The Elements of Boat Strength: For Builders, Designers, and Owners

Discussion in 'Boat Design' started by wardd, Oct 24, 2009.

  1. Paul Kotzebue

    Paul Kotzebue Previous Member

    Does Gerr describe how his methods are derived from sound engineering principles? I have not read the book, but I might have an interest in it if Gerr is able to relate his methods directly to first principles.
     
  2. daiquiri
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    daiquiri Engineering and Design

    I agree with you. Gerr's book doesn't contain any scientifically argumented explanation about how did he obtain his scantling equations and graphs. I have that book and did use it on several occasions, and in effect (as I stated in a similar thread here, plenty of months ago) all of us who are using it are actually relying on Gerr's word and professionalism.
    On the other hand, each time we use prepackaged scantling equations provided by some classification society we do exactly the same...
     
  3. Paul Kotzebue

    Paul Kotzebue Previous Member

    I do not agree. Class society rules such as ABS and Lloyd's are derived from first principles and do not represent one man's opinion. Shell plating thickness is usually based on flat panel theory and stiffeners are based on beam theory. The "prepackaged" scantling equations used by ABS and Lloyd's can be found in engineering textbooks not related to boat construction, and are the accepted means of calculating stresses from first principles.

    Modern class society rules using design loads, accepted engineering stress formulas, and accepted engineering failure modes should not be confused with cookbook scantling rules. They are not the same thing.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    I'm agreement with Paul
    "..Class society rules such as ABS and Lloyd's are derived from first principles and do not represent one man's opinion..."

    Most Class rules, for scantlings, are based upon many many years of data collection and model testing and real life-size model testing too, not one bloke who has designed a few boats. All the data that is collated is then run through endless statistical methods for trends and "averages" to arrive at a set of formulae that can be used in many situations.

    LR have gone slightly over the top with their latest SSC rules with formulae for formulae!!!...but they seem to take the view that everyone would just use their SSC software, ie punch in the numbers and out pop an answer...hmmm!

    However, no matter how one derives scantlings that is just half of the picture. Numbers for minimum modulus require proper analysis for load paths and access for welding, ease of fabricating etc etc etc
     
  5. MikeJohns
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    MikeJohns Senior Member

    I’d be cautious.
    I checked the design of a steel boat for someone who had used this scantling guide and although in the main things were ok I had two concerns.
    One was that much of his steel scantlings were on the heavy side for larger steel vessels. The other was that the designer had omitted the floors on the basis of his interpretation. He showed me the guide and I could see how he had been confused in the area of the keel.

    I read parts of the book then and I’d consider it as a rough design guide early in the design spiral and before refining the design to class scantlings. These days you’d be well advised to adopt an ISO compliant free guide such as provided by GL.

    I contacted Dave about his intended compliance and he said that several design aspects usually needed changing to meet various class requirements but that generally it was all robust enough in his opinion.

    The proviso that I would add is that it would pay to get the resulting structure checked by a class society (as Dave Gerr apparently does himself) or by an experienced qualified designer to make sure you made no oversights.

    My real moan with Dave’s otherwise informative books and articles is his ‘manglement’ of the SI system ….it’s truly horrible to behold (Sorry Dave :) ).
     
  6. Alik
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    Alik Senior Member

    Good book when it comes to details such as reinforcements, engine base, etc. Not really useful book for engineering, but maybe it will work with amateur designers designing limited range of craft...

    Gerr says it covers boats from 10 to 120', and to 45kts. I am sure that 120' and 45kts are far beyond where rules of thumb can be applied.

    Then, method for speed effect on scantlings is wrong - every knot above 10kts gives 1% increase of bottom thickness (p.30). This is nonsense - 10' boat at 10kts is already planning with hydrodynamic lift applied on bottom. But 120' at 10kts is still in displacement mode, mainly hydrostatic pressure is applied. One should deal with relative speed/Froude number to define whether loads are for planning or displacement craft.

    Hope they do not use this book as single source for students in Wesltawn… At least they should give alternatives.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    "...One should deal with relative speed/Froude number to define whether loads are for planning or displacement craft.."

    Rightly said. far too many people clearly read this and other similar books and hence influenced in the terminology/understanding and quote speeds in knots when talking about hull speed or planning etc. It is all about the Fn!
     
  8. Alik
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    Alik Senior Member

    Besides I really like last book from Dave Gerr - 'Boat Mechanical Systems Handbook'. In this format and scope of information it is always in use by our engineers.

    'Propeller Handbook' - we don't use it too much for calculations of props becasue we use software for that. But when it comes it dimensioning of shaft and P-bracket - yes, we use it.

    'The Nature of Boats' - I still use this book to dimension outboard engine mounts!

    There are few good books from Dave, in some points we would disagree but anyway it is valuable opinion.
     
  9. TollyWally
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    TollyWally Senior Member

    Life is often divided into 2 spheres, the labratory or office and the field. There is a built in tension between the two. Champions of each can point to a long list of failures in the other. Conservative rules of thumb have a better track record than envelope pushing engineering. It is interesting to note that bleeding edge optimization typified by successful racing teams consists of trial and error efforts conducted in the field.

    It's a good book, is it the last word? No, either is any other.
     
  10. lukedh
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    lukedh New Member

    Hi.

    Gurr's book may have some shortcomings but is a useful guide to put you in the right ball park. While this cannot replace sound design, a calculated figure is based on assumptions of the stresses a boat will encounter in use and include an additional factor for safety.

    Gurr himself is an experienced Naval Architect, I quote "A graduate of the Westlawn Institute of Marine Technology, Gerr studied physics at New York University and industrial design at Pratt Institute. A Fellow of the Royal Institution of Naval Architects (RINA), Gerr is also the director of the Westlawn Institute of Marine Technology".

    link: http://www.gerrmarine.com/dave.html

    So I guess the book is based on his experience of boat design and knowledge of marine technology. There are of course other books to choose from.

    I post this just in case anyone is under a missunderstanding that David Gurr isn't qualified.

    Cheers
    Lukedh
     
  11. daiquiri
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    daiquiri Engineering and Design

    Absolutely true. :!:

    As for classification societies' scantling rules, I was not implying that they have not been derived in a scientific way. That's more than obvious, considering the economic burden on them in case of eventual errors.
    I was referring to the fact that they will generally give you a ready-to-use formula for anything, and you either know which engineering theory it has been derived from, or you just trust them and use it as it is (which you will do anyways). The same can be said about Gerr's method, albeit with much smaller degree of confidence (especialy now that I've read MikeJohnes' post above) - it might also be a method based on a sound engineering principles, it's just that we (or at least I) don't know anything about it.
     
  12. Paul Kotzebue

    Paul Kotzebue Previous Member

    It doesn't have to be either/or. A good designer should have a solid engineering background and common sense experience. The tension we see in yacht design circles is between the amatuer hobbyist approach and the professionally engineered approach. This is not the case in commercial small craft design where most clients expect the engineering approach.
     
  13. frank smith
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    frank smith Senior Member

    Very interesting discussion .

    Just a few questions please. Would all engineering be done by an in house NA?
    Do NAs use out side engineering firms ? Is some kind of certificate for an engineered product required ? Could I as a hobbyist get a certified as built plan from a NA or engineering co. .
     
  14. Paul Kotzebue

    Paul Kotzebue Previous Member

    None of the above includes a mechanics of materials course as part of an ABET accredited engineering program. Mr. Gerr does not claim to have a degree in Naval Architecture or any other engineering discipline. He is not listed as a professional engineer in the state of New York. I recall that Professional Boatbuilder defined a Naval Architect as one who has a degree in Naval Architecture or holds a professional engineer's license to practice Naval Architecture.

    We have to take him at his word that he is an expert on the elements of boat strength.
     

  15. Alik
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    Alik Senior Member

    Maybe there is a tension in US and some other countries where there are no formal structural requirements for small craft. In Europe compliance to ISO12215 is required - there is no space for amateur hobbyst approach anymore.
     
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