SA/D

Discussion in 'Sailboats' started by TiPegleg, Sep 5, 2021.

  1. TiPegleg
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    TiPegleg Junior Member

    I’ve got a boat that I’m looking at that has me questioning a few thing, one of them was what I thought was settled - sail area displacement ratio.

    the boat is heavy and now the SA/D is making it just ridiculously so... or am I doing it wrong? I always thought SA for the calc was 100% fore triangle plus main, but the thing is that would give me a sail area of 625sqft for a boat that weighs 18t in the slings.

    I read Eric Sponberg’s paper on design ratios and he states that the “upwind sail area” should be used... so I should calculate with a 130% Genoa? That would give me 935sqft which put the boat in line with heavy cruisers. Eh?
     
  2. jehardiman
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    jehardiman Senior Member

    While SA/D can be used to compare the expected performance of like hull forms, it has very little to do with the ability to carry sail or the designed task of the vessel. Take for example old "letter" or meter boats that go to windward like they're on rails; the have very small SA/D ratios compared to modern broad flanked composite fin keelers designed for off the wind.
     
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  3. bajansailor
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    bajansailor Marine Surveyor

    Welcome to the Forum TiP.
    Can you tell us please what make / class this boat is?
    Or if she is a 'one off', can you post a photo of her?
     
  4. TiPegleg
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    TiPegleg Junior Member

    Agreed but I’m trying to compare apples to apples using this ratio. This boat in particular is a Joubert designed center boarder. The owner says that in slings it weighed in at 18t, the plans I saw say that it should displace 14t. Either way, for a lot of the center boarders I’ve see, 635sqft isn’t a lot of sail for a 42ft boat... using ratio correctly would help me out. This is very much a cruising boat.
     
  5. jehardiman
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    jehardiman Senior Member

    Yeah, I can believe a 42ft live-aboard cruising boat being 4 tons overweight...and lucky if only that. Given that a LOA of 42' is ~36' LWL, a designed displacement of 14t gives a mid 200 D/L. So it wasn't designed as a speedster at all. So given that it is a center boarder, it leads me to expect a shallow draft long keel cruiser hull form. That type doesn't need all that much sail area, and too much would be unsafe. What exactly is the boat and the rig? I'd expect a ketch rig to make sail handling easy.
     
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  6. bajansailor
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    bajansailor Marine Surveyor

    Is she similar in concept to the 42' Maracuja designed by Philipe Harle?
    Here is one for sale.
    MARACUJA 42-1993 https://www.ayc-yachtbroker.com/maracuja-42-0

    If she is, then I can say that they do sail well - some friends had one for a number of years.
    They crossed the Atlantic in 14 days flat from Tenerife to Barbados, averaging a steady 8 knots the whole way.
    And they collected some nice prizes when competing in the Bequia Easter Regatta over various years.
     
  7. TiPegleg
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    TiPegleg Junior Member

    I love the Maracuja. The one you linked to sold about 2 years back and I wasn’t quite ready to buy so definitely missed out. The boat I’m talking about is a sister ship to this Odd Life Crafting https://www.oddlifecrafting.com/ honestly, I think the Brazilian yard that build these boats used the Meta Dalu 40 as a template and built in steel instead. What’s really amazing about these hulls is that they are flame galvanized and even with an endoscope in all kinds of holes I found zero rust.
     
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  8. AJB
    Joined: Jul 2021
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    Location: 31 42S 152 04 E

    AJB Junior Member

    Tipeg,
    Perhaps equally important is the SA:Wetted surface, as around half of the drag at cruisng speeds - say 7.5 knots in your case - is viscous.

    Contemporary cruiser racers are generally in a range around 3.2. actual sail areas on non-overlapped jibs.

    Your wetted surface is probably around 350 sq.ft. ( say 33 m2) so a SA/WS of 3.0 would suggest 1050 ft2
     
  9. TiPegleg
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    TiPegleg Junior Member

    One of the documents that the current owner sent over is this. You can clearly see that the plan says meta Dalu, but the boat is a tropic 1200 483C9EB4-36C2-4FFF-8F92-1F9D836BA79B.jpeg
    The other doc was for when the original owner had the current sails made.
    FA71540F-0E97-4B61-9368-57FBCE64CD47.jpeg
     
  10. jehardiman
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    jehardiman Senior Member

    Ok, after seeing her lines (in the sister ship photos) I now see the issues. All steel...that shape....center board....yeah, someone would need to do some pretty sharp pencil work before increasing sail area.
     
  11. SuperPiper
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    SuperPiper Men With Little Boats . .

    Ti, just to be sure, the displacement is raised to the power of 2/3, right?
     
  12. sharpii2
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    sharpii2 Senior Member

    According to the drawing, I come up with a total of 78.1sm, which comes to about 840 sf.

    This includes the overlapping bigger jib and the main counted as a simple triangle. It probably has some kind of a roach, which would add around 10% more area to it at the least.

    This would increase the SA to 81.8 sm or 880 sf.

    At 18t, this gives an S/D of almost 12.0.

    At 14t,this gives an S/D of a little over 15, which would be typical for its rig type.

    I suspect that the 14t is the weight of the boat with empty tanks and no stores.
     
  13. sharpii2
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    sharpii2 Senior Member

    Yes.

    But the displacement needs to be in cubic units of volume, not pounds or kilograms.

    Since this is a metric designed boat, you can use cubic meters.

    This is very convenient, as 14t equals 14 cubic meters.
     
  14. TiPegleg
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    TiPegleg Junior Member

    So this boat will sail like a pig? That was my big fear.
     
    Last edited: Sep 6, 2021

  15. jehardiman
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    jehardiman Senior Member

    Not necessarily a pig. If you are part of the "fast is fun" crowd who wants to be doing sundowners in the yacht club by 1500, then yes...it could be seen as a pig dog. If you want to comfortably solo sail the Grand Bahama Bank without caring what day it is when you snuggle up behind that Exuma islet with the pure white sand beach and swimming pigs...then it is just perfect. Horses for courses....
     
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