carbon braced tabernacle

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  • 01 Jun 2011 03:04
    Reply # 609959 on 608802
    The Needlespar/Sunbird mast hinge is machined from two pieces of solid aluminium alloy bar, with a fork in one piece and a matching tongue on the other, just as you are proposing. The size, from memory, is about a 1" slot and a 1" pin, for a 4" dia mast. The tube that drops over it is not too tight, and you should also make a slack fit, by using a total thickness of plastic wrap of, say, 1/32" - 1/16".
    When the boat is on a trailer, you can have someone walk away forward on the ground with the halyard, whilst you support the mast athwartships; but if you do it afloat, you need an A-frame with pivot points in line with your tabernacle pin athwartships, to keep the mast under control, and to give you enough leverage. Take a line from masthead to frame, frame to bow. There are pictures of this in many old textbooks.
    I'd hesitate to say "just muscle it up", with a heavy wooden mast.
  • 01 Jun 2011 01:35
    Reply # 609829 on 609381
    Deleted user
    Mark Thomasson wrote:


    My guess is that it might not be as easy to slide up the carbon tube as you hope.  If you can live with something more bulky,  make as two halves bolted together (or wraped arounf with cord). If bulky, perhaps glass would suffice.

    Other questions are:

    how long?  perhaps at least 3x the diameter in each direction.

    support as to lower the mast.  The sunbird fitting is an expensive piece of engineered alloy.  Always seems a bit over the top when it does nothing onece the sleeve is in place.  That said there can be quite a force on the hinge.  Perhaps avoid the stresses by having soming with a bit of give, ie  a webbing hinge.

    Mark Thomasson.


    Interesting points. Leaving all the rest aside for a moment, I have never before heard of the Sunbird fitting. Can you tell me more or point me at more information?
    I would be afraid of making in very bulky, especially of putting bolts on it, under the feeling that they would be snag points. Perhaps I could mold a pry lip around it just above deck level so that if it stuck down I could start it with a pry bar.
    I would think 3x diameter would be a minimum, I'm thinking probably 4.
    I'm still very vague as to how I will apply the necessary leverage to hoist and drop the mast. I have no illusions that I might be able to muscle it up and down.
    I don't get a picture in my mind when you say webbing hinge, and the hinge does concern me somewhat. Could you please explain that a little more?
    Jeff
    Last modified: 01 Jun 2011 01:38 | Deleted user
  • 31 May 2011 13:30
    Reply # 609381 on 608802
    Jeff McFadden wrote:
    Seablossom has a wooden mast bolted to the step and very secure.  It is also, at this time, glued to the partners with a horrible substance known as SparTite, but that will be gone when I bring her home and replaced with normal wedges.  This is all well and good - except I want to set her so that I can transport her by trailer once or twice a year without it being a major hassle.  Therefore I am considering cutting her mast in two in such a way as to create a tabernacle, preferably leaving sufficient "stump" out of the deck that I can slip the sail bundle down and not have to remove it.

    My vision is that I would cut the mast in such a fashion that I had two longish sides, or "ears", growing up from the stump, and a blade extending from the round portion of the upper mast that pivoted down between them.  If, as I believe, this is a hollow birdsmouth mast, I will also have to fill and brace the ears and the tongue, probably with wood cut to fit and epoxied in place.

    In order to give this structure sufficient strength to support itself underway, I would stand the mast up, wrap it in plastic sheet, and wrap that in carbon fabric, one wrap, one coat of epoxy, another wrap, another coat of epoxy, and so on until I had sufficient wraps to give me the strength I need, quantity tbd.  It is my expectation that I would taper the layers of carbon toward the upper end so as not to create a concentration of force that might tend to break the mast.  At the completion of the carbon - epoxy application one should be able to remove the plastic sheet and have a freely sliding carbon sleeve over the tabernacle.

    I envision this carbon sleeve slipping upwards to fold the mast.  Given that the mast tapers going that direction that function ought to be automatically available. 

    Comments, anyone?

    Jeff


    My guess is that it might not be as easy to slide up the carbon tube as you hope.  If you can live with something more bulky,  make as two halves bolted together (or wraped arounf with cord). If bulky, perhaps glass would suffice.

    Other questions are:

    how long?  perhaps at least 3x the diameter in each direction.

    support as to lower the mast.  The sunbird fitting is an expensive piece of engineered alloy.  Always seems a bit over the top when it does nothing onece the sleeve is in place.  That said there can be quite a force on the hinge.  Perhaps avoid the stresses by having soming with a bit of give, ie  a webbing hinge.

    Mark Thomasson.

    Last modified: 31 May 2011 13:31 | Anonymous member
  • 31 May 2011 01:54
    Reply # 608846 on 608802
    Yes, good thinkin', Jeff, that will work. I don't think you need to taper the top for stress concentration reasons, though, just if the sail needs to drop down over the carbon tube. When this kind of tabernacle is made with alloy tube masts, it is usual to wind a lot of duct tape around the top of the tube after the mast has been erected, so as not to snag the sail.
  • 31 May 2011 01:30
    Message # 608802
    Deleted user
    Seablossom has a wooden mast bolted to the step and very secure.  It is also, at this time, glued to the partners with a horrible substance known as SparTite, but that will be gone when I bring her home and replaced with normal wedges.  This is all well and good - except I want to set her so that I can transport her by trailer once or twice a year without it being a major hassle.  Therefore I am considering cutting her mast in two in such a way as to create a tabernacle, preferably leaving sufficient "stump" out of the deck that I can slip the sail bundle down and not have to remove it.

    My vision is that I would cut the mast in such a fashion that I had two longish sides, or "ears", growing up from the stump, and a blade extending from the round portion of the upper mast that pivoted down between them.  If, as I believe, this is a hollow birdsmouth mast, I will also have to fill and brace the ears and the tongue, probably with wood cut to fit and epoxied in place.

    In order to give this structure sufficient strength to support itself underway, I would stand the mast up, wrap it in plastic sheet, and wrap that in carbon fabric, one wrap, one coat of epoxy, another wrap, another coat of epoxy, and so on until I had sufficient wraps to give me the strength I need, quantity tbd.  It is my expectation that I would taper the layers of carbon toward the upper end so as not to create a concentration of force that might tend to break the mast.  At the completion of the carbon - epoxy application one should be able to remove the plastic sheet and have a freely sliding carbon sleeve over the tabernacle.

    I envision this carbon sleeve slipping upwards to fold the mast.  Given that the mast tapers going that direction that function ought to be automatically available. 

    Comments, anyone?

    Jeff
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