Carbon mast to replace tabernacle

  • 29 Apr 2021 21:28
    Reply # 10405365 on 10348866

    All that is required is to put a good lashing around yard, sail and battens.  The parrels allow the sail to go along the mast quite happily once it is lowered. On a large boat, the sail bundle will be heavy and awkward to work around, of course, but that is a different issue.

    1 file
  • 29 Apr 2021 20:37
    Reply # 10405034 on 10348866

    I don’t see why the sail and lines would not drop and sit over a well designed three sided tabernacle...of course I have not done it, so maybe missing something.

    it would be my solution....when (if...!) I get around to it.

  • 28 Apr 2021 03:01
    Reply # 10378657 on 10348866

    The flaw of the three sided tabernacle is what to do with the sail, parrels and lines when stowing the mast. If the sail is to be kept rigged to the mast, then inevitably it will be crushed and stretched since it is designed to flake horizontally behind a vertical mast.

    If the JR control lines led though guides at the lower end of the upper section were kept somewhat tensioned while raising the upper section, easing them as the gap widened, the upper section would not swing greatly when it came free of the lower section.

    Similarly, these lines could be used to produce a close approximation when lifting the mast back into place, and the tag lines used for final adjustment. Attention to the joining/stiffening sleeve (such as a tapered tip) could further ease the process. And a simple carbon tube, unadorned with the spreaders and stays of Bermuda rigs, is a far simpler item to guide, especially if it’s 40% lighter than a comparable aluminum tube.

    Last modified: 28 Apr 2021 03:04 | Anonymous member
  • 26 Apr 2021 13:01
    Reply # 10358162 on 10348866

    The mast needs to be a reasonable close fit on the sleeve.  The chances of lining them up to slot over ...don’t think it will happen, and could damage the mast is attempting it.

    Why not make a three sided tabernacle as many have done already.  If a timber one would look to chunky, it could be in aluminium or GRP, with a curve to the front face. 

  • 26 Apr 2021 09:21
    Reply # 10356215 on 10348866

    Phil,

    I've often thought that your basic idea could be used - a tube permanently fixed into the boat, just as if it were the mast itself, over which the mast proper could be dropped - if there's a sound reason for wanting to do it. That tube, or an external tabernacle, or a conventionally stepped freestanding mast, all have exactly the same requirements in terms of reinforcement of the deck.

    But:

    • Aluminum and carbon shouldn't be put close together - they are at opposite ends of the galvanic table and the aluminum will corrode quickly.
    • The mizzen mast would be subject to a lot of bending, and the only way I can see to support it is an A frame on top of the wheelhouse.
    • Why does the mast need to be lowered and raised every year? Only if the winter is spent above a low bridge, or a boatyard insists on having the rig out for safety reasons, or something similar. Otherwise, the mast of a 40ft boat is usually left in place.
    • I can't see why a carbon mast is an essential part of the plan. It would seem much more reasonable to install a very thick walled aluminum tube into the boat, and then to drop an aluminum tapered flagpole over it, with suitable spacers to get the mating diameters right.
  • 26 Apr 2021 04:26
    Reply # 10353969 on 10348866

    Anonymous, you wrote "the lower section sounds like a tabernacle".

    I'm sorry I wasn't clear: the lower section is an aluminum pipe fixed at the hull and at the deck, ending with a inner section which mates with the bottom of the upper carbon tube. 

    As for the "professionally engineered and manufactured", I think this makes sense with Carbon Fiber masts. In this case, the strands required would be almost completely unidirectional on the long axis, but some consideration would have to be given to the abrasion the parrels would inevitably cause. Whether kevlar or other materials, the precise layup schedule, and myriad other details and quality of the end product would likely make the difference between this being a good idea & one that breaks and requires repair in some far off land.

    The intended boat is a 40' ketch, fitted out, as you surmise, something like a home. 

    The questions I am looking to answer are 

    1. Is this idea feasible at this size of boat? I've only seen stiffening sections join longer CF masts, and don't know the viability of this proposal

    2. What guidelines are there for stiffening sections? On the masts I've used the overlap was certainly not 10:1 as with tabernacles, although perhaps the length of the stiffening section itself was 1/10th the overall height of the upper section? 

    3. Carbon masts are excellent in compression. They are not as good at side loading. Does the 40% weight savings of usual CF masts necessarily translate to free standing masts for Junk Rigs?

    I've sketched out the basic proposal, and it did force me to think up some details. Thanks for the suggestion. For instance, at the bottom of the CF section, the lifting strop has to secure to a hardpoint in order to lift the top section. This hardpoint would need to be low profile (or removable) so as to not interfere with the sail and parrels.

    1 file
    Last modified: 26 Apr 2021 05:02 | Anonymous member
  • 25 Apr 2021 19:09
    Reply # 10349965 on 10348866
    Anonymous wrote:

    As a thought experiment, I'd like to pose the following.

    Drawings often help point out thought errors or improvements.

    There is a great deal of engineering, welding, materials and structural considerations involved with installing tabernacles for free standing JR masts. This is largely because the foredeck is bereft of structures to make such solutions fit with the purpose and aesthetic of most boats. 

    All of these engineering and material costs must be considered when considering the "price" of the mast.

    Most tabernacles I have seen have been wood, those of metal are generally made by people who have the tools already. So _costs_ is a word that is hard to define. Cash money, is for most of us, hard limited. So materials from the scrap pile, while having value, are free. Engineering, for most people is free too. They are already engineering their mast and sails. The time spent does have value but often it is available where cash is not. Trees for some people, can be cut and dried for no cost. Aluminum pipe is available ready to go and in some cases may be had for recycle prices.

    But assuming the mast and fittings are hired out:

    I have used several small boats which utilize two piece carbon fiber masts to permit easy stowage of a mast which can be raised by an individual for a trailer sailer. There is an internal aluminum sleeve over which the upper section that joins the two parts slides.

    And so my question: could the cost of an upper carbon tube be justified by creating a system using a mizzen mast which tilts forward to act as a crane which lifts the forward mast off a lower section braced at hull and deck partners?

    Why not use two batons to lift for a more generic solution?

    The lower section would be made of aluminum, and would be long enough to retain the sails, parrels and such of the sail when the upper section were removed.

    The lower potion in this case sounds remarkably like a tabernacle :)

    Obviously, the design of the two main mast sections and the sleeve between them would need to be carefully engineered and the mast professionally built. This cost would be either justifiable or not, and estimates would be crucial, but the result could be a way to avoid requiring a crane for a once or twice a year evolution.

    "carefully engineered and the mast professionally built." Also means impossible to repair on a deserted island... or maybe even not so deserted but lacking a carbon fibre builder.

    I think that a person used to working with carbon fibre would already use this for their mast because they are the professional builder and understand the material well enough engineer it.

    There are lots of people in these forums and not all of them are willing or able to trade time for cash. There are others who have the means to have various parts of their yacht built for them and not the time to do it themselves. For those people they would still need to do at least some preliminary engineering on both (or all three) kinds of mast just to get to the point where they would have enough information to find out what the parts would cost. So there is some up front cost (either time or cash) just to find out if it may be cheaper.

    My sense is that the missing part of the equation in the beginning is vessel size. For a 22 foot (6.75 ish meter) boat like the one I am working on, carbon fibre is just not in the cards... I started off with a free hull and intend to finish good enough for camping in. I am sure I can deal with things with no crane required... still not sure about the tabernacle :)

    For a 40 foot Ketch (I presume), the whole thing changes. No one is going to spend the money for a 40 foot vessel and _not_ finish it much more like a home. The size and weight of the mast(s) will be much bigger and the cost of the mast(s) while much more expensive than the 22 ft version, are a much lower part of the overall cost. Everything changes. Now the wood or aluminum mast cost has gone up, if a crane is needed the size of crane may be higher too though I expect the crane the fisher's use for lifting there load onto the local dock would lift a pretty big mast from the deck of a boat to vertical for a minimal cost... some places may not be so lucky to have something like that.

    So I think that there may be a particular size of vessel with a particular size of mast where carbon fibre may pay for itself. Smaller would be cheaper to use aluminum or wood and bigger may still need a crane even for carbon fibre. Each vessel would have to be assessed for it's own best solution.

    Last modified: 25 Apr 2021 19:11 | Anonymous member
  • 25 Apr 2021 16:51
    Message # 10348866

    As a thought experiment, I'd like to pose the following.

    There is a great deal of engineering, welding, materials and structural considerations involved with installing tabernacles for free standing JR masts. This is largely because the foredeck is bereft of structures to make such solutions fit with the purpose and aesthetic of most boats. 

    All of these engineering and material costs must be considered when considering the "price" of the mast.

    I have used several small boats which utilize two piece carbon fiber masts to permit easy stowage of a mast which can be raised by an individual for a trailer sailer. There is an internal aluminum sleeve over which the upper section that joins the two parts slides.

    And so my question: could the cost of an upper carbon tube be justified by creating a system using a mizzen mast which tilts forward to act as a crane which lifts the forward mast off a lower section braced at hull and deck partners?

    The mizzen would be on a tabernacle more easily supported by the coachroof of a 40 foot boat, and a halyard would pull upwards the carbon section of the main mast from its lower mate. The lower section would be made of aluminum, and would be long enough to retain the sails, parrels and such of the sail when the upper section were removed.

    Obviously, the design of the two main mast sections and the sleeve between them would need to be carefully engineered and the mast professionally built. This cost would be either justifiable or not, and estimates would be crucial, but the result could be a way to avoid requiring a crane for a once or twice a year evolution.

    This solution would have many limitations, especially the need for calm waters and several persons to facilitate safe handling of the mast once freed from the lower section. Regular lubrication of the joint would help prevent difficult extraction.

    Nothing is easy or perfect, and the user of this system would accept the constraints.

       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

                                                              Site contents © the Junk Rig Association and/or individual authors

Powered by Wild Apricot Membership Software