plating thickness?

Discussion in 'Boat Design' started by rlackey, Jun 24, 2005.

  1. Tad
    Joined: Mar 2002
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    Location: Flattop Islands

    Tad Boat Designer

    kmorin;

    As it is often mentioned in this forum I've been meaning to say something about the use of Dave Gerr's book, Boat Strength, for some time.

    Except for casual interest, the serious student of yacht design or naval architecture will find this book of no use. It's not intended for serious "real life" naval architects; it is intended for the interested amateur who wants to think about boat scantlings.

    One of the opening statements in the book is that, "scantling rules in this book were developed by the author, to cover only, vessels between 10 and 120 feet length overall." This would seem to put it out of contention for consideration as to Rich’s project.

    Another disclaimer is "The author and International Marine make no representations as to the suitability of these rules and procedures for determining the scantlings of any vessel." This, in my book, would put it out of contention for serious consideration in regards to any project.

    While the author lists several out of date classification rules in his Bibliography; ABS, Lloyd’s, and US Coast Guard guidelines. He gives no indication if these were consulted in any way with regards to his suggested scantlings. All his suggestions seem to be based on the size of the vessel, (his scantling number) rather than on expected loads. He gives no indication as to how his assumed loads were arrived at or what they might consist of and what the safety factor might be. There is no mention of global or longitudinal bending loads, thus the book is useless for larger vessels.

    Regardless of vessel size, if a naval architect is asked in a court of law, "how did you arrive at these scantlings?” You had better have something better to show than, "they came out of Dave Gerr's book".

    I would urge you to obtain a current actual scantling rule which the authors are willing to stand behind. Have at look at the Hull Construction, Materials, and Scantling Determination chapters in Larsson & Eliasson. For small fast boats ABS high-speed craft or Lloyd's SSC rules are a good start.


    All the best, Tad
     

  2. kmorin
    Joined: Apr 2005
    Posts: 185
    Likes: 18, Points: 0, Legacy Rep: 231
    Location: Alaska

    kmorin Senior Member

    design cycle

    Tad, I was responding to an inquiry at the most preliminary level of design! I didn't think the 52m boat was at the shipyard- nor do I think Rich is at the CNC output file level of his project. ( I think he intimated it was a possible future project.)

    This guided me to offer him a simple and inexpensive means to get in the "ball park". I'd say Dave's book was worth its wt in platinum for just exactly where this designer lists his project. All that was requested was a method to generate a rational number; not the final FEA for the panel stiffening webbing arrangements for his stabilizer fins' through hull.

    Without previous experience, and as a formative designer coming up with realistic hull materials to estimate one's way around the preliminary design cycle precludes sucessful use of the rules based investigation. A "life time boat", I think, may have dozens of iterations perhaps 100's? In your capacity as a practicing designer, how many refining cycles do you expect from concept to water? 10? 30?

    So, to respond to the original posted question within context of his circumstances; I still feel Dave's book is almost perfect for this level of Rich's project.

    Of course as he progresses in his learning with the great incentive of designing the best dream yacht he can, Rich will add more and more detail, just as you do when you refine your designs. As the details are refined the numbers will come along, the rationale behind each judgement will become more informed as the overall knowledge increases with time.

    However, for a 'first-around-the-cycle' -quick set of rules of thumb for initial calcs- nothing in the market place beats this simple approach- figure a common ratio "size" to other boats and pro-rate the entire scantling 'set' and frame organization to this ratio. I agree whole heatedly that its not an end-all last word design tool, but I really didn't think Rich was at that point in his design.

    As to applicability and disclaimers- everyone knows that publishers have lawyers who insert phrases in commercial literature! If you printed a copy of your design contract wouldn't it have some disclaimers that your lawyer insisted was "good business?"
    120' versus 150' - again all Rich asked about was how to get some START, initial guess, point of origin, something to hang his hat on.

    I think most readers here realize that the design cycle works from general to detailed and if Rich ended up with light or heavy plate, frames, and decks in his prelim estimates, they'd show up as unrealistic coefficients, too light or heavy a displacement and other indications he'd erred in the preliminary assumptions.

    After going around the design cycle a bit, Rich will arrive at a time when he'll have to start considering the overall structural design as opposed to a stick and frame approach, and when this comes I'd guess someone as thorough as he's showing himself to be- will either learn the needed numbers or hire it from a design office with experience in this size. (frankly, once he's at a stage of life that he can afford this yacht I feel he'd insure his investment by having ALL the calcs and overall design reviewed by NA, PE, and EE to insure it was up to Rules Standards. Feadship & Westport Ship don't offer too much in the 'owner/builder' line that I've heard about.)

    I've never designed or built anything larger than the life boats or tender for this "ship" so I'm out of my depth designing it. But for certain the Gerr book will provide a simple PRELIMINARY guide to help get Rich's ideas out of his imagination and onto the drawing board with some rough beginning of scantlings' values.

    Cheers,
    kmorin
     
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