Aftmast rigs???

Discussion in 'Sailboats' started by jdardozzi, May 28, 2002.

  1. CT249
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    CT249 Senior Member

    I've got my own copy of the memoirs.

    It's been explicitly said by people like Stearns since the '70s that a mainsail forms part of the structural support of a mast. That's one reason many offshore racers do not like sailing under storm jib alone.

    The weight of the mast alone was obviously not a huge issue, because Hoyt felt that once the mast had been moved forward and given a conventional rig, the boat was as fast as the competition.
     
  2. brian eiland
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    brian eiland Senior Member

    Let me ask you a question. Assume an analogous force-beam loading configuration.

    Lay that mast (beam) out horizontally as a cantilevered beam with one end stuck in a wall (the deck). Now pull upwards at the tip just as the forestay does. And then in an analogous situation load some distributed weights onto the beam (outwardly from the wall) just as the mainlsail pulls on the mast tube as it rises from the deck.

    Now ask yourself whether that beam (mast section on the horizontal) with the those extra weights has to be stronger than one with no added weights (no mainsail)??

    Now 'point loading' of the mast tube can be a different situation (where most of the loading is by a single or multiple forestays and backstays). If these are not lined-up in certain manner, then the mast tube itself has to be stronger to resist extra bending loads resulting from 'out-of-direct-alignment of the staying configuration. And if your mast is made of wood (likely for that boat), and desired NOT to be 'bendy' as most in those days were, ....then you need to build it heavier. And likely of a larger sectional shape, ....which would have been detrimental to the smaller mainsail's performance,... and a real drag factor up above the head of the mainsail (bare mast section).

    Furthermore, likely that mast section was NOT of a more aerodynamic shape that we get with modern materials, but probably a rectangular section characteristic of wooden mast of that era.
     
  3. sharpii2
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    sharpii2 Senior Member

    A better analogy would be to stand the mast up in a warehouse, put a huge weight on top, then run a bunch of strings, spaced evenly apart, along its length, to to the walls of the this warehouse. These strings would only be oriented for and aft. Along the side of this mast would be the usual set of shrouds and spreaders.

    Basically, the way I see it, is the sail cloth prevents the mast from buckling forward, as its attached to the boom and has some leech tension. So if the mast starts to bend, the sail flattens a bit until it can flatten no more. At this point, the mast is prevented from buckling further in that direction.

    The lift that the sail cloth provides arguably helps prevent the mast from buckling aft. But the main reason it doesn't buckle aft is because it is encouraged to buckle forward a bit, in the first place, and once buckling in one direction, it is very unlikely to buckle in the other direction.

    The thing about buckling is it takes a relative weak force, perpendicular from it, to stop it in its early stages.

    With a jib only, or a large jib and a very small main, The mast must be prevented from buckling at all. Hence it needs sections that are longer than normal, if not deeper too. So the mast begins to resemble a stove pipe, or wing. One possible solution is to run a second set of double lower shrouds up to the second set of spreaders, even if they have to attach to the same chain plates as the original set does.

    It is for this reason, IMHO, that mast aft and jib only rigs are never as structurally efficient as fractional ones are.

    Structural inefficiency usually means greater weight for the same SA. And this is absolutely fatal in competitive racing, except in situations where the CE of the rig must absolutely be moved aft.
     
  4. CT249
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    CT249 Senior Member

    /\ You nailed it, Sharpii.

    Brian, many racing masts in the era were pear shaped; see Uffa Fox's books. Others were oval shaped with heavily rounded edges. And to repeat myself, if the weight of the mast was such a problem with Atrocia, why did the boat perform well with the same mast in a conventional rig?
     
  5. Richard Woods
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    Richard Woods Woods Designs

    Long before that in fact. I remember being told in the 1960's that the OK mast needed the support of the sail, else it would break (I knew the worlds top wood OK mast maker and also one of the worlds best OK hull builders at that time)

    Richard Woods
     
  6. brian eiland
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    brian eiland Senior Member

    Before I proceed with this current question, I would ask either of you 2 gentleman if these were 'purposeful bendy mast' utilized in that wooden mast era we are talking about??
     
  7. brian eiland
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    brian eiland Senior Member

    Here is a few photos of another 6-meter of that era, and I am guessing we can assume Atrocia's mast was stayed somewhat similar
    http://www.6mrnorthamerica.com/clytie2.html

    Interestingly I also found this
    http://www.6mrnorthamerica.com/noreg.html
     
  8. brian eiland
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    brian eiland Senior Member

    Last edited: Mar 1, 2017
  9. CT249
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    CT249 Senior Member

    No, Sixes were probably not bending their masts as we do at that time.
     
  10. markstrimaran
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    markstrimaran Senior Member

    150% geona. aftmast, 24.5" Hobbie cat donor

    Just getting set up for the 2017 season. The debate continues. It's a 18" aft masted, trimaran, built to fish.
     

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  11. brian eiland
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    brian eiland Senior Member

    Sorry, suffered a total Hard Drive failure on my laptop while visiting Thailand,...lose a lot of data, photos, links etc. just got back to USA, so I will try to get reorganized,...after a lot of home projects.
     
  12. brian eiland
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    brian eiland Senior Member

    Wishbone Boom & Straight Standing Aftmast Thoughts

    I've recently run across a few of Catsketcher's postings, ....and I'm going to look for more. I found his use of a wishbone boom very interesting. I've sought to utilize one on the mizzen sail of my aftmast rig.

    Perhaps I need to relook at the possibility of utilizing an aft-mounted, straight-up-standing, wishbone controlled mainsail in place of my mizzen,....so of a modern version of the old Prout rig.


    http://www.boatdesign.net/forums/multihulls/open-bridgedeck-catamaran-design-53066-2.html#post734669



    http://www.boatdesign.net/forums/multihulls/open-bridgedeck-catamaran-design-53066-2.html#post734731


    [​IMG]
     
  13. markstrimaran
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    markstrimaran Senior Member

    storm rig

    Early season rig,
     

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  14. brian eiland
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    brian eiland Senior Member

    Gear Required to rig a Mainsail, and reef it

    This page (67) of this subject thread has a number of postings related to dealing with full battened mainsails.
    Page 67:
    http://www.boatdesign.net/forums/sailboats/aftmast-rigs-623-67.html

    Then just recently I ran across this video...
    Seawind Catamaran 1160 Reefing Explained

    Made me think back about the large numbers of gear we need to add to our boats to just rig a conventional mainsail,...and then the subsequent amount of extra gear to handle reefing it. :eek::rolleyes:
     

  15. brian eiland
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    brian eiland Senior Member

    Praise from a Crusing Sailor

    Ever once in awhile I receive a note like this one from a few days ago, on another forum, that buoys my spirit. And at my age right now, and having fought this battle for a long time, I need this sort of encouragement to continue on with this 'battle'.

     
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