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Sail entry and exit angle

  • 27 Dec 2016 15:19
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    Reply # 4478156 on 4475531
    Deleted user

    That's fair, David.

    Working the mainsail, it's clear that the exit angle on  a 7% camber sail at max camber of 37% chord can be no less than 6.3 degrees (straight line to the max camber point), and if I'm attempting to keep the aft 40% flat the exit angle can be no more than 12.7 degrees. So I split the difference.

    I chose a 30 degree entry angle.  Just because, I guess.  Then aimed to have a constant rate of change of the rate of change of the slope of the tangent line to the curve forward of the max camber point.  (Go ahead - try to read that and make sense of it.  I dare you.  Basically, the forward portions develop curve faster than the aft, up to the max camber point.  This is different than what Slieve suggests in one of his chapters on the split rig.  He suggests a constant curve - the arc of a circle, but I'm thinking the more we can develop early, we have a longer length to minimize the pressure gradient all the way out to the leach.)  I have no doubt that it would have been simpler and more practical to just bend a flexible batten around an actual series of points, but I got carried away on the spreadsheet and false precision.

    In any case, I get a curve where the greatest changes to the slope happen way forward and ease off as it gets to max camber, then slopes as gently as possible to match the aft flat run.  


    In the image above, you can see the slight bit of hollow in the aft run to keep the panel flat.  Too much?  Too timid?  I'm also planning on hollowing the leach (the true trailing edge, not the panel round) a bit to keep that last little bit good and tight.

    Last modified: 04 Jan 2017 14:23 | Deleted user
  • 26 Dec 2016 21:06
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    Reply # 4477545 on 4475531
    Anonymous member

    The only answer that I can find in my reading around the subject is that in theory, there is no single answer - the entry angle ought to vary with wind strength, sea state, shape of hull, cleanliness of hull, and doubtless several other factors.

    But with a junk sail, we can't vary the entry angle easily, so we have to choose one. I can only say what has worked for me - an angle somewhere between 25 and 30 degrees seems to be about right; I can't be more precise than that.

    The exit angle is easier. I try to keep the after half of the sail flat, no matter what camber I'm aiming for. 
  • 24 Dec 2016 18:23
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    Message # 4475531
    Deleted user

    In Appendix 5, Thoughts on Cambered Panels and the Split Junk Rig, Slieve lays out the arguement that we need to focus on four parameters when designing the sail shape for a cambered panel.  

    1. The amount of camber 
    2. The maximum position of the camber
    3. The entry angle
    4. The exit angle
    He also states, "All indications are that the entry angle should be as large as practical in en effort to combat the weaknesses in the junk rig's performance, and equally the exit angle should be as low as possible for the same reasons."



    Sold.  I'm in.  But, but...   what is the largest practical entry angle?  Is it a function of the max camber?  How about the smallest practical exit angle?  I'll make test cases to see if the actual sail hits these angles (with the use of hollowed leeches and concavity in the aft runs to make it flat), but what numbers am I shooting for?

    I've searched this one, but I may be missing a conversation in which this was discussed/resolved.  
    Last modified: 24 Dec 2016 18:25 | Deleted user
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