Chapter 11 - Stability

11.1 General

(1) This Chapter deals with the standards for both intact and damage stability.

(2) An intact stability standard proposed for assessment of a vessel type not covered by the standards defined in the Code shall be submitted to the Administration for approval at the earliest opportunity.

(3) If used, permanent ballast shall be located in accordance with a plan approved by the Administration and in a manner that prevents shifting of position. Permanent ballast shall not be removed from the ship or relocated within the ship without the approval of the Administration. Permanent ballast particulars shall be noted in the ship’s stability booklet. Attention shall be paid to local or global hull strength requirements from the point of view of the fitting of additional ballast.

(4) For the purpose of assessing whether the stability criteria are met, GZ curves shall be produced for the loading conditions applicable to the operation of the vessel.

11.2 Intact Stability Standards

(1) Standard Criteria:

• (a) The curves of statical stability for seagoing conditions shall meet the following criteria:

  • (i) the area under the righting lever curve (GZ curve) shall not be less than 0.055 metre-radians up to 30º angle of heel and not less than 0.09 metre-radians up to 40º angle of heel, or the angle of downflooding, if this angle is less;

  • (ii) the area under the GZ curve between the angles of heel of 30º and 40º or between 30º and the angle of downflooding if this is less than 40º, shall not be less than 0.03 metre-radians;

  • (iii) the righting lever (GZ) shall be at least 0.20 metres at an angle of heel equal to or greater than 30º;

  • (iv) the maximum GZ shall occur at an angle of heel of preferably exceeding 30º but not less than 25º;

  • (v) after correction for free surface effects, the initial metacentric height (GM) shall not be less than 0.15 metres; and

  • (vi) in the event that the vessels intact stability standard fails to comply with the criteria defined in (i) to (v) the Administration may be consulted for the purpose of specifying alternative but equivalent criteria.

(2) Vessels operating as Short Range Yachts:

• (a) Where Short Range Yachts are unable to meet the criteria above, the following criteria may be used:

  • (i) the area under the righting lever curve (GZ curve) shall not be less than 0.07 metre-radians up to 15º angle of heel, when maximum GZ occurs at 15°, and 0.055 metre-radians up to 30º angle of heel, when maximum GZ occurs at 30º or above. Where the maximum GZ occurs at angles of between 15º and 30º, the corresponding area under the GZ curve, A_req shall be taken as follows:-

    A_req = 0.055 +0.001(30º -θ_max) metre.radians

    where θ_max is the angle of heel, in degrees, where the GZ curve reaches its maximum;

  • (ii) the area under the GZ curve between the angles of heel of 30º and 40º or between 30º and the angle of downflooding if this is less than 40º, shall not be less than 0.03 metre-radians;

  • (iii) the righting lever (GZ) shall be at least 0.20 metres at an angle of heel equal to or greater than 30º;

  • (iv) the maximum GZ shall occur at an angle of heel not less than 15º;

  • (v) after correction for free surface effects, the initial metacentric height (GM) shall not be less than 0.15 metres.

(3) Alternative Criteria:

• (a) The curves of statical stability for seagoing conditions shall meet the following criteria:

  • (i) the area under the righting lever curve (GZ curve) shall not be less than 0.075 metre-radians up to an angle of 20º when the maximum righting lever (GZ) occurs at 20º and, not less than 0.055 metre-radians up to an angle of 30º when the maximum righting lever (GZ) occurs at 30º or above. When the maximum GZ occurs at angles between 20º and 30º the corresponding area under the GZ curve, A_req shall be taken as follows:-

    A_req = 0.055 + 0.002(30⁰ -θ_max) metre.radians;

    where θ_max is the angle of heel in degrees where the GZ curve reaches its maximum;

  • (ii) the area under the GZ curve between the angles of heel of 30º and 40º, or between 30º and the angle of downflooding if this is less than 40º, shall not be less than 0.03 metre-radians;

  • (iii) the righting lever (GZ) shall be at least 0.20 metres at an angle of heel where it reaches its maximum;

  • (iv) the maximum GZ shall occur at an angle of heel not less than 20º;

  • (v) after correction for free surface effects, the initial metacentric height (GM) shall not be less than 0.15 metres; and

  • (vi) if the maximum righting lever (GZ) occurs at an angle of less than 20º approval of the stability shall be considered by the Administration as a special case.

(4) For the purpose of assessing whether the stability criteria are met, GZ curves should be produced for the loading conditions applicable to the operation of the vessel.

(5) Superstructures:

• (a) The buoyancy of enclosed superstructures complying with regulation 3(10)(b) of the ICLL may be taken into account when producing GZ curves.

• (b) Superstructures, the doors of which do not comply with the requirements of regulation 12 of ICLL, shall not be taken into account.

(6) High Speed Vessels:

• (a) In addition to the criteria above designers and builders shall address the following hazards which are known to affect vessels operating in planing modes or those achieving relatively high speeds:

  • (i) directional instability, often coupled to roll and pitch instabilities;

  • (ii) bow diving of planing vessels due to dynamic loss of longitudinal stability in calm seas;

  • (iii) reduction in transverse stability with increasing speed in monohulls;

  • (iv) porpoising of planing monohulls being coupled with pitch and heave oscillations;

  • (v) generation of capsizing moments due to immersion of chines in planing monohulls (chine tripping).

(7) Sailing Vessel Monohulls

• (a) Curves of statical stability (GZ curves) for at least the Loaded Departure with 100% consumables and the Loaded Arrival with 10% consumables shall be produced.

• (b) The GZ curves required by (a) should have a positive range of not less than 90 degrees. For vessels of more than 45m, a range of less than 90 degrees may be considered but may be subject to agreed operational criteria.

• (c) In addition to the requirements of (b), the angle of steady heel should be greater than 15 degrees (see figure). The angle of steady heel is obtained from the intersection of a "derived wind heeling lever" curve with the GZ curve required by (a).

  • In the figure:-

    'dwhl'	=	the "derived wind heeling lever" at any angle θº
    	=	0.5 x WLO x Cos1.3θ

    where

• 

  Noting That:

  • WLO= is the magnitude of the actual wind heeling lever at 0º which would cause the vessel to heel to the 'down flooding angle' θ_f or 60º whichever is least.

  • GZ_f = is the lever of the vessel's GZ at the down flooding angle (θ_f) or 60º whichever is least.

  • θ_d = is the angle at which the 'derived wind heeling' curve intersects the GZ curve. (If θ_d is less than 15º the vessel shall be considered as having insufficient stability for the purpose of the Code).

  • θ_f = the 'down-flooding angle' is the angle of heel causing immersion of the lower edge of openings having an aggregate area, in square meters, greater than:-

  • = where Δ = vessels displacement in tonnes

• All regularly used openings for access and for ventilation shall be considered when determining the downflooding angle. No opening regardless of size which may lead to progressive flooding shall be immersed at an angle of heel of less than 40°. Air pipes to tanks can, however, be disregarded.

• As a result of immersion of openings in a superstructure, a vessel cannot meet the required standard, those superstructure openings may be ignored and the openings in the weather deck used instead to determine θf. In such cases the GZ curve shall be derived without the benefit of the buoyancy of the superstructure.

• It might be noted that provided the vessel complies with the requirements of (a) to (c) and is sailed with an angle of heel which is no greater than the ‘derived angle of heel', it shall be capable of withstanding a wind gust equal to 1.4 times the actual wind velocity (i.e. twice the actual wind pressure) without immersing the’ down-flooding openings', or heeling to an angle greater than 60°.

(8) Sailing Vessels Multi-hulls

• (a) Curves of statical stability in both roll and pitch shall be prepared for at least the Loaded Arrival with 10% consumables. The VCG shall be obtained by one of the three methods listed below:

  • (i) inclining of complete craft in air on load cells, the VCG being calculated from the moments generated by the measured forces; or

  • (ii) separate determination of weights of hull and rig (comprising masts and all running and standing rigging), and subsequent calculation assuming that the hull VCG is 75% of the hull depth above the bottom of the canoe body, and that the VCG of the rig is at half the length of the mast (or a weighted mean of the lengths of more than one mast); or

  • (iii) a detailed calculation of the weight and CG position of all components of the vessel, plus a 15% margin of the resulting VCG height above the underside of canoe body.

• (b) If naval architecture software is used to obtain a curve of pitch restoring moments, then the trim angle shall be found for a series of longitudinal centre of gravity (LCG) positions forward of that necessary for the Design Waterline. The curve can then be derived as follows:

• GZ in pitch =	CG’ x cos (trim angle)
  trim angle =
  where
  CG’=	shift of LCG forward of that required for design trim, measured parallel to baseline
  TFP=	draught at forward perpendicular
  TAP=	draught at aft perpendicular
  LBP=	length between perpendiculars

• (Approximations to maximum roll or pitch moments are not acceptable)

• (c) Data shall be provided to the user showing the maximum advised mean apparent wind speed appropriate to each combination of sails, such wind speeds being calculated as the lesser of the following:

  where
  vW =	maximum advised apparent wind speed (knots)
  LMR =	maximum restoring moment in roll (N.m)
  LMP =	limiting restoring moment in pitch (N.m), defined as the pitch restoring moment at the least angle of the following: a. angle of maximum pitch restoring moment; b. angle at which foredeck is immersed; or c. 0º from design trim
  A’S=	area of sails set including mast and boom (square metres)
  h=	height of combined centre of effort of sails and spars above the waterline
  φR =	heel angle at maximum roll righting moment (in conjunction with LMR)
  φP =	limiting pitch angle used when calculating LMP (in conjunction with LMP)
  AD =	plan area of the hulls and deck (square metres)
  b =	distance from centroid of AD to the centreline of the leeward hull

• This data shall be accompanied by the note:

  In following winds, the tabulated safe wind speed for each sail combination shall be reduced by the boat speed

• (d) If the maximum safe wind speed under full fore-and-aft sail is less than 27 knots, it shall be demonstrated by calculation using annex D of ISO 12217-2 (2002) that, when inverted and/or fully flooded, the volume of buoyancy, expressed in cubic metres (m³), in the hull, fittings and equipment is greater than:

  • 1.2 x (fully loaded mass in tonnes)

• thus ensuring that it is sufficient to support the mass of the fully loaded vessel by a margin. Allowance for trapped bubbles of air (apart from dedicated air tanks and watertight compartments) shall not be included.

• (e) The maximum safe wind speed with no sails set calculated in accordance with (3) above shall exceed 36 knots. For Part A Short Range Yachts this wind speed shall exceed 32 knots.

• (f) Trimarans used for unrestricted operations shall have sidehulls each having a total buoyant volume of at least 150% of the displacement volume in the fully loaded condition.

• (g) The stability information booklet shall include information and guidance on:

  • (i) the stability hazards to which these craft are vulnerable, including the risk of capsize in roll and/or pitch;

  • (ii) the importance of complying with the maximum advised apparent wind speed information supplied;

  • (iii) the need to reduce the tabulated safe wind speeds by the vessel speed in following winds;

  • (iv) the choice of sails shall be set with respect to the prevailing wind strength, relative wind direction, and sea state;

  • (v) the precautions shall be taken when altering course from a following to a beam wind.

• (h) In vessels required to demonstrate the ability to float after inversion (according to (3) above), an emergency escape hatch shall be fitted to each main inhabited watertight compartment such that it is above both upright and inverted waterlines.

11.3 Damage Stability

(1) The following requirements are applicable to all vessels, other than those operating as Short Range Yachts. Whilst Short Range Yachts are not required to meet the damage stability criteria defined below, it is recommended that they meet the requirements where practicable.

(2) It shall be noted that compliance with the damage stability criteria is not required for vessels that obtain full compliance with the ICLL conditions of assignment.

(3) The watertight bulkheads of the vessel shall be so arranged that minor hull damage that results in the free flooding of any one compartment, shall cause the vessel to float at a waterline which, at any point, is not less than 75 millimetres below the weather deck, freeboard deck, or bulkhead deck if not concurrent.

(4) Minor damage shall be assumed to occur anywhere in the length of the vessel, but not on a watertight bulkhead.

(5) Standard permeabilities shall be used in this assessment, as follows:

(6) In the damaged condition, considered in 11.3(3), the residual stability shall be such that any angle of equilibrium does not exceed 7º from the upright, the resulting righting lever curve has a range to the downflooding angle of at least 15º beyond any angle of equilibrium, the maximum righting lever within that range is not less than 100 millimetres and the area under the curve is not less than 0.015 metre radians.

• Notes:

• 1. Range of stability in “damaged” condition shall have regard, where appropriate, to truncation due to downflooding

• 2. The required properties of the “damaged” GZ curve, namely max.GZ≥ 0.1m and the area under the curve of ≥0.015metre radians. shall be achieved within the positive range of the curve taking into account any restrictions imposed by Note 1.

(7) A vessel of 85 metres length and above shall meet a SOLAS 90 passenger ship one-compartment standard of subdivision, calculated using the deterministic damage stability methodology. Such Vessels shall be provided with a Damage Control Plan and Booklet, in accordance with the requirements of SOLAS Chapter II-1, Regulation 19.

11.4 Elements of Stability

(1) Unless otherwise specified, the lightship weight, vertical centre of gravity (KG) and longitudinal centre of gravity (LCG) of a vessel shall be determined from the results of an inclining experiment.

(2) An inclining experiment shall be conducted in accordance with a detailed standard which is approved by the Administration and, in the presence of the Administration.

(3) The report of the inclining experiment and the lightship particulars derived shall be approved by the Administration prior to its use in stability calculations.

At the discretion of the Company and prior to approval of the lightship particulars by the Administration, a margin for safety may be applied to the lightship weight and KG calculated after the inclining experiment. Such a margin shall be clearly identified and recorded in the stability booklet.

A formal record shall be kept in the stability booklet of alterations or modifications to the vessel for which the effects on lightship weight and vertical centres of gravity are offset against the margin.

(4) When sister vessels are built at the same shipyard, the Administration may accept a lightweight check on subsequent vessels to corroborate the results of the inclining experiment conducted on the lead vessel of the class.

11.5 Stability Documents

(1) A vessel shall be provided with a Stability Information Booklet (and Damage Control Plan and Booklet if applicable) for the Master, that shall be approved by the Administration.

(2) The content, form and presentation of information contained in the stability information booklet shall be based on the model booklet for the vessel type (motor or sailing) published by/for the Administration.

(3) A vessel with previously approved stability information which undergoes a major conversion or alterations shall be subjected to a complete reassessment of stability and provided with newly approved stability information. A major refit or major alteration is one which results in either a change in the lightship weight of 2% and above and/or the longitudinal centre of gravity of 1% and above (measured from the aft perpendicular) and/or the calculated vertical centre of gravity rises by 0.25% and above (measured from the keel).

(4) Unless it can be clearly demonstrated that no major change has occurred, a lightweight check shall be carried out at the renewal survey.

(5) Sailing vessels shall have, readily available, a copy of the ‘Curves of Maximum Steady Heel Angle to Prevent Downflooding in Squalls’, or in the case of a multihull, the values of maximum advised mean apparent wind speed, for the reference of the watchkeeper. This shall be a direct copy taken from that contained in the approved stability booklet.

(6) The overall sail area and spar weights and dimensions shall be as documented in the vessel’s stability information booklet. Any rigging modifications that increase the overall sail area, or the weight/dimensions of the rig aloft, shall be accompanied by an approved updating of the stability information booklet.

(7) For Short Range Yachts, where the damage stability has not been assessed, the following note shall be added to the approved stability booklet;

11.6 Additional Equivalence Considerations

(1) None