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Contessa 26 project

  • 27 Sep 2017 21:13
    Quote
    Reply # 5283126 on 5281941
    Anonymous member
    Gerrit Louw wrote:

    ...When reefed only the lower section of the mast will be "in use".


    No.

    Reefed, the mast still has the halyard and lifts attached to its head, and its own mass rocking around up there interacting with boat motion and sail tugs - it is very much in use.

    Scarf joints to make long timber, are traditionally made with the longest overlaps... in masts. In any material, go for the best possible continuity. Welds are by nature discontinuous. Sleeves help; long inserts (if timber-Al hybrid) help; easing of sleeve / timber ends  to avoid hard spots, larger areas of contact at partners...

    You're only going to do this once, right?

    Cheers, Kurt
  • 27 Sep 2017 12:17
    Quote
    Reply # 5282156 on 5271442
    Anonymous member

    Hi Gerrit,

    Spruce is traditionally a timber of choice for making masts and spars, though I seem to remember that there are different grades of spruce and "spar grade" was always specified for masts, old growth with close grain from memory.  You could always reinforce it with epoxy and fibreglass.  Otherwise, Arne's suggestion of inserting a smaller top section and glassing the joint makes good sense.  I understand that welding aluminium substantially weakens and hardens it, and is undesirable for unstayed masts, but perhaps an engineer could give you more specific advice.  I have a 35 sq metre sail and my mast is a spun tapered single section, 200mm dia at the partners and 110mm at the truck, with a 5mm wall, made in France.  It cost a fortune, $4000 Aus.  Probably oversized for my 24ft boat though it weighs 5 tons.  I often drive the boat hard and feel no anxiety about the rig.  My yard, boom and battens are also alloy, though I like wood, and often dream of a nice straight tree!

    Good luck with your project.  I grew up in Durban but left in 1972 aged 20 and have been in the South Pacific, based in Australia ever since.
  • 27 Sep 2017 11:17
    Quote
    Reply # 5282020 on 5271442
    Anonymous member (Administrator)

    Hi Gerrit

    The yield bending moment of that 168.28mm tube is about 1890kpm if I got my sums right. That should be nearly twice the righting moment of your boat, so looks ok.

    If you don’t like to use wood, I suggest you buy a 100 or 120mm tube for the upper part. What is leftover of that upper part could even be inserted in the lower end of the mast, but I would not bother with that unless the original mast turned out to bend more than you like (which I doubt it will do).

    To make the thin upper section fit into the thick lower one, I suggest you make two ‘waistbelts’ of grp-tape on the thin one before inserting it  (adding epoxy) into the lower section. The necessary fairing above the joint will also ensure that the upper section cannot suddenly disappear into the lower one.

    I have made an aluminium-spruce hybrid mast for my Marieholm IF, which is similar to your boat.

    Good luck!

    Arne


    Last modified: 27 Sep 2017 18:40 | Anonymous member (Administrator)
  • 27 Sep 2017 09:47
    Quote
    Reply # 5281941 on 5271442
    Deleted user

    Hi Kurt,

    Thank you for your input. This is exactly why I joined the JRA. I am still exploring mast construction ideas. To splice two aluminum tubes is by far the most cost effective solution I have found so far. Decent timber such as Oregon pine (Douglas Fir) is very expensive in Cape Town. Spruce is slightly more affordable than Oregon pine, however I am unsure whether Spruce is a good material to use. My theory concerning the spicing of two aluminum tubes is based on the fact that when hoisting the full sail only the top two panels will be above the position where the two tubes were joined. As the size of the sail will be roughly 35 sq. meters I think that this will only happen in fairly light winds, and therefore present less danger. When reefed only the lower section of the mast will be "in use". 

    I am not adverse to hybrid masts so I will explore that a bit more.
  • 27 Sep 2017 06:33
    Quote
    Reply # 5281633 on 5271442
    Anonymous member

    I can't lend my encouragement to the idea of a mast welded partway up. Nor to a spiral-welded mast. In a stayed rig, maybe, but not for a junk. That doesn't mean it can't be done, but I like the inherent sense in a vertically-one-piece mast, whatever suitable material it's made of.

    I know Paul Thompson here in NZ has experience designing vertically-seamed steel masts. (LaChica, Aphrodite/Sky...) Heavier, in steel, but he can calculate that relative to your boat. Any prospect for a timber alternative?

    Outside the one-piece principle, some people are using aluminium tube up to a point, (for you, 6m) then deeply socketing a timber topmast, well-fit, into that, and for boats about your Contessa's size. (Fantail, Tystie... and others?)

    In smaller boats, there are some hinging masts knocking around. Two-piece with a sleeve.

    Cheers, Kurt
  • 20 Sep 2017 16:07
    Quote
    Message # 5271442
    Deleted user

    Dear All,

    I am new to the association and junk rigs, but from what I have read I believe I am in the right place. :) I have a 1967 Contessa 26 (hull #16) that is turning 50 in October and instead of replacing the old original mast with a new one I want to convert her to a junk rig. I have bought Hasler and Mcleod's book and I have studied some of Arne's writing.

    I have been searching for an aluminum pole for a mast, but here in Cape Town the picking is dismal.  What I have come up with is as follows: 2 x 6 metre 6063-T6 alloy with a 168.28mm diameter and a 4.2mm wall thickness. I know this is not the best alloy to use, but it is all that is available that would be suitable size-wise. My plan is to have the two poles professionally joined, shortened, tapered and strengthened with a spine running length-wise inside the mast. According to my calculations the final mast should weigh roughly 75kg.

    For the sail I was thinking of something similar to Ingeborg's sailplan, however I would like to lengthen the boom and shorten the mast a bit. To support the compression of the mast on the un-cored deck I want to move the forward bulkhead aft to just forward of the new mast.

    Well that is all I have for now, if anyone can provide some insight at this stage I will greatly appreciate it. I know little to nothing about sail design and any comments or advice in this area would help a lot.

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

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