Critique my sail design?

Discussion in 'Boat Design' started by dustman, Nov 27, 2022.

  1. dustman
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    dustman Senior Member

    Please give the image a look and give your input. Not really to scale, just a sketch...

    Somewhat of a combination of a gaff and junk rig with wishbone boom and battens. Freestanding mast.

    Thinking of either lazy jacks or some kind of rods sticking up off the boom to collect the sails and battens when reefing. Should be able to reef by easing the halyard and taking up the sheet like a junk rig.
     

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  2. gonzo
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    gonzo Senior Member

    I would say that trying to reef that sail in a storm will be extremely difficult and dangerous. Instead of a single boom there are a bunch of wishbones flapping around. As far as performance, all that extra weight and turbulence can't be any help either.
     
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  3. dustman
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    dustman Senior Member

    What specifically about the design leads you to believe it would be hard to reef?

    Kind of the point of this sail design is that it would be(hypothetically) exceptionally easy to reef and manage generally. There would be no need go forward, and there would only be a sheet and halyard. If in a strong wind and the battens aren't coming down for some reason then just ease the sheet to weathercock to take the pressure off then let out the halyard. With no standing rigging there will be nothing in the way of easing the sails way past 90 degrees. The lower batten should weigh less than 15lbs and the upper ones even less, for an 80ft2 sail.

    As far as the turbulence, just like with a junk rig I am accepting a little less efficiency in trade for ease of use. The battens will be faired as much as possible. With camber and fewer battens should be much more efficient and lighter than a standard junk rig, with much less complexity.
     
  4. Robert Biegler
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    Robert Biegler Junior Member

    I have seen a picture of a junk rig with wishbone battens, but I don't remember where. Likely either in the Junk Rig Association Newsletter, or somewhere on The Junk Rig Association - HOME https://junkrigassociation.org/ The battens did not go around the mast, presumably because part of the sail extended to in front of the mast, and the designer wanted the sail to take a smooth curve on both tacks instead of lying against the mast on one. I don't remember how the battens were prevented from turning vertical.

    I am not sure whether the bit that goes from the front of the batten, round the mast and to the luff of the sail is meant to be rigid or rope. If rigid, then you risk it binding on the mast whenever a batten is not quite horizontal.

    The tapering planform means that when you reef, the aft end of each batten is further forward than the aft end of the batten below, which is likely to foul the sheetlets.

    You have drawn the panels between the battens as having a fairly large span/high aspect ratio. When you reef such a sail, the reefed panels can therefore blow out further to lee, likely over any rods, unless you make those rods very long. The panels are likely to flap around, and impale themselves on the rods. Lazy jacks would be preferable.

    The junk rig does not normally have a downhaul. If the sail and battens were weightless and frictionless, then wind pressure would blow each panel into a tube shape (if you don't cut camber into each individual panel). The higher the aspect ratio of each panel, the more that will affect sail shape.

    I am not aware of anyone having tried a boom at a constant vertical angle to the mast in connection with a junk rig. What is the purpose of this feature? If you expect that leach tension helps sail shape, then this could be useful when the sail is fully hoisted, but would lose its function once you reef.

    I once made a junk sail with battens going round the mast. You can see pictures in AYRS Catalyst #8 (Catalysts – Amateur Yacht Research Society https://www.ayrs.org/catalysts/), in an article summarising what I knew then about attempts to improve the aerodynamics of the junk rig. The whole issue is mostly about rigs, including Jeff Doyle's very interesting approach to a soft wing junk rig.

    Phil Bolger drew a rig intended to combine the best of gaff and junk, and it shares some features of yours. As far as I know, at least one customer tried it, but eventually replaced it, I don't remember whether with a standing lug or a conventional junk rig, and I don't remember reading any analysis of why the rig was replaced. Anything you can find on that should be useful.
     
  5. gonzo
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    gonzo Senior Member

    Having years of experience sailing in rough weather. Large spars are a bear to handle. How do you propose to handle three wishbones is say 70 knot winds and high seas?
     
  6. HJS
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    HJS Member

    The free-standing mast will bend under the slightest load so that the sail takes on an extremely inefficient shape. The whole concept will never work as described.
     
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  7. dustman
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    dustman Senior Member

    Thank you for the considered and thorough reply. Sorry for the late response, high workload.
    It is meant to be rigid. I had concerns about binding as well. The batten would be 1.5 inches thick, and the thought was that if the hole was cut to a tight enough tolerance then the batten wouldn't have the opportunity to droop enough to bind. I had also considered using a short section of tubing epoxied into the hole of the batten that fits precisely around the mast. Also thought of using pillow block bearings bolted to the top and bottom of the batten, but they are heavy and expensive and wouldn't be so great aerodynamically. My original thought was to use strap or rope but I want to eliminate as many potential failure/maintenance items as possible.
    Great point, to be honest I hadn't even thought of that, a big oversight. I could make every batten the same length or make the bottom panels the same size with only the top panel tapered. Not sure if straying from a more elliptical shape would have significant aerodynamic consequences?
    The rods were planned to reach almost all the way up to the next batten, slightly veed out. I was hoping to not have to use lazy jacks because of the increased complexity and potential for failure and would be another maintenance item to worry about.
    The sail shape in this instance will come from the fact that luff and leech will attached to a rigid batten, so when the sail fills it will take the shape determined by how loosely the fabric is layed between those two points, and it will also be contained by the inner shape of the battens. I would be relying on the weight of the battens to maintain tension when reefed. I could add more attachment points to the mast or more battens. Would like to keep that to a minimum as to decrease complexity and negative impacts on aerodynamic efficiency, as well as keeping weight aloft to a minimum.
    The purpose would be to eliminate the need for any extra doodads to maintain the orientation of the boom, such as lazy jacks. The boom would just stay where its at no matter what, wouldn't go up or down or tilt. With the proposed design this seemed important.
    I realize this. I considered making the yard vertically stable as well, so the luff and leech can't change orientation relative to each other. In this design all the vertical tension in the sails will be carried through the reinforced luff and leech by the way.

    I have poked around on the JRA website, and all over the place, and haven't found anything quite like what I'm thinking. I was hoping somebody already went through the trial and error! I found your article very interesting.
     
  8. dustman
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    dustman Senior Member

    The battens on this sail would extend beyond the mast no more than 6 feet and weigh 20lbs or less. How would this be any harder to handle than a standard junk rig in that regard? Can you be more specific about what exactly the problem would be? My first boat will not be sailed in those conditions, I would just be island hopping around the Bahamas.
     
  9. dustman
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    dustman Senior Member

    I reject your hypothesis on the grounds that it totally depends on the rigidity of the mast relative to the load imposed upon it.
    Care to articulate as to why you believe this to be so?
     
  10. gonzo
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    gonzo Senior Member

    Nothing is rigid, there are degrees of stiffness. You need to calculate the forces in compression to design the battens. They are thin columns out of alignment.
     
  11. clmanges
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    clmanges Senior Member

    @dustman, you say you've 'poked around' the JRA site. If you sign up (dirt cheap) you'll have free access to an enormous volume of information, including an entire copy of Practical Junk Rig by Hasler & McCleod. I'd suggest you start with that book; it tells you step by step how to design and build an entire rig. Their final product is kind of a bread-and-butter sail which will be usable in a broad variety of conditions. Once you understand that rig you can explore variations.
     
  12. Robert Biegler
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    Robert Biegler Junior Member

    The engineers here will correct me if I'm wrong: I don't think the tolerance is what matters. Say wind pressure lifts the end of the batten, with a torque of 1 Nm (Newton metre; substitute lbsft at will). Your 1.5 inches are about 4 cm or 0.04 m of vertical separation between the upper and lower edge. To counter a torque of 1 Nm with an 0.04m separation there needs to be a force of 25N on each edge, no matter whether you get that 0.04 m separation from a tight or a loose tolerance. The section of tubing looks to be a better solution, but would interfere reefing, unless, perhaps, you let the tubes be cone-shaped, so that the tubes can slide into each other. Or you allow for movement.

    With only three panels in the sail, that is nearly a third of the height of the mast above deck. That is a lot of weight and windage. And unless you sheet all the way out before you drop the sail, it is likely to still fall outside the rods.

    You are thinking of the shape along the chord. I meant the shape along the span, between one batten and the next.

    Bolger's Chinese Gaffer is the closest I know:
    [​IMG]
    Note that a junk sail may have as few as two pieces of running rigging, the halyard and the sheet, along with two kinds of standing rigging, the lazy jacks and Hong Kong parrels that hold the battens to the mast. If you want more control, you could use double sheets, either one or two luff hauling parrels that let you adjust how much of the sail is in front of the mast, and perhaps a yard hauling parrel. So up to six lines, if you want to get fancy.

    Bolger eliminates the luff and yard hauling parrels, but then needs both a throat and a peak halyard, he has three sheets, and he also has two slab reefing lines, presumably because once he reefs, first sheet 2 and then sheet 3 start pulling battens up. And there is a vang. Bolger ends up with eight lines. I haven't read "103 Small Boat Rigs" or the essays in which Bolger explains the rationale for his choices, so I can't comment.

    Still, sticking with conventional junk sheeting should let you do much the same job without the vang or reefing lines, so you may be fine with three pieces of running rigging. I am pretty sure you would need both throat and peak halyard, though. Both the lug and the junk sail align the halyard with the mast, so that the angle at which it pulls doesn't change much as you raise or reef the sail, and they use the proportion of sail before the halyard and mast to determine what proportion of the vertical load is carried by the luff compared to the leach. You propose to do away with the part before the mast, so you can't use the same method, and I think you will need two halyards.

    You haven't said why you don't want any part of the sail in front of the mast. The most common motive is to avoid having the mast to disrupting the flow on the lee side of the sail on one tack. If that is it, you don't need your wishbone battens to go around the mast. Let each terminate in a fork or jaw, like this:
    [​IMG]
    Tie a rope from one tine of the fork around the mast to the other tine. You could even let the luff of the sail slide on a little rod so that it is always blown to be in line with the lee side of the mast. Otherwise, use Bolger's planform with conventional junk sheeting. Try it at model size, and if that works, at dinghy size.

    The wishbone booms will let you have camber, but unless you let the sail lie on the leeward part of the boom, I don't think you can control how that camber is distributed along the chord. Conventional sails do that by sail cut and tension, but the point of the junk rig is to reduce loads, and so you don't have that much tension to play with. There may be an alternative.

    You may have noticed that I proposed, in the article, curved, rigid battens, basically making each batten a Bierig CamberSpar. It looked very good on the model sail. When I tried it at dinghy size, the battens didn't flip as they had in the model, and I never worked out why (I didn't have the boat for long, and was focused on other experimental bits). Nils Myklebust built a c. 25 sqm sail for his motorsailer, chose to flip the battens by 180 degrees instead of about 90, and because they were never going to do that on their own, he rigged a line tied to each batten at the point of maximum camber, and pulled on that to flip the battens. That did work, and he got a good sail shape.

    I think the curved, rigid battens could be applied to the gaff/junk hybrid. Take the above picture of the jaw of a wooden gaff. Give each batten some vertical curvature relative to the orientation shown in the picture. Shape the batten pocket to allow the batten to flip around 45 degrees to either side. I am not aware of anyone ever having tried this, so I make no promises, but I think there would be less risk of binding, and better control over the distribution of camber along the chord. And you could make the jaws wide enough that the sail's luff is just behind the lee side of the mast.

    In either case, the higher the aspect ratio of each panel, the less the battens will influence sail shape away from the battens. The goals of control over sail shape and simplicity by having few battens are in conflict, and you will need to decide on a trade-off.

    Have you had opportunity to sail with either a gaff or a junk?
     
  13. dustman
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    dustman Senior Member

    The thinnest part of the batten on each side would be slightly less than an inch and a half. A standard 8' 2x4 can withstand nearly a thousand pounds of compressive load. The compressive load on the battens would be minuscule compared to their strength and relative rigidity. A worthwhile consideration though, thank you for bringing it up. I am more worried about weathering causing splitting and deterioration of the wood.
     
  14. gonzo
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    gonzo Senior Member

    That is not correct. A thin column will buckle and break. Check Euler's theory column buckling. It is a standard method to calculate failure of thin columns.
    Secondly, there is no such thing as rigidity or a rigid material. There is a material property called stiffness.
     
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  15. dustman
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    dustman Senior Member

    I think you know exactly what I mean.
    Yes, you are correct from a real world standpoint, just meant to illustrate how strong wood is in compression.

    To my understanding, every square foot of surface area will experience a load of .256lbs at 10mph, every doubling of wind speed 4 times as much pressure, so at 80mph it would be 16lbs per ft2, assuming all the wind energy is be perfectly transferred to the sail. Any one batten would be responsible for 25ft2 at the most, which would mean a load of 40olbs, but realistically much less than that, and the compressive load some fraction of that. In conditions like that I would have long ago reefed down as much as possible(more likely with sails down and a drogue out), meaning that I would have less than 20ft2 of sail shared between the yard and a batten(the columns of the batten would be less than 3' long at this point). 20x16=320 divided by 2 = 160lbs. I have used 3' or longer sections of 2x2 to temporarily support loads much greater than that and noticed little to no deflection.

    If you are so inclined to show that my battens would fail you are free to do the calculations. The battens would be made of western red cedar, the maximum span of a batten column would be about 60", each of the two batten columns would be a square of approx 1.45", at the widest point the columns of the batten would be separated by 12", a semicircle meeting at both ends of the span.

    I'm never going to find myself in those conditions on this boat in the first place.
     
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