Assessment of Seaworthiness - ISO 12217-1/2 STIX

Discussion in 'Stability' started by ram68ocean, Jul 27, 2021.

  1. ram68ocean
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    ram68ocean Junior Member

    Hello, I am after some advice and direction on how to assess seaworthiness for a 'motor yacht with a keel bulb'. In other words, a sailboat without sails, or a motor yacht with a sailboat's keel. Any advice?

    It looks like ISO 12217-1 for Power Boats is not fully applicable. And ISO 12217-2 for sail boats also not.

    How could an equivalent STIX number be obtained? Aiming for highest category.

    Are there other ways available of assessing seaworthiness and motions? Especially to assess how good a design is. And also to compare multiple designs and effect of some appendages such as bilge keels.

    The aim is to have it 'Area Category 0 – Unrestricted service'. as stated here five-minute-briefing-mca-coding https://www.rya.org.uk/e-news/up-to-speed/five-minute-briefing-mca-coding
    Thanks.
     
    Last edited: Jul 27, 2021
  2. TANSL
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    TANSL Senior Member

    I believe that ISO 12217-1 for Power Boats is fully applicable. The norm does not ask anywhere about the shapes of the hull or its keel. Just ask the sail area, if it exists.
    There is nothing for motor boats, equivalent to STIX. The 12217-1 will tell you the highest category your boat can achieve.
     
    Last edited: Jul 27, 2021
  3. Alik
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    Alik Senior Member

    ISO or RCD categories are for certification and marketing purposes only. They should be not treated as assessment of seaworthiness!
    Say, category A specified significant wave 7m. However, for most of yachts with that category 7m wave will be survival condition, not fully operational condition.
    Many 8m boats certified to category B can't go to 4m wave; they are hardly usable in that conditions.

    It is completely incorrect to apply sailing craft criteria to motorboat with keel, too.

    Resume: look at other criteria of seaworthiness, for Your case.
     
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  4. ram68ocean
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    ram68ocean Junior Member

    Hi TANSL thank you for your reply. I have downloaded your ISO 12217-1 spreadsheet and started completing it.
    It looks amazing, very useful, thanks for sharing, but I am not quite sure how to use it right. Do I have to complete the tab General, then the tabs No1, No2 and so on?
    It would be nice to have a short instruction on the cover.
    I am aiming for highest category (A).

    upload_2021-7-27_15-23-45.png
     
  5. TANSL
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    TANSL Senior Member

    Hi @ram68ocean ,
    The spreadsheet that you have downloaded corresponds to the old version of the standard. Some time ago, the version of 2013 was released, which is currently in force. I'm just working on updating my spreadsheet right now. It is not finished because not all the sheets are automated, each one can be automated, but the set is already available in Excel and works up to sheet 6.
    The rest must be completed by hand. So I think it can be very useful to you as it is right now.
    The operation is very simple, as can be seen on the sheets, the green background cells must be filled in by hand by the user. The rest of the boxes, by pressing the "Complete Cells" button, will be calculated and filled in by the application. In the sheets that do not appear that button, it is not necessary to calculate anything.
    In sheet "No2", with the data entered, the program decides the option to select and certain sheets, not applicable for that option, are automatically classified as "n.a." , facilitating the user's task.
    I hope you find it useful and do not hesitate to ask me anything you need.
     

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  6. Alik
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    Alik Senior Member

    For the current spreadsheets, they can be downloaded from IMCI website...
     
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  7. TANSL
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    TANSL Senior Member

    I did not know it. Thanks for the info. It saves me a lot of work.
     
  8. ram68ocean
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    ram68ocean Junior Member

  9. ram68ocean
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    ram68ocean Junior Member

    Get your point, totally.
    But the STIX must be at least an indicator up to certain point.
    For info, in Larsson's Yacht design book, there's a subsection called 'assessment of seaworthiness' where it explains and calculates the STIX index.
     
  10. Alik
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    Alik Senior Member

    Well, seaworthiness is much more complex than a STIX, and it only applies to sailboats.
    For powerboats, there are few problems with ISO methods. Say, they assess the stability, but do not assess the accelerations at speed.
    There is also 'built in' wave height for the assessment of structure; if the craft occasionally exceeds that structural damage is likely.
     
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  11. Ad Hoc
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    Ad Hoc Naval Architect

    Correct.

    Well, let's look at the question:

    It is, at times, very subjective and requires far more 'data' than a simplified spreadsheet of a compliance check, as Alik notes.

    Since if your vessel is in waves of length that match your vessel's length, it wont be comfortable. If the time between wave crests at the speed which you are travelling and encountering waves matches the natural period of your vessel, it wont be comfortable. If your vessel encounters both of these at the same time, it will be extremely uncomfortable... and so on.

    Yet take that same vessel and alter course or speed or both, then it is a totally different scenario...same boat!

    Seaworthiness requires much much more than a simple ticking the boxes of spreadsheets.
     
    Last edited: Jul 27, 2021
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  12. TANSL
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    TANSL Senior Member

    The "Calculations Worksheets" proposed by ISO 12217 are nothing in themselves, they are mere formats to present the results derived from a series of naval architecture calculations that must be carried out. Not even the ISO mentions Excel spreadsheets. Another thing is that with Excel you can create macros that facilitate data entry. That is known to everyone with a little experience in vessels governed by ISO standards. The results can be presented in many ways, but adopting these "Calculations Worksheets" makes it much easier to get approval from the regulator.
    Apart from in the design category, in which the height of the wave is discussed, no regulation on "Stability and buoyancy assessment and categorization" speaks of wave lengths. So, in my opinion, use these spreadsheets extensively and, if you need any other study on some other subject not covered by the ISO, use the technician, the regulation, or the appropriate software for that study.
     
  13. ram68ocean
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    ram68ocean Junior Member

    Could you expand on that a bit more please?
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Sure.
    Seaworthiness comprises many disciplines and elements that collectively form the "design" of the vessel. A simple, but not an exhaustive list begins with:
    1 Safety
    2 Strength
    3 Stability
    4 Motions

    1 Safety - do you have an EPRIB, flares, radios, signals, lifejackts, radar onboard, for example?
    If no, to any of these, one could not consider the vessel seaworthy. This is in the sense it is not "safe" to go to sea. Why?..well if in an emergency situation, can you contact anyone..er... no radios.. nope, bummer!
    Ok...can you let others nearby know....er... no flares, bummer, nope! Ok...if your vessel is swamped an capsizes, can you automatically inform anyone anywhere in the world....er... no EPRIB.. nope... bummer!
    so, if you cannot tick any of these boxes, one cannot consider the vessel safe to go to sea ergo its "seaworthiness" is poor, i.e. not safe!

    2 Strength - is the structure designed for the routes and sea conditions that are planned. If not, then you may well experience structural failure of some sort. Whether it be minor or major - either can lead to a catastrophic failure of the hull girder. So, unless you can satisfy this to yourself with surveys/certificates to demonstrate that the vessel is 'structurally' sound and suitable for such sea conditions, again, its 'seaworthiness' is poor...i.e not safe.

    3 Stability - this is in the statical sense. Is there sufficient restoring moment in the vessel that she can always return to the upright under all conditions of loading and in increasing sea states?
    If all the crew/pass are on one side watching whales go by and the fuel capacity is low (so a higher KG) and then the weather is blustery, and you happen to be beam on..and some swells are also approaching beam on too... is there sufficient restoring moment to withstand these events all occurring concomitantly... if not, then the vessel has deficient stability, again, its 'seaworthiness' is poor...i.e not safe.
    If you then add into this, say one of the voids is breached, owing to the whales striking the hull, as you got too close...is she still able to withstand the damage and ingress of water and loss of stability and remain upright in said conditions?...if not, then the vessel has deficient stability, again, its 'seaworthiness' is poor...i.e not safe.

    4 Motions - if the routes you intend to ply have a dominant wavelength of X and it matches or is very close to the length of your vessel, then the motions will become uncomfortable, and this also leads into #1 and #3, for potential broaching and deck diving. If the waves you are expecting on said route are of a period Y and the speed at which you wish to traverse the route results in the encounter period matching the natural periods of motion of your vessel, the motions will be terrible. To the point of being dangerous for crew and pass, as they will be unable to perform their duties. Poor motions = uncomfortable accelerations, which can lead into #2 excessively higher loads on the structure and also = seasickness = fatigue. So, if you are left not being able to perform your duties owing to the poor motions, again, its 'seaworthiness' is poor...i.e not safe.

    This is just a 'flavour' if the topics that must be considered and all fall into the category of seaworthiness.
    Ticking boxes for a rule compliance, may make one feel better - yeahhh, I got a good 'rating'....however, if you are unable to satisfy any of the items listed above, then one can only conclude, again, its 'seaworthiness' is poor...i.e not safe.

    A good skipper/master may well be able to "paper over the cracks" by their skill in navigation on rough routes. But as soon as an emergence situation occurs, can you grab for the radio, or is the bulwark going to hold, or vessel roll too excessively anyway etc etc...again, its 'seaworthiness' is is now suddenly in question!
     

  15. ram68ocean
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    ram68ocean Junior Member

    Thank you for your explanation Ad Hoc, very interesting and easy to grasp.
     
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