Revision of ISO 12217 - Small craft - Stability & buoyancy

Discussion in 'Stability' started by Convenor 12217, Jan 18, 2013.

  1. Alik
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    Alik Senior Member

    Regarding revised ISO12217-1, I noticed that offset load test procedure is very different now. What surprised me is that test weights are now 85kg, and should be placed 0.1m from deck or seat. This is OK for testing, but a nightmare for calculations - we have to create fake load cases LC1 and LC2 where displacement exceeds maximum loaded displacement.

    Before we calculated only 2 load cases - LDC and MOC for every boat; now we haver to calculate 5 cases, also more cases if we have recesses that can be filled with water.

    I think the developers of 2013 option did not make the job properly. Not all the stability software is capable to calculate what they want!

    My opinion, there should be procedure for testing - used for boats below 6-12m, and procedure for calculations using common design cases in naval architecture for bigger craft. These inventions with CG of a person 0.1m from deck are complete absurd!
     
  2. Claus Riepe
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    Claus Riepe Junior Member

    Alik,
    Luckily you are wrong. 1 crew of 75 kgs suffices to right an inverted 6m BayRaider dinghy. So why be defeatist and not accept the design challenge to copy that? C.
     
  3. Alik
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    Alik Senior Member

    Well, You reference something that is raider boat, no side decks that will give a huge stability when inversed, water ballasted, spar of floating material... Yes, if You call it a dinghy, You might be right :)

    I was sailing a lot - both 6m dinghies and 18-20' cats. I would say I was a bit lighter those days :D but bringing them back was not easy if not impossible. Try to bring Tornado or similar catamaran back - I theory one can do it, in real conditions at sea - hardly possible, without another boat.
     
  4. Convenor 12217
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    Convenor 12217 Convenor 12217

    ISO 12217-3:2013 in respect of undecked sailing boats

    ______________________________________________________

    Dear Claus,

    ISO 12217-3 has three options for the assessment of sailing boats that are not fully enclosed: 7, 8 and 9. Option 7 is the Capsize Recovery Test to which you refer, Option 8 is the Knockdown Recovery Test which ensures that the boat will right itself from a knockdown without the help of the crew, which is obviously for boats with an external ballast keel, and Option 9, the Wind Stiffness Test, which assesses the wind strength needed to cause stability or swamping issues.

    The first two options ensure those boats can be recovered to upright. The third option was developed for boats that cannot comply with the other two, and yet which have a satisfactory safety record by virtue of their modest sail area and inherent stability and freeboard. Examples of such boats can be seen at http://www.cornishcrabbers.co.uk/index.cfm/boat/Crabber.Shrimper19, http://www.drascombe.co.uk/drascombe-lugger-pr-52.php, and http://www.neilthompsonboats.co.uk/norfolk-range/norfolk-gypsy/

    It is important to note that Option 9 relies on the user to sail the boat appropriately and for this reason Safety Signs are required to be displayed on the boat and information given in the Owner's Manual regarding the wind strength at which plain sail should be reefed. This Option is analogous to the way sailing catamarans are assessed, for which, because many such boats cannot be righted after a capsize, great importance is placed on providing the user with the information to avoid such events occurring.

    The requirements for Option 9 were amended in the recent revision following the incident described at http://www.maib.gov.uk/publications/investigation_reports/2006/Mollyanna.cfm
    It seems likely that the casualty to which you refer did not comply with ISO 12217-3:2013 in respect of safety signs and owner's information.

    Convenor 12217
     
  5. Convenor 12217
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    Convenor 12217 Convenor 12217

    ___________________________________________________________

    Dear Alik,

    A mass of 85kg per person is used in ISO 12217:2013 largely as a result of an accident investigation (visit http://www.maib.gov.uk/publications/investigation_reports/2004/breakaway_5.cfm). Investigation showed that the standard ISO figure of 75kg per person, being taken from very large populations of data represent a reasonable overall mean figure, but does not reflect the probability that a small group of people may weigh more than this on average. In fact in the subject boat, the ten persons on board comprised 4 adults and 6 children, but they weighed on average 85kg each. 85kg is a compromise figure but in fact is a fair average for Western adult males.

    Another lesson from this same incident was that in small craft the effect of longitudinal positioning of the crew strongly affects the trim and so the static stability. This is why the Offset Load Test looks at a variety of positions for the crew. As vessels get bigger the standard ship practice of neglecting trimming effects becomes more acceptable, but the smaller they are the more significant this becomes. The subject boat would not have capsized if the crew had been evenly distributed.

    In general the most stability-critical loading case (especially for the Offset Load Test) is not fully loaded but what is called the Loaded Arrival Condition, ie: fully loaded less 95% fluids and stores. But while this is usually the worst case in respect of solid VCG, half fluids is worse as regards free-surface-effects. This is why half-fluids was adopted. So it is incorrect to say this test is applied at a displacement greater than that when fully loaded - it is slightly lighter.

    The explanation for the adoption of a person's VCG being taken as 0.1m above deck is less obvious. In ships, this VCG is usually taken as 0.3m above a seat or 1.0m when standing. These figures reflect the truth if the people do not react to the heeling of the vessel, which in ships is often the case. However in small boats people most certainly do not react in this way, but they endeavour to stay with their trunk upright.

    However so the 'ship' figures do not reflect the forces when a standing person remains upright. In this case their "effective" VCG is at the ankles - their pivot point. The ISO working group found that if the 'ship' figures are used then in smaller boats the crew limit was drastically reduced from that found to be satisfactory from experience. In fact this figure is easier to apply if the assessment is by calculation rather than when using test weights - it is just a number change in the moments table.

    The effect of variation in the VCG of a person on the stability of a ship is modest. But, especially when boats are less than about 10m long, the effect of the assumed VCG of each person is very pronounced - just consider a 3m long dinghy.

    The problem with varying the assumed VCG of a person with boat length in the manner you suggest, inevitably leads to a sudden change in crew limit as the length boundary is exceeded - not a very desirable outcome. For simplicity the ISO working group decided to adopt the lower figure as more generally representing reality for the smaller, more critical boats. In any case this is just one factor in the assessment in which the overall safety margin was judged against practical experience.

    Finally, it was considered more important to write the standard on the basis of what is technically appropriate than on the basis of what software is available at any particular point in time. Software is continually developing and changing all the time to meet developing needs.

    I hope that these explanations make sense to you, even if you do not necessarily agree with them.

    Convenor 12217
     
  6. Alik
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    Alik Senior Member

    Thanks, understood. But for practical use IN DESIGN PROCESS this creates a lot of artificial problems, including those related to software use. Now we have to study 5-7 load cases instead of 2 previously, because criteria are applied to those different cases; some of these cases are purely artificial and causing confusion. Actually, now for pleasure craft we have to study more loading conditions than for small commercial/special craft, though design budgets for pleasure craft design are much smaller. On other side, there are still no special criteria for power catamarans (which we design a lot), and we have to use roll angles from monohull formulas - very unfavorable and not representing the reality.

    Regarding CG of a person, I doubt there was a need to differentiate from standard IMO values which evidently represent worst case, at least for calculation option. Moreover, for the same size of boat classification societies would still use 1.0m for a standing person.

    Another problem is load cases - now full load condition is 95% of liquids (see 3.4.4), this makes specification of full load displacement different under ISO and common procedures used in naval architecture!

    This statement is incorrect, because to new 2013 standard the craft should be offset tested at maximum load condition (B3.1.3) plus increased crew weights, this exactly creates situation when displacement at LC1 and LC2 exceeds mLDC, because mass of crew taken is 85kg. Unless any tank is wider than 0.35BH, otherwise it is taken 50% of liquids. In standard, there is no offset test in loaded arrival condition foreseen, but I agree it should be - logically! I believe the worst condition for offset load test will be '10% of liquids' condition, i.e. waste tanks are also almost empty.

    I would recommend to increase mass of person to 85kg for all load conditions (in US, as I know, they use similar value since 2010), and take common CG values. Offset load test should be checked at loaded arrival condition, not at loaded or 50% where tanks are almost full. Then, it would be sufficient to study 2 load cases: loaded arrival (likely to be the worst case for stability, as tanks are at bottom and still there is free surface) and full load (worst for downflooding angles, freeboard and AVS). Loaded arrival or better to say '10% of liquids condition' should be then taken with 5-10% of liquids in waste and grey water tanks. Margins for possible free surfaces at full load should be fitted into criteria, and full load should be 100% of liquids.

    My opinion (as member of few technical committees) - 2013 version could be done better/simpler. There is a system with criteria (inherited from previous versions of standard), but there is no consistency with load cases to study.

    We did the study of significance of criteria based on our designs, and the outcome that some criteria can be simplified or dropped, demanding on size. I am preparing to publish those studies. But it is evident that from 10-12m the offset load test criteria is not the priority, for monohull craft.
     

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  7. Claus Riepe
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    Claus Riepe Junior Member

    Dear Convenor,
    last time I looked, boat manufacturers can still freely cherrypick between different testing options, whichever test suits them best. (IF they take the trouble of practically testing at all, as there are still no independent third party checks required for Cats. D and C.)

    And what you say about Option 9 really troubles me, as the criterium "a satisfactory safety record by virtue of their modest sail area and inherent stability and freeboard" lacks precise clarification and specification inside 12217, so still does allow older design sailboats to get a Cat. "C" marking as long as their sailarea is "modest". What when those boats do capsize?
    Remember what they rightly say on page 27 of the "Molyanna" report: "Notwithstanding a dinghy’s categorisation, it is recognised that most, if not all, dinghies are prone to capsize in any conditions ..." .
    -I hold that any boat which is is prone to capsize must be practically tested for capsize recovery. No exceptions.

    I realise that contrary to widespread public belief RCD was/is NOT primarily intended to be for consumer information or protection, but for the harmonisation of international trade. However, the ensuing categorisations of recreational crafts have factually become a marketing feature. Manufacturers use the RCD categories to promote their boats with consumers thus potentially -and legally- lulling them into a false sense of safety at sea.
    Still, RCD, and ISO 12217, does neither recognise nor appreciate different levels of safety.: There are sailing boats which can pass all three tests, 7, 8, and 9, while some others merely comply with a single one of them, and fail the others. Still, legally under RCD and ISO the boats cannot / mustnot be marked for their specific levels of safety. I think ISO should allow for such distinctions though I realise RCD does not allow this at present.

    And there is one other issue where 12217-3 is causing a safety problem: Sailing boats which are also fitted for rowing and/or motoring must comply also with all details for non-sailing boats. That is dangerous. Sailing boats are for sailing heeled, non-sailing boats are for best stiffness, swamped stability. Such stiff boats do not recover easily from inversion.
    ISO should allow boats under 6mtr. LOA to be tested only for their intended MAIN propulsion, as are boats of 6 mts. and longer.

    All in all, I think ISO 12217-3 must again be reviewed for closing arbitraryness loopholes in Option 9 , and to allow for meaningful consumer safety differentiations.
     
  8. gonzo
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    gonzo Senior Member

    I have successfully recovered boats of 16 feet on my own. For example, a Snipe. With two people it is really easy. As teenagers we used to do it for fun.
     
  9. Richard Woods
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    Richard Woods Woods Designs

    So it seems you are saying that all powerboats (of any size) have to be crew rightable from capsize???

    Unless you really do mean "prone" as opposed to "can". In the former case the fact that if an old boat design rarely capsizes (like the Drascombe for example) then it is not "prone" to capsize and thus doesn't need testing for capsize recovery. Which I believe is the current requirement

    And you are right, the classic Mirror dinghy was in Cat D if considered a rowing or motor boat, but in C as a sailing boat. All because it had two drain holes in the transom at seat level

    Richard Woods
     
  10. SukiSolo
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    SukiSolo Senior Member

    Still has the two drains when I last looked....;) at least the wooden ones.

    Tricky subject, but several NA's I have discussed 'recovery' with, reckon the best test is still the real world one. Despite all the calcs and the best intents, refining a prototype is probably the easiest to ensure good behaviour in real conditions.
     
  11. meren
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    meren Junior Member

    Downflooding openings is one tricky part for example in case of offset load test. I don't remember wording but something like "where water may enter cockpit trough drains, wait until downflooding stops..." This is same in parts 12217-3 and -1, but there is more accurate explanation only in part 1 where it is a small printed NOTE that water should be let in through ALL openings when offset load in question. I find wording very confusing in part 3 where it is just "where water may..." Designer might found that drains with non-return flaps can be kept watertight during test if water may not enter the cockpit due good seal under water pressure. Or is it just up to designer to deside which opening is one where water might enter cockpit.:confused:
     
  12. gonzo
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    gonzo Senior Member

    I think that, like with all new regulations, they will eventually clarify several of these questions.
     
  13. TANSL
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    TANSL Senior Member

    Gonzo, please, what do you mean new regulation ?. The ISO 12217 standard can not be considered as "new". A first edition of 12217-1 October 2002, and in 2013 came a new version.
    What question will they eventually clarify?.
    Could you, please, clarify what you mean by these things you say?
     
  14. gonzo
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    gonzo Senior Member

    This thread is about changes in a regulation. Maybe you didn't get the memo about the thread :rolleyes:
     

  15. TANSL
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    TANSL Senior Member

    A new regulation is not the same as a new release of an existing one. You talked about a new regulation. Which one?.
    In any case, I think it would be interesting to know what you think are the questions that should be clarified.
    This thread is trying very delicate, very interesting topics, and if you think there is something unclear in this "new" rule your contribution in this regard is necessary.
    (Your experience with snipes can be very instructive.)
     
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