Another write up by Arne Kverneland

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  • 19 Sep 2018 17:58
    Reply # 6676909 on 869421
    Anonymous member (Administrator)

    Chapter 6, The (wooden) Mast

    Now, while I was 'up at speed', I had a go and added som newer info to Chapter 6, and I also gave the SA/Displacement write-up a make-over.

    Arne

    PS:
    (when the links dies, just look up http://www.junkrigassociation.org/arne   )



    Last modified: 19 Sep 2018 19:38 | Anonymous member (Administrator)
  • 17 Sep 2018 17:42
    Reply # 6673081 on 869421

    Hear, hear,

    a supporting vote from one summer's experience (and no theoretical understanding). I have an aluminium/aluminium mast of 6+3,2 meters and 600-700mm bury, glued using grp belts and epoxy. 

    I have not been kind to my mast on this first season, but I have had zero doubt about the integrity of the joint.

  • 17 Sep 2018 16:34
    Reply # 6672947 on 869421
    Anonymous member (Administrator)

    Here is the updated draft, version 20180917A. See p.6 with the new 'rule' about the top-mast's bury.

    A.

  • 17 Sep 2018 14:09
    Reply # 6672689 on 869421
    Anonymous member (Administrator)

    James,

    An engineer’s job is to make use of the scientists’ basic data and formulas, and try to make useful rule of thumbs out of their information  -  plus from practical experience. This lets them later design things much faster than if they had do go into 'deep science mode' all the time. I can imagine, when real new ground has to be gained (like when building the Concord SST) that scientists and engineers had to work hand in hand. We are not there with our junkrigs...

    Engineers often get it right, but it has happened that bridges, houses, aeroplanes and even sailboats’ masts have fallen down because the formulas and rules have been stretched too far.

    Still, I feel confident that my home-made mast joint rule is conservative enough to be safe. Thanks to Graeme K, who did that testing of epoxy on aluminium, I am sure the mast will not suddenly ‘go telescopic’, even with no screws or bolts added.

    Cheers, (..and cheers and good luck to Mark...)
    Arne

     

     


    Last modified: 17 Sep 2018 18:08 | Anonymous member (Administrator)
  • 17 Sep 2018 13:42
    Reply # 6672681 on 869421

    Arne, great guide, just what is needed,  I will sit down with a whisky one night to have a go at the calculations!

    Tusen takk. 

  • 17 Sep 2018 11:50
    Reply # 6672561 on 869421

    Thanks, that makes sense.  I wasn't expecting "high science" but really what gave you a feel that this was the right amount.  Unfortunately I'm no engineer, so I was quite nervous designing my mast. 

    I'm trying to think what the failure modes are.  What I can come up with is:

    (1) topmast breaking/deforming at the top of the main mast

    (2) topmast trying to pivot around the top of the main, and deforming the main in the area of your lower GRP belt (heel of the topmast pushing out the alloy tube of the main)

    (3) topmast trying to pivot around its base, and deforming the top rim of the main mast, effectively ripping apart the alloy tube

    (4) differential flexing between top and main, leading to failure of the glued joint

    Any other failure modes? 

    I think the PJR formula is against (1), as the partners are assumed to be solid and immovable.  I think I was more worried about (3) and (4).  One reason I chose to have a close-fitting topmast, glued the entire length of the overlap, was to try to make them as much as possible behave as a single, solid unit.  (Another reason was that I didn't think of using GRP belts!).  I agree with you that the longer the overlap, the greater protection against all of those failures.

    Sorry if I seem to be thinking aloud at you - I'm finding it very helpful to understand what's going on with a hybrid mast!


  • 17 Sep 2018 09:20
    Reply # 6672408 on 869421
    Anonymous member (Administrator)

    James,

    there is no high science behind my suggested bury of 4-5 times the inner diameter of the lower section. Still, if we regard the top section as a balance scale with the ‘buried’ end as one arm, it is obvious that the contact forces between the upper and lower section will double each time we half the length of the bury. At some point the material, aluminium or wood, will be deformed.

    Only after I had decided on the bury of Ingeborg’s top section did I realise that this bury also would exceed 10% of the length of the top mast. I guess I will edit the write-up to say that the top section’s bury into the lower section should be..

    ..at least 5 times the inner diameter of the lower section and at least with a 10% bury...

    Arne

     


    Last modified: 17 Sep 2018 09:20 | Anonymous member (Administrator)
  • 17 Sep 2018 08:31
    Reply # 6672302 on 869421

    Thanks, Arne, for another excellent technical guide.  I don't know about armchair reading, but it's a good breakfast table read!

    I'm interested in your guidance for the amount of bury between the topmast & main part.

    The mast for River Rat is very similar to what you describe, but I made the topmast from aluminium tube half the diameter of the main part, sheathed in wood:  

    I didn't know how much overlap to allow. My first thought was that if you regard the topmast as a mast, and the main part as a tabernacle, then I should use the PJR suggestion of at least 10%.  The length of the top part above the main part is 3m, so should be 30cm.  But I got nervous, as I wasn't sure how the forces would affect the 100mm dia tube of the main part, so went for a full 1 metre.  In hindsight, this is far too much, creating a lot of extra weight aloft.

    Your rule of thumb suggests a bury of 4-5 times the inner diameter of the lower tube.  This would give 40-50cm.  This feels about right to me.  But I'm wondering whether there's any science behind your suggestion? 

    (I should warn you I have some particularly mad mast ideas for my next conversion - but they're best kept for another thread!)

    James


    Last modified: 17 Sep 2018 08:33 | Anonymous member
  • 16 Sep 2018 19:15
    Reply # 6671518 on 869421
    Anonymous member (Administrator)

    The Cambered Panel Junk Rig (TCPJR), Chapter 6b,
    The Hybrid Mast (aluminium-wood)

    I just uploaded a draft of Chapter 6b, The Hybrid Mast to 'my page'. I decided to make it as a separate chapter instead of adding it to the end of Chapter 6, which is about the wooden mast.

    I am afraid this is not easy armchair reading. One should be quite keen on building a mast to bother with it. No great prose, but hopefully helpful to keen wannabees. In the end, the chapter will be proof-read and any un-clear points will hopefully be put right. Have a look.

    Arne 

    Last modified: 16 Sep 2018 19:34 | Anonymous member (Administrator)
  • 05 Sep 2018 15:25
    Reply # 6654639 on 869421
    Anonymous member (Administrator)

    A (final?) appendix on my Fan-up Preventer (FUP) write-up

    After having played around with my FUP on my present boat, Ingeborg, and after final tests of it yesterday, I have written some illustrated notes on it and added it to the end of the FUP write-up of 2014. I think I have solved the friction problems with it. At least it works well on Ingeborg.

    If you have any comments, please jump to the Ingeborg topic as it was there the FUP was last discussed, early this year.

    Cheers,
    Arne

    Last modified: 05 Sep 2018 19:24 | Anonymous member (Administrator)
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