Freestanding Rig Effect

Discussion in 'Sailboats' started by SuperPiper, Dec 29, 2011.

  1. SuperPiper
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    SuperPiper Men With Little Boats . .

    Listen carefully.

    At the end of the Canadian sailing season, I conducted a rig tension test. The shrouds were tensioned far greater than I would normally consider. The shroud-spreader combination induced some pre-bend into a normally rigid, normally straight mast. I was afraid that something was about to break. To my amazement, the tension on the forestay did not seem to follow.

    It was almost as if the shrouds and the sprung mast was a free-standing system.

    So my question to you: is there a configuration of shroud tension, spreader compression, and mast pre-bend that renders the whole system independent of the forestay?

    I've tried drawing vectors for this and I have created some theories, but I can't prove it to myself.

    I believe the pre-bend is the secret.

    What are your experiences?
     
  2. Petros
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    Petros Senior Member

    could you describe the rig in more detail? size, LOA, mast height, rig configuration, back stay?

    It seems to me that if the mast is bowed backward it would act like an archer's bow, the bend of the mast would put tension on the shrouds the way a bow puts tension on a bow string.
     
  3. SuperPiper
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    SuperPiper Men With Little Boats . .

    Petros, I like your bow analogy. It is an easy image to understand.

    Here is a cartoon of my pre-bent mast with the shroud tension causing the spreader to push the mast into the bow-shape. I put a lot of tension on the shrouds, but the forestay tension did not increase in the same proportion. It got me thinking that the pre-bent mast, the spreaders, and the shrouds were some kind of free-standing structure.

    There was NO backstay in this arrangement.

    Look at the cartoon. Could that arrangement stand independent of the forestay tension?
     

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  4. Doug Lord
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    Doug Lord Flight Ready

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    Seems like it would be real simple to build a model of the rig and test it. Seems to me that there is nothing preventing the rig from going backwards w/o the forestay.
     
  5. Stumble
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    Stumble Senior Member

    In most racing rigs the forestay really doesn't do much to keep the rig up while at the dock. The side shrouds, and the mast itself do most of that work. Where the forestay comes in is when working to windward where the main sail leech is also pulling the rig back, and the backstay loadst of course are pulling back the rig. This is how you control the headstay sag, and thus the angle of the jib to the wind.

    On really fun off shore racing boats you actually detach the forestay while jibeing the spinnaker pole....
     
  6. Ilan Voyager
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    Ilan Voyager Senior Member

    The "effect" is well known since more than 100 years on sloops. It's basic geometry and trigonometry. The angle of the shrouds with the forestay is too small to have a significant effect on the forestay as with more tension you'll get mainly a compression than will be bend the mast (the good old Euler)...while the mast bends the fixation of the forestay will go a bit down (toward the deck, less height if you prefer) so it will mostly cancel the gain in tension of the forestay and a new static equilibrium will be obtained, where the shrouds have more tension, the mast more compression (that won't last a long time...) and the forestay has virtually no changed of tension.
    To change the tension of the forestay efficiently you have 2 possibilities; shorten the forestay (hydraulic of mechanic) ou to have a better angle. Sorry my english vocabulary on sailboat rigging is very short and I'm too lazy to spend more time looking for it. A scaled drawing will show you the explaination: keep the length of the mast (but the height of forestay fixation) will be less as the mast bends) , and the length of the forestay are invariable. The shrouds can change of dimension...
     
  7. Petros
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    Petros Senior Member

    In normal sailing conditions the sail drives the boat forward and there is not much tension on the fore stay. The two shroud lines are placed aft of the mast step, so when the mast is pushed forward the shrouds are in tension, mast in compression. The fore stay is just to prevent the mast from falling over backwards, or to adjust the amount of curve in the mast (to change the shape of the sail, flatten or give it more camber).

    if the mast step is holds the mast to the deck, or if the mast it goes through the deck with the step near the keel, than the mast will stand free and the shrouds will bow it over backwards without any tension on the fore stay.

    by slackening the two side shrouds and shortening the fore stay you also will move the mast forward, moving the center of effort on the sail forward. This can be used to balance the tiller and alter the way the boat handles. if the mast is canted fare enough forward the weight of the mast will put tension on the shrouds and leave the fore stay slack.
     
  8. MalSmith
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    MalSmith Boat designing looney

    No.

    The bow analogy is right. Most of the extra tension was going into compressing the mast to create the pre bend. But with a deck stepped mast you will still need the forestay to hold the mast in position.
     
  9. sharpii2
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    sharpii2 Senior Member

    To get better fore stay tension, you really need back stay(s) of one sort or another.

    They come in two flavors:

    1.) Standing, where the back stay connects with the top of the mast and leads back to the transom on the center line. With this kind, your mainsail has to have a triangular head, so it can be set within the triangle, aft the mast, it creates.

    2.) Running, where you have two that connect to the mast at the same height as the fore stay and come down to your side decks or transom corners. On every tack, the lee one is severely slackened, or released, and the windward one is tightened. These are used on rigs that have rectangular heads to their mainsails that can not swing past a standing back stay.

    What you appear to have are shrouds with some aft drift. These, in concert with a fore stay, will hold up a deck stepped mast. This set up is quite popular on trailerable sail boats. To get the best fore stay tension, you want the mast as straight as possible. Even then, your head stay tension will be adequate at best.

    When I owned a boat with that kind of rig, the jib was first to come down, before anything was done to reef the main. This was because I knew the fore stay would sag more and more as the wind increased.
     
  10. Silver Raven
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    Silver Raven Senior Member

    "Petros" - Is it possible that "In normal saling conditions the sail drives the boat forward" might be - 'that in normal (whatever the heck that is) sailing conditions the sail 'pulls' the boat forward?? Does not a airplane get lifted into the air & not - in fact pushed up into the air??

    I've designed, built & raced catamarans with large - percentage wise - wing-masts since 1967 with some notable (in our small puddle - out here in Australia) success & obtained excellent windward ability by - over-rotating the wing - to the aft soft-sail flap & to the course. Pressure variance.

    However - the question asked was would a - pre-bend mast work without a forestay - yes if Aladin had a rope from on high holding it up - otherwise it would just fall down backwards - not so good for sailing - me thinks. james
     
  11. Perm Stress
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    Perm Stress Senior Member

    *In normal upwind sailing conditions internal forces in rig and sails is an order of magnitude bigger as driving force created by them.
    *On jib, tack is pulled back and up, entire length of luff is pulled back, to the side and down, head is pulled almost directly down and a bit to the side, clew is pulled forward and aft; when summing up the forces, we find that only some surprisingly small percentage of sheet tension is actually driving the boat, best part of it is "consumed" to counteract tack and luff forces, directed backwards.
    *On mainsail, picture is in principal the same, with a difference that only small part of boom push to the mast is actual driving force (here i assume the mainsheet is vertical, as it is mostly the case)
    *As luff of jib do pull the headstay back and to the side, forestay tension create some forward force; however, if this force do actually MOVE the attachment point forward, depend on relationship between headstay and mainsail leech tension and rig configuration.

    *with masthead rig, main has to be ~1.5 times bigger in area as jib, to make headstay and mainsail leech forces balance. As it is rarely the case in reality, top of masthead rig is normally pulled forward from static position.
    *with fractional rig, there is lever advantage for mainsail leech to act, not to mention "naturally" bigger main relative to headsail. So, with fractional rigs, it is not unusual for headstay attachment point to be pulled back from it's static position when sailing. And the more you increase leech tension in main, the straighter the headstay will be.

    ______________________________

    To SuperPiper

    First and foremost, I assume here that your rig is fractional (maybe bakstayless), with really large main, as used on sportboats and dingies.

    Increase in tension of shrouds as you describe, actually do little to increase static tension of forestay, because of large difference in angles involved. Normally angle between shrouds and axis of mast in side view is ~5 degrees, while angle between axis of mast and headstay is in the order of ~15...20 degrees. Even from here, if you increase tension in shroud (as projected to CL of boat!), only ~1/3 of this increase will vent to the forestay. More so, because shrouds are angled more to the side as to the aft from the mast, necessary increase in actual shroud tension become once more much bigger. At some point non-linear behavior of mast and all the system will kick in, reducing forestay tension gain even more. Exactly what you did observe.

    More important is, however, the other effect.
    The more you tension the shrouds the way you describe, the more rigid the whole assembly becomes when sailing.

    With the rig I did assumed, shrouds could be completely slack when at rest, and you will be still able to hoist your sails and go. BUT, with every puff of wind, with every pull on mainsheet, the mast will bend aft and to the side, flattening the main, opening it's upper part => dumping the power. It is perfect for strong winds, especially so when the wind is extremely gusty. (like wind in the lee of hills or industrial buildings, here we call those condintions "hammers" :) ) Very little operator input is required to react to the gusts, and keep the boat on her feet. On the other side, if wind is not so gusty, or not so strong, you are at great disadvantage, as "power" of the rig is reduced.

    With shrouds tensioned "as wires of piano", the assembly became rigid. So, the mast bend much less to the puffs of wind, nor to the pull on the mainsheet. Than you are at full power in light winds. At cost of much less "automatic" responsiveness to gusts. When sailing with highly tensioned rig in gusty conditions, it will be up to you to keep the boat on her feet by quickly and accurately working her mainsheet and traveller.
     
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  12. dinoa
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    dinoa Senior Member

    Intuitively I can see that the mast SuperPiper describes can be supported by mast bending, shrouds and spreaders alone. Working it out is more difficult but here is my perception of it:

    Take a strung bow and stand it on one of its tips on deck and it will fall over.

    Now detach the string from one end and split the string into 2 threads. Step the unstrung end of the bow on deck. Restring the bow attaching the 2 free ends of the thread as shrouds aft of the mast step. The mast now stands but is free to rotate on an axis joining mast tip and step.

    Add the spreaders and it is now self supporting.

    Dino
     
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  13. philSweet
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    philSweet Senior Member

    Please post three or four sets numbers you recorded and the length measurements between mast and shouds/stay. Best guess is you goofed. Different forestay size compared to shrouds? The proportion of stay tension to shroud tension should be close to constant over any practical range of tensions. How were you gaging tension.
     
    Last edited: Jan 3, 2012
  14. SuperPiper
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    SuperPiper Men With Little Boats . .


    Dino, that is exactly the image that prompted me to ask the question in the first place.

    Since posting and then reading all the responses, I've realized how easy it is to confuse me. The fact that increasing the tension in the shrouds did not seem to increase the tension in the forestay led me to the deduction that the rig was independent of the forestay and was therefore freestanding. As described by a number of you, it's all a matter of proportions. I've redrawn the cartoon for the extreme case (Dino's bow) where forestay tension is absolutely unaffected by shroud tension.

    I did not have a tension gauge to quantify any of my observations.

    The rig in the original sketch might have stood by itself if the centre of gravity of the mast was forward of the mast step and the mast wanted to fall forward due to its own weight. This might have been handy while adjusting the rig, but it would not have been sufficient to eliminate the forestay while sailing.

    Do you think?
     

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  15. MalSmith
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    MalSmith Boat designing looney

    This discussion seems to suggest that the longer you make the spreaders, the more prebend you will get but the less forestay tension you will have. To have prebend control and forestay tension, you would need to add a set of lowers from the chain plate to the spreader attachment point on the mast.
     
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