"Easy" mast stepping - alternatives for a tabernacle?

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  • 14 Mar 2018 23:51
    Reply # 5979319 on 4611808

    The triangular section wooden bocks.

    David wrote:

    The problem with the wooden blocks would be to hold them in the correct position, but if you can solve that, I don't see any reason not to use them. 

    How about just stick them to the mast with Simpsons?

    Purely for volume displacement, slightly shorter than the surrounding urethane casting so they would be entirely immersed.

    And would that add anything to the adhesion to the mast, of the finished casting?


  • 13 Mar 2018 00:04
    Reply # 5968381 on 4611808

    Making a design is, for me I am afraid, a rather long-winded process.

    Each new idea that clever people give me, seems to suggest another, and sometimes the need to revise a decision previously made. One could always do it better next time.

    Temporary mast clamp for jumper strut – and possible tabernacle hinge

    Another matter which I now realise should have been more carefully considered when designing the tabernacle. If the mast is to be raised or lowered from the deck, then a temporary pole (I’m calling it a jumper strut) needs to be rigged at approximately right angles to the mast, a bit like a spinnaker pole.

    The lifting line (halyard?) goes from somewhere around the mast head, over the end of the strut and down to a turning block placed as far forward as possible at deck level.

    For me, this strut can be only about 2.5m (which means it might also serve well as one of a pair of drying-out legs, later.)

    (An aside: A longer strut would be better, but we are a bit compromised with junk rig. The forward placed mast and lack of a bowsprit precludes a more ideal geometry for the lifting arrangement.

    A rough back-of-an-envelope statics calculation (here) shows that at mast horizontal, the position of maximum tension, the tension in the lifting rope is less than 150kg. That’s still quite a lot. My windlass has a working load rating of 180 kg. However, I was going to use a 2:1 tackle anyway, just to slow down the rate of pull.)

    I was going to make a rotating mount for the temporary strut, and have it carried by the same pin which also makes the lashing point for the mast. However I see now that with castable urethane technology, the temporary strut could, alternatively, have a “jaws” fitting and bear on the mast, like a spinnaker pole does. It does not need to be clamped as it is mainly only in compression, but a clamp is convenient, and with the casting technology suggested by David, it seems worth while to cast a “jaws” – or even make a full clamp – at the same time as casting the mast fitting blocks.

    That being the case, we are now more than half way to making the other half of a proper clamp-on tabernacle hinge – a fitting which I had been intending to avoid by using a rope lashing.

    It is time to stand back and review the entire assembly: tabernacle, fitting of the mast via blocks cast from urethane, hinge, jaws/clamp for jumper strut.

    I got the tube welded to the top of the tabernacle yesterday (here it is) and then immediately realised how easy it would have been (and possibly still could be) for that tube to be part of a 12mm-pin butt hinge. The other part being a similar tube welded to a flat plate fastened to one side of a mast clamp – the same clamp could carry the jumper strut. The pin can be knocked out and all these parts removed and stowed away when not in use. It would take only a couple of minutes to re-assemble the clamp, insert the pin and mount the strut. Here is a rough sketch.

    I wish I had made the tube a bit bigger now. It is 12mm ID with a 4mm wall thickness. It looks a bit light for a load-bearing hinge, and I wish I had made it heavier now. But maybe it is still strong enough to be part of a butt hinge.

    Anyway, I think I will cast a couple of “halves” while doing the other casting. It will allow me to make a “jaws” and be still not too late to make a proper clamp-on tabernacle hinge if decided at a later date to dump the rope lashing idea and make a full clamp-on hinge fitting.

    In my case now, I think there are still two options:

    (1) the simplest most stripped down option – a rope lashing and pin for the mast hinge, and a jumper strut carried by a “jaws” bearing on (or clamped to) the mast

    (2) a tempting alternative - removeable integrated butt hinge/strut mounting clamped to the mast

    The flanged channel with its bolt-on 4th side provides the obvious mounting points for halyard and other turning blocks. I think this, and the above, covers all the required functions of a tabernacle, but a little messy, and perhaps there are still some elements which could be removed.

    Next time, always next time! I hope these lengthy and tedious questions and answers might be of some help to the next person who decides to make a tabernacle. It would be nice to be able to draw up a fully integrated simple design for tabernacle, hinge and lifting gear based on a folded aluminium channel, wood, and castable urethane fittings, with welding required only for the removable clamp-on butt hinge.

    PS you can get lucky sometimes. It seems that there was a very slight inaccuracy in the folding of the tabernacle. The sides are not quite parallel, by about 1 or 2 mm as far as my eyes can determine, when wearing the right glasses. The open sides flare out very slightly.

    Last modified: 13 Mar 2018 04:43 | Anonymous member
  • 12 Mar 2018 09:25
    Reply # 5966762 on 4611808

    Maybe they would be fixed permanently to the plywood dams, which would be left in place and coated with epoxy

    Something like that. Thanks David, for all your help.

    We can get PMC (smoothon) 780 here but not 790. But the 780 carries a built-in release agent. I need the urethane to stick to the mast but not the tabernacle. However I have located an alternative called Simpact 85 which has the same or slightly higher shore hardness and looks good, it requires a release agent. Much higher tensile strength and lower viscosity to pour.It only has a pot life of 4 minutes though. They are all about NZ$50 per litre. 

    It will be interesting to try this (to me) new material.

    Last modified: 12 Mar 2018 09:27 | Anonymous member
  • 12 Mar 2018 08:28
    Reply # 5966690 on 5959773
    Graeme Kenyon wrote:

    And one more question David: PMC780 is available in NZ (evidently not 790). But it is expensive, I may decide to fabricate with wood and epoxy rather than cast. Still weighing it up. Question: can you confirm 100mm is long enough to spread the load, for the top and bottom blocks? And if casting in urethane, would there be any problem with reducing the volume of material required by permanently immersing 4 wooden triangular fillets in each of the blocks?

    100mm is the depth of the partners that I used on Tystie, with a much bigger mast.
    The problem with the wooden blocks would be to hold them in the correct position, but if you can solve that, I don't see any reason not to use them. Maybe they would be fixed permanently to the plywood dams, which would be left in place and coated with epoxy? If epoxy costs less than polyurethane, the wooden blocks would decrease the volume such that exotherm wouldn't be much of a problem if you poured the blocks with liquid epoxy.
    Last modified: 12 Mar 2018 08:31 | David
  • 11 Mar 2018 21:34
    Reply # 5965934 on 4611808

    Thanks Dave I never considered for one moment welding to the mast. But I do propose to weld a heavy wall tube to the top edge (at the back) of the tabernacle.

  • 11 Mar 2018 21:28
    Reply # 5965925 on 4611808

    Hi Graham,

    don't even think of welding to the mast, as soon as the welder touches the aluminium you have lost almost half of the strength of the section at that point and at the most critically stressed location on the mast. Use a clamp made of two semi circular bent strap pieces with bolting tabs  at the ends, then weld your hinge pieces to this. 

    David.

  • 11 Mar 2018 10:11
    Reply # 5959773 on 4611808

    David wrote:

    Ideally, the sides of the channel would not be parallel, but would be spread apart a degree or two so that the square block touches the sides only at the same moment as it bottoms out.

    (And also to allow easy popping out of the mould)

    Its another excellent point - I wish I had thought of it. The people who made the channel seem to be able to work to a high degree of accuracy, I think they could have done that.

    Oh well... next time!

    Could probably have made the closed part of the tabernacle hexagonal instead of square too – or even multi-fold close to semi-circular.  Less material (cheaper and lighter.)I did think of that, but decided square would be simpler to fit the mast to. But anyone who decides to make the mast fitting pieces out of castable material won't have that concern, and might well consider other shapes because they also require less volume.(I am assuming this stuff is not cheap.) I am not sure how these other channel profiles would affect strength, and also the pin-and-lashing hinge might need a re-think - but worth considering, perhaps.

    Leaving aside the question of the hinge, maybe the most efficient folded aluminium section is not the the top hat, but something more like a bowler hat, with slightly flared rather than parallel sides. 

    And one more question David: PMC780 is available in NZ (evidently not 790). But it is expensive, I may decide to fabricate with wood and epoxy rather than cast. Still weighing it up. Question: can you confirm 100mm is long enough to spread the load, for the top and bottom blocks? And if casting in urethane, would there be any problem with reducing the volume of material required by permanently immersing 4 wooden triangular fillets in each of the blocks?

    Last modified: 11 Mar 2018 21:31 | Anonymous member
  • 11 Mar 2018 09:36
    Reply # 5959421 on 4611808

    Yes, I can see how method 1, a square block around the mast, helps with making the hinge. I'd still go with poured polyurethane as being the quickest, easiest way to make this. It might be just a little too tight at the sides, as the mast is being stepped, but that would be easily fixed. Ideally, the sides of the channel would not be parallel, but would be spread apart a degree or two so that the square block touches the sides only at the same moment as it bottoms out.

  • 11 Mar 2018 01:17
    Reply # 5955040 on 4611808

    The hinge

    It seems that fitting out the square tabernacle to take the round mast boils down to two approaches: Build up the mast to a square section (the packing being part of the mast) – call this method 1.

    Or, as David is suggesting, make a cast packing, in two parts, which acts as a clamp, one part attached to the tabernacle and neither part attached to the mast. – method 2.

    Both are practical and (on thinking about it) neither is difficult to do.

    I can see the beauty of method 2.

    If neither approach has any potential disadvantages, then, before deciding, it is also necessary to consider the type of hinging system to be employed, because there are implications. It is a given that there will be no welding or drilling of the mast at this point, or pins through the mast.

    Method 1 allows a simple and cheap hinging arrangement. For example (breaking the no-weld rule just once) a piece of heavy-walled tube can be welded across the aft upper end of the tabernacle, stiffening it, providing a roll-over surface to bear the flat aft of the built-up mast as it falls backwards over the tabernacle – and allows for a temporary pin to be pushed through, protruding each side, to make a temporary “sampson post” to which the mast can be lashed when raising or lowering. The lashing will rotate with the mast as it rolls back over into the lowered position, as the centre of rotation is the centre of the pin. I don’t think anything could be much simpler (apart from a suggestion David has made in the past which is to face the tabernacle aft and hinge the mast at the deck, with no fitting at all. I would certainly do it that way if I were not encumbered with a raised cabin preventing the mast from reaching the horizontal.)

    (Method 1 allows the attachment of a normal type of hinge to the mast too, as there would probably be enough meat in the packing to take screws - perhaps applicable to a smaller set up. But I won't be doing that.)

    David’s Method 2 does not, as far as I can see, lend itself so readily to such a simple hinge as the pin and lashing allowed by method 1.

    I am inclined to think that if there are reasons why method 2 is preferable, then it would pay to cast an extra pair of semicircle packers, to enable the making of a temporary clamp to be able to put around the mast just above the tabernacle, and a custom made fitting which can make this clamp the second part of a two part hinge, the first part being the removable pin through the welded-on tube, as described in method 1. It needs to be in place only when actually raising or lowering the mast.

    I think this would be very nice, but it might call for some more workshop engineering and a bit more cost and time. Just considering the hinge on its own, I think the rope lashing is a bit more KISS and I like it more.

    My guess is that one method is as good as the other, and hopefully not much difference in cost, so perhaps it just boils down to personal preference and what skills and materials are at hand.

    But I am not sure.

    Method 1 requires a nice fit, with the possibility of some clamping in the fore-and-aft direction by the use of a gasket placed under the bolt-on 4th side. (Thin polyurethane foam would be good here, I think.)

    Method 2 simply allows a good tight clamp all round (though with immediate release the moment the front is removed and the mast allowed to rotate backwards.)

    Are there any disadvantages to method 1?


    Last modified: 11 Mar 2018 03:52 | Anonymous member
  • 10 Mar 2018 23:17
    Reply # 5953797 on 4611808

    Thanks again for your technical advice, David, so generously given.

    I have not used castable polyurethane before, but will see if I can find out what it costs and where to get it here. I would like to learn how to use it, because it may well turn out to be the best way to join the two aluminium mast sections together, later. For the moment I feel a little under-confident in my ability to do it the way you have suggested and am still a little attracted to the less elegant approach, using the materials I am familiar with. Still thinking. A built up mast for a close fit - not hard to do - as opposed to David's suggestion of a tight clamp. Hmmm, might be better. I will report back when I have got my hands on some of this castable stuff and had a play with it.

    (David's advice to go for the larger version of the folded aluminium section has already proved advantageous. The 11mm clearance all round gives a useful working space in which to carry out the fitting of the mast to the tabernacle. I am continually grateful, and amazed at the value of this forum as a melting pot for ideas.)

    Last modified: 11 Mar 2018 01:19 | Anonymous member
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