A question of philosophy

Discussion in 'Class Societies' started by DUCRUY Jacques, Feb 1, 2011.

  1. fcfc
    Joined: Feb 2005
    Posts: 781
    Likes: 29, Points: 28, Legacy Rep: 399
    Location: france,europe

    fcfc Senior Member

    ISO is not a classification society. It is just the bare legal minimum to have the right to sell the boat on the EU market.

    Think of cars. All cars sold in EU do pass all EU legal safety requirements. But when a driver association test them for security, some cars have no stars, other got 5 stars.

    ISO, being more recent and derived from ABS, has "more" engineering inside than ABS (curvature etc ...), allowing lighter structures.

    ISO seems also to provide adequate basis for racing boats. Now ISAF asks for ISO structure in OSR 0 1 2 . No longer ABS. NB ISO upper length limit is 24m 80ft. Above that, maxi racing yachts need a "normal" classification society.
    But ISAF asks more for safety than ISO (rerighting test for OSR0, front and rear watertight bulkheads, struts on stanchions, etc etc etc ...). They are also asking for category A (Ocean) compliance since OSR-2 (Offshore races, that would normally match category B ISO).


    Strictly nothing prevent you from using more than the ISO legal minimum. Either another rule such as Germanisher Lloyd or Veritas. Or doing your ISO calcs by multiplying your ISO hull pressure by 1.2 . Or assuming your boat has a "virtual " weight in ISO calcs of 1.5 the intended boat weight, or changing the value of Kdc coefficient (The one depending on the design category ABCD intended). Or of course any other safety coefficient you want
     
  2. Perm Stress
    Joined: Sep 2009
    Posts: 554
    Likes: 24, Points: 0, Legacy Rep: 323
    Location: Lithuania

    Perm Stress Senior Member

    Yes I do.
    But I talk here about side effect to Open 60 class, which was never connected to IOR in any way.
    Those ketches did demonstrate the possibilities of two mast rig under IOR rule, and some people did try to translate advantages to Open 60. Some attempts were, from NA side just stupid, as one I mentioned.
     
  3. Alik
    Joined: Jul 2003
    Posts: 3,075
    Likes: 357, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    Actually, for composites ISO produces heavier structures compared to ABS (and also compared to LR SSC) by assuming lower properties of materials. And curvature was/is in ABS as well.

    There is no need for such tricks; structure designed to ISO is already quite conservative (if built properly). The only thing I change after ISO calculations is topsides lamination; we tend to add more laminate there just for practical reasons.
     
  4. fcfc
    Joined: Feb 2005
    Posts: 781
    Likes: 29, Points: 28, Legacy Rep: 399
    Location: france,europe

    fcfc Senior Member


    Not sure, in ISO, you have three methods to determine material properties (composite) to use in computations. (Table C-1 in annex C).
    EL-c is using only tabulated data in the norm. OK, it is quite conservative. But there is also EL-a, where you do measure actual mechanical properties on "representative" samples and use the figures you ve found in ISO calcs.
     
  5. Alik
    Joined: Jul 2003
    Posts: 3,075
    Likes: 357, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    In theory it is, but we never used method 'a' and and never seen anyone using it :D
     
  6. fcfc
    Joined: Feb 2005
    Posts: 781
    Likes: 29, Points: 28, Legacy Rep: 399
    Location: france,europe

    fcfc Senior Member

    I guess it is used either by top racing boats or high volume production. To shave material either weight or cost.

    I cannot imagine a TP52 built according ISO 12215. But TP52 classe rule mandate ISAF OSR-1 section 1 2 3 compliance. And ISAF OSR1 section 3 mandates ISO 12215 category A compliance. So TP52 (at least the recent ones), are built according ISO at least. I do not think they use tabulated data provided in ISO annexes for mechanical properties.
     
  7. Alik
    Joined: Jul 2003
    Posts: 3,075
    Likes: 357, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    For TP52, there are just few offices who design them; I am sure not all of them do engineering in-house :) Besides as I understand for those boats certification (to ISO) or plans approval (to ABS) is not required, just statement of compliance...

    For mass production boats - we often find them made of strange materials. Better to not test those materials as specs can be lower than one expects :)
     
  8. fcfc
    Joined: Feb 2005
    Posts: 781
    Likes: 29, Points: 28, Legacy Rep: 399
    Location: france,europe

    fcfc Senior Member

    ABS is no longer used by ISAF. Only remains ISO.

    And yes. It is not full CE marking, just some ISO rules compliance.
    But compliance must be stated only by ISAF recognized notified bodies.
    There are currently 31 notified bodies that can deliver CE marking. But only 8 recognized by ISAF.
     
  9. Alik
    Joined: Jul 2003
    Posts: 3,075
    Likes: 357, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    Correct. Racing boats are very small niche and there is no real funds behind to develop own rules or control compliance. So they force designers use ISO12215-5 now; if designers use method a) we all understand that mechanical properties of materials can be different in tested sample panels and in actual hull.

    Today's racing sailboat design is mainly about making lighter boat hull than competitors. So where we are in terms of weight, and how find loopholes in structural rules? :D
     
  10. Perm Stress
    Joined: Sep 2009
    Posts: 554
    Likes: 24, Points: 0, Legacy Rep: 323
    Location: Lithuania

    Perm Stress Senior Member

    With careful building ($) selected materials ($) detailed engineering ($) much lightening is possible without searching for loopholes. Even in commercial shipping. GL rules, for one, state permissible tension/compression stress 150N/mm2 for mild steel. But when using FEA, permissible stress is ~220N/mm2 for the same steel, same application. I am sure that in composites even greater differences are possible -laminates are much less standard-strength product as steel.
     
  11. Alik
    Joined: Jul 2003
    Posts: 3,075
    Likes: 357, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    That is true for commercial boat projects. Now we have to look at racing boat design/construction budget... Believe me, in reality in racing yacht design (and in small craft design in general) there is much less advanced engineering and FEA than one used think (or used to claim in press-releases).
     
  12. fcfc
    Joined: Feb 2005
    Posts: 781
    Likes: 29, Points: 28, Legacy Rep: 399
    Location: france,europe

    fcfc Senior Member

    It is not a loophole. It is a giant hole.

    Racing boats structures have no other issues than full Finite Elements Analysis.

    Instead of using 10.2.2 equation simplified thickness for composite, use annex H laminate anlysis. Or even more, use validated CLT software (ie agreed with the notified body) to analyse your composite panel and to show it will sustain the required hull pressure. And use mechanical properties determined from experiments. All this is already valid per ISO 12215.

    NB if you design a TP52 and use the method a) , you will have to prove to Germanisher Lloyd that the samples you have derived mechanical data from are representative of a hull panel ie per ISO12215-5 rule ie "same typical workshop conditions using the same material, fibre contents and sequence, methods of lay-up, thermal treatment and time sequence" and tests done according to international standards.

    It is even more visible in ISO 12215-9. The norm just defines the forces your keel must sustain. It is up to you to compute an adequate structure, and prove it to the notified body.
    There is also an easy simplified case with more or less tabulated data. But if you design your boat according to the simplified case, it is not even worth to come on the race start line.
     
  13. Alik
    Joined: Jul 2003
    Posts: 3,075
    Likes: 357, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    This is how we do in our office.
     
  14. Perm Stress
    Joined: Sep 2009
    Posts: 554
    Likes: 24, Points: 0, Legacy Rep: 323
    Location: Lithuania

    Perm Stress Senior Member

    I was inside "Ambersail" -an old Whitbread-60 boat, Bruce Farr design. I had an opportunity to look at some structural drawings, that came with boat. Having experience in FEA optimization, there is a lot of obvious clues to FEA optimization used to design it.
    I would not think TP52 today are designed less carefully as W60 almost 20 years ago.
    I would not think, that "loophole hunting" is still a productive way to design and build competitive boats in box rule class today. If you are too conservative, your boat is too heavy and you loose. If you are too optimistic, your boat breaks apart and you lose. And there is only so much optimization (read shaving off unnecessary material) possible with simple beam theory based engineering.
    The only possibility where it is really not necessary to apply advanced engineering is when minimum weight requirement is more than generous.
    But alas, TP52 vent to dramatically reduce their minimum weight limit, so it looks like no one in high profile racing want to sail heavier boat, in order to simplify engineering. They go for lighter weight and heavier designers fee... :)
     

  15. Alik
    Joined: Jul 2003
    Posts: 3,075
    Likes: 357, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    I would say that was experience, not just FEA. If it was only FEA anyone who can run the software could design light structure.

    I have worked with leading boat structural design companies; many of them still use rulebook, not FEA.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.