Sailing boats' Stability, STIX and Old Ratios

Discussion in 'Stability' started by Guillermo, Sep 3, 2006.

  1. Guillermo
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    Guillermo Ingeniero Naval

    I wrote Nauticat to ask about this, and here the response:

    "Hello Mr. Gefaell
    The stability of the boat is more than enough for category A, but it is the side doors, which makes it extremely difficult to get it in category A, as those are considered as flooding openings.
    I hope this explains the situation. The same thing with our Nauticat-331 and Nauticat-44.
    best regards
    NAUTICAT YACHTS OY
    Veli Kärjä"


    Here we have a decent builder and another STIX 'strange' situation. Thanks a lot Veli.
     
  2. Mikey
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    Mikey Senior Member

    There we are... also the Nauticat 44 is a category B boat but Multichine 28 is a category A boat :(

    Something is just not quite right here.

    Mikey
     
  3. fcfc
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    fcfc Senior Member

  4. Guillermo
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    Guillermo Ingeniero Naval

    There are several models of Ribs around there categorized as B. There is not an STIX calculation and number for them, but an specific norm: ISO 6185-4. But I'm interested to know how a 5.35 m length one is able to be B categorized, as there is not a norm usable for that under 8 m length, if I'm not wrong.

    But a 3,5 tonnes one will only need 0,2 m. Weight, always weight...! :)

    Cheers.
     
  5. Guillermo
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    Guillermo Ingeniero Naval

    Too busy these days to actively participate in the Forums.
    Just a quick post to inform I have posted an improved release of the STIX spreadsheet calculator, available at the "Spreadsheet Library". I'll keep working on that (as time allows) to make it more complete and useful.
    Cheers.
     
  6. Guillermo
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    Guillermo Ingeniero Naval

    Brian Trenhaille of Hawaii Marine has developed a very complete spreadsheet to estimate a lot of your sailing boat's characteristics. It doesn't calculate STIX, but a lot of other useful info.

    http://hawaii-marine.com/templates/Products/ASail-e/description.htm

    From there:
    "This spreadsheet takes you input values for your sail boat and automatically calculates many of your sail boat's numerical characteristics (ratios, coefficients and parameters). Then this spreadsheet presents various target values allowing you to quickly check and compare your sailboat's characteristics with other similar successful sail boats.

    There are several advantages and benefits to this calculative approach. Some of these advantages are listed below.

    First, this method saves time. Many of computations are quickly generated behind the scenes on your input data. Also to modify, simply change a value in the spreadsheet and it will automatically and immediately recalculates all affected values.
    bullet

    Second, this approach provides clear and neat documentation.

    Third this approach is cost effective because the calculative approach is already developed for you, research time is minimized to the familiarization of concepts when necessary and not to time consuming development activities.

    Fourth, this template is kept simple, it contains no Excel macros and there is no Visual Basic code utilized in it's creation. Also advanced Excel features such "Goal Seek," "Solver" and "Scenarios" are not used.

    Fifth, because this is a spreadsheet and not a program, the users can easily modify it to suit their particular needs."
     
  7. Guillermo
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    Guillermo Ingeniero Naval

    I like this 40 footer
    http://www.nordship.dk/index.php?id=351&main=Yachts&menu=Nordship DS40&menuid=344

    Specification here: http://www.nordship.dk/nordship/files/File/PDF/40_UK_Total_07-02-2006.pdf

    Some estimated numbers for her:

    Basic data from Yachting World and Nordship's site.

    Overall Hull Length = 12,2 m
    Length Waterline = 10,8 m
    Flooded Buoyancy = (Y/N) N
    Beam Waterline = 3,51 m
    Beam = 3,9 m
    Displacement MOC = 11100 kg (best guess)
    Displacement Max = 12948 kg (best guess)
    Height of CE above DWL = 7,52 m
    Height of CLR below DWL = 0,8 m
    Angle of vanishing stability = 140 deg
    Downflooding angle = 120 deg (best guess)
    GZ at downflooding angle = 0,25 m
    GZ at 90 degrees = 0,67 m
    Sail Area = 78 sqm (site) & 95,9 sqm (YW) -- I,J,P,E missing
    Area to flooding (Agz) = 68 m.deg
    Area to AVS = 70 m.deg


    STIX factors and figure for MOC and 78 sqm SA:

    Base Length Factor (LBS) = 11,267
    Displacement Length Factor (FDL) = 1,040
    Beam Displacement Factor (FBD) = 1,061
    Knockdown Recovery Factor (FKR) = 1,435
    Inversion Recovery Factor (FIR) = 1,186
    Dynamic Stability Factor (FDS) = 1,231
    Vaw (Not aplicable)
    Wind Moment Factor (FWM) = 1,000
    Downflooding Factor (FDF) = 1,250
    Delta = 0

    STIX = 54,984 (Wow!)
    Note: Maybe this figure is some enters lower due to the 120º guess on downflooding angle. If it were 110º, STIX would be around 53 and around 49 if 100º.

    Ratios related to 10500 kg lighship condition, 3700 kg ballast and 2 m keel. Figures in () are for full load:

    Length/Beam Ratio L/B = 2,88 as: (0,7*LWL +0,3* LOA)/Bmax
    Lwl/Bwl Ratio Lwl/Bwl = 3,08
    Ballast/Disp Ratio W/Disp = 0,35 (0,29)
    Displacement/Length Ratio D/L = 232,49 (around 280?)
    Sail Area/Disp. Ratio SA/D = 16,53 (14,37)
    Sail Area/Wetted surface SA/WS = 2,33 -- best guess for WS: 33,44 sqm
    Power/ Disp. Ratio HP/D = 2,41 HP/ton (1,95)
    'Hull speed' HSPD = 7,98 Kn
    Potential Maximum Speed PMS = 9,15 Kn (8,1)
    Velocity Ratio VR = 1,15 (1,02)
    Efficient motoring speed (1.1 S/L) EMSPD = 6,55 Kn
    Capsize Safety Factor CSF = 1,8 (1,68)
    Motion Comfort Ratio MCR = 32,35 (39,90)
    Heft Ratio HF = 1,1 (1,36)
    Roll Period T = 3,67 Sec (4,41)
    Roll Acceleration Acc = 0,08 G's (0,05)
    Stability Index SI = 0,94 (1,13)

    Something like a 10% more of sail area would have been nice to my taste, just to bring SA/D ratio to around 18 for lightship and 16 for full load, to make her more agile in light winds. STIX would still between 48 to 53 (depending on Dfl angle) which are yet very nice figures for this size of boat nowadays (Anyhow I would have not been discontent with present SA figure).
     

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  8. Vega
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    Vega Senior Member

  9. Guillermo
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    Guillermo Ingeniero Naval

    OK. I'll buy this one and you buy the Pogo 40 :cool:
     
  10. Guillermo
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    Guillermo Ingeniero Naval

    Some interesting (estimated) numbers for CIGALE 14 (http://www.alubat.com/voilier alu cigale14.php):

    Lightship (7000 kg)

    Length/Beam Ratio L/B = 3,03
    Lwl/Bwl Ratio = 3,33
    Ballast/Disp Ratio W/Disp = 0,47
    Displacement/Length Ratio D/L = 79,36
    Sail Area/Disp. Ratio SA/D = 25,56
    Sail Area/Wetted surface SA/WS = 2,23
    Power/ Disp. Ratio HP/D = 3,26 HP/ton
    Hull speed HSPD = 8,92 Kn
    Potential Maximum Speed PMS = 11,16 Kn
    Velocity Ratio VR = 1,25
    Best motoring speed (1.1) CSPD = 7,32 Kn
    Capsize Safety Factor CSF = 2,37
    Motion Comfort Ratio MCR = 14,65
    Heft Ratio HF = 0,48
    Roll Period T = 1,86 Sec
    Roll Acceleration Acc = 0,36 G's
    Stability Index SI = 0,41
    Angle of Vanishing Stability AVS = 116 º


    Full load (9300 kg?) 6 pax. No water ballast considered.

    Ballast/Disp Ratio W/Disp = 0,35
    Displacement/Length Ratio D/L = 105,43
    Sail Area/Disp. Ratio SA/D = 21,15
    Sail Area/Wetted surface SA/WS = 2,13
    Power/ Disp. Ratio HP/D = 2,45 HP/ton
    Hull speed HSPD = 8,92 Kn
    Potential Maximum Speed PMS = 10,4 Kn
    Velocity Ratio VR = 1,17
    Best motoring speed (1.1) CSPD = 7,32 Kn
    Capsize Safety Factor CSF = 2,16
    Motion Comfort Ratio MCR = 19,46
    Heft Ratio HF = 0,64
    Roll Period T = 2,38 Sec
    Roll Acceleration Acc = 0,22 G's
    Stability Index SI = 0,53
    Angle of Vanishing Stability AVS = 114 º

    More info at: http://www.northseamaritime.com/Page/ALUc14.htm

    A pity not having a GZ curve for this boat.
     

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  11. Guillermo
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    Guillermo Ingeniero Naval

    And now an excellent design from Heyman Yacht Design, in Sweden, very much of my content! :)

    One of the boats I'd buy or built, if only I could! :cool:
    (SA sems somewhat low to me, but her ketch rig allows for an extra mizzen staysail)

    Some preliminary numbers for her, asuming the stated displacement of 36 tonnes is for the MOC, and downflooding occurs at 115º.
    Cp is asumed as 0,56 and Cwp as 0,7

    ATOA 64
    Loa = 22,30 m
    Lh = 19,50 m
    Lwl = 17,32 m
    Bmax = 5,40 m
    Bwl = 4,86 m
    Draught T = 1,60 m
    Body draught Tc = 0,95 m
    Disp = 36000 kg (Asumed to be MOC)
    Ballast = 10000 kg
    Sail area = 146,4 m2 (estimated measuring I, J, P...etc, from drawings)
    Power = 140 KW
    Heeling Arm = 8,97 m
    WS Appendages = 13,18 m2
    WS Hull = 65,89 m2
    Wetted Surface = 79,07 m2

    Length/Beam Ratio L/B = 3,33
    Lwl/Bwl Ratio Lw/Bw = 3,56
    Ballast/Disp Ratio W/Disp = 0,28
    Displacement/Length Ratio D/L = 193,26
    Sail Area/Disp. Ratio SA/D = 13,64
    Sail Area/Wetted surface SA/WS = 1,85
    SA (metric)/ Power (Imp.) SA/HP = 0,77
    Power/ Disp. Ratio HP/D = 2,40 HP/ton
    Hull speed HSPD = 10,10 Kn
    Potential Maximum Speed PMS = 9,8 Kn
    Velocity Ratio VR = 0,97
    Best motoring speed (1.1) CSPD = 8,29 Kn
    Capsize Safety Factor CSF = 1,65
    Motion Comfort Ratio MCR = 44,87
    Heft Ratio HF = 1,24
    Righting Moment/Beam RMB = 0,8
    Roll Period T = 5,21 Sec
    Roll Acceleration Acc = 0,06 G's
    Stability Index SI = 0,97

    Displacement MOC = 36000 kg
    Displacement Max = 42295 kg
    Height of CE above DWL = 8,33 m
    Height of CLR below DWL = 0,64 m
    Angle of vanishing stability = 132 deg
    Downflooding angle = 115 deg (best guess)
    GZ at downflooding angle = 0,24 m
    GZ at 90 degrees = 0,61 m
    Sail Area = 146,4 sq.m
    Area to flooding (Agz) = 55,48 m.deg
    Area to AVS = 58,67 m.deg

    Base Length Factor (LBS) = 18,047
    Displacement Length Factor (FDL) = 1,067
    Beam Displacement Factor (FBD) = 1,061
    Knockdown Recovery Factor (FKR) = 1,500
    Inversion Recovery Factor (FIR) = 1,288
    Dynamic Stability Factor (FDS) = 0,795
    Vaw = Not aplicable
    Wind Moment Factor (FWM) = 1,000
    Downflooding Factor (FDF) = 1,250

    Delta = 5

    STIX = 75,140


    I would like to clarify the displacement and downflooding angle doubts. Somebody?
    More info at:
    http://www.atoa64.com/

    Cheers.
     

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  12. Vega
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    Vega Senior Member

  13. Guillermo
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    Guillermo Ingeniero Naval

    Excellent! Do you have the corresponding GZ curve?
     
  14. Guillermo
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    Guillermo Ingeniero Naval

    Coming back to the very initial purpose of this thread:
    From a Dave Gerr's article at the Westlawn Institute Magazine's September 2007 issue:
    "Because complying with the STIX under ISO is now the law in Europe, there is a tendency to interpret STIX as a good indicator of seaworthy stability in the U.S. I personally don't believe that STIX is all that a reliable indicator. Nevertheless the STIX number is a reasonable marker, but it is not as reliable -in my opinion- as simply applying the basic concepts above (he's referring to several of the 'old' ratios and parameters) along with one or two others involving dynamic stability...for really accurate results by designers"
    http://www.westlawn.edu/news/WestlawnMasthead03_Sept07.pdf

    (Italics are mine. Thanks a lot, Perry!)
     

  15. YachtManuals
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    YachtManuals Junior Member

    Heel on wave surface


    I have seen this mistaken argument in several places. In fact any object floating on the inclined surface of a wave is accellerated sideways (in the direction on wave travel) so that apparent gravity is perpendicular to the surface of the water. The experiment would show that a pendelum suspended from anything floating on waves will always point straight down to the "deck" of the object. This means that deep keels and wide hulls both "heal" the same amount on the side of waves (assuming only the wave action). This can also be explained by the motion of a water particle on the surface of waves: they describe a circular motion. Otherwise the water on the wave surface would be rushing down to the bottom of the trough. In fact the water on the surface is perfectly happy to stay put, because it experiences the force of gravity as always pointing down perpendicular to the SURFACE of the wave, just like the vessel.
     
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