The elements of boat strength

Discussion in 'Boat Design' started by Mik the stick, Dec 18, 2013.

  1. Mik the stick
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    Mik the stick Senior Member

    This my most recent purchase allows me to work out plank/plate thicknesses and the size of other bits ultimately allowing me to predict the weight of any boat I might want to design. I used this in my recent posts "Perhaps Beebe got it wrong". My weight estimates may be quite inaccurate, this is not important to me as I think I tend to overestimate. Some have said boats built to these rules are likely to turn out on the heavy side.
    Michael Kasten uses ABS scantling rules. I have the greatest of respect for these two designers, I have learned more from Gerr books than any other single source. However if ABS are the rules prescribed by the establishment how do you get insurance for a boat built to anything else.
     
  2. Tad
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    Tad Boat Designer

    If you are in the UK then ISO 12215 applies. You can also build (under survey) to Lloyd's, Germanischer Lloyd, DNV, RINA (Italy), and others.

    ABS stopped supporting rules for anything less than 79' about 20 years ago.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Geer's "scantling rules" are specifically intended to suit the novice builder/designer, so they can have a "wholesome", well founded structure. This is just fine for a cruiser and really all these "rules" are meant to offer. If you're building a racer or are interested in a performance oriented craft, you'll have to crunch the numbers yourself, to get the most from the materials and structure choices you've made. You can safely knock 10% off most of the scantlings, "scaled" with Geer's book, knowing the result will not be as robust, but likely strong enough for the average skipper, without having to crunch numbers.
     
  4. MikeJohns
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    MikeJohns Senior Member

    ISO 12215 is basically ABS brought up to date.

    I'm not sure what material you are designing with but presumably steel or Al alloy. Dave Gerr's Elements isn't the best design guide for metal boats which will can be overly heavy especially if you follow the steel guide. It omits a bit of detail that can trip a novice designer as I have found reviewing others designs. It also fails to explain that some of the members may not even be required like failing to give an option for a plate only stem ( no stem bar).

    I'd recommend GL for metal boats, it complies with 12215 and is a superior standard with full class support for smaller craft. The rules are available for free download see http://www.boatdesign.net/forums/cl...r-lloyd-gl-rules-free-28404-2.html#post647307
     
  5. TANSL
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    TANSL Senior Member

    I wouldn´t say that ISO 12215 is basically ABS because I think it is not, but that does not matter. All standards use the formulas of the strength of materials, so yes, from this point of view, one can say that they are all basically the same.
    Advise to use certain standards because it complies with ISO is a bit strange. Would not it be better to advise to use ISO?.
    What you need to do depends on what you need for your boat. If the ship should comply with any CS then you use the rules of that CS. If your ship should reach the "CE" mark, the easiest way is to use the ISO 12215 standard and other ISO standards for small boats.
     
  6. Mik the stick
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    Mik the stick Senior Member

    Thanks there is much to look at, Gerrs rules seem simpler if less complete. There seem to be so many rules to conform to I don't understand how Naval Architects cope. I am using Gerrs rules to help me predict a design's finished displacement as an alternative to finding similar designs and using cubic numbers.
     
  7. Alik
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    Alik Senior Member

    But be careful to use Gerr's rules for semi-planing and planing craft. The way he is adding thickness as an addition to absolute speed is completely incorrect.

    I would say Gerr's rule only works for heavy displacement boats well below the hump. Anything close to the hump and above will have significant effect of slamming, thus one needs to find vertical accelerations and work from those.

    To me, it is much easier to understand the principles of defining the design loads (hydrostatic and hydrodynamic), and principles of calculation of panels and beams. With this basic knowledge, one can engineer any boat (given some experience, of course).
     
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  8. TANSL
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    TANSL Senior Member

    I totally agree. Furthermore, it is the way to get the structure of minimum weight.
     
  9. MikeJohns
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    MikeJohns Senior Member

    ISO 12215 is a design standard while class societies produce a design and construction standard which is significantly different, it’s easier to follow and more thorough IMO. Class will also advise on their standards for free and have a good plan approval service.

    ABS OSRY approach was largely incorporated into 12215 wasn’t it?
     
  10. MikeJohns
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    MikeJohns Senior Member

    Which satisfies class too if you use their loads and engineer the structure to suit. which for small craft is all design pressure and no global loads so easy to check rule based scantlings against a basic structural spreadsheet, then you can see their FOS and pick up any errors quickly.
     
  11. Alik
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    Alik Senior Member

    Design loads from ISO12215-5 are identical to GL's Boats and Yachts Rules (see section Structure > Advanced Composites). But strength criteria are different!
     
  12. TANSL
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    TANSL Senior Member

    I have not spoken at any time how CS or ISO standards operate. Nor do I speak of the powers of each. They are totally different, but if you want a boat rated for BV, you can not use ISO 12215 to scantilling it. Similarly, if you want to get the "CE" mark for your boat the best you can do is use the ISO standard as far as boats are concerned. And the ISO standard also says a lot about materials, joining methods, construction details, testing equipment, etc..
    As far as I know the ISO has not largely incorporated ABS. Maybe, I'm almost certain, that both may have common things but nothing more, and the same to be common things with GL or DnV. Reveal that ISO 12215 has copied ABS, if that's what you mean, it does not seem right.
     
  13. daiquiri
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    daiquiri Engineering and Design

    Since ISO compliance is required for commercialization in the EU (and, for the reason of reciprocity, is adopted in the US too), the logic tells me that class rules have to satisfy the ISO, but the opposite is not necessarily true. Hence, class rules must be more stringent than ISO. Is this true for the CS rules you know?
    If it is true, than by adopting the GL rules (for example) one should be on the safe side. Am I correct?
     
  14. TANSL
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    TANSL Senior Member

    You are right, daiquiri, in my opinion. But one designer should be on the safe side but not too safe side
     

  15. Alik
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    Alik Senior Member

    The book 'Elements of boat strength' was the biggest confusion to me. The principles of design loads are not explained - local and global strength; though great explanation of those is given say by Larrson, Eliasson in 'Principles of yacht design'. There is no mention of material properties, properties of panels and beams (EI, SM, etc.) so the reader is not taught to make justified choices to design efficient structures. Seems the book was not peer-reviewed by naval architects (You know what I mean!), as most of Westlawn materials, so it contains number of incorrect statements. So it is a 'cookbook' not an engineering science, and it gives an illusion of simplicity on really complicated subject.
     
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