"Easy" mast stepping - alternatives for a tabernacle?

  • 01 Mar 2018 07:20
    Reply # 5883262 on 4611808

    With all this talk about tabernacles, I thought you might be interested to see one in the process of being installed:

  • 28 Feb 2018 22:08
    Reply # 5882735 on 4611808
    Anonymous member (Administrator)

    Graeme

    Right to you three points:

    1.      All good here.

    2.      I too was thinking of 200mm, but 300mm will certainly not hurt.

    3.      What you are describing is very close to what I tried to suggest. I suggested ‘up to’ 10mm clearance, and then I meant 10mm on all sides so that should mean a square section with up to 170mm sides, inside dimensions.

    Arne


    Last modified: 28 Feb 2018 22:10 | Anonymous member (Administrator)
  • 28 Feb 2018 21:47
    Reply # 5882696 on 4611808

    I am sure you are right Arne, on all three points.

    1. Marcus did also stress the need for drainage when we discussed it the other day – same as you. Any wood down there will be thoroughly saturated with epoxy. And Marcus suggested cutting in some small gutters -  your suggestion of rubber pads might be a good one too.

    2. Yes, the wood block does indeed take the full weight of the mast. If I was making the tabernacle in steel I would certainly weld a plate there, or at least use fastenings - but the challenge here is to avoid welding or drilling at that critical point. I thought 200 – 300mm might give enough gluing surface for the wooden block to get by without any welding (or fastenings.) David seems to agree and has suggested 200 would be plenty, and recommends gluing with Simpsons. I might just go 300 anyway. And keep an eye on it, as I will with all these first-time-trial things.

    3. I guess it goes without saying that the mast heel plug should have a drain hole, a good point for reminder.

    Arne, in a previous post you seemed to be suggesting plenty of clearance (ie over-size tabernacle) would add to the strength of the tabernacle – if I understood you correctly. It makes sense to me, could you clarify please if that is what you meant?

    A half sheet of aluminium (600mm wide) will fold into a 160 x 160 channel, with “top hat” flanges of about 25mm – I am not sure at this stage what to allow for the folding, but the offcut (from 600mm) will be too small to be of any use for anything. To save an extra cut and make things quicker (and cheaper) for the engineer – why not use the entire half sheet and make a tabernacle channel of about 170 x 170.  Since the mast will be packed out to fit the tabernacle anyway, why not go the extra 10mm, use all the material, quicker and cheaper – and even more cross-sectional area (strength in bending.) ?

    Any problem there?

    By the way, I think now it makes sense to forget all about filleting and coving the inside corners of the tabernacle etc to fit the mast – just use epoxy, glass tape, filler and some judicious use of encapsulated wood, and build the mast up square at the top of the tabernacle, and square heel plug at the bottom – and sand the resulting square block sections to fit exactly inside the square tabernacle channel. And make it around 170 x 170 to save a guillotine cut, and use up all the material.

    (Maybe some of the clearance space could also taken up by bonding a liner inside the tabernacle, at least where the square blocks will bear - a bit of hard sheet plastic of some kind? A small teflon cutting board will go through a thicknesser quite nicely to make packers of the right thickness - for example...? (though I am not sure if even Simpsons will stick to teflon!))

    What say you, David and Arne?

    (PS Thanks too, David, for your most recent post. All points clear and understood.)

    Last modified: 28 Feb 2018 23:02 | Anonymous member
  • 28 Feb 2018 11:21
    Reply # 5881265 on 4611808
    Anonymous member (Administrator)

    David wrote:
    « "Marcus made the further suggestion that at the bottom of the mast, a wooden heel plug inserted and bonded into the round tube, could extend a little outside of the tube and be made square – to fit exactly the tabernacle – and also to prevent rotation of the mast." Good thinking - that will be a good alternative to blocking within the tabernacle. «

    All this looks good. If I should dare add something, it would be:

    ·         If I understand it correctly, the mast with its heel plug will rest with all its weight onto that wood block. Wood against wood = trapped moisture. I would glue four little rubber spacer pads (hard gasket rubber, 4-6mm thick) on the top of the wood block. This would let the spot dry out between rains, if correctly made. Remember, moisture traps = rot traps.

    ·         I would make the wood block long enough to give a large glue surface. The whole weight of the mast (including all extra down-pulling forces) will rest on it.

    ·         Should there be a drain hole in the mast heel plug?


    Cheers, Arne


  • 28 Feb 2018 08:22
    Reply # 5881213 on 5880522
    Graeme Kenyon wrote:

    To clarify: wooden diaphragms, with a 200mm diaphragm at the deck, not enough to resist the potential for the sides to buckle?

    [An afterthought: Such an external reinforcing strap, if packed with suitably-shaped packing on all four sides, could encompass the flange and the other three sides, and result in a rounded (near-circular interesting-shaped) clamp. It could be pulled tight with one bolt.

    A few vertical holes through it in the right places and it might look like a pin rail!

    (Or provide the attachment points for turning blocks etc.)]


    Rather than a wooden diaphragm, I would use a spacer tube with a stud and nuts, or a strap bolted across the flanges. The wooden diaphragm would depend on the holding of fasteners into it, if the sided tended to buckle outwards.

    Yes, your afterthought is one valid solution. 

    I would go for 160mm x 160mm, and then add semicircular blocks of wood or plastic to the tabernacle at the top and bottom of the bury, such that the clamps, of whatever form, will hold the mast very tightly, and these blocks would spread the load evenly onto the faces of the tabernacle. [line loading = cylinder touching flat surface; surface or area loading = two flat surfaces touching].

    "Make sure that the tabernacle is strong throughout its length, with no weak point at deck level."  Yes - discontinuities, corners and notches are the places where fatigue cracks start in aluminium. Carry the flange on through the deck.

    "Marcus made the further suggestion that at the bottom of the mast, a wooden heel plug inserted and bonded into the round tube, could extend a little outside of the tube and be made square – to fit exactly the tabernacle – and also to prevent rotation of the mast." Good thinking - that will be a good alternative to blocking within the tabernacle.

    "Last on my list of priorities (though still a requirement later) will be a watertight detail where the tabernacle goes through the deck – without the need for any horizontal flange welded on at that critical point. There will be more than one way to achieve a water-tight detail there."  You'll need partners with enough depth for the tabernacle to rest against, just as with stepping a mast. This could be built up with layers of plywood, or blocks of hardwood.

    "When it comes to bonding epoxy or Simpson’s goop to aluminium, is it better to remove the anodising at that point, or to glue directly to it?"  Avoid epoxy. Simsons will bond to almost anything, and won't mind the anodising.

  • 28 Feb 2018 03:07
    Reply # 5880929 on 4611808

    Thanks Arne. By filleting and coving the inside corners I was planning to fit the tabernacle to the mast – rather than building up the mast to fit the tabernacle in the way you suggest. But I had a long discussion about this with Marcus this morning and he pointed out to me the same point that I think you are making, that it is necessary only to have a close fit at the bottom and the top of the tabernacle, in between can be clearance. That is a good point, which I had not thought of, which only makes the job easier.

    (Marcus made the further suggestion that at the bottom of the mast, a wooden heel plug inserted and bonded into the round tube, could extend a little outside of the tube and be made square – to fit exactly the tabernacle – and also to prevent rotation of the mast.

    Your other point, which is equally of interest, is that the tabernacle can be made larger than the minimum required to accommodate the mast, and thereby the cross-sectional area (and strength) is increased. Have I understood you correctly? If so, then I can see a good case for making the tabernacle with a very large clearance to start with, even 10mm clearance or more. Thank you - again, you make the job simpler (as well as stronger) because there is now less need for accuracy in the dimensions of the channel.

    I am confident now that no welding will be needed, and following David’s suggestion of taking the “top hat” section for the full length of the channel, the tabernacle will be full strength from top to bottom. A suitable size wooden plug bonded into the channel at deck level, with no drilling or welding, will provide a step for the mast and not create any weakness there.

    Last on my list of priorities (though still a requirement later) will be a watertight detail where the tabernacle goes through the deck – without the need for any horizontal flange welded on at that critical point. There will be more than one way to achieve a water-tight detail there.

    The making of a tabernacle now seems to have been reduced to simply cutting a 1200 x 2400 sheet of aluminium vertically, folding it into top hat section channel with plenty of clearance for the mast, bonding a wooden block into the channel at deck level - and taking it all through the deck. A few more details bolted on or around, as necessary, and "Robert is your father's brother" as they say.

    Here is another little question which I suspect might be easily answered by David. The mast pole is anodised – and Marcus suggests getting the tabernacle anodised too. When it comes to bonding epoxy or Simpson’s goop to aluminium, is it better to remove the anodising at that point, or to glue directly to it?


    Last modified: 28 Feb 2018 03:31 | Anonymous member
  • 27 Feb 2018 23:49
    Reply # 5880707 on 4611808
    Anonymous member (Administrator)

    Graeme,

    since I am used to rather sloppy workmanship (lack of precision  -  my own), I would not worry so much whether the clearance ended up with 1 or 3mm between mast and tabernacle. Before erecting the tabernacle I would shim out the mast pole at both ends of the tabernacle, using some 100mm wide ‘waist belts’ from glass tape in epoxy. This should make a good fit with sufficient contact area. To make sure that the tabernacle is strong enough, I would rather give it up to 10mm clearance around the mast.

    Make sure that the tabernacle is strong throughout its length, with no weak point at deck level.

    Arne


    Last modified: 28 Feb 2018 08:17 | Anonymous member (Administrator)
  • 27 Feb 2018 23:06
    Reply # 5880641 on 4611808

    Goodness of fit beween mast and tabernacle

    I would surely appreciate some advice here.

    I am about to order materials, and the engineer wants a drawing giving measurements for the top hat channel. He says he can (using some cad software to calculate for the fold, and an offcut as a trial-and-error sample) fold to an accuracy of 1mm.

    (From my fairly limited experience with light-duty sheet metal equipment, I know that folding is not as simple to do accurately as one might think, and I find the above claim rather surprising.)

    Anyway, he wants accurate instructions and I will get only one shot at this, so a bit nervous about what sort of clearance to specify, between the 152.4mm OD tube, and the channel which will enclose it.

    I was hoping not to have to bother with packers or liners, but just make a snug fit with the 4th side bolted on.  

     

    I propose a 154mm x 154mm inside dimension channel, to give 1 or 2 mm clearance to start with. Does this sound about right?

    Next, I was going to glue fillets into each of the four corners (The two fillets at the front bonded to the removable 4th side only, the other two fillets glued into the corners of the channel.

    Then cove the fillets with epoxy filler, glass the coving and while the glass is wet, clamp the whole assembly onto the base of the mast, over a release coating – thus casting a (hopefully) near-perfect fit, which can be dis-assembled.

    Does that sound like a plan? At this stage I just need to know what clearance I should specify between the aluminium pole and the inside of the channel.

    Thoughts?


  • 27 Feb 2018 21:43
    Reply # 5880522 on 4611808

    David, thank you for pointing out these details. I do value this generous sharing of experience from you guys. I would like to visit each of the points you have made, for clarification.

    I think you're on the right track with the top hat section. Yes, I am hoping this is the detail which will make a non-welded aluminium tabernacle simple, strong and achievable.

    Ideally, I'd extend the flange from top to bottom… A channel that's under compression on the open side will try to buckle at its edge, either inwards or outwards, and the point of maximum load is at deck level, as with a mast. I thought the tendency for the sides to buckle could be taken care of below deck by using (wooden) diaphragms. This would make a neater and more compact arrangement, and require a more simple (square hole) detail at the deck. However, if you think the flange is necessary below deck then that can be done. I have pretty much decided now on a split junk rig so the mast or taberncle will be clear of bunks and there is room for the flange. I like your suggestion of tapering the flange as I suppose the need for it would decrease progressively from deck down to heel.

    To clarify: wooden diaphragms, with a 200mm diaphragm at the deck, not enough to resist the potential for the sides to buckle?

    I'd put in a wooden block at deck level, about 200mm long, with Simsons marine glue. That paragraph is clear and understood.

    At the top, I'd add some reinforcing, as the round mast will lie against the square box with line loading, not area loading, and will try to bend it outwards. I am not sure that I understand the technical terms “line loading” and “area loading”. I had planned to fillet and cove the inside four corners of the box (above deck) to fit the mast snugly all round. Despite that, are you still suggesting external reinforcement is desirable at the top of the tabernacle? If so, would it be sufficient to use a removeable metal strap at the top, which goes right around the tabernacle, packed out where necessary, and fastened (clamped) back to itself by a bolt or two, in a manner which makes it possible to remove – or tighten?

    [An afterthought: Such an external reinforcing strap, if packed with suitably-shaped packing on all four sides, could encompass the flange and the other three sides, and result in a rounded (near-circular interesting-shaped) clamp. It could be pulled tight with one bolt.

    A few vertical holes through it in the right places and it might look like a pin rail!

    (Or provide the attachment points for turning blocks etc.)]


    Last modified: 27 Feb 2018 23:28 | Anonymous member
  • 27 Feb 2018 11:56
    Reply # 5879590 on 5879533
    Graeme Kenyon wrote:

    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.)


    I think you're on the right track with the top hat section. If it couldn't be done by bending, I'd want to weld or screw on a square section alloy bar.

    Ideally, I'd extend the flange from top to bottom, but if that's inconvenient at berth level, I'd make a gradual transition over half a metre from full flange at deck level to minimal flange. A channel that's under compression on the open side will try to buckle at its edge, either inwards or outwards, and the point of maximum load is at deck level, as with a mast.

    I'd put in a wooden block at deck level, about 200mm long, with Simsons marine glue. I always prefer this over epoxy where there are two materials with different elasticity, and where there will be bending loads (it's the same as when making a hybrid mast). Coat the top and bottom of the block with epoxy, but leave the sides uncoated to provide moisture for the Simsons to cure. The bonding area is huge, and fasteners shouldn't be needed.

    At the top, I'd add some reinforcing, as the round mast will lie against the square box with line loading, not area loading, and will try to bend it outwards.

    Last modified: 27 Feb 2018 11:58 | Anonymous member
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