Main-less rig

Discussion in 'Multihulls' started by Spiv, Feb 10, 2008.

  1. Nordic Cat
    Joined: Jan 2008
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    Nordic Cat Senior Member

    I agree with you Phil.

    That is why I have chosen to use a bi-rig design with fully rotating unstayed masts, so the scene from the Lagoon described above will not happen. Let go of the main sheet, the sail and mast will rotate to feather, drop whatever sail you want, tighten the sheet and be on your way!

    All this without loosing any performance, and by splitting the sail area in 2, you get manageable sail sizes. Yes, use a powered winch, or design it so you can use the anchor winch to raise the main.

    For a tri, I would consider a Ballestron rig instead.

    For Stefanos requirements, I would suggest 2 A-masts, one on each hull, with a furling headsail. Shorter masts = easy handling. An alternative could be a hinged mast on each hull, with stays down to the hinge that also rotate. Then a forestay and aft stay, maybe a bit to stiffen the mast and keep it in column. Even lighter and better performing.

    Someone mentioned "slot effect" - this myth seems to perpetuate - can we agree that there is no slot effect, either on a fore/mainsail or a bi-rig boat!
    Flow goes from high to low pressure, and a sail is 3 dimensional.

    Regards

    Alan
     
  2. brian eiland
    Joined: Jun 2002
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    brian eiland Senior Member

    You are certain of this statement, are you??
    It’s really difficult to believe the number of accomplished sailors who still do not fully understand the ‘slot effect’

    I might suggest you have a look thru some other material I've presented in support of this 'slot effect' and how many known sailors can not properly define it. The material I present is supported by some very respected persons.

    Same Old 'Slot' Misconception
    http://boatdesign.net/forums/showpost.php?p=83339&postcount=121

    JavaFoil reference:
    http://boatdesign.net/forums/showpost.php?p=89766&postcount=148

    ..some pictoral references:
    http://www.boatdesign.net/forums/showpost.php?p=110983&postcount=24



    http://boatdesign.net/forums/showpost.php?p=5685&postcount=3
    http://www.arvelgentry.com/techs/The%20Aerodynamics%20of%20Sail%20Interaction.pdf
    http://www.arvelgentry.com/techs/A%20Review%20of%20Modern%20Sail%20Theory.pdf


    ...maybe just do a "search" on this forum with the search button above for "slot effect"
     
  3. yipster
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    yipster designer

  4. MAINSTAY
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    MAINSTAY Junior Member

    I second what Pericles wrote:
    "Although the A frame mast may have more drag, the main sail running up the vertical wire luff will not suffer the 30% wind shadow effect of a conventional mast and better performance from the sail area should be the result." And add that a half A-frame would reduce drag significantly. The half beam of a tri is on the same order as the beam a mono. The lateral base, between the beam-end shroud and a centerline mainsail luffstay, is about the same as between chainplates on a mono, so does not have unusual design parameters.

    And if Brian Eiland's mast is stepped about twice the distance from the transom and raked aft so the mast head is above the aft vertical stay, his design would not need a Y backstay or jumper.

    If this forward step is done on Yipster's rig, the rig is lighter and simpler, but the overhang is increased for mast lowering. The discussion about cabin interference is interesting, but perhaps a lowered mast could be horizontal with headroom or higher to act as support for an anchorage awning for sipping in comfort.

    If the stayed mast is wimpy when lowered, check the rotation at the chainplates. If it is fore or below the hinge of the mast, it will get looser and wimpy. And if aft or higher, may get too tight and overstress something. But if the the rotation points are in line with the hinge, the rig will be stable going up and down.
    Larry Modes
     
    1 person likes this.
  5. yipster
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    yipster designer

    ok i'll step fwd, raking rig back you mean?
    half an A frame, drag lighter and simpler i dont see
    plz elaborate a little or add a rough doodle
    for the drinks i figger a bimini better than mast
    but ok, balljoints on longt adjustable plates?
     
  6. MAINSTAY
    Joined: Feb 2005
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    MAINSTAY Junior Member

    Half A-frame mast

    The attached Boat10Dwgs.xls file shows a beam and headon view of the half-A-frame with a mainluff stay on a trimaran and a catamaran.

    In both cases the max stress in the cross beam supporting the maststep is reduced since the step is moved close to the hull side which has or can have deep-beam strength and stiffness. The stress in the crossbeam for the forestay is unchanged. The stress in the crossbeam for the mainstay is up, where normally this supports the maststep and is down.


    The attached BtFm3.xls file shows that the mainstay rig can have the mainluff stay: 1)near the transom like a mizzen, 2) with a mid-boom mainsheet to the tramsom, 3) similar to a current sloop, or (not shown) any where between and beyond.

    It should tack like a rig with a staysail and swept-back shrouds. But, with less chaffing during tacking, no running backstays, and little chance that the boom or main will rest against a shroud while running.

    Larry Modes
     

    Attached Files:

  7. masalai
    Joined: Oct 2007
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    masalai masalai

    I fail to see any advantage in setting the mast to one side? Leave the mast midships but further back or raked and run the sails from the bows of the cat or tri hulls... John Hitch has it all figured out & it works... Good enough for me.
    Cheers...
     
  8. MAINSTAY
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    MAINSTAY Junior Member

    Masalai,
    The rig leaves the masthead in the same location, and the mainsail is not on the mast, but on a mainluff stay, therefore the sail plan is unchanged. If your foresails go to the bows, that is unchanged.

    There are several advantages to a canted rig on a multihull.
    1) It gets clear air for the mainsail luff, increasing the drive of the sail.
    2) the step is at or close to the side of a hull. The side of a hull is a deep, stiff beam, or could be made so, for better stiffness of the rigging.
    3a) On a catamaran, the mast step is not at the weakest point of the crossarm.
    3b) on a tri, the mast is not blocking the accomodations in the main hull.
    4) it reduces the complexity of the rig, by eliminating the backstay and one shroud, which means fewer parts to maintain or fail, less windage, less weight aloft.

    Larry
     
  9. Spiv
    Joined: Jun 2007
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    Spiv Ancient Mariner

    OK. but then why not have an A frame, it would be a lot more rigid, the whole rig will be on compression, so the forces on the hulls will be the same.
    Additionally, the A frame is very easy to lower, giving you access to rivers and canals with low bridges.
     
  10. Pericles
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    Pericles Senior Member

    Rather than an "A" frame mast why not give consideration to a catenary arch mast? The catenary is the ideal curve for an arch which supports only its own weight. When the centerline of an arch is made to follow the curve of an up-side-down (ie. inverted) catenary, the arch endures almost pure compression, in which no significant bending moment occurs inside the material.

    http://en.wikipedia.org/wiki/Catenary


    Regards

    Pericles
     
  11. masalai
    Joined: Oct 2007
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    masalai masalai

    I like that shape for a mast - it looks quite "sexy" - what looks good is good? - - For a "hitch-hiker type rig, I suppose the stays would hang below the midpoint - rather than a "masthead cap" type of assembly... - Thanks Pericles - I must sit and quietly think about this one...
     
  12. brian eiland
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    brian eiland Senior Member

    I'm sorry, I do not see enough support (backstay, or aft lead shroud) to keep good tension in the forestay. Am I missing it?
     
  13. brian eiland
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    brian eiland Senior Member

    Hello Stefano,
    Are they still building this vessel you posted photos of?? Isn't it about the size vessel you were seeking?
     
  14. MAINSTAY
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    MAINSTAY Junior Member

    Spiv wrote
    There is no reason that an A-frame mast could not be raked aft and used with a vertical or near vertical aftmost stay. I believe that that could be soundly designed.

    But consider:
    1) An A-frame rig has almost twice the parasitic drag of the half-A-frame rig. The shroud that replaces one leg has some drag, so the difference is not quite twice. The additional drag means that the A-frame will not be as fast nor point as high.

    2) What does the stay tension need to be to prevent the windard mast from going into tension? Does this tension preload of the masts require additional measures to prevent column failre in the masts?

    3) Why not allow the windard mast to be in tension? In most rigs the mast has a larger cross section than any stay on the same rig. The larger cross sectional area of the mast would better resist the strain. In addition, a base that's twice as wide, generally means the forces in the lee mast are half as much.

    4) How are the forces the same on both hulls, except at rest? In a sloop rig the mast is in compression, the winard shroud is in greater tension, and the lee shroud is in lesser tension. In an A-frame rig, the lee mast is in compression, the stays are in tension, and the windard mast may be in tension or compression.

    The primary advantage of the A-frame or half-A-frame is that the mainsail luff gets clean air. The Procydon, and other A-frame rigs, claim around a 15% increase in speed. That is such a desireable gain, I do not understand sailors who spend money on computer-designed laser-cut exotic-material multi-ply sails to get a 5% increase, when for the same money they could get three times the gain.

    Larry Modes
     

  15. brian eiland
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    brian eiland Senior Member

    When I first looked at this, my reactions were that there was insufficient support to the mast sections being pushed forward by the 'V' aft jumper, and that jumper did not bisect itself to the backstays.

    Normally we ask the ends of spreaders and jumpers to intersect the rigging wires in a manner than equally bisects the total angle. In this way we are not further asking the spreaders or jumpers to also carry big bending loads caused by unequal angles and resulting moments. But rules are there to be broken on occassion, and this aft V jumper might have to be built to absorb this extra bending as it will also see unequal forces in the 'discontinous backstay(s)' loading, as well as the forestay loading by the staysail. Interestingly the geometry will allow for a cross brace between the two mast legs at this 'hounds' location without interfering with the forestay of the staysail. So you would end up with a more robust triangular structure at this hounds location. this might even counter some twist loading that could occur here.

    It would be interesting to 'map' this rig configuration and play with sutble modifications.

    You still need forward jumpers of some sort to counter the push of the aft jumper structure.
     
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