Backstay problem on Gunter rig

Discussion in 'Sailboats' started by laukejas, Oct 19, 2013.

  1. laukejas
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    laukejas Senior Member

    Hello all,

    Some of you may know, that I mentioned a while ago that I'm planning folding skin-on-frame sailboat, 4-meter. Since Bermuda rig requires tall mast, and in this case, it means it has to be composed of many segments, I decided to limit mast to 4 meters, and add some more height with Gunter rig, since I want high aspect mainsail - upwind sailing is what I'll do most.
    However, there is a problem. Gunter rig doesn't allow traditional backstay. I could go with running backstays, but I think this is too complex in 4-meter, takes too long in a jibe, too hard to reach while hiking.

    Then, I thought, I could go with triangle system - forestay, and two shrouds, angled towards stern. However, from my sad experience I know that in light winds, when it's necessary to sheet out a lot, these angled shrouds interfere with sail shape, or sometimes even block the boom, and in high winds the shroud angle may be too low to prevent dismasting. There isn't a universal angle that works in both conditions.

    But I actually have an idea to improve this angled-shroud system. Quite complex, but maybe still better than running backstays. How about an angled shroud that's angle could be adjusted? It would attach not to the hull directly, but to a pulley system that is attached on 2 points on the hull - one almost by the mast, and one further back. By adjusting this system, shroud could be rolled forward, to act like a shroud, or backward, to act like a stay. This would be done depending on conditions - windspeed, course, sail camber, required boom angle.

    Advantages over traditional running backstay system - there wouldn't be a moment when there is no "back" support for the mast, reducing chance of losing it in a gust. This is a dinghy, after all. Things happen fast.

    Attaching crude images of what I mean.

    [​IMG]

    This is shroud (blue) rolled forward (light winds, when less back-support is needed);

    [​IMG]

    And this is that shroud rolled backward, providing more back-support to the mast. Could be used in stronger winds, particularly on downwind.

    Two more advantages - it could still be used as a traditional double-backstay system (rolling aft only the leeward shroud), and also, due to 2 attachment points on the hull, it would distribute the load better.

    But this is just an idea. I'm not really good with pulley systems, and I'm cracking my head trying to figure out how this rolling system could be rolled back and forth while keeping shroud tensioned. If it is rolled backwards, it naturally has to be longer, so it means the pulley system has to let some of the shroud out. And when rolling forward, it has to be shortened. And all that done in reasonable time, comfortable enough to do while hiking.

    I don't know. Can this be done? Can anyone suggest a pulley system that could do this job - roll shroud back and forth (fixing it in place) while keeping it at constant tension?


    If this is a bad idea, could someone suggest any better solutions for Gunter rig to provide back-support for the mast?

    P.S. This is not a plan of my planned sailboat, just something I sketched for reference - but mine will be similar.
     

    Attached Files:

  2. philSweet
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    philSweet Senior Member

    You need to start by actually calculating the loads in pounds for the rig in it's simplest form and the look at the numbers and see if you really have a problem. Some pretty heavily loaded boats get by with a simple tricycle staying. I've flipped my 16'er straight over the bow when I got hit by a microburst. That was with two beefy guys crawling over the transom. You could figure on the boat full of water and rolling over the bow and still have a buildable rig. The sail interference is minimal, particularly on a twisty gunter rig, just twist the top off and keep the bottom sheeted in. Jib cut can help with this. A bit of lap at the bottom of the main lets you twist both sails more for better stay clearances.

    There have been some systems that do what you are thinking, but for the most part, they are on boats which have people who are being paid to sail the thing.

    For what it's worth, I designed a running backstay that was integrated into a boomvang tackle and was entirely self tending. There were 2 vangs, one for each side, and they only operated when the boom was more than about 10 degrees off centerline. It was for a fifty-footer with wooden mast.
     
  3. alan white
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    alan white Senior Member

    The backswept shrouds should be adaquate for your rig. The boom will contact the shrouds but only broad reaching to running, where sail shape is more about area than shape. It's an old tried and true method of rigging that's still used on most all small sailboats, so why would your boat be any different?
    Don't overthink the design. Complication is often worse than slight inefficiency. Especially in this case. Also, a skin-on-frame boat is going to make best use of a shroudless rig rather than stressing and distorting the hull (as it is, you probably have to build a cross-beam athwartship at the mast to support shrouds, negating some of the value of an intentionally light boat.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Why would you need to stay a modest rig on a folding SOF boat? The rig looks to be oddly proportioned, particularly for a SOF and you don't appear to have nearly enough purchase for that gunter. How have you sized the rig for your boat? What's it WS/SA ratio, for example?
     
  5. laukejas
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    laukejas Senior Member

    philSweet, could you please show your self-tending backstay? It sounds intriguing.

    alan white, I don't get why shrouded system would increase load on hull. My common sense says that pressure acts on the sail, especially at the top of the mast. If it is unstayed, the force goes along the length of the mast, to the attachment point on the hull, and then sideways to where sailor is heeling over. In other words, load travels long and curved path, like an L shape. With addition of the shrouds, the force from the top of the mast goes directly to where sailor is heeling out. I may be wrong, but I thought that with shrouds, loads are supported more directly, therefore, less stress on mast, hull, and so on. Unless shrouds are over-tensioned.
    I'm trying to design this sailboat that employs both - mast would be fixed strong enough, but supported additionally with shrouds, so the load would be distributed. Correct me if I'm wrong.

    PAR, I haven't yet came up with exact proportions and ratios, since I have no similar boat for reference (couldn't find SOF sailboat of 4 meters that folds into 1.33m pieces), so I'm making a crude model right now, so that I can estimate displacement. After that I'll start theoretical calculations.
    My estimates so far:

    LOA 4m
    Beam 1.4m
    Displacement 160kg (1 person) 240kg (2 persons) (VERY crude estimate)
    Gunter/gaff mainsail 7.8m^2
    Jib 3.2m^2
    Mast height 4m

    I know that sail area is a bit on the high side, considering narrow beam, but I expect sailing in light winds, and there will definitely be at least 2 reef points. No trapeze.

    As I said, that sketch here isn't of my own boat proportions, just painted it to show the idea of rolling shroud, so don't hurry to judge! :p

    What did you mean by "you don't appear to have nearly enough purchase for that gunter"?

    And answering your question - well, apart from what I said about path of load along the boat, I just really don't trust the mast to hold such loads with no support. I consider 7-8cm thick aluminum mast, composed of 3 parts - so it will be even thinner in joints. Apart from that, if I tension forestay for jib, mast will automatically bend forward. And having sailed a bit with cat and sloop rigs, I really, really need that jib.

    Please correct me if I'm wrong about that stayed-versus-unstayed thing. Don't mean to sound omniscient, I just figured that this is more logical.
     
  6. alan white
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    alan white Senior Member

    For one thing, a stayed mast pushes DOWN while the chainplates are pulled up and inward. Unstayed, the weight of the mast is all that is felt by the keel, and the gunwales are completely unstressed except laterally by a presumed thwart. You could, as I said, build a frame (an inverted triangle) to support the mast and the shrouds, but why? The boat is a skin on frame presumably to save weight. Why choose a source of additional weight?
    A bendy mast will still allow the boat to tip over without breaking. Some very fast boats have no shrouds,
    If you were suggesting to build a 200-300 lb boat of glass or wood, shrouds would be expected. But skin on frame boats are so light that your cheif advantage of low weight is lost and the boat becomes an oxymoron.
     
  7. PAR
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    PAR Yacht Designer/Builder

    How can you stay a mast on a SOF build? One quick look at load paths and you will fold your boat, with modest rig tension alone. I'm not sure of your gunter attachment, but clearly there's not enough purchase to hold it up, let alone make it stand upwind. Reconsider your yard purchase percentage. A gunter rig is a very poor choice for a boat like this. It's comparatively heavy, displays excessive windage, has a poorly shaped leading edge, tends not to stand well upwind, even on highly strung version (which this can't be for obvious reasons), places the CE too high for a boat like this, plus other issues.

    Lastly, not "trusting" a material or engineering concept, suggests a lack of understanding in both, which isn't the best way to engineer things. Free standing rigs have been around for centuries (many) and can be not only trustworthy, but a reasonable safety feature too. There's several good reasons you don't see many gunter rigs around. You'd be best advised to use a more conventional, preferably lower aspect rig, of which there are several that will permit spar stowage within the boat's length. The sprit or lug seem obvious choices and is a jib really useful in this size, particularly considering the additional loads it'll impose, on what can only be described as a delicate build method?
     
  8. michael pierzga
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    michael pierzga Senior Member

    Upwind a boom topping lift will create headstay tension
     
  9. PAR
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    PAR Yacht Designer/Builder

    This is a SOF boat Michael, so how much tension do you think he should use on that topping lift?
     
  10. Perm Stress
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    Perm Stress Senior Member

    used optimist rig would be a perfect choice for a first try:
    cheap, effective, readily available, perfectly sized for your boat.
    Most probably, you will stay with it after some practice.
     
  11. PAR
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    PAR Yacht Designer/Builder

    The Opti rig is a good choice, though possibly a little small in light air, certainly hits all the major considerations - low aspect, spars store in the boat, free standing, simple, offers reasonable performance, etc.
     
  12. laukejas
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    laukejas Senior Member

    All right, you convinced me. I'll drop the Gunter rig then, and consider something else. But what is there to choose from? Lug or sprit have the problem of performing differently on different tacks. Is there a symmetrical rig that could be used with short mast, and still provide enough sail area? Higher aspect ratio would be still preferable, but it doesn't seem possible without a yard... At least to my limited knowledge!

    EDIT: How about simple Bermuda with large roach and battens?
     
  13. PAR
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    PAR Yacht Designer/Builder

    A tall aspect sail is the last thing you want to consider. First you should determine how much area this puppy can carry. The differences you might experience from one tack to the other on a sprit or lug isn't really significant, unless you're racing in light air.
     
  14. TeddyDiver
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    TeddyDiver Gollywobbler

    Dipping lug (no bad tack!) gunther or sprit, all of them are perfect for unstayed rigs and best for your boat. You can have all of them just pick the one suits the weather best for the day..
     

  15. laukejas
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    laukejas Senior Member

    Well, I still can't figure it out with these righting moments and all. I always come up with weird numbers, so for now, I aim at about 10m^2 sail area, which seems reasonable for boat of such size.
     
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