How do you decide if your small water craft is stable?

Discussion in 'Stability' started by Furkan, Nov 21, 2022.

  1. Furkan
    Joined: Nov 2020
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    Location: Turkey

    Furkan Junior Member

    Hello all,

    My question is about stability calculations for a small watercraft.
    Which calculations do you make and which rules you look for when you carry out stability calculation for your own design?

    Any method to follow? Or any recommended software(with the theory of the used method)?

    Thanks in advance and have a great day.
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Do you mean something like a canoe?
  3. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Hi Furkan,
    I presume that you have designed a small water craft and you now want to calculate the stability for it.
    If so, can you post more details about the boat please?
    Are you familiar with stability calculations? Do you know the basic formula BM = I / V ?
    Where BM is the distance from the vertical centre of buoyancy to the metacentre, I is the second moment of area of the waterplane, and V is the volume of displacement at this waterplane.
  4. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    At first, for small boats, say < 5-6 m, the crew weight is an important part of the total weight, and so the stability is closely dependent of your assumption for the crew weight and position.
    Once a "design" crew weight and position are set, you can compute both the initial stability (i.e. the GM) and the dynamic stability (i.e. the righting moment energy within a rolling event of say 0-25 deg of heel, or less if the watercraft is open deck and the freeboard is too low). I did the exercise for a dinghy and proposed scales of stability for the two criteria, see here :
    About dinghy intrinsic stability : proposition for a standard assessment
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  5. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Stability on a small watercraft has to include the skill of the crew, since the weight of the crew can be over 60% of the displacement.
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  6. jehardiman
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    jehardiman Senior Member

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  7. darkstar71
    Joined: Jan 2016
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    Location: United Kingdom

    darkstar71 powered by understanding

    Hi Furkan,
    There are two perspectives of your query. First, basic engineering checks i.e. how will you know your boat is stable enough or not? Last, which regulatory requirements you will need to comply? Regarding the first, follow @Dolfiman procedure. But, regarding the rule compliance, it depends on many things, location of operation of the vessel, owner requirement etc. But a good starting point should be as @jehardiman said. I knew there are some ISO and Norwegian standard for small vessels.

    GM = (KB + BM) - KG >0.1
    KB = (2T/3 ~ T/2)
    BM = IT/displacement

    Regarding static stability, (KB + BM), you can control during your design. For existing vessel, you will have no control over them. However, you can play with the vertical distribution of weight and ballast to make the vessel to have sufficient stability.

    Wish you all the best.

  8. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    @Furkan , a boat is stable or not if it meets certain stability criteria. To do this, the first thing you must do is choose the criteria, standard or regulation, that must be applied to your boat. Since you are in Turkey you may want to design your boat according to ISO standards for small craft. They give the necessary procedures and calculations for the stability and buoyancy assessment and categorization.
    Do not forget that, in addition to stability, you must also analyze buoyancy.
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