center of flotation calculation and implications?

Discussion in 'Boat Design' started by capt vimes, Jan 7, 2010.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    In a word, yes.

    They are simplified!

    These formulae are based upon what, measurements (empirical), of a small range of vessels, but are treated as all encompassing. They are not and cannot be. These formulae are just “tools”, nothing more.

    If I wish to drive in a nail into some wood, which ‘tool’ would I use? Would I select a saw or a chisel?..no. I could drive it in, eventually, but the right tool is the hammer. But then which type of hammer, a mallet, a claw hammer, rubber (soft afced) hammer etc.

    Trying to treat simplified formulae as absolute, is incorrect. There are always caveats. One needs to understand what these caveats are.

    Are these tools useless then…no. Not at all. They are very useful indeed. But, they just provide guidance and a “ball park figure”, that is all. To assume more than this is ignorance of what said formulae represents and the end goal.
     
  2. Alik
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    Alik Senior Member

    Should You have range of tools then? I think You should be given a choice :) But in 'Propeller handbook' there is only one formula for displacement craft - formula A. No other tools provided.

    In power prediction, different tools are different prediction methods, for different purposes, conditions and types of hulls. And the purposes and conditions are to be specified. In simplified formulas in question they are not.

    I have seen many times how people make mistakes with prediction methods (using software) without knowledge, and more often - using simplified formulas they do not fully understand. On previous pages I referenced Westlawn student making calculations for his powerboat with Holtrop (well outside limits of method), and school publish it in their newsletter! What kind of faculty is there?? Amazing...
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    One should not be given a choice.

    One should learn what the choices are available. Without learning what there is out there and how these are derived and hence used, one is making the same mistakes as someone using a simplified formula and treating it as an absolute.

    Just because only one is provided does this make it wrong?

    If there is no caveat about its applicability and that the reader should research more for others that may be and provide references for them (as a start), then yes it is misleading the inexperienced designer.

    There are many books with just one formula. Since the author is ostensibly promoting ones own work. But those books that refer to many other ‘works’ on the same matter have much more credibility.
     
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  4. Alik
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    Alik Senior Member

    Students should be given a choice and study all spectrum of tools. Professionals can find those tools themselves.

    If You have read the Bibliography at the end of that book, can note that no publications on resistance calculations methods are referenced there. Not a single one - no Savitsky, no Gerritsma, no Blount... Not even PNA. Isn't it misleading?

    That's the point, promoting!

    Those with single formula are not engineering books. Meanwhile preface says it is 'easy-to-use guide for... naval architects'. Have You read it?

    Yes, that is how it should be.
     
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  5. Eric Sponberg
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    Eric Sponberg Senior Member

    Keith's Identity Discovered!

    To all readers,

    Thanks to intrepid observer Vincent Guilbault in France, we now know the identity of the Mr. Keith of Keith's Formula. You may recall that in the last chapter of The Design Ratios, we reviewed Crouch's Formula, Keith's Formula, and Wyman's Formula. We knew who Crouch was and who Wyman is, but Keith was an enigma--mentioned only in Skene's Elements of Yacht Design--and no other information about him or his formula was readily known. Well, thanks to Mr. Guilbault's discovery, we now know that he was Mr. H.H.W. Keith, a professor of Naval Architecture at MIT, and his formula was published in a 1912 book, "Propellers," by a fellow professor at MIT, Cecil H. Peabody.

    I took the liberty to update The Design Ratios with this information and am uploading the revised document here. I made a few other minor editorial changes, but basically, it is all the same. The note below was added to the bottom of page 54, so if you would like to simply copy and print the following text and add it to your current copy, you can. Otherwise, simply download this latest version (same file name) from here. This new version is also posted on my own website for free download, in the Articles Section.

    Eric

    The following text is added to page 54:

    NOTE: Update, January 2011: Observant reader Vincent Guilbault of La Baule, France, discovered who Keith was. H.H.W. Keith (I don’t know what names the initials stand for) was a professor of naval architecture at Massachusetts Institute of Technology (MIT). He contributed his formula for speed prediction in a book called “Propellers”, written by Cecil H. Peabody, also a professor at MIT, and published by John Wiley & Sons in London, England, in 1912. Other records discovered on the Internet reveal that H.H.W. Keith was an author and member of SNAME up until at least 1941. You can see a digital copy of “Propellers” at the following link: http://www.archive.org/stream/propellers00peabgoog#page/n0/mode/2up. Go to page 28 and you will see Keith’s Formula as he originally wrote it:

    Speed, V = C x ((L x P)^0.333)/B

    Where:
    V = speed in miles per hour or knots
    L = Length overall, feet
    P = Break Horsepower of engine or engines
    B = Extreme Beam, feet
    C = Coefficient that ranges between 7 and 11 for different length/beam ratios:
    Type of Boat
    Cruiser: For L/B = 3 to 5: C(mph) = 9 to 11; C(knots) = 8 to 9.5
    Runabout: For L/B = 5 to 7: C(mph) = 8 to 10; C(knots) = 7 to 8.5
    High Speed: ------------------: C(mph) = 8 to 9; C(knots) = 7 to 8

    Notice that instead of Displacement, the original uses Beam overall. L is also within the cube root. The C coefficients are correspondingly different, so you cannot mix the two versions—be clear as to which version of this formula you use. The C coefficients are still based on experience—that is, you should know some boats that perform according to these coefficients in order to use them reliably. Special thanks for Vincent Guilbault for this discovery!
     

    Attached Files:

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  6. Fanie
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    Fanie Fanie

    Good grief, 1912 eh !

    You mean to tell me they already knew back then what we don't know today in 2011 :eek:
     
  7. kroskris
    Joined: Mar 2010
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    kroskris Boat designer

    Thanks a lot!

    I have just quickly revised the document you attached, and it looks prety good.
    I will take the time to read trough it thoroughly!

    This will dramaticaly help me to completely understand hull design.:)
     
  8. kroskris
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    kroskris Boat designer

    Hello Eric!

    I saw an article in the Feb/March 2011 issue of the Proffesional Boatbuilder Magazine, regarding the S number and also about this thread.:idea:

    Congrats:)
     
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  9. Eric Sponberg
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    Eric Sponberg Senior Member

    Thanks! Yes, the article is on page 10 in the Rovings column by Dan Spurr. I asked Dan if he would consider publishing a short piece on the S Number and this thread, and he consented to do so.

    Eric
     
  10. CarlC
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    CarlC Junior Member

    Dr. Sponberg

    Dear Eric,

    We always do what you tell us to, don't we?

    My best wishes to you, Carl
     
  11. Eric Sponberg
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    Location: On board Corroboree

    Eric Sponberg Senior Member

    Carl,

    I never take it for granted and understand that you have a schedule and an editorial agenda to meet. But I must say you treat me very well, and thank you for that.

    Eric
     

  12. oldsailor7
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    oldsailor7 Senior Member

    This whole thread has been a mine of useful information and technical discourse, particularly in the realm of desired features in a multihull.
    Can anyone tell me then why Lock Crowthers Buccaneer 24 is held in such high regard by sailors who have owned, built or sailed one.
    On the face of it it doesn't seem to have many special features.
    It has a low B/L ratio of 8/1, when the late Edmund Bruce says the optimum for a cruising multi is nearer to 10 or 11/1, and a racing multi to 12 to 16 /1, or more.
    It has a chine bottom, whearas round bottoms are considered more efficient.
    It has a poorly designed daggerboard. The acute upturn of the keel at the stern causes a severe stern wave which is much criticised, and it's masthead bermuda rig is outdated.
    However people seem to love it.
    Even Gary Baigent refers to it as "The Marvelous B24".
    Curiously enough it will move out in light airs when other boats seem anchored to the water, and there is no record of one ever capsizing, pitchpoling or breaking up. But above all it is fast, in an effortless way.
    It has been hugely over canvassed by some racing sailors but seems to love it.
    What is it about this Trimaran that makes it so special ???
     
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