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Thread: Questions on sheet geometry

  1. #1
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    Default Questions on sheet geometry

    This is for a sharpie skiff, cat-ketch rig and Leg-O-Mutton sails. Drawings by Chapelle from the Smithsonian, so please don’t suggest consulting the designer. Thinking about the geometry of the mainsheet, specifically the anchor point to the hull.
    This is what I think I know: It is my understanding that given the geometry of the spars, and the sheet attached to the boom at the extreme outboard point, it is essentially “self- vanging”. Using a rope or wire horse with a loop in the center appears to put the pull of the sail to the windward side and raises the point of effort, negating the self-vanging. Clearance of the rudderhead and tiller is not an issue here.
    Using a rope or iron horse with a sliding ring or block appears to put the pull of the sail to the leeward side.
    Anchoring the sheet block on the centerline puts the pull of the sail, well, on the centerline.
    Then, there is the sheet arrangement illustrated in G. Sherman’s wonderful UNA thread. This appears to split the difference of sail pull points, but limits the travel towards the centerline.
    What are the advantages or negatives to having the sail pull points , windward, leeward or center.
    I came here of my own free will looking for an education, so feel free to skool me and correct the errors in my thinking. Thanks in advance.
    On a similar note, would the fore sail be sheeted like a jib, or sheeted as the main? The same questions apply.

  2. #2
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    Default Re: Questions on sheet geometry

    The sail is not self-vanging but when you're hard on the wind with the main sheet pulling from the leeward rail, the boom will be pulled down a bit. The problem is that you will be oversheeting if you drag the boom in so it's right over the rail.

    So no matter what you do, there's some boom lift. Which you'll find just fine for most sailing as the more wind, the more boom lift, which means the more the head of the sail sags off depowering the upper part of the sail so you're not heeling as much as you would were the sail fully full.

    A leg'o'mutton sail is not happy when sheeted to weather.

    For simplicity, I'd sheet to the center so you don't have a traveler (in this case a rope gunnel to gunnel) in the middle of things inhibiting movement.

    G'luck

  3. #3
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    Default Re: Questions on sheet geometry

    Boom lift creates upper sail twist to leeward. This can be bad at times, as the top of the sail will be spilling some wind power, but it can also be good if you are trying to depower a bit in heavy air. The wind itself is also slightly twisted up higher, and a sail which is properly sheeted down low would usually be over-trimmed up high if it had the same angle of attack without a few degrees of twist up there. End-boom sheeting is not self-vanging unless you happen to have some sort of curved traveler track or horse. On most boats without that stuff, easing the main out will also increase twist, sometimes greatly, and you may still want some sort of simple vang up front. If you're serious about maintaining the best possible sail shape and efficiency, a simple boom vang is probably going to be well worth having.

    The biggest drawback to mid-boom sheeting with long booms is that it will usually bend the boom when sheeted hard. This uses up the sail's draft-creating foot curve and can yield an overly flat lower third or so of the mainsail. The amount of draft lost (or not) will depend on boom length and flexibility.

  4. #4
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    Default Re: Questions on sheet geometry

    To clarify, do you have booms or sprits on this rig? If similar to this boat, they are sprits and are indeed self vanging, in which case the sheet anchor is not near so critical.

    -Dave

  5. #5
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    Default Re: Questions on sheet geometry

    Bending the boom with mid boom sheeting is interesting. When I first got my old gaff rigged iceboat I didn't understand why there was a wire bridle on the boom with turnbuckle to tighten it, as well as a padeye fastened to the boom at the mid point of the bridle. I found that when you set it all up right you distributed the mainsheet tension over three points rather than a single point. Very little boom bend when horsing down on the 4 x 1 mainsheet tackle at iceboat speeds.
    Ben Fuller
    Ran Tan, Liten Kuhling, Vernon Langille, Josef W., Merry Mouth, Imp, Macavity and a quiver of unamed 'yaks.
    "Bound fast is boatless man."

  6. #6
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    Default Re: Questions on sheet geometry

    Doesn't the amount of mast rake also figure into what advantages and disadvantages there are in the choice of sheeting arrangement?
    "When I was a boy with never a crack in my heart."

    -W. B. Yeats

  7. #7
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    Default Re: Questions on sheet geometry

    Thank you Gentlemen for your informed replies. This is the information I am looking for. Mr. Woxbox, yes the rig shown above is THE rig I am referring to. Yes they are sprits. I think I have a handle on my original questions, that anchor point is governed more by what is necessary to make it work within the surrounding hardware. I am concluding to go with the single centerline mount, no bridle or horse, for simplicity sake. I'm still trying to wrap my head around the physics tho, for knowledge sake. Given Mr. Bradshaw's lesson on the sail twist issue, and the apparent "self-vanging" of the sprit geometry, is twist a desired feature or no? This photo,shamelessly borrowed from the thread Charles W Morgan Restoration; A Volunteer's Perspective-1 post# 2178 , by Morgan Volunteer (Thank You), illustrates the twist you describe. https://c1.staticflickr.com/5/4230/3...6af4d02b_b.jpg As seen, the boom is riding quite high towards the lee, and the top of the sail spills the wind. Is this a good thing, or no, and why? Would you vang it or no, and why? Again, Thank you all for your patience. I'm learning.

  8. #8
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    Default Re: Questions on sheet geometry

    Thank you Gentlemen for your informed replies. This is the information I am looking for. Mr. Woxbox, yes the rig shown above is THE rig I am referring to. Yes they are sprits. I think I have a handle on my original questions, that anchor point is governed more by what is necessary to make it work within the surrounding hardware. I am concluding to go with the single centerline mount, no bridle or horse, for simplicity sake. I'm still trying to wrap my head around the physics tho, for knowledge sake. Given Mr. Bradshaw's lesson on the sail twist issue, and the apparent "self-vanging" of the sprit geometry, is twist a desired feature or no? This photo,shamelessly borrowed from the thread Charles W Morgan Restoration; A Volunteer's Perspective-1 post# 2178 , by Morgan Volunteer (Thank You), illustrates the twist you describe. https://c1.staticflickr.com/5/4230/3...6af4d02b_b.jpg As seen, the boom is riding quite high towards the lee, and the top of the sail spills the wind. Is this a good thing, or no, and why? Would you vang it or no, and why? Again, Thank you all for your patience. I'm learning.

  9. #9
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    Default Re: Questions on sheet geometry

    Sorry guys, for the duplicate post. having trouble with my connection. Must be a twist in the link.

  10. #10
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    Default Re: Questions on sheet geometry

    There are (at least) three reasons why a certain amount of twist is desireable.
    - Spilling some wind prevents excessive heeling.
    - The jib "bends" the air, so that the wind on the lower part of the mainsail is slightly more against you than the wind higher up.
    - Ever noticed that you can go higher up against the wind in a gust? Usually, the wind higher up is stronger than the lower down wind.

    /Mats
    My blog about my time as a boat building student and as a rigger apprentice http://kaptenmohsart.blogspot.se/ in Swedish only, but there are many pictures :-)

  11. #11
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    Default Re: Questions on sheet geometry

    If your sail does happen to be a sprit-boomed sail, as shown above, then it will be self-vanging to a great degree. Those sails operate with a triangle of tensions, with the triangle's edges being along the sprit boom, along the sail's foot (which should be cut pretty straight) and along the sail's luff between the tack and the mast/boom intersection. This triangle effectively locks the sail into a pretty non-twisting mode, and there isn't much you can do to generate more twist, even if you want to. You won't need a vang, as it wouldn't do anything.

    Is this situation a good thing or a bad thing? Sometimes both. You'll probably get a few unavoidable degrees of twist, which will help to offset the wind up there being slightly twisted, which is fine, and the lack of excessive twist will make sure that all of your upper sail area is working for you. On the other hand, you won't be able to induce more twist to depower in a blow the way you could with a conventional boom, which on some boats at some times can be very helpful.

    The gaffer in that photo is very close to having a seriously undesirable situation going on. When you ease out a gaff sail, you generally want to pay close attention to the gaff, and it is generally not wise to let it go any farther forward than 90 degrees to the keel line max (straight out to the side). This may limit how far out you can ease the boom, because the gaff goes first and farther. A boom vang can help in that situation. Those guys are sailing awfully close to the point where bad things can start to happen, in terms of sail and boat control.

  12. #12
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    Default Re: Questions on sheet geometry

    A bit of twist is desirable because the wind is fairer aloft. This is the reason that square riggers had a twist in the set of their squares.
    Jay

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