In need of enlightenment (SCP)

Discussion in 'Sailboats' started by sawmaster, Sep 30, 2010.

  1. sawmaster
    Joined: May 2010
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    sawmaster Senior Member

    hey folks,
    Iwas reading one of doug lords post-I ran into an abbreivation I'm not familiar with.Doug says,Bethwaite reports that boats with an scp/total wt ratio above 30% will plane upwind.Praytell what is SCP?
    thanks,
    saw
     
  2. Olav
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    Olav naval architect

    From this post:

    ;)
     
    Last edited: Oct 2, 2010
  3. PAR
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    PAR Yacht Designer/Builder

    Sustainable Consumption and Production . . .
     
  4. Paul B

    Paul B Previous Member

    The SCP is one factor that might indicate a boat will plane upwind. Of course there are other factors that have to be "right" as well.

    Here is the page that explains the term and the calculation from Bethwaite's High Perfromance Sailing.
     

    Attached Files:

  5. CutOnce

    CutOnce Previous Member

    It would seem to me that this may be a little simplistic. Hull shape is kind of important. So is the rig. So are the foils. The problem with oversimplified equations is that playing with one variable leads to adjusting another to achieve a desired mathematical result - but that kind of play can lead to fundamentally unsailable boats.

    Above all else, keeping weight to a minimum, righting moment within the realm of realistic possibility and rig power manageable is more important than chasing ratios.

    I could see heavier extremists creating 16' designs needing 8-10' of lever arm from centerline, and overpowered rigs requiring the reaction time of a Red Bull-soaked Hummingbird - all to meet a ratio for upwind planing. In reality the boat would be unsailable by normal humans.

    Equations only work in closed loop, steady state environments - but we sail in fluky, shifty winds, random sea states and not everyone has the reaction time (or stamina) necessary to go from fully out on the wire to on centerline every ten seconds.

    It would be silly (and I'm sure Bethwaite would agree) to design a boat to a specific SCP instead of designing a boat for the real world.

    As far as I read from High Performance Sailing, Bethwaite's use of SCP was for forensic evaluation of what actually worked in the real world - not as a design tool to guide the architect.

    I hope I'm not offending anyone if I'm failing to treat their sacred cow with adequate reverence. No offense intended.

    --
    CutOnce
     
  6. Doug Lord
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    Doug Lord Flight Ready

    Hey saw below is a PDF done by Eric Sponberg with many useful comparative ratios that can be used on a number of boats.
    One other ratio that is interesting is W/SA where W equals weight in pounds and SA equals sail area in sq.ft.. It is primarily used to compare small boats-say from 26' down to see if they will foil. This is just a quick look and after this looks positive there are a lot more factors to consider. But if this doesn't come up golden then the boat most assuredly would need to be modified to have a chance.
    The range(currently) is between 2.4 and about 3.2. The closer to or below 2.54 you are means that not only will the boat foil but is likely to take off in lightwind. A couple of examples:
    Mirabaud-2.4
    Moth with Veal or Payne about 2.54


    pix-Mirabaud and Moth-almost identical in speed:
     

    Attached Files:

  7. Paul B

    Paul B Previous Member

    I believe you are correct.
     
  8. Doug Lord
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    Doug Lord Flight Ready

    =============
    I completely disagree with this statement. This is one of the best ratios around to evaluate upwind performance in a breeze for small high performance non-keelboats. It is an excellent designers tool!
    Bethwaite says under the heading of "The dominance of ratios", section 16.11,p 178 that:"This does not mean that the effects of the displacement /length ratio and speed/length ratio cease to operate. It is just that they cease to be of much importance. And again,for the same reasons as previously, for the fastest boats, only the 'upwind' ratio remains relevant, because they now sail with the Apparent Wind forward of the beam for almost all the time."
    No ratio answers all questions, no ratio is a "sacred cow" and the designer still has to design. Ratio's are tools that it takes some experience to learn how to use, but they can be a big help in comparative analysis and in helping to check whether a design is close to a target range or not.
    SCP/total weight and W/SA are small,high performance boat ratios and they can be real useful-but again-the designer still has to design.
     
  9. Paul B

    Paul B Previous Member

    What cutOnce wrote is pretty accurate. Maybe you should read what Bethwaite has actually written. Copies of the book are available at Amazon.com.
     
  10. CutOnce

    CutOnce Previous Member

    I'm in complete agreement with this! But evaluation is an after-the-fact process, not a predictive process.

    Accountants are the best reporters of business results - and generally accepted accounting practices (GAAP) forms a trusted consistent framework from which results can be generated. Accountants are perhaps not the best predictors of the future and don't (or should not) provide vision of what will work in the future for a business.

    What I'm saying is your SCP (Moo!) is a valuable analytical tool for looking at completed boats, but it may not be a major factor in predictive design - there are many other factors that are more important to get right before you can run the numbers.

    The great thing about places like this is that many people may have many opinions - it is nice to hear yours. Thanks for your thoughts.

    --
    CutOnce
     
  11. sawmaster
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    sawmaster Senior Member

    thanks to all

    thanks to everyone who responded to my question about SCP.
    Not only was my question answered,but I received numerous opinions as to its value and relevancy in boat design-always an interesting and spirited debate in boat design forums.Special thanks to Doug Lord for Sponbergs Pdf file.
     
  12. Doug Lord
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    Doug Lord Flight Ready

    ==============================
    MISTAKE! I've used this ratio dozens of times and yet when it came to describing it in the MPX thread I wrote it incorrectly -I substituted "into" for "by"!
    It should be this:

    SCP(sail carrying power)= the RM in ft.lbs divided by the distance in feet between the CE and CLR. To get Bethwaites ratio this number is then divided by the total weight in pounds.SCP/Total Weight- A ratio of 30% or better permits upwind planing.
    ==
    My apologies to anyone who has tried to use this in its mistaken form!! And thanks to Olav for posting it which encouraged me to take a second look tonight. UPDATE: Thanks to Olav for correcting my mistake in his original post.
     
  13. sawmaster
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    sawmaster Senior Member

    scp revisited

    Hey Doug:
    Am I correct in assuming RM is distance from CB to rail (or hiking rack) times weight of crew-for unballasted boats?
     
  14. Doug Lord
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    Doug Lord Flight Ready

    -------------------
    For the purposes of Bethwaites formula, RM for a trapeze boat(as an example) is obtained by multiplying the weight of the crew(at their CG) by the distance from the centerline of the boat(which intersects the CB)*. The boats weight plays no role assuming the boat is sailed flat.
    * for this formula the centerline can be assumed to be the centerline of the mast and the distance to the crew CG measured at 90 degrees to the mast as viewed from aft.
     

  15. sawmaster
    Joined: May 2010
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    sawmaster Senior Member

    rm clarification

    Thanks,Doug,
    that makes sense.
    Saw.
     
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