APPENDIX 12 - USE OF ISO “FIRST OF TYPE” RIGHTING MOMENT CURVE FOR STABILITY ASSESSMENT
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Statutory Documents - MCA Publications - The Workboat Code Edition 2 - APPENDIX 12 - USE OF ISO “FIRST OF TYPE” RIGHTING MOMENT CURVE FOR STABILITY ASSESSMENT

APPENDIX 12 - USE OF ISO “FIRST OF TYPE” RIGHTING MOMENT CURVE FOR STABILITY ASSESSMENT

 1. Introduction

1.1 Where the stability of a Small Workboat is assessed using the righting moment curve prepared to show compliance of the design with ISO 12217, this curve shall be subject to verification and, if necessary, correction, as set out below.

1.2 ISO 12217 normally requires the stability to be assessed in the Minimum Operating Condition. However, where the Loaded Displacement Mass is more than 15% greater than the former, the stability also has to be assessed in this heavier condition.

1.3 Where data is available for both conditions, the heavier should be used for the purposes of this Code.

2. Stability Verification Test

2.1 The vessel to be certified shall be subjected to the stability verification test when as close as practicable to the loading condition used for the righting moment curve to be checked, as defined in ISO 12217. The purpose of the test is to verify that the stability of the vessel is adequately described by the righting moment curve of the “First of Type” used for ISO 12217 assessment. Where this is not adequately demonstrated, this curve shall be corrected as described below before reassessment of the stability compliance.

2.2 The test shall be conducted in calm conditions. The vessel shall be heeled to both port and starboard as much as practicable by the application of a heeling moment sufficient to produce a heel angle of firstly at least three degrees in either direction, and secondly at least five degrees, i.e.: two heeling moments and four heel angles in all. The heeling moments and angles of heel shall be recorded as precisely as practicable.

2.3 The heeling moments shall be applied using weights that are part of the loaded condition of the vessel, and moved through a known amount. The righting lever deduced for that angle of heel is given by:

  • where: GZ = righting lever (m)

    • w = mass moved to produce the heel angle (kg)

    • h = distance parallel to design waterline mass was moved through to produce heeling moment (m)

    • ϕ = angle of heel produced (degrees)

    • Δ = displacement of vessel as used to derive the GZ curve in question (kg)

2.4 Where the weights used to create the heeling moment have to be moved vertically from their normal location in order to generate the necessary heeling moment, the resulting measured righting moment should be corrected for the change in the vertical centre-of-gravity of the craft. The correction = GG1sinϕ, added to the measured GZ when the weights were raised during the heeling test, where GG1 is the shift in vessel VCG due to the weights moved.

2.5 The First of Type righting moment curve shall be considered acceptable for stability assessment if the average deviation of the four values obtained at 2.3 and corrected according to 2.4 below the righting moment curve is equal to or less than 5%. Where the deviation is above the curve, no limit shall apply.

2.6 Where the average deviation of the four values obtained at 2.3 above from the righting moment curve is more than 5%, the First of Type righting moment curve shall be corrected throughout the range of heel angles by an amount equal to GG1sin ϕ, where ϕ is any heel angle. The value of GG1 used for this correction shall be obtained as follows:

  • GG1 = { (δ1/sinϕ1) + (δ2/sinϕ2) + (δ3/sinϕ3) + (δ4/sinϕ4) } / 4

  • where: δn = difference in measured GZ from First of Type GZ curve at angle ϕn, for the four values obtained according to 2.2 and calculated according to 2.3 and 2.4 above.

2.7 Where the righting moment curve is modified in this way, the stability shall then be reassessed using the requirements of chapter 11 of the Code.

Parent topic: The Workboat Code Edition 2
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