb 2018 23:40
    Reply # 5874880 on 4611808

    Scott wrote:

    I've just finished doing the math for my boat; same problem, different scale.  I elicited the input of a couple of marine engineers that I admire, and the same answer came back:  make it out of aluminum or stainless steel.  It'll be lighter, stiffer, and less bulky than a wooden one.  Costs can be reasonably had.  A good engineer can even work in lightening holes if it's designed in from the beginning.  Though with your smaller scale, I don't think you'll need them, nor is there room.

    For your scale, I'd lean toward in a simple u-shaped tabernacle, that becomes a closed box below decks.  Scantlings need research, or a smart guy like Arne or David to chime in.

    I had planned on making a “Pete Hill” type tabernacle, in fact I looked closely at the one he made for Annie, since my mast length, scantling requirements etc are similar to those of the SIBLIM design. However the source of timber that Pete used had dried up and I could not find an economical source of suitable wood, so I gave up on the idea.

    My aluminium tube arrived a few days ago, and the idea has revived. I found a local engineer who can guillotine and fold aluminium sheet, and the cutting and folding to make a 160mm x 160mm channel is only of the order of about $30. Cutting and folding 6mm is certainly feasible locally, and maybe thicker, I am not sure. I was wondering if 6mm would be thick enough.

    So I am now looking at the cost of the material, aluminium sheet (plate?). Starting with a length of channel and some pre-cut flat plate it should be possible to use hand tools and make something simple and square, similar to the wooden tabernacle, the only welding required would be lugs for bolting on the 4th side of the box, attachment points for turning blocks, and flange at the deck.

    (I would add to Scott’s suggestion and make it a closed box above deck as well as below.

    The hinge could be a simple temporary lashing to a horizontal round bar fixed at the top rear of the tabernacle – temporary because when the mast is erect it is enclosed by the bolted up 4th side of the box and no need of a hinge until next time it is lowered.

    Or the tabernacle could be reversed, with open side facing aft, and the mast “hinged” at the deck with no fitting at all, as suggested by David Tyler. Once again, the 4th side of the box, above deck, bolted on after the mast is raised. Still considering the pros and cons of those two options.)

    So the question is scantlings and I was hoping someone smart about these things would “chime in”, as Scott puts it. If the tabernacle can be made from sheeting light enough for cutting and folding, but strong enough for the Pelorus, then this could be a runner. (If not, the idea might still be useful for a smaller boat.) 

    I seem to recall Pete used the reasoning that the wooden tabernacle scantlings could be derived from the Hasler-McLeod scantlings for a hollow wooden mast (I can’t find the quote now) and I think he used 50mm timber. My hollow aluminium mast will be the same at the partners as SIBLIM’s (aluminium 152mm OD and wall thickness 5mm.) The height above deck will be about 9m and the boat displacement is about 3.5 ton or thereabouts. Could I use similar reasoning and fabricate an aluminium tabernacle from 6mm plate? It will essentially comprise a U-section channel with 4th side bolted on to make a box, both above and below deck, with rounded fillets moulded inside the box above deck to stiffen the structure a little, and snugly fit the mast. Does 6mm plate thickness sound enough?

    I would be interested to see the numbers Scott came up with for his boat


    Last modified: 24 Feb 2018 00:40 | Anonymous member
  • 04 Apr 2017 10:58
    Reply # 4711071 on 4611808
    Anonymous member (Administrator)

    JRA member Lex Hodgkinson in Newsletter 20 described a clever tube-type tabernacle for his 17-footer which would allow the heel of the mast to be well below deck level. 

    Chris

  • 04 Apr 2017 10:42
    Reply # 4711057 on 4611808

    I had same kind of thoughts, but didn't dare to say them "aloud" :)

    Back to the thinking mood, then!

  • 04 Apr 2017 10:40
    Reply # 4711053 on 4611808

    Yes, 10% of the mast should be within the tabernacle, but I see no reason why you can't make Arne's concept in wood, and then the heel could be lower than deck level. In fact, it's probably easier to work in wood, as you can use a large proprietary stainless steel butt hinge.

  • 04 Apr 2017 09:49
    Reply # 4710984 on 4611808

    Just checking: The height of the tabernacle should be at least 10% of the mast length (over the deck), right?

    (As I stated below, it's not possible for me to build Arne's excellent tube tabernacle. Instead it seems that I have to go with the traditional, wooden tabernacle, which puts the boom higher than would be desirable on my low-ballast ratio boat.)

  • 17 Feb 2017 11:37
    Reply # 4614280 on 4611808

    Economically there seems to be one problem at least for me: the aluminium tubes sold here seem to be 6000mm long. That would mean buying a more expensive tube than the mast itself, and only use maybe 1500 mm of it. Ouch.

  • 17 Feb 2017 10:41
    Reply # 4614273 on 4614186
    Anonymous member (Administrator)
    David Tyler wrote:... ...It's not necessary to let the mast go right down to the bottom of the tabernacle, only to the usual 10% bury, so overall, there's probably the minimum increase in sailing weight.

    Yes, I can see your point, David. All one has to do then is to move that bolt shown in the bottom end of the tabernacle, upwards until the bury of the mast gets just right. That would save some mast length and quite some lifting of the mast as well.

    I guess I drew that bolt in the bottom end because I felt that the bending load was lowest there.

    Arne


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