Recommended thickness for partners/mast-support?

  • 13 May 2019 17:43
    Reply # 7338510 on 7325237

    Thanks David, that's good to know. I think I am going to make the pivot out of 50mm   aluminium plate, and make the support at the foot out of a Urethane Casting as you suggest.  See you in Brixham hopefully.

  • 11 May 2019 08:26
    Reply # 7335076 on 7325237

    Malcolm, the thickness of the polyurethane that I've got at deck level is about one third the diameter of the mast, and that is fully proven to be OK now. That would be about 50mm for your mast?

  • 10 May 2019 21:32
    Reply # 7334344 on 7325237

    In that case, Malcolm, maybe your original question does need to be answered. 

    So, my question is how thick should these "C" pieces be? I am thinking 30mm. These are acting like the wedges in a normal mast partners, and I am wondering if this is enough to ensure I don't put "point loadings" on the mast and risk a kink/bend. 

    Putting the full load of the mast at the tabernacle on 30mm of the mast wall, if that is what you mean, does rather seem to me to be "point loading". I would have thought a somewhat wider support would be necessary, even for your relatively short mast.

    But I just realised: if your C-clamps only come into play during the raising/lowering process (now you are talking about putting supports at the heel and at the top of the tabernacle) then it seems the C -clamps are not "acting like the wedges" at all, and may well be sufficient to support the mast just while it is being raised.

    I am only guessing. The detail seems a bit complicated to me. Maybe you should submit a sketch and hope that someone better educated will chime in from here on.

    Last modified: 10 May 2019 21:54 | Anonymous member
  • 10 May 2019 14:53
    Reply # 7333607 on 7325237

    Thanks again Graeme, David's articles made a deal of sense. I think I will try the cast Urethane moulds too - as half the tabernacle is already in the boat I will have to cast all four in the top half, but it should go OK.

    I think I am going to stick with my idea for the hinge as the existing pivot points are kind of geared up for this approach.

    Thanks again!

    M

  • 09 May 2019 11:30
    Reply # 7331004 on 7325237

    Malcolm wrote:

    I like the idea of the resin cast internal packing. Was this a filled-epoxy item or machined from a solid piece? I see in your album a picture entitled "urethane failure", was this an earlier/aborted attempt to make these blocks?

    I supported the mast at the top of the tabernacle with a clamp made in two halves from a 2-pot pourable, castable urethane resin called Simpact 85A. It sets into a hard rubbery compound. It was (I thought) a cheaper alternative to a well-known product called Spartite (you can Google it.) In doing the casting, I followed advice given by David T who is probably the best go-to person on castable resins. There is a thread called “Easy” Mast stepping- alternatives for a tabernacle? – you can click here and you will see a lot of tedious stuff by me, I am afraid, but if you persevere and read through the thread you will find the very useful advice from David as to how to go about doing the casting. It was successful, but I made a mess of the last casting by using the dregs, and the failure you refer to was a report back to David on that bit.

    At the bottom of the tabernacle (the heel of the mast) I wanted a square block, and part of that was done with epoxy resin. I glued four triangular section fillets to the mast, filled and faired with epoxy and microballoons then wrapped in a fibreglass bandage. Then a bit more filling and fairing in order make it fit nicely in the tabernacle. I could have done the top the same way – alternatively I could have done the bottom with cast urethane half-clamps. They are just different ways of achieving the same result.

    HOWEVER: Part of the reason for doing these things was in order to learn about the materials. I don‘t think it is particularly necessary, and if I were doing it again I might consider just gluing triangular section wooden fillets into the four corners of the tabernacle and its (4th side) gate to fill the gap between the circular mast and the square tabernacle, as snugly as possible. This much simpler method is, I believe, how a wooden tabernacle might be done on the inside. I have done it this way on the recent smaller tabernacle and it was much easier of course, and I can’t see anything wrong with just doing that.

    Similarly, the above description of making a square block at the heel is also not the easiest way to do it. Easier to start with a square block of wood, and turn one end in a lathe so it is round and can be fitted into the bottom of the mast. Here is a wooden square-to-round heel plug with a drainage hole drilled through it.

    However you do it, the mast needs to be not only supported but also fixed so it can not jump up, or rotate.

    As you say the mast is about 30kg, I am hoping it will be possible to walk it up to the point where the halyard can be routed through the bow-roller  to complete the lift.

    My recent mast is about 30 kg and a little taller than yours – without too much difficulty I can walk it up all the way to vertical, without needing any pull from the bow. (By the way, it gets easier as you get closer to vertical, so if you can get it to a point where you can rout the halyard through the bow roller - then you are almost there, and won't need to.)

    Warning For anyone using a tabernacle or any system of raising a mast - especially trailer boats - ALWAYS look aloft first before raising the mast. Overhead power lines are ubiquitous, we tend not to notice them. This type of fatal accident has happened in the past.

    Last modified: 10 May 2019 04:53 | Anonymous member
  • 09 May 2019 10:28
    Reply # 7330954 on 7325237

    Graeme, thanks for taking the time to respond.

    I must confess that I like the simplicity of your weld-less design.

    In actuality Selway/Fisher also suggested the Wherry approach, with a large "centreboard-style" case for the mast inside the cabin. I concluded that this approach was just too obstructive, and having a long slot through the fore-deck seemed to work against the desire for structural strength.

    So, in the end, I opted for a tabernacle that sat above the cabin roof with a forward-facing open box inside the cabin to provide the necessary depth of bury, with the steel "knees" supporting this, and a square "pole" welded to the bottom of the box set into the hog. This means I am not relying on the cabin structure to take any of the supporting forces - these being transferred from the tabernacle to the deck clamp and transverse deck-beams by aforesaid knees. The net being that my solution was (as you suggest) influenced by the original wherry-based proposals!

    In this way I can just have a short slot in the cabin roof, which I agree would be best sealed with a mast boot along the lines of Arne's suggestion.

    I agree that the C piece blanks will cost some to be cut, but I have the engineering facilities to machine the "sub axles" directly from the blank and arrange the securing bolts etc., so not too bad.

    I like the idea of the resin cast internal packing. Was this a filled-epoxy item or machined from a solid piece? I see in your album a picture entitled "urethane failure", was this an earlier/aborted attempt to make these blocks? 

    As you say the mast is about 30kg, I am hoping it will be possible to walk it up to the point where the halyard can be routed through the bow-roller  to complete the lift.


    Thanks again, Malcolm



  • 08 May 2019 00:35
    Reply # 7328413 on 7325237

    Malcolm I hope you don't mind me coming on to this thread, because I can't actually answer your question. But I am thinking that your idea of C-clamps with stub axles sounds rather expensive and complicated, and may not be necessary. 

    Most people are familiar with the fascinating system used by some of the old wherries and barges in England to allow lowering a heavy mast with minimum of effort, to shoot under bridges. The part of the mast below the fulcrum was ballasted and it swung up through an opening in the foredeck - features which are both probably out of the question in most modern yachts today. But the image has left us with a mind-set which has re-ocurred over and over again in nearly all the tabernacle designs we see - that the opening of the tabernacle must face forward, and there must be a pin through the mast, allowing it to rotate about an axis which is some distance above the heel. Neither of these things is actually a necessary part of a tabernacle, and a pin through an aluminium mast, as you know, is undesirable. I wonder if your "solution" has arisen as a result of this familiar paradigm.

    David T has pointed out that a tabernacle may, with advantage in some cases, have its opening facing aft. In that case there is no need for a pin or hinge of any kind, as when raising or lowering, the mast rotates about its heel, and is held in place by the socket which the tabernacle naturally provides. Nothing could be simpler. The axis of rotation is now at the bottom of the mast instead of just a little way up, but that barely affects the leverage required to raise an unballasted mast. Provided you have enough clearance for the tabernacle to give full "bury" above deck, this system might suit Malcolm because there seems to be no obstacle such as raised wheelhouse aft of the tabernacle, which would otherwise complicate things a little (and then the forward-facing tabernacle might make more sense.)

    The following may be of less help to Malcolm but I am placing it here because it might provoke some more ideas for Malcolm to consider. Scott Y recently posted on another thread: "If I recall correctly you were also fitting an aluminum tabernacle to your boat without any welding. I would appreciate it if you could take some photos of the tabernacle setup and provide some more details on how it was fitted to mast step, partners and the mast."

    The aluminium tabernacle was for a little motor sailer, and was simply a sheet of aluminium folded into a top hat section. In New Zealand, having it made this way by the local engineering shop proved to be a cheaper option than purchasing the quantity of good quality timber required and making it myself. No welding necessary. The scantlings, or thickness of the material, was equal to or slightly greater than the wall of the mast it would enclose. (That useful criterion was suggested by Pete Hill in an an article he wrote about wooden tabernacles.) A "fourth side" bolts on to the open face, afterwards, in the obvious way, and may be a structural necessity.

    null

    Some internal packing is required to house the circular mast inside the square tabernacle, and I chose to make a resin casting, although triangular section wooden fillets would be easier and probably just as good. The plan was to have the tabernacle with the opening facing forward, and to have the mast rotate back over a pin which does not go through the mast but passes behind it. It is fixed to the upper rear wall of the tabernacle. When raising or lowering the mast, it would be simply belayed to the horizontal pin with a temporary rope square lashing, which will rotate with the mast, about the pin. The mast and tabernacle are made and ready to deploy, but I must confess I have not yet installed and tried it in practice, because the project has been delayed while I try out some of these ideas on a smaller scale, on a little trailer yacht.

    Accordingly, I made another folded aluminum top-hat tabernacle, this time smaller, and slightly rectangular rather than square. The rectangular section allows a little fore-and-aft "wriggle room" should the rig need to be moved forwards or aft slightly, simply by altering the internal packing. It would also allow a little adjustment of mast rake if required. The intention was again to have it facing forward, with a fixed rear pin and a rope lashing for a hinge. However, Marcus took one look at the proposal and pointed out a much simpler alternative, which probably will not be possible in most boats, but works on this little boat because the little cabin has an open top. It was pointed out that a small slot cut in the open cabin top would allow the mast to be heel-stepped in the conventional way, and simply walked up over the heel in the manner suggested by David T.'s aft-facing tabernacle idea. In fact, in this case, a tabernacle as such would not even be necessary, but I have used the aluminium tabernacle anyway, because it was already made and it does still make the operation of raising and lowering the mast a little easier. Here is a picture of the little tabernacle, dry-fitted in place.

    To answer the rest of Scott's question, I glued plywood onto the three sides of the tabernacle where it goes through the deck, offered it into the slot, and then fitted a rectangular wooden partner around it which is glued and fastened to the fibreglass deck, and also glued to the plywood facing of the tabernacle. It was all fairly quick and easy to do. This little boat won't be stiff enough to put too much loading onto the partner, and the fibreglass layup is relatively heavy in that area. If a laminated beam and knees prove over time to be necessary, they can be added.

    To raise the mast, the heel is simply placed into the tabernacle then the mast is walked up until it is vertical and then closed off with a gate which bolts across at the aft face of the tabernacle and also completes the aft part of the partner. It is simple, quick to deploy and works very well. The heel of the mast is square so it can not rotate, and it is pinned in place down there so that it can not jump out. The mast itself is more than 6m in height, and weighs about 30kg. A bigger mast may require a lifting apparatus, as would be the case with any tabernacle. Malcolm's mast is relatively heavy, but it is less than 6m tall so quite possibly he might get away with just pushing it up, as I (an old-age pensioner) can do without too much difficulty with the above little set-up.

    If necessary to make the whole thing leak-proof (which I don't need to bother about in this case) the answer would have to be a tailor-made fabric skirt which is duct-taped to the mast and flashes over any openings - a fairly conventional idea which is often used on through-deck masts, and Arne has documented a very good pattern for a round opening, in one of his many practical writeups (here); that idea could be adapted to the rectangular tabernacle. I guess something like this will always be necessary for a "sunk tabernacle", but, of course, if there is enough room for the working part of the tabernacle to be entirely above deck (as in Annie's new boat) then there can never be a problem with deck leaks.

    I am sure there are better ways, and I hope the above bounces a few ideas for Malcolm.

    Last modified: 08 May 2019 05:55 | Anonymous member
  • 06 May 2019 18:18
    Message # 7325237

    I am building a Junk Rigged Steam Yacht with a folding tabernacle. (3-sided, braced box with pivot near top, and set into the hog/keel).

    The mast is aluminium with base diameter of 177mm dia (of 4mm wall thickness) and 5.8M height above partners and ~620mm bury. Sail area circa 16sqM.

    My original plan was to put a 25mm pivot pin through the mast, but reflecting on this idea suggests that this would be a bad idea because of the stress point and weakness this would cause in the mast.

    So, my revised plan is to fit two "C"-shaped aluminium pieces around the mast and have "stub axles" attached to these to provide the pivot, and bolt the two pieces together to cause this to be tight around the mast.

    So, my question is how thick should these "C" pieces be? I am thinking 30mm. These are acting like the wedges in a normal mast partners, and I am wondering if this is enough to ensure I don't put "point loadings" on the mast and risk a kink/bend. My intent is to get this laser cut from plate, so thickness has large cost implications.

    ...I am sure this is a "how long is a piece of string" question, but it is the information I have :-/ 

    (If the above words do not make sense, then perhaps this post dealing with the steel work might help... the top part of the tabernacle is not attached in the pictures (it sandwiches the cabin roof) but can be seen on the floor in the picture of the galvanised parts (all in 5mm steel plate)....

    https://sy-befur.co.uk/2016/02/25/fuel-tank-mast-structure-and-galvanizing/

    thanks! 

    Last modified: 06 May 2019 18:20 | Anonymous member
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