Sailing boats' Stability, STIX and Old Ratios

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

  1. Tad
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    Tad Boat Designer

  2. lausl
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    lausl carpe diem yacht design

    Inclining experiment Spreadsheet

    I have had a spreadsheet for solving the VCG from incling experiment data, its gone in the nirvana somehow, and I thought it was from the spreadsheet page here, but a cannot find it,,

    Does anybody know of a source or wants to share?
    Regards,
    Klaus
     
  3. Guillermo
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    Guillermo Ingeniero Naval

  4. Nick.Rollings
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    Nick.Rollings New Member

    Old Boats

    Hello List

    Just reading this thread on stability and old boats.

    Suzi and I are looking at a Valiant 40 for liveaboard cruising on the East Coast of Australia and would value your oinion on this design. We plan to undertkae coastal cruising for the first year, and then venture to fiji or similar.

    thank you

    Nick and Suzi


     
  5. Guillermo
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    Guillermo Ingeniero Naval

  6. Nick.Rollings
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    Nick.Rollings New Member

    Valiant 40

    Greetings from Aus,

    Thanks for your reply and links, yes I have seen thess sites and used them (and the Valiant owners group) as the basis for my decision for a Valiant (or similar). I have also been over 2 Valiants in the last year and like them very much.

    However, this site is a little different to others I read in that you guys are into boat design and the various methods of assessing a yachts suitability for a particular purpose. I guess I was after a professional opinion in terms of the yachts design to back up the positve stories we read in these other sites, particularly in light of V40's being old boats. I use Carl's sail calculator for basic stuff.

    I think if I read between the lines however, you are saying the Valiant 40 is fine for what we want to do?

    thanks


    Nick
     
  7. Guillermo
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    Guillermo Ingeniero Naval

    Hi Nick,
    As I'm traveling I have not had time do an in-deep look to the Valiant ratios. Could you please post the ones you got using Carl's calculator?

    I have only got this:

    Sail Area/Displacement....15.2
    Displacement/Length.........255
    Ballast/Displacement..........34

    Those are nice numbers for a short-handed globetrotter, to my taste. Ballast/displacement could have been somewhat higher, but it's good enough.

    I also like her long keel and protected rudder, as well as her handsom canoe stern.

    Her relatively low freeboard will probably make her downflooding angle (to the companionway) somewhat low by today's standards, which can negatively affect her STIX, as it happened in the commented case of the Hans Christian 48T

    Having said that, I have to say I love the boat and I find it pretty adequate tou your intended plans. :)

    Cheers.
     
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  8. Nick.Rollings
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    Nick.Rollings New Member

    V40 and Carl

    Thanks again for your advice on this, particularly since you are busy travelling.

    Its just great to hear a professional dicussion on a yacht I (think) I know to be a good balance for our requirements. And, yes, she does have a nice rear end!

    Carls Sailing Calc gives

    Valiant 40
    LOA 39.34
    LWL 33.88
    Beam 12.32
    Disp 22306
    SA 768

    Disp to LWL 256
    LWL to Beam 2.75
    Hull Speed 7.8
    Motion Comfort 33.96
    SA to Disp 15.51
    Capsize Ratio 1.75
    Pounds / Inch 1491
    Sailing Category - Cruiser

    I'll do a bit of swatting up on STIX as well, its always fun to learn new things.

    thanks

    Nick

     
  9. Guillermo
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    Guillermo Ingeniero Naval

    Here the numbers I've worked out for the Valiant 40. All to be taken with perhaps something else than a grain of salt, as I have taken rough measurements on the avilable plans in internet.
    (Mix of metric and imperial data, sorry)


    INPUT

    Hull length (m) = 12,16
    Waterline length (m) = 10,36
    Maximum beam (m) = 3,76
    Floatation beam (m) = 3,38
    Total draught (m) = 1,83
    Body draught (m) = 0,76
    Moulded depth (m) = 1,72
    Freeboard (m) = 0,96
    Displacement (kg) = 10.206
    Ballast (kg) = 3.500
    IJPE area (m2) = 71,7
    Air draft (m) = 15,66
    Heeling arm (m) = 13,30
    Engine power (HP) = 54,4
    I (m) = 14,30
    J (m) = 5,6
    P (m) = 13,5
    E (m) = 4,7
    Height keel (approx.) = 1,07
    Length keel (approx.) = 3,78


    OUTPUT (with some assumptions)

    Block coeffcient = 0,37
    Prismatic coefficient = 0,56
    Waterplane coefficient = 0,7
    Main section coefficient = 0,67

    (2Lwl+Lh)/3Bmax Ratio = 2,91
    Lwl/Bwl Ratio = 3,06
    Length/Draught Ratio = 6,64
    Beam/Draught Ratio = 2,05
    Beam waterline/Body draught = 4,45
    Ballast/Displacement Ratio = 0,34
    Displacement/Length Ratio = 256
    Sail Area/Disp. Ratio = 15,49
    SA (metric)/ Power (Imp.) = 1,32
    Power/ Disp. Ratio (HP/Ton) = 2,42
    Hull speed (kn) = 7,81
    Potential Maximum Speed (kn) = 8,19
    Velocity Ratio = 1,05
    Efficient motoring speed (kn) = 6,41
    Capsize Safety Factor = 1,75
    Motion Comfort Ratio = 34,0
    Heft Ratio = 1,16
    Moment of Inertia (kg*m2) = 46232,43
    Roll Period (sec) = 3,86
    Roll Acceleration (G's) = 0,07
    Stability Index = 1,03
    Calculated Angle of Vanishing Stability (º) = 121

    Upright Heeling Moment (kg*m) = 4831,20
    Righting Moment at 1º (kg*m) = 217,16
    Heeling Moment at 20º (kg*m) = 4106,61
    Righting Moment at 20º(kg*m) = 2021,94
    Dellenbaugh Angle (º) 14kn wind = 22,25


    Initial Metacentric height (m) = 1,2
    Asumed AVS (º) = 121
    Asumed downflooding angle (º) = 90
    GZ at downflooding angle (m) = 0,44
    GZ at 90 degrees (m) = 0,44
    Area to downflooding (m*deg) = 39,03
    Area to AVS (m*deg) = 45,85

    STIX (to input displacement) = 29 (B category)


    Most probably the STIX is higher, but anyway it seems to be not very high, as it was the case of the HC 38T. If real downflooding happens around 100º then STIX goes up to 31. If it is 110º, then STIX rises to 33, just within the A category. But we have to consider these boats were designed without STIX in mind. Are they less safe because of that? No, in my opinion.

    Again: Please be VERY cautious about all this STIX thing. It's based in very, very rough (and may be wrong) approximations.
     

    Attached Files:

  10. Crag Cay
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    Crag Cay Senior Member

    With all due respect Guillermo, I think you should consider removing those figures for a number of reasons.

    Firstly, the designer is still active and if anyone wants the full calculations for the Valiant 40, Bob Perry does a consultation service where he would be more than happy to answer any question.

    Secondly, although you cage your calculations with plenty of caveats, the STIX figures fly in the face of all established experience with this particular boat. I have never heard of any 'seaworthiness' problems with the Valiant 40, so to even suggest it is CAT B would appear to be number crunched nonsense. Either there are gross errors in your assumptions, or the credibility of the ISO standard takes yet another blow to its credibility.

    The big issue with buying a Valiant 40 is paying the right price for them. The combination of their wide age range, their quite small accommodation for their size (without the cute / silly stern they are only a ~37 foot boat) and the wide variation in the quality of repairs to the ones with Uniflite fire retardent resins, has to all be set against their reputation and cult status.
     
  11. Guillermo
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    Guillermo Ingeniero Naval

    Crag,
    I respect your opinion, but after considering it I think I'm going to keep the post as it is. This is a boats design forum and any of us has the right to analyze whatever boat if done with the proper respect to its designer. Specially if the designer is of the size of Bob Perry.

    I am not disrespecting him at all as I'm clearly expliciting my numbers may be wrong and also I'm explicitly saying I do not care if the STIX is low or not to like the boat and find it a lovely and excellent one. One of those I would like to own myself.

    In my opinion this case is, as in the case of the HC48 T (STIX of only 32.5 as per the info provided by the builder himself) one more example of why STIX has to be taken with a grain of salt when used to judge a boat and do not take it as the "word of God" as some do.

    Read again my first post in the thread, please. From there:

    "....STIX provides not enough information about the seaworthiness of a boat..... and may even be a tricky and dangerous number. Seaworthiness is a complex matter, involving stability, all around scantlings, quality of movements, and a long etc."


    I will write Bob Perry and make him aware of these posts just in case he wants to make any kind of comment or contribution and even any kind of criticism to my numbers and opinions. I think that's fair enough.

    All the best. :)

    P.S.: Done. I have sent a message to Perry Design to make them aware of these posts.
     
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  12. Nick.Rollings
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    Nick.Rollings New Member

    Calcs

    Thank for taking the time to run these figures. I fully take on board the uncertainty in the figures and their subsequent interpretation.

    They further suggest that the Valiant 40 is a fine yacht and would suit us perfectly for our intended use.

    Unfortunatel;y the one I was goig to look at has sold!

    Happy sailing.

    Nick






     
  13. George Keune
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    George Keune New Member

    Area to flooding

    Caro Mr. Guillermo,

    Can you help? I believe I understand the calculation as per you xls. My question how to calculate "area to flooding Agz" and "area to AVS"
     
  14. Guillermo
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    Guillermo Ingeniero Naval

    You need the GZ curves of stability in the MOC and MLC conditions (this last if applicable).
     

  15. nikmik
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    nikmik New Member

    Mr. Guillermo,

    I started with the construction of the following characteristics yacht:
    -Desing length 8.975 m
    -Lengthover all 8.975 m
    - MAx beam 2.500 m
    - Desing Draught 0.530 m
    - Displacment 5.25 t
    - Block coeff 0.40307
    - Prismatic coeff 0.5469
    - Vert. prismatic coeff 0.5290
    - Lenght on watherline 8.287 m
    - Beam on waterline 2.118 m
    - Entrance angle 18.868 deg
    - waterplane coeff 0.6228
    - midsship coeff 0.7876
    Conditions must be beam max 2.5m.
    Is it possible with this trough will be STIX class A
     
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