weight or sailarea?

Discussion in 'Multihulls' started by Brorsan, Oct 4, 2012.

  1. Brorsan
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    Brorsan Junior Member

    Which 8.5m cat would be the fastest in about 10-16 knot wind:
    Disp 900kg and 40sqm sail upwind or 1000kg disp and 48sqm sail upwind?
    Any guesses?
     
  2. PAR
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    PAR Yacht Designer/Builder

    Which car would be faster up hill, a 1,500 kg with 400 HP or a 2000 kg with 450 HP?
     
  3. upchurchmr
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    upchurchmr Senior Member

    No guess, not enough to even start with.
     
  4. Richard Woods
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    Richard Woods Woods Designs

  5. Doug Lord
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    Doug Lord Flight Ready

    --------------------
    A factor will be the sail loading(works for foilers and some multihulls):

    1) 900kg and 40sqmSA=22.5 kg per sqm

    2) 1000kg and 48 sqm =20.8kg per sqm

    If L/B ratios, OA beam, sail planform shape, waterline length, were the same, and crew weight proportional, #2 might have a slight edge.....
     
  6. idkfa
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    idkfa Senior Member

    1)Higher L/disp with lower SA/disp

    2)Lower L/disp with higher SA/disp

    I'll go with option 1), efficiency beats power, and especially in moderate(enough) wind situation.
     
  7. upchurchmr
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    upchurchmr Senior Member

    Actual condition and build of the sails, hulls and blades, and the sail control setup will make more difference than a simple ratio.

    Sailors ability have something to do with it also.

    Take all the junk out of the boat also.
     
  8. Doug Lord
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    Doug Lord Flight Ready

    Thought I'd use the accepted ratios for design comparisons(see pdf below) plus what I did in the previous post-except in lbs and sq.ft this time(c. below)-and see what showed up:
    A note on DLR and SA/D:
    --DLR lower is better
    --SA/D higher better
    --W/SA lower lbs per sq.ft.SA better(not in Design Ratios pdf)

    1) LWL-27.9', Displ-1980lb, SA-430sq.ft.
    a. SA/D=43.61
    b. DLR= 40.52
    c) W/ SA= 4.6lb per sq.ft SA
    --------
    2) lwl-27.9, Displ-2200lb, SA-516sq.ft.
    a. SA/D=48.72
    b. DLR=46.04
    c. W/SA= 4.26lb per sq.ft SA

     

    Attached Files:

  9. upchurchmr
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    upchurchmr Senior Member

    Doug,

    Which one had the lifting foils?
    Which one had a wave piercing bow?
    Which one had a taller mast?
    Which one had a full deck?

    Why are you supporting something with no real answer?

    I guess I should ask my self, why do I care about this question everyone on this forum would normally say more detail is needed?
     
  10. Doug Lord
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    Doug Lord Flight Ready

    --------------------------
    I did say more detail is needed* and I provided more-but not enough. Just thought I'd see how far I could go w/o more info. Good practice.
    * "If L/B ratios, OA beam, sail planform shape, waterline length, were the same, and crew weight proportional, #2 might have a slight edge..... "
     
  11. Brorsan
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    Brorsan Junior Member

    Upcurchmr, if you have nothing more to say, then please keep quiet. Thank you.

    I guess a lighter boat will have an adventage in smooth waters due to less wetted surface, and better wl/disp ratio.
    But in choppy conditions with lulls and puffs the heavier boat with more power will push through that chopp and keep the speed more stable through the lulls and puffs.
    When the big sail area boat reef, the bennefit is lost and the lighter boat would go better, at least downwind.
    Thats my thoughts, whats yours?

    Idfka, i like that argument. I dont know, but i wish it is true.

    Mr. Woods, do you consider your Gwahir and Wizzer designs as mid sail area, low disp designs? (they are rather fast designs as i have understiod from yor site)
     
  12. idkfa
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    idkfa Senior Member

    The heavier boat does have a better SA/disp ratio, and in light wind, (they) will be travelling at low Fn, and option2 will the advantage, it also has a better SA/WA ratio, which is also significant at low Fn.

    Doug why assume similar l/b, less weight for same length gives lower l/b, and especially as drag is non-linear at higher Fn!

    upchurchmr, look at the pics of the cars, which is faster? sure there are other factors but if you had the hp/weight one could make an educated guess? Certainly other factors like Cp, and LCG (wave piercing bows??) can change things.

    Adding weight means you have to find power to accel that weight to punch through, so if that weight powers more sail area, fine, like an additional body on the wind rail, if not then the additional drag can be more sig at higher Fn.


    When the wind picks up, or speed picks up, the world is no longer linear.
     
  13. Doug Lord
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    Doug Lord Flight Ready

    -------------------
    I'm not so sure we have enough info to say that: most hull sections taper as they come down from the sheer to the waterline and below, so it would be likely that the waterline beam was less on the lighter boat IF the sections tapered like that so L/B might go from 10.1/1 to 10. 2/1. But,If the transom and bottom intersection was right at the waterline on the heavy boat, when weight is removed the waterline would be shorter which MIGHT make the Length/Beam ratio the same! Also assumes a vertical stem... Not enough info to draw that conclusion..
     
  14. idkfa
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    idkfa Senior Member

    How does the A-cat do against the F18?
     

  15. Doug Lord
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    Doug Lord Flight Ready

    I found this-the builder of the DNA A Class cat says: http://www.racingcats.com/
    " The extra power keeps the boat in the groove in the lulls, where you previously had to steer up to keep the hull flying. DNA's with the new rigs are beating the F18's around the course now. Trapezing downwind will be the norm for the 2011 season, something we have never seen before in the A-class. "

    -----------
    A Class cats have no spin but are much lighter than an F 18(less than half the weight).

    A Class Cat
    Dimensions
    Length: 5,49 m (18 ft)
    Width: 2,30m (7.5')
    Weight: 75 kgs -minimum weight A-class hull weight= 165lb
    Sailing weight@ 1-175lb crew=340
    Sail area: 13,94 m2 (150sq.ft.)
    Mast length: 9,00 m
    W/SA-2.06

    ============================
    F 18 Cat
    F18 Capricorn cat developed by Martin Fischer: http://www.f18.nl/index.php?Itemid=46&id=30&option=com_content&task=view

    SPECIFICATIONS
    Length 5.52 m / 18.1'
    Beam 2.60 m / 8.5'
    Mast 9.08 m / 29.78'
    Total Hull Weight 180.00 kg / 396 lb.
    Sailing Weight(2@ 175lb crew)= 746lb
    Sails: Mainsail 17.00 m2 / 182.9 sq. ft.
    Jib 3.45 / 4.15 m2 / 44.65 sq. ft.
    Upwind total: 227sq.ft.(big jib)
    Spinnaker 19.00 / 21.00 m2 / 225.9 sq.ft.
    Downwind total: 452 sqft
    W/SA
    --upwind-3.29
    --downwind-1.65
     
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