ABS scantling rules

Discussion in 'Class Societies' started by StrandedMariner, Sep 27, 2007.

  1. StrandedMariner
    Joined: Sep 2007
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    Location: Shanghai, China

    StrandedMariner Steelboatsailor

    I have been doing quite a lot of research in preparation of getting a steel Dix 43 built. The description of the plans says: "Construction is steel, designed to the ABS scantling rules for ocean racing yachts."

    Now somebody told me that the ABS rules expired many years ago (1994 ?), and that the structure needs to be re-calculated with ISO or RINA or another international standard.

    Why would this be necessary? Is it a requirement for insurance? Excuse my ignorance.

    Thanks in advance,

    Andreas
     
  2. MikeJohns
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    MikeJohns Senior Member

    Andreas

    ABS has not 'expired' see www.eagle.org .
    Acceptable standards worldwide tend to be Lloyds, DNV, ABS, and lately ISO. Construction to any of these standards will be sufficient for a steel vessel.

    Are you building locally (Shanghai) ?
     
  3. Landlubber
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    Landlubber Senior Member

    StrandedMariner,

    I read some of your website, ABS is certainly not your only problem eh!
     
  4. StrandedMariner
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    StrandedMariner Steelboatsailor

    Thanks Mike. I am not sure yet if I will build in China or elsewhere. Right now I am trying to get as much information I can, to make the right decision.
     
  5. StrandedMariner
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    StrandedMariner Steelboatsailor

    Landlubber, we rather tend to call it 'challenges' in stead of 'problems' ;)
     
  6. marshmat
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    marshmat Senior Member

    ABS standards are still valid, as far as I know.
    Probably the two biggest factors that determine which rules to build to are:
    - What the designer says
    - What your insurance company says
    If there is any chance at all you'll want to charter the boat in future, the insurance company will likely need proof that it meets the rule they like best. To be on the safe side, there seems to be a growing trend in metal yacht construction to build so that, on any given aspect of the design, the strictest of the common rules is satisfied- thus the vessel will be better than any of the rules require. Whether this slightly more time consuming and expensive approach is for you, depends on what you plan to do with the boat.
    The designer says its structure is engineered to ABS standards. You'll have to build and outfit the boat to the same standards if this is to mean anything. Check with your insurer before you start to make sure they're OK with that, it won't cost much to make the phone call but it could cost you a fortune in insurance later on if you don't check.
     
  7. StrandedMariner
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    StrandedMariner Steelboatsailor

    Thanks for the good advice Matt!
     
  8. Dudley Dix
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    Dudley Dix Designer

    Our designs have been built in many countries around the world and structural acceptance by the authorities has never been a problem for builders, to my knowledge. Some require a letter confirming that the design is suitable for offshore use. Some require a copy of the ABS results printout.

    ISO and CE standards seem to be based on ABS calcs anyway.
     
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  9. Robert Gainer
    Joined: Jul 2004
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    Robert Gainer Designer/Builder

    While we are on the subject of scantling rules what is the factor of safety that is considered appropriate with today’s materials? Do you use a much larger factor of safety for an offshore sailboat boat then an inshore sailboat? Or does the larger displacement of the offshore boat build into the equation a larger factor of safety. Do you use the same number for all aspects of the design or do you design the rig with a smaller factor of safety then the hull girder.
    All the best,
    Robert Gainer
     
  10. Dudley Dix
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    Dudley Dix Designer

    The ABS Rules set the safety factors in the calculations, which are quite complex if done by hand. Using a computer program the results are the minimums that are needed to fit the rule. The results are supplied as mimimum Inertia and Modulus values. The designer must design panel and stiffener sections that at least equal those values if the design is to meet the requirement of the rule.

    Category 0 racboats have higher safety factors. For inshore boats the ABS rules may be used as a datum and you may decide to go a bit lower on strength if other regulations don't force you to do otherwise.

    Scantling rules don't normally control rig design. This is normally up to the designer and is based on empirical calcs developed through many years of sailboat history. An inshore rig may be less robust than an offshore rig.
     
  11. Robert Gainer
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    Robert Gainer Designer/Builder

    On 8-16-05 on the CSBB forum Bob Perry said in response to a question about the factor of safety in sailboat design,

    “We use safety factors varying from 2.00 to 4.00. We never use more than four and we never use less than 2.00. It depends upon the boat and it's use. For our typical cruising boats of moderate displacement we go with four. For our "cruising sled" series of light cruising boats we drop the safety factor in order to help reduce weight. It's my attitude that there are components of your cruising boat you should be able to take for granted: rig, rudder and laminate schedule.”

    Is this mainstream thinking or does this only represent one designer’s opinion. I understand that the ABS rules establish a minimum but what is considered best practice today. What should the factor of safety be for different categories of use?

    Two recent keel problems immediately come to mind. One was a Bavaria 38 Match and the other was a Max Fun 35. In both cases a life was lost when the keel dropped off. The design was deficient in the case of the Max Fun 35 and the subcontractor who made the weldment changed it which made it even weaker. The boat was designed to the ABS rule and that specified a minimum safety factor of 2 using the yield point of the steel as the design number, It was designed in error to have less then that safety factor and the designer used the ultimate tensile strength of the steel which is a larger number but the wrong number to use in the calculation. To compound the problem more ballast was added to the bottom of the after the boat was placed in service and that only increased the strain on the hull keel interface. Even if the calculation was done correctly was 2 too small a number for the factor of safety?

    The Bavaria 38 Match keel problem was different in detail as far as the mode of failure but it happened because of the same chain of errors. The designer designed something weak and the builder made changes to make it even weaker. Again what was the designed factor of safety and was it large enough?

    Of course the underlying problem is that a light boat is a fast boat and there is no incentive for a builder to add structure (weight) when that adds expense and impairs performance. Does the scantling rule need to include limits for ballast/displacement ratios and minimum hull weights for boats? If it was required to carry around the weight in the hull instead of in the keel it would go into improving the strength of the hull girder.
    All the best,
    Robert Gainer
     
  12. Dudley Dix
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    Dudley Dix Designer

    I can't comment on what Bob Perry wrote, I have not seen the full context of what he said. I also do not want to get into a public discussion about design, I really don't have the time to do it justice.

    I made my earlier post primarily because the original question was about one of my metal designs, the Dix 43. That design has had many boats built and the only structural problem to date has been a cracked skeg when a stiffener was omitted in error by the builder. Even that boat had covered many thousands of miles before the problem appeared. That design was calculated to ABS without increasing the safety factors.

    Before I bought the ABS program I did comparative calculations of a design that I had done based on Lloyds calculations. The ABS calcs came out slightly lighter in some respects and slightly heavier in others. Overall there was not much to choose between them.

    Lloyds calcs have never been disputed, so why does this happen for ABS? I think that it is because of a few high profile cases of racing boats that have suffered structural damage in racing or unusual conditions. Crews must consider the boat when sailing in rough conditions but big sponsorships and prize money encourage them to carry on racing when looking after the boat should be of prime importance. The result is that these boats are subjected to more load than they may have been expected to previously.

    All scantling rules have been developed empirically, based on past successes and failures. In the past the crew racing a carvel boat had to look after it to survive if conditions became extreme. They think that modern materials are indestructible so they want to keep racing in case the opposition is still racing. If you load any material beyond what it is capable of withstanding it will fracture.

    The loads used in ABS are based on testing in certain conditions and on assumptions that have proven safe in the past. If the crew sail outside of those assumptions and conditions then the safety factors are rapidly diminished until they disappear altogether.

    Racing boats fly off the tops of waves, with massive pounding loads. Cruising boats are sailed much more conservatively, with lighter loads in most conditions. It would be very unusual for a cruiser to suffer the same damage as has happened to BOC or Whitbread raceboats.
     
  13. Bad_Trim
    Joined: Feb 2006
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    Bad_Trim Junior Member

    ABS Scantlings

    Hi Dudley,

    What program are you using to develop ABS scantlings?
     
  14. Robert Gainer
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    Robert Gainer Designer/Builder

    Thank you for your comments but it was not my intention to cast doubt on the ABS rules. Instead I was trying to get an idea of what was considered a reasonable factor of safety to use for different services. And I was very interested to see how much a designer increased the factor for offshore boats compared to inshore boats. The offshore boat is heaver for the most part so even with the same factor the structure is heaver in proportion to the boats increased displacement. Does the increase from added displacement add enough strength or do the prudent designers increase the factor of safety? I hope that made sense.

    Bringing the keel problems into the discussion just mudded the waters. This is what often happens when on the rare occasion I have more then one idea in mind at the same time. Everything gets put into one mega thought instead of being placed where it belongs.
    Thanks for your thoughts and all the best,
    Robert Gainer

    By the way I have looked at some of your plans and was very impressed with both the plans and the finished boats.
     

  15. Dudley Dix
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    Dudley Dix Designer

    I use a little-known program, which was sold by ABS themselves but is no longer available. I seem to be one of very few people who have it. Dave Vacanti also has an ABS program for sale.

    To answer your safety factor questions more directly, I have always used the ABS results as my guide and designed to be just above them in my panel and framing calcs for the worst locations in the boat (most heavily loaded due to unsupported lengths and location due to slamming loads). That puts most of the structure well in excess of ABS.

    Working on this basis, if any failures occur then the design must be modified to remedy the situation and the knowledge that is gained goes into future designs as well. This is the empirical way that marine design has developed over centuries.

    Even now, with all the computer software and computing power that is available, the same principles apply. That was proven by the damage to "Golden Aluminum" in the 90s. That boat proved that modern flat BOC type hulls might be loaded very differently in a short and sharp sea from what is normal for more moderate boats. That knowledge will have affected other BOC boats that were designed later.

    Based on this it seems that the lighter the boat the bigger the safety factor that it needs. The heavy boat will not launch itself out of the water, so does not have to endure the same slamming loads. By its nature it already has higher safety factors than the light boat.
     
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