Sail Loading on Rig, Rig Loading on Vessel

Discussion in 'Sailboats' started by brian eiland, Oct 14, 2003.

  1. MikeJohns
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    MikeJohns Senior Member

    Has anyone worked through Guttelle's long method yet? (posted by Tony )I am just looking at it now, one minor sign error so far and a confusing collection of labels but basically a sound method. Interesting that he takes RMmax or 1.5 times RM30.
     
  2. brian eiland
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    brian eiland Senior Member

    Sail Aero-Structures: Computing Primary Load Paths

    WHAT IF WE WERE WRONG ALL ALONG?
    For more than 25 years sailmakers have pursued the "load path model."
    Envisioning continuous paths of primary stress radiating out of the three
    corners, creating the ultimate sail shape-holding structure amounted to an
    exercise in carefully placing fiber, along these paths.

    At the 17th
    Cheasapeake Sailing Yacht Symposium, Dr. Ranzenbach and Dr. Xu present the latest research into the topic, and reveal a much more nuanced picture. For a fascinating, academic look into the current understanding of sail structures, read "Sail Aero-Structures: Studying Primary Load Paths and Distortion

    Brian added: I was unable to attend this symposium, but I saw this interesting notation by Quantum Sails. However their referenced website posting was not useful.
     
  3. Ssor
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    Ssor Senior Member

    I haven't rewad the entire thread but I worked with a man who had sailed most of his life in the cape cod area. When I asked him about these design problems his response was that practically everything about a sail boat was empirical, if the sticks came off the wire wasn't strong enough, if the sticks broke they weren't strong enough. Since it is the shrouds that heel the boat they must be at least strong enough to support half the weight of the boat. A mathamatician will tell you that the value of Pi is 3.14159, an engineer will tell you that the value for Pi is 3.14159 but to be safe use 4.
     
  4. brian eiland
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    brian eiland Senior Member

    Well this sort of thinking sure would not advance the state of the art
     
  5. Ssor
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    Ssor Senior Member

    When you have completed all of your calculations and assembled the pieces and taken it out and sailed it you will have gained empirical knowledge. If it all stayed together then your numbers were correct, if it failed, then you missed some factor. when you are trying to optimize the weight/strength ratio you can miss on either side of optimum, too much weight and you loose the race, too little strength and you loose the rig. Out on an ocean, with help a week away the consequences are more dramatic. Some rigs are never intended for storm survival. Testing sailing rigs requires time and conditions that can not be modeled with a large degree of certainty. you not only have to deal with the forces of wind but also with the rolling, pitching, and yawing couple from the waves.
     
  6. zerogara
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    zerogara build it and sail it

    It would be nice if we can take all scientific knowledge, load it all up in computer models and make practical interfaces for practical solutions.
    The reason it is not done in 99.9% of all models is that models never reach a final form. There is always more to learn, more to try, more to break and start over again.

    So we keep breaking and improving, breaking and improving over and over again, just as scientists create models only to find weaknesses and go back to more reasearch and remodeling.
    The big difference is that from practical trial and error (a single hull whose rig is constantly redesigned) there can only be refinement for that specific example and will not apply as a formula to all other states (hulls, rigs, circumstances).

    What are we looking for and why are we looking for it? The world's most expensive sailboats in design and construction are the ones breaking their rigs more often. Cost is a huge issue in all of this, and to have the ultimate you pay the price.
    What good is it if you had a computer model loaded in your PC and designed and built your rig, if for ANY given rig their can be a set of circumstances extreme and rare enough to break it in pieces.
     
  7. Ssor
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    Ssor Senior Member

    After all of the fine calculating we add a safety factor of four or five, and then go to the next size commercially available components.
     
  8. zerogara
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    zerogara build it and sail it

    Is that 4-5% above calculated size? If not how is this factor computed in let's say wire size?

    In many small dinghies and skiffs things are simpler. If for a given size/rig/sail area/weight most people use 2.5mm and get very few failures, some might use 3mm which might give a longer service than the hull. Some might pay more and use dyform of 2.5mm which has a breaking strength higher than the 3mm usual wire. There is not much else of a choice out there except for a severe overkill.

    When boats get larger and more complicated the commercially available choices become a more sophisticated issue. For a while sailmakers where dictating rig design. Then rig design and new materials start being demanding for the sailmaker. With electronics and hydraulics evolving we might see sailboats in the future with a stick shift for changing gears/speeds for the rig and then automatics.

    At some point we might miss "sailing" and start looking for slim canoe stern heavy cruisers with maybe a little bend on the stick and some fancy sail that changes shapes because we are now too spoiled to go back to straight sticks and saggy sails.

    Remember carburators that would flood, and have flat spots, and non synchronizing straight cut gears, and mechanical clutches? Some don't!
     
  9. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    I haven't read everything here, BUT remember that it's the boat stability, the rightening moment that will limit the forces. there is no need to make the rig much stronger than what's needed to heel the boat :)
     
  10. zerogara
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    zerogara build it and sail it

    I disagree as a generalization. This may be fine for coastal sailing, but in long passages through the high seas where conditions force a boat to sail under extreme conditions forces amplify.
    Someone mentioned of breaking waves and sliding backwards down a slope, and I would add gusts at crests of breaking waves etc. etc. Yes a boat will heel but how fast can it heel to avoid breaking? A sphere or a cylinder may be able to heel pretty fast with minimal resistance, but sailboats rarely look like cylinders, especially in long passages where all sorts of stuff are strapped on deck.
    An other issue is that rigs don't only break around the rolling axis, they break in the fore/aft direction too.
    The worst weak link in the whole chain would be the hull and deck. Assuming they are not you carry wire cutters to save them.
     
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  11. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    OK, I see. If you want the rig to withstand a 360 degrees roll.
    BUT rightnening moment and the boats moment of inertia CAN be calculated for a maximum possible loading condition. Compare those numbers with different rules for rig dimensions. Whn you order a mast from Selden, they ask for RM30.
     
  12. skab81
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    skab81 New Member

  13. jarhead
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    jarhead Junior Member

    maybe you dont need to know anything about sails. it might be that loading a rig with strain gauges at all the points on a mast where loads enter, recording these stresses over a range of windspeeds and directions, would result in a pretty good resource to calculate rig specifications. surely someone has done this?
     
  14. brian eiland
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    brian eiland Senior Member

    Vessel Substructure to Support Rigging Loads

    The Boat ‘Is’ the Structure

    Who would have ever dreamed that a lake sailing, ultra light-weight, racing catamaran would encounter, and be capable of sustaining rigging loads comparable to those of an America’s Cup boat??

    From a recent article in Seahorse magazine, “Alinghi, Birth of a Crazy Boat”, this paragraph emerges, “The boat ‘is’ the structure. These boats are now so complex, and the loads so high, that structural aspects take on a particular high importance: imagine that on a 1.2 ton boat you can reach 23.7 tons of mast compression…levels seen on an IACC design weighing over 24 tons! Also one cable in the substructure is sized at 56 tons…it is amazing that such light boats can produce such enormous loads.” (let alone absorb them, BE)
    Alinghi Catamaran
    http://www.boatdesign.net/forums/showpost.php?p=98787&postcount=39
    ….attached PDF

    The point is ‘the boat is, or should be, designed as a structure’ to carry the rigging loads. In far too many cases with production vessels and custom vessels this is not the case. Most are concerned with a maximization of the interior layout, and bulkhead placements are supplemental to this interior layout configuration. Yes, care is taken with the placement of the mast and the need to support the compression loads of the mast, but seldom are the shrouds and stays tied to a 'designed substructure’ of the vessel…they are simply and traditionally attached to the outer hull skins and ends of the vessel’s shell.

    This ‘flaw’ can be even more of a problem with a multihull vessel. The catamaran platform lacks the ‘backbone’ structure of the traditional keeled monohull, or central hull structure of the trimaran. Yet seldom is there any serious thought given to some sort of supplemental frame structure in these vessels. No wonder we see so much headstay sag. No wonder these boats can not carry a nice masthead genoa. Added to this equation, the shrouds are attached at a ‘shallow angle’ (half that of the forestay), so their aft-pulling capability is limited by both this shallow angle and the 3 point (tri-angled) configuration. Then at their point of attachment to hull sides (skins) there is likely no firm bulkhead backing, just skin. The shell of the hull is being asked to absorb the load, which it does in a ‘forgiving manner’ resulting in more forestay sag. It’s no wonder these vessels can’t carry a decent size headsail.

    Now some would say this is not important, just utilize a fractional rig and smaller headsails. Rather interestingly this same Seahorse article discusses the preferred selection of multiple headsails and overlapping foresails for this optimized race boat. Naturally I found this observation rather interesting due to my preference for headsails, but I won’t rehash those arguments here, just reference a few subject threads I’ve contributed to these discussions. **see below

    From another portion of the Seahorse article, “Alinhgi, Structural design:
    The below-deck cable and beam truss structure is clearly visible in these two pictures, tying in the torsional loads that cause conventional cats to twist under sailing loads. The first boat sported a full carbon truss/tie rod structure on the centerline, looking somewhat like one side of a mast. This takes the place of a trimaran’s main hull, to carry the very high fore-and-aft loads generated by the forestay, mast and mainsheet.”


    I have suggested such a fore-to-aft truss member be incorporated in the central wave splitting nacelle structure on my cruising designs:
    A Longitudinal Beam & Headsails/Headstay Load http://boatdesign.net/forums/showpost.php?p=111971&postcount=71
    …excerpt..“Imagine a flat, on edge, mounted down the centerline on the underside of the bridge-deck. This flat plate will act as a rib to strengthen the fore-to-aft rigidity of the vessel. If a tow-bundle of hi-tech fiber was laid along the bottom edge of this flat plate, the rigidity could be even greater, akin to a bottom truss structure”…(or one side of a mast).

    Back to the Sea Horse article,"On Alinghi the concept was taken an important step further. The aft portion of the central longitudinal truss member was divided into two separate members that ‘Y’ out to the stern beam. This completely solves the Achilles’ heel of catamarans, which is torsional deformation of the platform. Alinghi’s windward hull trims down by less than one-degree compared to the leeward hull at maximum load.
    Finite element analysis and a specially written truss-analysis program were used to calculate the loads on every structural member and optimize their stretch behavior under all conceivable loading conditions as some of these loads reach 40 tons."


    This torsional deformation subject matter is certainly an important one, not only in race boats, but cruising ones as well. It can prevent us from carrying our sailing rig in a most efficient manner, and it can rack a boat to a slow death.

    The big Polish RACE catamaran "Warta Polpharma" added a cross X bracing of hi-modulus cable between the inner corners of its fore-beam and aft-beam in an attempt to stiffen itself in torsion and racking. Team Adventure was also contemplating the same arrangement (in fact I believe they did such an installation right after the finish of the RACE in preparation for a trans-Atlantic speed run). Of course these are non-bridgedeck cats.

    Solid bridgedeck designs that we find with cruising cats are advantaged over the trampoline cats with respect to this cross-brace stiffening. But particular attention must be paid to the ‘flat-plate nature’ of most bridgedecks, and their construction composition, and their positive attachment to the main crossbeams in order that they are utilized most effectively.

    Many cats are seen to have basically a flat bridgedeck structure with some attached fore-to-aft stiffening beam/ribs on the bottom side. These attached beams/ribs do act to cut down the unsupported panel size and give a stiffer walking deck to the saloon, but they only act in a small manner to prevent the bridgedeck from bending fore-to-aft. And they don’t contribute to the athwartships or diagonal bending problems at all. If we give a little camber shape and/or add a corrugated channel shape to the flat panel deck we improve things a little bit further.

    Athwartships bending is a most serious concern as our rig’s shrouds are always acting to bend our vessel up in half around the mastbase pushing down. In open-deck cats the stiff crossbeams along with their dolphin strikers and gull strikers resist these bending loads. In our cruising cats it’s the main bulkheads we rely on to do this job. But so often we see vessels with less than desirable bulkhead arrangements. I believe it is very important that their be a minimum of two major bulkheads, one fore, one aft, and that akin to the open-deck boats these two bulkheads must be continuous across the whole beam of the vessel. And these bulkheads need some ‘beef’ rather than just be made of some ‘flat sheet’ of ¾ or 1 inch plywood or the sort. What’s wrong with a good stiff 2 - 5 inch thick panel of hi-tech sandwich cored material. Now you have a ‘bulk’head! Make sure you have a good bond between this super bulkhead and your bridgedeck and you are on your way to a stiff boat. Throw in a couple of ‘diagonals’ of hi-tech yarn (possible buried within the other structures, skins, and panels of the boat) to triangulate things.

    Now you’ve got a stiffer boat and some good substructure to mount your sailing rig to.

    In the particular case of my rather unusual mast-aft design, I tie the shrouds directly onto the same ‘super’ bulkhead that my mast is mounted on. And one of my backstays (the mizzen forestay) is tied directly to the same super bulkhead right at the base of the mast. Its upward force is countered by the downward force of the mast. The other two backstays are tied to another bulkhead at the stern, while the forestays are tied to the ‘flat plate’ rib structure down the centerline of the boat.


    **
    Sail Aerodynamics
    http://boatdesign.net/forums/showthread.php?t=457
    Mastheaded Vs Fractional rig
    http://boatdesign.net/forums/showthread.php?t=14089
    Mast-Aft Sailing Rig
    http://boatdesign.net/forums/showthread.php?t=623
    Cutter Rig to Windward
    http://boatdesign.net/forums/showthread.php?t=5596
     

    Attached Files:


  15. water addict
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    water addict Naval Architect

    This does not seem true to me. I cannot think of any sizeable modern boat where the shroud and stay attachment is not a specified structure to handle the rig load. Can you give an example of this? I'll be sure not to buy that type of boat.
     
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