In-depth | SUPERYACHTS


should be revisited in the future, as it is almost 50 years old and less suitable for some more recent ship types. Recognising this fact, MSC.1/Circ.1200, “Interim guidelines for alternative assessment of the weather criterion”, contains a specification of the limitations of applicability of the current weather criterion.


Pure loss of stability Te upper parts of a ship’s bow and stern section are usually wide due to bow flare and wide transom. As a result, the submerged part of the hull varies when the ship is sailing through longitudinal waves and the resulting stability variation may result in stability failure caused by pure loss of stability. Tis is a single wave event. A Level 1 criterion for pure loss of


stability was accepted during the 54th SLF Subcommittee meeting in January 2012. However, some pending matters were to be resolved by the ISCG. Such matters are marked below by text in [square brackets]. Te Level 1 vulnerability criterion for pure


loss of stability should be applied only if the service speed Froude number exceeds [a standard determined, between 0.2~0.31]. For a given loading condition, a ship is


considered vulnerable to pure loss of stability if: GMmin


< [r]


where: GMmin


is the minimum occurring value


of the metacentric height when the ship is sailing through longitudinal waves, obtained by direct GM calculation; [r=0.05 with allowance for heeling


moment depending on Froude Number]. For ships without a tumblehome hull form, can be estimated using the following


GMmin


approximate formula: GMmin


= KB(d) + IL /V(d) – KG (1)


where: d is the draught of the considered loading


condition; KB(d) is centre of buoyancy above base; IL


plane at a lower draſt dL dL


[δdL SW


/2)]; [SW


as SW is determined as dL = d - δdL ; is determined as δdL = Min(0.75d,L. =f1


is reference wave steepness determined (L)];


V(d) is the displacement volume at


draught d; KG is height of centre of gravity above base.


38 The Naval Architect September 2012


is a moment of inertia of the level water ;


Parametric roll The variation of stability in waves, as described for pure loss of stability, is also the physical basis for another mode of stability failure: parametric roll. However, parametric roll is a multi-wave event and the resulting periodic stability variations may result in the development of parametric roll resonance. A Level 1 criterion for parametric


roll was accepted during the 54th SLF Subcommittee meeting. However, some pending matters were to be resolved by the


ISCG. Such matters are marked below by text in [square brackets]. For a given loading condition, a ship is


considered vulnerable to parametric roll if: ΔGM / GM ≥ [[0.5] or [0.34 with allowance


for bilge keels]] where: ΔGM is the amplitude of occurring


metacentric height variation when the ship is sailing through longitudinal waves, obtained by direct GM calculation; GM is the metacentric height calculated in


calm water.


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