Seakeeping standards

Discussion in 'Boat Design' started by amateur mariner, Sep 30, 2009.

  1. amateur mariner
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    amateur mariner Junior Member

    Does anybody make me understand what al standards are being used for the sea keeping analysis of any vessel.what all we need to satisfy while designing the boats?

    Looking forward for views and comments
     
  2. gonzo
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    gonzo Senior Member

    Your question covers dinghies to supertankers. What kind of vessel are you needing standards for?
     
  3. amateur mariner
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    amateur mariner Junior Member

    I m looking for yachts
     
  4. Perm Stress
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    Perm Stress Senior Member

    I don't think you will find much quantitative answers on this.

    Seakeeping as such, is not so standardized, as Ultimate safety .

    For commercial ships, for example, there are a few:

    1) Load Line Convention requirements on minimum freeboard depending on:
    * type of ship
    * type of weathertight closures of decks
    * length


    2) Load Line Convention requirements on minimum bow height, depending on:
    *type of ship,
    *length
    *speed
    *area of waterline forward of amidships

    3) lower limit for transverse stability -angle of maximum, angle of vanishing, area of stability curve, weather criteria (ability to survive a squall)

    4) upper limit to metacentric height, (used in design of ore carriers as excessive initial stability can make rolling motion so violent, that people are killed on impact to bulkhead)

    From all of them, only #4 is directly related to seakeeping qualities. ##1,2,3 mainly govern ultimate safety, seakeeping only indirectly.


    I do not know about many similar codified standards for yachts (I assume sailing yachts) :

    1) Capsize screening indexes of various handicap systems
    2) Minimum vanishing stability angle (different requirements by EU RCD, organizers of some big ocean races, etc.)
    3) minimum freeboards (like for Volvo 70, if I am correct)

    All of them again mostly govern Ultimate Safety, but not seakeeping.

    I believe good yacht design companies have some quantified criteria, for example:
    * minimum extra buoyancy forward (as multiples of displacement or whatever ) to avoid nose diving for certain type of yacht at certain speed for yacht of certain length and displacement in certain sea conditions
    * freeboard/length and shape of topsides to make deck "dry" (while definition of "dry" is still open to anyone's interpretation)
    * deck beam/waterline beam ratio
    * character of cross sections to achieve acceptable level of pounding in seaway (again, for certain type, at certain speed, certain length, certain displacement... )
    * character of cross sections to achieve acceptably "soft" rolling in seaway (again, at certain speed, certain length, certain displacement...), while definition of "soft" is again open to anyone's interpretation.
    * and so on...

    ...but normally they do not publish it.


    The fundamental work on the subject is Ch. Marchays "Seawortiness -forgotten factor". You may find useful information there.

    And to finish a rather long post, my basic approach when designing a new boat would be:
    - choose a boat with seakeeping characteristics you need (best sources of info are: 1) personal experience; next go 2) relaxed chat with guy with whom you have personal experience at sea, who has personal experience with boat you need; than 3) personal experience of others; 4) magazines, books, internet... ; ##3 & 4 are basicaly not reliable, because you rarely know the criteria used to separate "good" from "bad", how far the boats are really pushed, and so on...)
    - choose a boats with seakeeping characteristics you need to avoid (same rules for sources of information)
    - calculate above mentioned or any other ratios you believe are important, for reference boats and make a guess how they create those seakeeping characteristics
    - calculate same ratios for your design and compare them with those of reference boats
    - make a guess what differences in ratios are important, and what are not
    - correct ratios you guess have to be corrected
    - finish your design and have it built
    - go to sea trials in weather you have been designing for (and experience a lot of nasty surprises about your creation's qualities - that's a joke :) ), best of all together with reference boats (in race, club race, cruising rally, whatever...)
    - take lessons and make adjustments to your next design...
     
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  5. amateur mariner
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    amateur mariner Junior Member

    i have read some where that some ISO standard is being used for this purpose.
     
  6. Perm Stress
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    Perm Stress Senior Member

    The problem is, the seakeeping qualities are extremely difficult even to describe in simple words, let alone in some quantities (read numbers).

    A far as I know there is STIX (Stability index) calculation in ISO.

    But this is more about ULTIMATE SAFETY trhan about seekeeping as such.

    Any standard, by the way, has to be as simple as possible. This way, not any standard is going to be too accurate, and always severely limited to type of boats it was developed from.

    Regards
     
  7. Perm Stress
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    Perm Stress Senior Member

    Quantitative seakeeping

    quote taken from : http://www.tedbrewer.com/yachtdesign.html


    COMFORT RATIO (CR): This is a ratio that I dreamed up, tongue-in-cheek, as a measure of motion comfort but it has been widely accepted and, indeed, does provide a reasonable comparison between yachts of similar type. It is based on the fact that the faster the motion the more upsetting it is to the average person. Given a wave of X height, the speed of the upward motion depends on the displacement of the yacht and the amount of waterline area that is acted upon. Greater displacement, or lesser WL area, gives a slower motion and more comfort for any given sea state.

    Beam does enter into it as as wider beam increases stability, increases WL area, and generates a faster reaction. The formula takes into account the displacement, the WL area, and adds a beam factor. The intention is to provide a means to compare the motion comfort of vessels of similar type and size, not to compare that of a Lightning class sloop with that of a husky 50 foot ketch.

    The CR is : Displacement in pounds/ (.65 x (.7 LWL + .3 LOA) x B1.333). Ratios will vary from 5.0 for a light daysailer to the high 60s for a super heavy vessel, such as a Colin Archer ketch. Moderate and successful ocean cruisers, such as the Valiant 40 and Whitby 42, will fall into the low-middle 30s range.

    Do consider, though, that a sailing yacht heeled by a good breeze will have a much steadier motion than one bobbing up and down in light airs on left over swells from yesterday's blow; also that the typical summertime coastal cruiser will rarely encounter the wind and seas that an ocean going yacht will meet. Nor will one human stomach keep down what another stomach will handle with relish, or with mustard and pickles for that matter! It is all relative.
     

  8. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Can you define "sea keeping"
     
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