Main-less rig

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

  1. brian eiland
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    brian eiland Senior Member

  2. brian eiland
    Joined: Jun 2002
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Wishbone Aftmast Rig & Aftmast Rig Subjects

    Stefano,
    I know you had written asking about this wishbone shaped aftmast and its designer. Here is a subject posting and thread on that subject. Sorry I thought you were aware of it.

    Wishbone Aftmast
    http://www.boatdesign.net/forums/showpost.php?p=146538&postcount=51
    http://www.boatdesign.net/forums/showthread.php?t=1999

    ..and my latest single aftmast posting
    Aft-mast Development & Justification
    http://www.boatdesign.net/forums/showpost.php?p=198605&postcount=98
     
  3. Spiv
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    Spiv Ancient Mariner

    A-Frame Etchells

    I just found this article I thought I'd lost.
    It was printed in Yachting World in 2000; in about 2001 I wrote to the inventor and he said he was rigging a 40' cat with his system, he had a web site with some pictures, but it eventually disappeared from cyberspace.

    Some interesting points from the article:
    • Designer Bill Dixon and engineers High Modulus had an input
    • (Contrary to unstayed masts),[FONT=&quot]"With the A-frame system the main loads are kept within the A-frame structure and are not transferred into the hull in the form of high bending moments about the deck. The result is that we can build a surprisingly light but robust structure."[/FONT]
    • The masts sit on the original chainplates and can be raked simply to obtain good balance.
    • Once you adjust the sails for the wind speed, you just let the 'boom??' swing to maintain the best angle of attack.
    Has anybody heard of Mark Wagner?
    Did he really patent his rig?
     

    Attached Files:

    Last edited: Jul 1, 2008
  4. wittytutoi
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    wittytutoi New Member

    Hi everyone,
    I have just joined having come accross this site while looking for further info on bi-pole rigs. The idea is not entirely new of course. Some 30 years ago Sail Magazine ran a quite comprehensive article about the idea. They were talking about monhulls of course but there were some nice ideas.
    On the issue of weight. We are talking of plain round aluminium poles. Standard issue and cheap. If there is some doubt about sideways stress the poles can be sleeved in the middle (standard round tube again) over about 50% of their length. The stays can be rigid. There is such a large daysailing cat lying at Port St Johns in South Africa, with solid stays running up halfway the masts. The mainstay can also be rigid. All that needs to happen is for the triangulation to be balanced. No need to put excessive stress on anything. A gooseneck off the mainstay takes care of the boom, if you insist on such a thing. Essentially a balanced triangle is such a beautifull thing that numerous variations can be thought out. Another important point was/is the joining of the masts on top. Here the suggestion was to install some sort of a circular platform to which the two poles would be connected. It could be used to install all sorts of things; running lights, radar, reflectors and even an airbag to stop the cat from going turtle if capsized. To me the bipole rig seems extremely logical when compared to the one mast rig where on a monohull one is trying very hard to push the mast though the keel. Effectively on a cat the whole boat would "hang" from the rigging triangle. Obvously the design would have to be modified to accomodate the base of the triangle but I am sure any architect worth his salt could come up with a half dozen suggestions in no time.

    Now for the issue of lowering the mast. Here one has to go and have a look at boats in Holland. As they have to lower their masts for every bridge - of which there are thousands accros their rivers and canals -their systems are smart and above all simple. Essentially the mast is stepped in a tabernacle that sits on top of the coachroof. This makes it easy for the mast to lie flat when lowered. They use one of three methods to lower the mast: The bowsprit - which is also hinged so that they can lift it in harbour to shorten LOA - is used as a lever; a wisbone or triangular contraption that starts ahead of the mast and lies along the gunwales is hinged to the deck and used as a lever; a post is fixed to the bows, fitted with a hand operated winch and used as a lever. All very primitive but oh! so clever.
    Lowering the mast on a cat would of course be another problem since the mast is stepped well below the height of the coachroof and, lowered, would interfere with all sorts of things. Using a bipole, one would have to hinge the rig at some point up the mast - complicated but not impossible, remember the sleeves.
    Wittytutoi
     
  5. Spiv
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    Spiv Ancient Mariner

    Thankyou for your contribution Wittytutoi.
    Do you have any pictures of the cat in Port St Johns or of those rigs in Holland?
     
  6. wittytutoi
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    wittytutoi New Member

    Hi!
    Unfortunately I do not have any pics of the cat but I will try and do a simple drawing in the next couple of days. For the Dutch rigs please go to http://barges.apolloduck.com. and look up the sauling barges. It is a bit laborious to blade through all the pics but some of these classics are just too beautifull. Do not miss "Bruine Beer", she's a real beaut although quite recent.
    Happy viewing
     
  7. Manie B
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    Manie B Senior Member

    Roller main sail

    Gents with your experience and experimentation

    has anyone ever tried to roller reef a main sail on its vertical axis directly behind a single normal mast - similar setup to a jib foresail arrangement just straight up (vertical) behind = next to the mast

    in other words a mast furler that is outside the mast instead of inside the mast

    strictly for easy cruising ???
     
  8. Fanie
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    Fanie Fanie

    Hi Manie,

    You may probably do better when you stay it down just in front of the mast, this way you'll have a clean leading edge whick may make a wee bit better performance. Tacking would mean the sail has to come around the mast, a bit uncomfy and no auto tacking.

    With the stay just behind the mast in a close haul the mast may affect the leading edge some resulting in poorer performance. This would make for some easy and auto tacking. Both would be easy to furl the main sail.

    Personally I like the idea.
     
  9. Nordic Cat
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    Nordic Cat Senior Member

    I bought an Iroquois Chieftain that had this kind of setup, it was a roller furling forestay mounted around 80 mms out from the mast.

    There was no way to really keep the stay well tensioned, so when hard on the wind, the middle part of the sail would bulge out about 20-25 cms from the mast.

    Terrible for pointing ability as well as efficiency. OK for broad reaching and downwind work.

    What it needed was a way to stiffen the mast better in the fore/aft direction. This was a masthead cutter style rig with a baby stay, but no way to push the middle of the mast aft....

    With the right mast set-up and maybe a couple of flaps to close the gap between mast and sail it should make an OK cruising system.

    Alan
     
  10. sandy daugherty
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    sandy daugherty Senior Member

    The main-on-a-vertical-roller-behind-the-mast idea is not new; several can be seen in Marinas around Annapolis, and are a possible explanation of sunbrella strips on the leach of some mainsails I've seen on the bay. Vertical battens can support a modest leach.

    I'm glad to see this discussion of a self lowering A-frame for a catamaran. The individual legs could support sails, and might be foil shaped and rotate for windage considerations. Setting the mast somewhat aft would allow multiple vertically-furling sails, and the suggestion from someone above about taking backstays to a targa arch solves several delimmas for me. What remains to be solved is maintaining forestay tension while the boat wracks in seas or wakes. Rigidity in pitch between the hulls of a large catamaran has been a major engineering hurdle for high-speed catamaran ferries.
     
  11. catsketcher
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    catsketcher Senior Member

    a few thoughts

    Hello all

    My two cents on the mainless rig as I see this dicussion heading

    - the bi-pole rig has one very severe engineering compromise it must cope with. It has to do with the compression loading on the rig.

    The basics of compression loading are worked out using a formula called Eulers (say Oilers) column buckling formula. It takes into account the length and the cross sectional area of the column in the following way

    P(critical load) = Pi Squared x EI/L squared

    E is the Youngs modulus of the material and
    I is the second moment of area of the section
    L is length

    Increasing the E or stiffness helps buckling resistance in a linear way. Twice as stiff gives twice as much buckling resistance BUT
    Calculating I is tricky and basically it goes up exponentially (measured in the fourth power) as you increase the size of the section. A section only slightly larger can have twice the buckling resistance of smaller section.

    The result of all this is that it will always be lighter, cheaper and less windage to support a compression load in a single compression member - one mast. Bipole rigs will have to be heavier than a single mast and weight up high is a no-no as well as increased windage. At least in all the cruising cats I know of there have been no issues with the main box beam. It is very easy to reverse engineer what works and fit this in with the accommodation so getting away from a single mast form an engineering standpoint is not necessary.

    As to the stay behind the mast. I have seen this done with two cranes hanging out the back of the mast - really bad idea. It was on my mums cat and if I did the stay up tight then the mast would threaten to come down as it was heavily loading up the back of the mast. It also was a dog to windward. She got a normal fully battened main and the boat sailed much better.

    I think it is a really good idea to have a rig that works super well to windward. In the 5 trips north I have done it is always a little hard to get south. On the Ozzie east coast you wait until the northerlies come. Just a couple of hundred miles south they are getting northelies but up at Cairns or Lizard island you still get southerlies. We have often used the good windward performance of our cat (and previous tri) to get south when the trades have eased and gone slightly east. Once south we have better chance of getting tailwinds. My wife prefers sailing to windward on our cat (as long as we can lay the course) So I would counsel a careful overview of the positive aspects of the normal sloop rig. I certainly enjoy my windward sailing and have not enjoyed sailing on cats where this ability has been compromised.

    cheers

    Phil Thompson
     
  12. bill broome
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    bill broome Senior Member

    there is something to be said for an a-frame rig on a cat. i'd think about hanging a mast spar from the a-frame both to reduce the catenary tension in the luff and give you fast furling action.

    forward canted masts will generate mammoth tensions, really scary numbers, and their reaction in a seaway might be interesting, too. i'd think twice and then not do it.

    there was a wharram up in queensland a few years ago with a-frame rig, a bit of searching might get you some useful feedback.
     
  13. Day Tripper
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    Day Tripper Junior Member

    View photo on my posting

    The boat I am trying to convert to convential mast used to have an A-frame design.
     
  14. Fanie
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    Location: Colonial "South Africa"

    Fanie Fanie

    bill broome,

    Care to elaborate on this ? Maybe a drawed picture ?

    Exactly why the aft mast on my little tri sits upright. Forward canting shortens the leading edge slightly, and the vertical angle is not reduced by much. Makes almost no difference on the sail area too.

    The mast I have is unfortunately not stiff enough, but I'll have one soon to play with again. If I don't get one I'll make my own. Stiff as a ... oh never mind :D

    One thing about the aft mast is the lift is great. Even on a broad reach the leeward amas doesn't get much submerged...barely - for the hour of wind I had the last two trips :rolleyes:
     

  15. MAINSTAY
    Joined: Feb 2005
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    MAINSTAY Junior Member

    Main on stay just aft of mast

    ManieB
    Nordic Cat
    catsketcher

    Your experience that a sail on a stay right aft the mast has "terrible pointing ability" and is a "dog to windward" supports the modeling results shown in one of C. A. Marchaj's charts which shows that a gap of 10% of the chord of the sail caused a 15% loss of force at the luff.

    Putting the main on a stay right behind the mast results in a loss of power because the gap allows air on the higher pressure weather side to flow to the lee side of the sail which in turn causes the the mast's wind shadow to elongate and delay the reattachment of laminar flow.

    So a small gap is worst than no gap. And both are worse than no mast and clear air.
    Larry Modes
     
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