The elements of boat strength

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

  1. Ad Hoc
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    Ad Hoc Naval Architect

    D

    The main point about ISO rules is that it is a "catch all". For those that are not building to Class and anything else that may fall into this category.

    There is no ISO surveyor to check the build quality...there is no ISO plan approval head office. The ISO rules are for those that needs a reference to apply a measure of compliance to "something". A flag state will need assurances that the boat being built, is not done by some fly-by-nighter and when it goes horribly wrong....whose to blame?

    When designing to Class one generally builds to Class rules too. Thus there is the whole safety of the vessel aspect. Flag sate will accept any Class approved design and build, period.

    But the safety of the vessel (i.e ostensibly a Class function) is not the purview of the Flag state. Flag rules default to "some other" for structure etc. This "some other" is now ISO rules. Since flag states do not, in general, produce their owns set of rules for designing structure etc. They do however, produce their own set of rules for crew/passenger safety and safety of passengers coupled with their systems on-board to enhance this side. (Such as life rafts, CO2 release, SFP etc). This too is now being harmonised..albeit very poorly.

    So, if you design a vessel to Class rules and have a Class stamp on your dwgs..perfect. If you design to Class rules but not have a Class stamp on them...you can submit them to Flag and then they will review...since they too can get a copy of said Class rules and check for compliance.

    Thus if you're not designing to Class rules....in steps ISO. That's it.
     
  2. Alik
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    Alik Senior Member

    There are differences, actually I did a comparison in some of my papers.

    ISO and class rules loads are similar. But for compotes ISO takes safety factor of 2 for tensile strength, though classification societies take 2.5-4 (3-3.33 say, are for bottom) depending on the area. For structural properties of composites same formulas are used by ISO and LR SSC for number of values, but ISO method 'c' then deducts 20% for uncertainty of glass content.

    Actually, we usually design all structures to ISO as first approximation as they are easy to follow, and then at advanced design stages we go to class rules as those are more time consuming. Compliance is about 90%. Say, the bottom floors of high speed craft to class rules are usually heavier than ISO. There are also some slight difference in definition of pressure points, span, etc. in ISO and class rules but those can be handled.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    I think it is a case of what you're used too as well.

    I can do a midship section, to Class, in about 5-10mins..knowing which rules are the dominant ones is the key. I don't bother checking endless odd ones since knowing what type of vessel it is and where/how it is operating enables me to short circuit the design process immediately.
     
  4. Alik
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    Alik Senior Member

    This is because of the material; You do most of designs in alu! For composite/sandwich is not that easy; yes I can assign the materials and number of layers from the experience but to minimize the laminate it takes few steps with software.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Correct....about 90% of my stuff is ally.

    A composite midship section takes me a bit longer as i have to double check some rules, as I tend to forget over time and is more hand calc based too. This takes me around 30mins or so...or may be a bit longer if I've forget some rules.
     
  6. rjwintl
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    rjwintl Junior Member

    just another thing to remember : when the Vikings built their boats 1200 years ago they never read Dave Gerr's Boat Strength !!! ... nope, they weren't using epoxy or gel-coat either !!! ... and more recently Chris-Craft, Penn-Yan and Lyman boats were never "designed" on these theoretical/experimental "expert" parameters !!!
     

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  7. BlueBell
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    BlueBell . . . _ _ _ . . . _ _ _

    Oh Those Vikings

    Different times, different materials, different locations, different geographies, different technologies.
    Differing levels of knowledge and understanding.
    Differing motivations.
    A lot of differences.
     
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  8. Rumars
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    Rumars Senior Member

    You really don't know how human knowledge works. Let me assure you that there was a "Gerr" in viking times, and he had a "book" of what works and how to do things. Just like all the other boats you mentioned the viking boats were designed according to the knowledge standard of their time and place combined with their builder experiences. Some where bog standard boats, and some where innovative in form, building technique, etc., and their inventors contributed to the general knowledge. Sometimes knowledge is lost and stays lost, or it gets reinvented sometime and somewhere else.

    Just for your info, class rules, ISO, and all other boatbuilding standards developed by consensus between industry professionals and are updated regularly to reflect our current knowledge. While many seem overly conservative their aim is not to smother progress, it's to ensure safe boats for the average user.
    Innovation and experiment are done all the time, and their results incorporated into the next iteration of the rules, so that "normal" users may benefit, but only after sufficient evidence has been gathered so that enough people can agree "this is a good thing". Contrary to what you may think, all this rules are not imposed by an outside authority on the poor boatbuilders, they are the ones who banded together to create them and pressured "the authorities" to give them the power of law. As long as you are willing to take the risk and pay for the damages to others, you can build and operate whatever boat you can dream of, nobody will say anything.
     
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  9. rjwintl
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    rjwintl Junior Member

    … guess you told me, huh? … rjw
     
  10. rjwintl
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    rjwintl Junior Member

    … power of law, which means control. I get it. As for not understanding development of human knowledge, I do know that I’ll continue to learn my whole life and use what works and disregard the senior member know-it-alls! rjw
     
  11. Rumars
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    Rumars Senior Member

    Yes control, that's how society works, you give up some of your rights for the benefit of the whole community. Case in point, you don't want someone's supertanker breaking in half in front of your beach so you agree that anyone (and that includes you) building and operating such a thing does it to a certain standard and be insured for the damages, and you actually demand from your government to make it law and enforce it.
    Other times the community imposes something not the law. Many countries have no insurance requirements for private boats, yet marinas always want you to be insured. The insurer demands some proof that your boat won't break up first thing when it hits the water, and the simplest way to do this is build to some accepted standard.
    Right now a guy I won't name sits in a parking lot on his big boat and scratches his head. The port won't launch him without insurance or a big cash bond, the insurers won't touch his boat without comprehensive survey, no pro wants to sign of on his boat without stability analysis, and he has no money for such a thing.

    As for learning and using what works, how do you do that without books? Oral lore is not sufficient anymore, our knowledge base is to extensive for this kind of transmission.
    Rules are just a type of codification of our collective experience, not the boogeyman. You can ignore them and do your own research from scratch by trial and error, and decide for yourself. It's called research, innovation and first principles engineering an is done all the time. Sometimes it works and sometimes it breaks, and you have to pay the bill and start over.
     
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  12. rjwintl
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    rjwintl Junior Member

    Yes indeed … I get it. Studied through college design courses and (not bragging) graduated at the top of my class … statics & strength of materials, wood , steel and concrete design, estimating & drafting, physics and surveying… even though Jack Kemp sponsored me to go to the USNA, I backed out and used my degree to build houses like my great-grandfather! I understand Established thought; I learned and retained a helluva lot, but, practical experience for 45 years in business really trumps many established knowledge. I built a boat this summer, used math to figure the bow arch (who does that) , had the VA DWR review my design and estimates along with an exact material list … submitted it all and was granted title & registration. How many folks do that for personal use and save thousands of dollars like I did … insurance, definitely if I went into business selling a better boat than most could build on their own … I ‘m not the dummy you seem to infer !!! … rjw
     
  13. DogCavalry
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    DogCavalry Senior Member

    Woah! No one is inferring nor implying you are a dummy. But this forum is awash in folks claiming knowledge and credentials they don't have. We constantly get folks claiming to be professional Naval Architects or shipyard project managers who ask beginner questions, or argue petulantly with fellas who really are NAs and PMs, because the fellow generously explained something the newb bloody well should have known by the end of first-year. Sometimes they lose patience.
     
    Last edited: Dec 13, 2021
  14. TANSL
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    TANSL Senior Member

    Many times a question can seem stupid or beginner's to those who are not trained enough to understand what is behind it or the intention of the questioner.
     

  15. DogCavalry
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    DogCavalry Senior Member

    It's true that a professional would never use Gerr's Elements, because they use other procedures. He acknowledges that in the book, and in other articles over the years, worked through problems using various professional standards and rules, exactly as a credentialed professional would, because he is one, and regularly does so. Then he works through using the book and gets substantially the same answers, at a fraction of the effort. Or tests the assumptions of the rules and demonstrates internal inconsistencies and irrational unintended consequences.

    But the single most important attribute of his book was that they had several copies in my local library.
     
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