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Square Top Mainsail, Shaping with less Mast Bend  Bottom

  • Wondering if anyone has tips for shaping a sq top mainsail for a Nacra 5.2. The mast wants to bend to the shape of the original Dacron main sail luff, whereas the Slo Sail square top is a much straighter cut along the luff. So in a close haul, even with tight diamond wires, the top third of the mast (beyond the diamond wires) bends a bit. Are people compensating by not rotating the mast as much?

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    ***********************
    Pairajacks
    1985 Nacra 5.2
    Corvallis Oregon
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  • Compared to how tight the diamond wires were under a soft rig, are the wires now scary tight?
    Are they tight enough to clearly see aft pre-bend in the mast without your sail up?And you moved your spreaders significantly aft?
    Yes, most cats with pre-bend use less rotation successfully.
    Hopefully you'll get a more specific response on the 5.2 from the NACRA members here.
    Bob

    --
    Sheet In!
    Bob
    ___/)________/)_______/)__/)____/)_____/)/)__________/)__
    Prindle 18-2 #244 "Wakizashi"
    Prindle 16 #3690 "Pegasus" Sold (sigh)
    AZ Multihull Fleet 42 member
    (Way) Past Commodore of Prindle Fleet 14
    Arizona, USA
    --
  • sails are typically cut to follow the mast/prebend



    Edited by MN3 on May 28, 2019 - 11:22 PM.
  • I have a 5.5 SL with square top. Following the sail makers recommendations (Skip E). I set up for prebend.
    1.5 to 1.75 inch spreader rake with 1 inch of prebend. The mast seems to be working fine with the sail.
  • I have a 5.5 SL with square top. Following the sail makers recommendations (Skip E). I set up for prebend.
    1.5 to 1.75 inch spreader rake with 1 inch of prebend. The mast seems to be working fine with the sail.
  • geepaksI have a 5.5 SL with square top. Following the sail makers recommendations (Skip E). I set up for prebend.
    1.5 to 1.75 inch spreader rake with 1 inch of prebend. The mast seems to be working fine with the sail.


    That's the same numbers Skip recommended for the stock pin head main on the 5.5 SL.
    I think it was 550 lbs on the loose gauge.

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    Ron
    Nacra F18
    Reservoir Sailing Assn.
    Brandon, Mississippi
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  • To answer klozhald, i could probably try to tighten up the diamond wire a little more. Im just not expecting much benefit because much of the bend occurs above the diamond attachment...about a third is above the diamond. Note that this is a new square top main sail that wasnt part of the original equipment. My 1985 nacra 5.2 has a straight post for the diamond..theres no rake. I would like to hear from some of the nacra folks starting to experiment with sq tops on their boats.

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    ***********************
    Pairajacks
    1985 Nacra 5.2
    Corvallis Oregon
    ************************
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  • The way a modern rig works is that the sale has extra luff curve compared to the pre-bend of the mast. In your case the mast is straight and the square top has a little bit of curve to it which will give it some extra girth. When you apply downhaul or Main sheet the mast will start to bend, and the sail will flatten out. Eventually the mast bend will be great enough the sail is as flat as a pancake. So when I sail my A-cat I now consider myself manipulating the mast to control the shape of the sail as an integral part of the rig.

    It’s not shocking at all to me that the mast will behave quite a bit differently with the new square top presumably made of modern laminate materials compared to a 30-year-old Nacra sail. It sounds to me like the mast is bending to flatten out your sail while you’re sailing in upwind. That sounds quite normal to me. So in my conclusion I’m not sure that you have a problem? Unless you have something else going on here?



    Edited by bacho on May 30, 2019 - 01:10 PM.

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    Greenville SC

    Offering sails and other go fast parts for A-class catamarans
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  • The question is what happens when your mast bends far beyond the point where the sail flattens out. My unerstanding is that if the cut of the main is 3 inches off vertical at the top, then the mast bend to flatten the sail would be 3 inches. What happens to the sail performance when your mast too easily bends to 8 inches?

    --
    ***********************
    Pairajacks
    1985 Nacra 5.2
    Corvallis Oregon
    ************************
    --
  • pairajacksThe question is what happens when your mast bends far beyond the point where the sail flattens out. My unerstanding is that if the cut of the main is 3 inches off vertical at the top, then the mast bend to flatten the sail would be 3 inches. What happens to the sail performance when your mast too easily bends to 8 inches?

    If I may paraphrase Bacho, your mast is doing what your sail is telling it to, through the cut of the material, and controls like downhaul and the mainsheet.
    If it doesn't look or feel right to you, this is the conversation to have with your sailmaker.
    Send or bring pictures.

    --
    Sheet In!
    Bob
    ___/)________/)_______/)__/)____/)_____/)/)__________/)__
    Prindle 18-2 #244 "Wakizashi"
    Prindle 16 #3690 "Pegasus" Sold (sigh)
    AZ Multihull Fleet 42 member
    (Way) Past Commodore of Prindle Fleet 14
    Arizona, USA
    --
  • QuoteThe question is what happens when your mast bends far beyond the point where the sail flattens out.

    wouldn't this be in direct relation to the amount of downhaul you set?
    couldn't you just reduce the amount of downhaul?
  • pairajacksThe question is what happens when your mast bends far beyond the point where the sail flattens out. My unerstanding is that if the cut of the main is 3 inches off vertical at the top, then the mast bend to flatten the sail would be 3 inches. What happens to the sail performance when your mast too easily bends to 8 inches?


    When too much (or too little) mast bend occurs you are at less than optimum trim for that given mainsail shape. There would have to be some force applied to make the mast bend past the cut of the luff curve. This could be any combination of diamond wires, downhaul (cunningham), Mainsheet, or even too much rig tension.
  • pairajacksThe question is what happens when your mast bends far beyond the point where the sail flattens out. My unerstanding is that if the cut of the main is 3 inches off vertical at the top, then the mast bend to flatten the sail would be 3 inches. What happens to the sail performance when your mast too easily bends to 8 inches?


    There is a little more to it than simply the luff curve. You’re also pulling back on the mast which opens up the leech at the same time.

    --
    Greenville SC

    Offering sails and other go fast parts for A-class catamarans
    --
  • pairajacksThe question is what happens when your mast bends far beyond the point where the sail flattens out. My unerstanding is that if the cut of the main is 3 inches off vertical at the top, then the mast bend to flatten the sail would be 3 inches. What happens to the sail performance when your mast too easily bends to 8 inches?


    What happens is the leech will open up and the top of the sail will twist off to leeward. This can be a good thing to some degree since a flat and twisted sail has a lower center of effort and reduced drag, so good in high wind applications. The problem is that if the mast bends too easily, the top of the sail will twist open in light wind reducing power and hurting your upwind performance.

    sm
  • And that is why modern rigs are far stiffer side-side then old rigs. More power and more tunable. The 5.2 rig is like a wet noodle in comparison. Increasing diamond wire tension may help but has limited impact on the top 1/3rd of the rig above where the wires stop.

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