Junk Rig Catamaran Questions

Discussion in 'Boat Design' started by cdubb, Jun 9, 2016.

  1. cdubb
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    cdubb Junior Member

    I am rather fond of the junk rig sail and also am quite fond of catamarans, the two together suits my tastes very well. While though web searches I've only seen the "biplane" design of this combo. That being said is there a option to have a single sail junk set up on a catamaran? I admire the ingenuity the biplanes but I can not help but wonder if a single sail is possible. And if it isn't feasible I am quite curious as to why not, as I have never seen a explanation on this. The attachment is what finally drove me to ask this, it's a design and build from the 70's from a gentleman by the name of Hermann Otto.

    Regards
     

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  2. Skyak
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    Skyak Senior Member

  3. cdubb
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    cdubb Junior Member

    Good link skyak, good info tossed about on that thread. If I was to specify what I want out of the rig I would have to say ease of use and being comparatively inexpensive to make and maintain. The junk rig has many upsides for use of it for me, along with a unique look. I see in the thread my question about a junk sloop is briefly mentioned and as I suspected the I un-stayed mast is a issue on a catamaran but it is doable(?). On a one off build this I would assume this issue could be addressed tactfully, although the only way of doing this that is immediately springing to mind is a trimaran set up to house that mast.
     
  4. serow
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    serow Junior Member

    I'm guessing that if the wind is square to the direction of travel one of the sails is largely useless, a bit like having you wind stolen during a race. Is that so?
     
  5. Skyak
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    Skyak Senior Member

    Surely it is 'doable' but the hope here is that we steer you toward the best solution and at the very least can predict the result. Multihull boats have nice wide structures that make it easy to add tall light stayed rigs. An unstayed rig of similar height will be much heavier.

    The strengths and weaknesses come down to very simple points
    -aspect ratio
    -mast drag
    -control

    Bermudan sloop is obviously much easier to make tall and skinny, the main sail cuts the drag of the mast, and there are highly refined used rigs coming off boats all the time.

    The junk rig has lots of things going for it. If i was only given a bunch of fabric and wood and told to cross an ocean with it there is a good chance I would make something that most resembles a junk rig.

    The Biplane rig is a little bit more efficient in structure, and a lot less efficient in aerodynamics.
     
  6. cdubb
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    cdubb Junior Member

    A more conventional Bermudan sloop set up would be my second choice, I have zero opposition to this rig. But for the sake of the idea at hand and with the factors stated, what would be a end result of a sloop junk set up? I realize information to go by is scarce but a educated guess would suffice here. I realize that a Bermudan rig is the far more traveled path but if you are going to build a boat you may as well make her what you want her to be. That's why I wonder what the result would be of this concept.
     
  7. Rurudyne
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    Rurudyne Senior Member

    One thing to consider about the junk rig is that usually the masts are unstayed while centerline mounted cat masts tend to need stays.

    The junk rig is one people love to hate, however, the reasons for them, their strengths, also create folks who love them. Internet mayhem ensues.

    Junk rigs such as the shaped type do achieve better performance without much increase in complexity or handling. I've seen comments to the effect that their performance on the wrong tack, when the mast is disrupting the airfoil shape, is not as greatly diminished as some assert it should be. But the big advantage of the junk rig to me seems to be ease of handling. Yes, the Bermuda rig is easy to handle with good gear (and you can find good gear surplus), so that's not a world beating advantage of the junk rig.

    In the end, what you like, what looks good, is what matters.

    I, myself, like paddlewheels. People will go on about how inefficient they are but we live in an era relatively small high speed props that do good to do much better than 50% efficiency by some estimations. Few small high speed props able to go in shallow weedy waters will match a well sorted out feathering paddlewheel in those conditions and no paddlewheel will be as good in rough seas as a comparable sized yacht with relatively small high speed props. These days people lust after fast boats too (with just enough cruising range to get back to the dock in some cases) and fast and paddlewheel (at least in any ordinary sized pleasure craft) just don't tend go together well. But people are still build paddlewheel boats, mainly with fixed rather than feathering floats, and are deliriously happy with them.

    If you like junk rigs go for it!

    If I was able to build a large paddlewheel yacht I'd use junk rigged get-home sails if only to be contrary, and not just because I like the rig too. On a smaller boat with the attendant windage I'd use a kite ... maybe one inflated with hot air and string of LED running lights to scare the willies out of folks at night who thought they just saw a UFO. ;)
     
  8. peterAustralia
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    peterAustralia Senior Member

    Has been done

    Note that Junk Rigs are generally unstayed. Thus it makes no sense to put them in the middle of the cat. If going Junk rig, usually best to put them in the hulls, one mast in each.

    Example Dragon Wings by Gary Lepak
    http://www.duckworksmagazine.com/04/s/vintage/multihulls/index.htm

    there is this
    http://www.duckworksmagazine.com/13/projects/oryx/index.htm#.V1u826IRPcs

    and something a bit different
    http://www.tacking-outrigger.com/baltic_proa.html

    from memory there was a Wharram Cat with twin Junk sails, called P.H.A.


    http://wharrambuilders.ning.com/group/junkrigged
     
  9. cdubb
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    cdubb Junior Member

    I've seen a formula on another site in regards to making a unstayed mast secure. As I recall it was no less than 10% of the mast height or boat length of the mast needed to be "buried" for it to be adequately strong. Could this structure be built around the mast above deck in order to get the right distance of the mast buried? Here's some chicken scratches I drew so maybe what I'm getting at is better understood.
     

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  10. cdubb
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    cdubb Junior Member

    I guess what I am wondering is with all things considered with the junk rig, what would need to be done to make it work on the centerline of a cat?
     
  11. Stumble
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    Stumble Senior Member

    You would need a really really strong mast base. It would be a pure cantilever, so it isn't like the engineering is all that difficult, but the cost in materials and weight would be severe. Probably the best option would be a monolithic crossbeam/mast that are one piece....

    Again just because someone can be done doesn't mean it should be done. I am not an engineer, or a boat builder, so my guess here isn't very good, but I would guess what you are talking about would cost 3-4 times what a Bermuda rig would cost for worse performance.
     
  12. serow
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    serow Junior Member

    You wouldn't have much mast compression, just the weight of the mast etc, there being no shrouds, but all the wind forces would have to be taken in bending and/or torsion by the supporting beam. With the wind blowing from the side of the boat the cross beam would be in bending and with the wind blowing from the stern the cross beam would be in torsion. Of course generally both these cases will be combined to a greater or lesser extent.
    If this cat has a cabin located where the mast goes then provided its made robust enough the mast can go through a hole in the roof into a socket in the floor in the same way it would in a monohull. Calculations required.
    If there is no cabin suitably located then best way IMO to deal with this is to use a suitable cross tube with a tee piece or equivalent where the mast goes. If you want a wooden mast this would have to be the right diameter to allow the mast to slot in. A circular tube is the best shape for resisting torsion but there are practical advantages in using a square or nearly square rectangle. If you wanted an ally mast I suppose you could do a similar thing or flange it and bolt a suitably designed and detailed mast on. Avoid like the plague a temptation to use parallel beams close together with the mast cobbled between them; the torsional forces will make them bend in opposite directions and the mast will flop about like you are trying to prize floorboards apart. Bear in mind the loadings on all this will happen frequently and all the while and any inadaquacies will soon be found out sooner or later.
    Of course having sorted out the restraining forces at the base of the mast you then have to transfer them efficiently to the hulls which unless they have been designed with this in mind may have it's own problems.
    Welding in the middle of an aluminium tubular major structural component would not be first on my list of things to do, especially if going afar.
    The cross beam might be of quite large dimensions to take the torsion.
    But there's no doubt a neat solution could be found, almost certainly at greater weight and expense than mast and shrouds.
    As always the devil is in the detail.
    This is the sort of thing which doesn't show any fixings. I'm assuming you can work out where the hulls go.
     

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  13. cdubb
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    cdubb Junior Member

    I like the way you think man! Good points earlier in the quote too. Cheers!

    Thats a nice bit of hardware in the attachment good to know that is a option by the through the cabin route seems more viable. As you said there is far more to consider than just the rigging. That being said is there a designer you could recommend? Any one versed in cats and junk rigs too?
     
  14. FAST FRED
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    FAST FRED Senior Member

    The main reason for building a cat is it can have faster speeds than a monohull.

    Faster speeds mean that there is much mote windward work , is the cat is at all fast.

    A sloop or cutter rig is faster than a rig designed for downwind.
     

  15. Rurudyne
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    Rurudyne Senior Member

    One can also build a cat for the accommodations on a limited length.

    There's a water taxi the looks like an old Star Trek shuttle that looks like it would make the nice basis for a houseboat, for example, especially if one could convince their state to issue a hull number that began with NCC. ;)
     
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