The elements of boat strength

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

  1. Ad Hoc
    Joined: Oct 2008
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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
    Joined: Jul 2003
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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
    Joined: Oct 2008
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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
    Joined: Oct 2008
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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.
     
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