Calculating sail area

Discussion in 'Sailboats' started by Mikthestik, Jun 2, 2016.

  1. Mikthestik
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    Mikthestik Junior Member

    I have researched the best proportions for a Gaff sail to be Luff =1 Head =0.883 Leach =1.73 Foot = 1.02. I don't know why that should be but Gaff sails are tricky for some people to calculate. So I thought I could help. It is possible to draw it out and make triangles and calculate 1/2 * base * hieght that could get tiresome for a Junk sail. Here is my alternative.
    1/ The Circumferance (outside edge) of a circle is C = PI*D PI =3.1427.
    2/ Sails are not circular.
    3/ worked example:- A 20 foot LWL boat needs a Gaff sail. The designer Desides on a 55% boom(foot) = 11ft therefore the other dimentions are luff = 10.78ft leech =18.65ft head =9.52ft. The circumferance of the sail is 49.95ft it is not a circle but if it were it diameter would be C/PI or 49.95/3.1427 = 15.9ft.
    area of a circle is PI * r * r radius = D/2 = 7.95
    3.1427 * 7.95 * 7.95 = 198.6 sqft.
    The Gaff sail is 198.6 sqft and I didn't have to draw it.
     
  2. PAR
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    PAR Yacht Designer/Builder

    I don't think there's any "best proportions" for a gaff sail, though there certainly are very general guides for traditional shapes. Trapezoidal shapes aren't very hard to figure area on, though if doing it manually, a little tedious, compaired to a Bermudian or jib. The real key is to use a shape that'll place the CE where you need it. You can simply move the mast, but this can present issues with where a berth might live, bulkhead locations, etc., so it's best to adjust sail shape or rig proportions.
     
  3. Mikthestik
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    Mikthestik Junior Member

    Yes Par its on the net, I didn't believe it either.mik
     
  4. PAR
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    PAR Yacht Designer/Builder

    I don't know who published "ideal gaff dimensions" but there's also evidence of alien abduction online too, so take it for what it's worth, which isn't much in my opinion. I've designed no small number of gaffers, over the years and not one of them is the same proportionally or dimensionally. We all have preferred things to do, in regard to gaff length and angles, luff to foot percentages, etc., but most of the time, the elements of the design force our hand in regard to these figures, rather than a formula making us reconsider were a mast might live.
     
  5. DCockey
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    DCockey Senior Member

    The area of a quadrilateral is not the same as the area of a circle with the same circumference. In fact the area of the circle will always be larger than the area of the quadrilateral. Consider a square; each side 1 long for circumference of 4. Area of of the square is 1. A circle with a circumference of 4 has an area of 1.27.
     
    Last edited: Jun 3, 2016
  6. gonzo
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    gonzo Senior Member

    Are you proposing a circular sail?
     
  7. gggGuest
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    gggGuest ...

    Not in any case if my memory of long ago lessons serves me correctly.

    And I also make the area of the example sail 132 sqft (assuming a 90 degree angle mast to boom, which might not be true, and a completely flat sail, which ought not be true).

    I too find it very hard to believe there can be one ideal planform for all gaff mainsails.
     
  8. DCockey
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    DCockey Senior Member

    I corrected by post above. A circle always has the maximum area of any planar shape for a given circumference.
     
  9. Mikthestik
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    Mikthestik Junior Member

    Please accept my apologies I'm guilty of bashing the wrong calc buttons and not double double checking results. However I didn't dream up my research.

    The best proportions for a gaff mainsail are luff 1.0, head 0.833, leech 1.73, foot, 1.02. The angle of the gaff to the centerline of the mast should be 30 degrees. The rake upward of the boom should be 6 degrees. If a topsail is to be carried, then the angle of the gaff should be eased to about 42 degrees. John Leather. Comment: I seem to remember that Bolger favours 45 degrees for a gaff, while it is clear from drawings and photos that Nigel Irens likes them to be nearly vertical. You choose. :eek: mik
     
  10. Doug Lord
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    Doug Lord Flight Ready

    Gaff

    This is a gaff main I really like-it also happens to be on one of the fastest boats of its time. The proportions might be worth some study.......

    [​IMG]
     
  11. CT249
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    CT249 Senior Member

    I'm no gaff expert, but I've had reservations about Leather since I read his comments in the reprint of Dixon Kemp's Manual, where he is so critical of the works of others. Given that I've never been able to find a John Leather design that could win significant races, it seems odd that he appears to believe that he is correct and Herreshoff, Nicholson, Fish, Fife, Watson, Crane, Burgess and all the dinghy designers were wrong.

    As others have noted, there was a wide range in the proportions of gaff mainsails. Something like a 50' fishing boat that spends of its time reaching around dragging a net is obviously going to have different ideal mainsail proportions to something like a Renjolle, which had no restrictions and found that an almost vertical gaff was faster.

    [​IMG]

    As another example, I think it was Warrington Smyth who pointed out that in some Dutch craft, the proportions of the sail was designed to allow for the fact that they sailed in narrow waterways where much of the sail was hidden in the lee of the trees. When a sail's proportion has to vary so much according to use and environment, there is no rule that is always right.
     
  12. gonzo
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    gonzo Senior Member

    Doug Lord: that is not a gaff but a gunter.
     
  13. PAR
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    PAR Yacht Designer/Builder

  14. gonzo
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    gonzo Senior Member

    This can turn into a debate like the one about when does a boat start to plane.
     

  15. DCockey
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    DCockey Senior Member

    My understanding is the upper spar is parallel to the mast in a gunter rig, and angled away from the mast in a gaff rig.
     
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