Chapter 1 - Buoyancy, Stability and Subdivision

1.1 General

 1.1.1 General

1.1.1.1 A craft should be provided with:

• .1 stability characteristics and stabilization systems adequate for safety when the craft is operated in all modes except of displacement including adequate provision for the safe landing of the craft in case of any system fault;

• .2 additionally, buoyancy and stability characteristics adequate for safety where the craft is operated in the displacement mode, both in the intact condition and the damaged condition; and

• .3 stability data providing detail of the craft's buoyancy and stability characteristics in all operational modes, taking account of the craft's manoeuvring characteristics and local increases in wave height and wind strength expected to be encountered in the operational area under the conditions applying to the certification of the craft under these Guidelines.

1.1.1.2 Account should be taken of the effect of icing in all stability calculations for loading conditions where icing may accrete. An example of an established practice for ice accretion allowances is given in annex 3 for the guidance of the Administration in terms of application and adaptation as appropriate for a specific craft. The application of such allowances to individual craft may be dependent on the performance of any de-icing system that may be fitted.

1.1.1.3 Unless the contrary intention appears, for the purpose of this chapter the term "stability" should be taken to include longitudinal stability as well as transverse stability.

1.1.1.4 The provisions of this chapter may be supplemented where considered appropriate:

• .1 by the Administration;

• .2 to cover special modes of operation, such as amphibious mode;

• .3 to address weather and sea conditions in the craft's area of operation; or

• .4 to address hazards identified in the safety assessment for the craft.

1.1.1.5 For the purpose of this chapter, unless expressly defined otherwise, the following definitions apply:

• .1 Downflooding point means any opening through which flooding of the spaces which comprise the reserve buoyancy could take place while the craft is in the intact or damage condition and heels to an angle past the angle of equilibrium;

• .2 Permeability of a space means the percentage of the volume of that space which can be occupied by water;

• .3 Watertight in relation to a structure means capable of preventing the passage of water through the structure in any direction under the head of water likely to occur in the intact or damage condition; and

• .4 Weathertight means that water will not penetrate into the craft in any wind and wave conditions up to those specified as critical design conditions.

1.1.1.6 Conditions of sufficient stability

In all relevant operational conditions of loading, craft should comply with the following provisions:

• .1 in the displacement mode, withstand simultaneous action of the dynamically applied wind pressure and the craft rolling (weather criterion) according to 1.1.3.5;

• .2 in the transitional and take off/landing modes of operation, the provisions of 1.1.3.4;

• .3 the static stability curve in the displacement mode in calm water should comply with 1.1.3.2;

• .4 stability of passenger craft should meet the provisions of section 1.2 and of cargo craft the provisions of section 1.3; and

• .5 in all modes of operation should have static and dynamic stability in yaw, pitch, roll, and heave, surge and sway. This stability should be proven by appropriate tests and trials, theoretical calculations, simulation analysis, model tests or sea trials.

 1.1.2 Intact buoyancy

1.1.2.1 All craft should have a sufficient reserve of buoyancy at the design waterline to meet the intact and damage stability requirements of this chapter. The Administration may require a larger reserve of buoyancy to permit the craft to operate in any of its intended modes. This reserve of buoyancy should be calculated by including only those compartments which are:

• .1 watertight;

• .2 accepted as having scantlings and arrangements adequate to maintain their watertight integrity; and

• .3 situated in locations below a datum, which may be a watertight deck or equivalent structure of a non-watertight deck covered by a weathertight structure.

1.1.2.2 Arrangements should be provided for checking the watertight integrity of those compartments referred to in 1.1.2.1.

1.1.2.3 Where entry of water into structures above the datum as defined in 1.1.2.1.3 would significantly influence the stability and buoyancy of the craft, such structures should be:

• .1 of adequate strength to maintain the weathertight integrity and fitted with weathertight closing appliances; or

• .2 provided with adequate drainage arrangements; or

• .3 an equivalent combination of both measures.

1.1.2.4 The means of closing openings in the boundaries of weathertight structures should be such as to maintain weathertight integrity in all operational conditions.

 1.1.3 Intact stability

1.1.3.1 General

1.1.3.1.1 It should be shown by calculations and/or by trials that in all operational modes and load cases within its operational restrictions a craft will return or can be readily made to safely return to the initial position of draught/altitude, heel and trim when displaced during roll, pitch, yaw or heave motions or when subjected to a transitory force or moment associated with such motions.

1.1.3.1.2 The roll and pitch stability on the first and/or any other craft of a series should be qualitatively assessed during operational safety trials as required by chapter 16 and annex 8. The results of such trials may indicate the need to impose operational limitations in relation to any operational modes, operational areas and loading conditions.

1.1.3.1.3 Suitable precautions of arrangement, equipment or operational procedures should be taken against the craft developing dangerous attitudes, yawing, inclinations or loss of stability subsequent to a collision with a submerged or floating object in displacement, transitional, take-off/landing, planing and surface effects modes, particularly in modes where any part of the craft or its appendages is submerged.

1.1.3.1.4 When turning in calm water the inner angle of heel should not:

• .1 induce instability in the craft;

• .2 exceed the angle at which the wing makes contact with the water surface necessitating corrective control action when in the ground effect mode in calm water at the design altitude; and

• .3 exceed the angle at which the skeg makes contact with the water surface when the craft is in the ground effect mode.

1.1.3.2 Intact stability in the displacement mode

1.1.3.2.1 Craft of all types should comply with corresponding provisions of annex 5 in all permissible loading cases.

1.1.3.2.2 If characteristics of a craft do not suit the application of 1.1.3.2.1 the Administration may allow alternative criteria which are equivalent to those in 1.1.3.2.1 as appropriate for craft type and region of operation.

1.1.3.3 Stability in the ground effect mode

1.1.3.3.1 Definitions

• .1 The focus of altitude is the point in the wing chord of action of the increment of lift force caused by the change of the altitude.

  .2 The focus of attack angle is the point in the wing chord of action of the increment of lift force caused by the change of the attack angle.

1.1.3.3.2 General

• .1 The WIG craft should have stable flight in all normal operational conditions and should return to the original state after influence of short vertical, longitudinal or lateral forces and moments;

• .2 Stable flight should be verified by the leading craft test flight: The WIG craft under the scheduled operating conditions can be stable flight in the ground effect mode.

1.1.3.3.3 Longitudinal stability

• .1 The longitudinal static stability of craft in flight should be calculated. The following two criteria should be satisfied at the same time:

  • .1 the focus of altitude should be in front of the focus of attack angle , which is - ＞0; and

  • .2 the focus of attack angle should be behind the centre of gravity of the craft , which is - ＞0.

• .2 The dynamic longitudinal stability of craft in flight should be verified by calculation, simulation test or Actual craft trial. The longitudinal dynamic stability should be satisfied that the motion of the disturbed craft is oscillatory attenuation.

• .3 If equipped with automatic motion control system, the verification of the dynamic longitudinal stability of craft should consider the influence of automatic motion control system.

1.1.3.3.4 Lateral stability

• .1 The dynamic lateral stability of craft in the ground effect mode should be verified by calculation, simulation test or actual trial. The lateral dynamic stability should be satisfied that the motion of the disturbed craft is oscillatory attenuation.

• .2 If equipped with automatic motion control system, the verification of the dynamic lateral stability of craft should consider the influence of automatic motion control system.

• .3 The craft can fly stably in the ground effect mode should withstand crosswind corresponding of the service weather restriction, and that should be proved by actual trials of the leading ship, and the heeling angle of craft does not exceed the immersion angle of the skeg. That should be proved by actual trials of the leading ship.

• .4 The limit of ultimate heeling angle of craft turning on the calm water in the ground effect mode should be measured by actual trials, that to avoid the instable motion due to wing or skeg contact with the water surface. In addition, the limit of trim angle of craft turning should be measured, that to avoid losing stability due to turning with excessive attack. The limit of heeling angle and trim angle of craft turning in the ground effect mode should be clearly specified in the operation manual of craft.

1.1.3.4 Controllability and stability in other modes should comply with chapter 16.

1.1.3.5 Stability verification

1.1.3.5.1 If the craft is fitted with a system for directing sprays of air engines under a wing or other craft structures to create a static air cushion or for other purposes, then the effect of that system on craft stability should be taken into account.

1.1.3.5.2 For a craft which is designed and certificated to be capable from the displacement mode wholly or partly to mount to a gentle slope shore (and to come down backwards) and to operate in amphibian mode, the maintenance of satisfactory stability during such manoeuvres should be verified by trials and documented in the operational procedures, including transit through the wave breaking zone in all conditions up to the worst allowable conditions for those manoeuvres.

1.1.3.5.3 Validation of the transverse stability of the full scale craft in calm water should be conducted.

1.1.3.6 WIG craft weather criteria

1.1.3.6.1 Craft operation, depending on the operational modes, should be restricted by the worst intended conditions and critical design conditions specified according to the results of trials conducted with the craft itself or with one of the craft of a series of identical craft.

1.1.3.6.2 In the displacement mode of operation, stability is considered to be sufficient if the following conditions are observed when the dynamically applied heeling moment M_v due to the beam wind pressure (in the loading condition with least reserves of stability and subjected to the critical design conditions) is equal or less than the capsizing moment Mc,

• M_v ≤ Mc or K = Mc/M_v ≥1.0

1.1.3.6.3 The ability to keep the planing and ground effect modes under the worst intended conditions should be confirmed experimentally during the delivery seakeeping test of the first craft in series.

1.1.3.6.4 The heeling moments due to the wind pressure should be taken as constant during the whole period of heeling and determined as follows:

• The heeling moment M_v (kNm) in the displacement mode of operation is calculated as follows:

  • M_v = 0.001 P_v A_v Z f

    where:

    • P_v = wind pressure (N/m²);

      A_v = windage area (the projected lateral area of the portion of the craft above the acting waterline) (m²);

      Z = the windage area lever equal to the vertical distance to the centre of windage from the centre of the projected lateral area of the portion of the craft below the plane of the acting waterline (m); and

      f = streamline factor, f ≤ 1, determined by model tests in a wind tunnel (f=1 if such data are lacking).

The value of P_v should be determined according to table 1.1.3.6.4 for wind force corresponding to the critical design conditions. This wind force should be at least one Beaufort Scale number higher than that corresponding to the worst intended conditions.

Table 1.1.3.6.4 – Wind pressure P_v, in Pa

1.1.3.6.5 Amplitudes of WIG craft motion for application of intact stability criteria

• .1 The amplitudes of rolling for the displacement and planing modes of WIG craft should be calculated according to the methods of annex 4 or otherwise as agreed by the Administration.

• .2 The amplitude of rolling Θ_Z is determined in accordance with 1.1.5.3 of annex 4, with a sea state corresponding in displacement mode to the critical design conditions.

• .3 The amplitude of rolling in the displacement mode of operation is determined for the craft in beam seas and with propulsion and stability equipment inoperative.

1.1.3.6.6 The recommended scheme for determination of the capsizing moment Mc in the displacement mode of operation is given in 1.1.5.1 and 1.1.5.2 of annex 4. For this purpose, the angle of flooding should be taken as the lowest angle of heel corresponding to residual freeboard of 0.3 m below:

• .1 the lower window sill;

• .2 the upper edge of the coaming of the outside entry door; and

• .3 other points of flooding.

 1.1.4 Buoyancy and stability in the displacement mode following damage

1.1.4.1 The provisions of this section apply to all permitted conditions of loading.

1.1.4.2 Except as provided in 1.1.4.3, for the purpose of damage stability calculations, the volume and surface permeability should be as follows:

1.1.4.3 Notwithstanding 1.1.4.2, permeability determined by direct calculation should be used where a more onerous condition results, and may be used where a less onerous condition results from that provided according to 1.1.4.2.

1.1.4.4 Administrations may permit the use of low-density foam or other media to provide buoyancy in void spaces, provided that satisfactory evidence is provided that any such proposed medium is the most suitable alternative and is:

• .1 of closed-cell form if foam, or otherwise impervious to water absorption;

• .2 structurally stable under service conditions;

• .3 chemically inert in relation to structural materials with which it is in contact or other substances with which the medium is likely to be in contact; and

• .4 properly secured in place and easily removable for inspection of the void spaces.

1.1.4.5 Any damage of a lesser extent than that postulated in 1.1.4.6 and 1.1.4.7, as applicable, which would result in a more severe condition should also be investigated. The shape of the damage should be assumed to be a parallelepiped.

1.1.4.6 The following side damages should be assumed anywhere on the periphery of the craft:

• .1 the longitudinal extent of damage should be 0.1L, or 3 m + 0.03L or 11 m, whichever is the least;

• .2 the transverse extent of penetration into the craft should be 0.2B or 0.05L, whichever is the least. However, where the craft is fitted with inflated side-skirts or with non-buoyant side structures, the transverse extent of penetration should be at least 0.12 of the width of the main buoyancy hull or tank structure; and

• .3 the vertical extent of damage should be taken for the full depth of the craft.

1.1.4.7 Damages should be assumed anywhere on the bottom of the craft as follows:

• .1 the longitudinal extent of damage should be 0.1L or 3 m + 0.03L or 11 m, whichever is the least;

• .2 the transverse extent of damage should be the full breadth of the bottom of the craft or 7 m, whichever is the less; and

• .3 the vertical extent of penetration into the craft should be 0.02B or 0.5 m, whichever is the less.

 1.1.5 Inclining and stability information

1.1.5.1 Every craft, on completion of build, should be inclined^footnote and the elements of its stability determined. Alternatively, the mass and centre of gravity of the craft may be determined by weighing methods. When it is not possible to accurately determine the craft's vertical centre of gravity by either of these methods, it may be determined by accurate calculation.

1.1.5.2 The master should be supplied by the owner with reliable information relating to the stability of the craft in accordance with the provisions of this paragraph. This information should clearly show all constraints to the loading of the craft in accordance with these Guidelines, including the ranges of acceptable vertical transverse and longitudinal centre of gravity. The information relating to stability should, before issue to the master, be submitted to the Administration for approval, together with a copy thereof for their retention, and should incorporate such additions and amendments as the Administration may in any particular case require.

1.1.5.3 Where any alterations are made to a craft so as to affect materially the stability information supplied to the master, amended stability information should be provided. If necessary, the craft should be re-inclined or re-weighed.

1.1.5.4 A report of each weighing, inclining or lightweight survey carried out in accordance with this chapter and of the calculation therefrom of the light-ship condition particulars should be submitted to the Administration for approval, together with a copy for their retention. The approved report should be placed on board the craft by the owner in the custody of the master and should incorporate such additions and amendments as the Administration may in any particular case require. The amended light-ship condition particulars so obtained from time to time should be used by the master in substitution for such previously approved particulars when calculating the craft's stability.

1.1.5.5 Following any weighing, inclining or lightweight survey, the master should be supplied with amended stability information if the Administration so requires. The information so supplied should be submitted to the Administration for approval, together with a copy thereof for their retention, and should incorporate such additions and amendments as the Administration may in any particular case require.

1.1.5.6 Stability information demonstrating compliance with this chapter should be furnished in the form of a stability information book which should be kept on board the craft at all times in the custody of the master. The information should include particulars appropriate to the craft and should reflect the craft's loading conditions and modes of operation. All watertight and weathertight structures included in the cross curves of stability and the critical downflooding points and angles should be identified.

1.1.5.7 The operating company of every craft should establish and implement documented operational procedures to ensure that the craft's operational mass and longitudinal centre of gravity can be maintained within the certified limits for the craft. Methods reflected in these procedures may include marking and use of draught marks, regular operational weighing of the craft or marking of the waterline corresponding to the maximum take-off mass and permissible longitudinal centre of gravity for which the craft is certified.

1.1.5.8 Calculations of form stability levers for craft in all operational modes should be carried out on a basis of actual longitudinal centre of gravity.

 1.1.6 Loading and stability assessment

On completion of loading of the craft and prior to its departure on a voyage, the master should determine the craft's trim and stability and also ascertain and record that the craft is in compliance with stability criteria of the relevant requirements. The Administration may accept the use of an electronic loading and stability computer or equivalent means for this purpose.

 1.1.7 Marking and recording of the design waterline

The design waterline should clearly be marked amidships on the craft's outer sides and should be recorded in the Wing-in-Ground Craft Safety Certificate. This waterline should be distinguished by the notation WIG.

1.2 Provisions for passenger craft

 1.2.1 General

1.2.1.1 Where compliance with this chapter requires consideration of the effects of passenger weight, the following information should be used:

• .1 The distribution of standing passengers is 4 persons per square metre.

• .2 Each passenger has a mass of 75 kg.

• .3 Vertical centre of gravity of seated passengers is 0.3 m above seat.

• .4 Vertical centre of gravity of standing passengers is 1.0 m above deck.

• .5 Passengers and luggage should be considered to be in the space normally at their disposal.

• .6 Passengers should be distributed on available deck areas towards one side of the craft on the decks where muster stations are located and in such a way that they produce the most adverse heeling moment.

1.2.1.2 The stability of the craft should be verified, taking into account the assumptions in 1.2.1.1, for each of the following loading conditions:

• .1 with full number of passengers and cargo and full provisions on board craft;

• .2 with full number of passengers and cargo and with 10% of provisions; and

• .3 without passengers and cargo and with 10% of provisions.

1.2.1.3 The stability of the craft in all modes of operation, except aircraft mode, should be additionally verified in calm water under the loading condition described in 1.2.1.2.2 but with 50% of the passengers located in their seats on one side from the craft centre line. The remaining passengers should be located in their seats and/or passageways and other spaces not allocated to individual passengers so as to result in maximum heeling moment towards the side on which passengers remain seated.

1.2.1.4 If in the normal operation of the craft a loading condition is used in which the reserves of stability are less than for the loading conditions described in 1.2.1.2 and 1.2.1.3 then the stability for that loading condition should be verified as well.

 1.2.2 Intact stability in the displacement mode

1.2.2.1 Craft should have sufficient stability in calm water in all possible and permitted conditions of cargo stowage and with uncontrolled passenger movement so that a residual freeboard of 0.1 m is maintained in way of the datum described in 1.1.2.1.3 and all parts of fixed airfoils excluding flaps and ailerons.

1.2.2.2 The angle of heel under the combined action of heeling moments due to passenger crowding according to 1.2.1.3 and the greater of the moments due to wind and turning being determined experimentally should not exceed 8 degrees or the angle of entrance of the wing into the water, whichever is the less.

 1.2.3 Buoyancy and stability in the displacement mode following damage

1.2.3.1 Following any of the postulated damages detailed in 1.1.4.5 to 1.1.4.7, the craft should have sufficient buoyancy and positive stability in still water to simultaneously ensure that:

• .1 after flooding has ceased and a state of equilibrium has been reached, the final waterline is not less than 300 mm below the level of the openings described in 1.1.3.6.6;

• .2 the angle of inclination of the craft from the horizontal does not normally exceed 10° in any direction. However, where this is clearly impractical, angles of inclination up to 15° immediately after damage but reducing to 10° within 15 min may be permitted provided that efficient non-slip deck surface and suitable holding points, e.g. holes and bars, are provided;

• .3 there is a positive freeboard from the damage waterline to survival craft embarkation positions;

• .4 any flooding of passenger compartments or escape routes which might occur will not significantly impede the evacuation of passengers;

• .5 essential emergency equipment, emergency radios, power supplies and public address systems needed for organizing the evacuation remain accessible and operational; and

• .6 the residual stability complies with the appropriate criteria as laid out in annex 5.

1.2.3.2 When damage occurs which does not prevent the safe transition of the craft from the displacement to the planing or ground effect modes, such that the craft may safely proceed without assistance to a port or place of refuge or to meet with a salvage ship, the operational procedures for the craft should not prevent such actions.

1.2.4 Inclining and stability information

1.2.4.1 At periodical intervals not exceeding annually, a weighing or lightweight survey should be carried out on all passenger craft to verify any changes in lightweight displacement and longitudinal centre of gravity. The passenger craft should be re-weighed or re-inclined whenever, in comparison with the approved stability information, a deviation from the lightweight displacement exceeding 0.5% or a deviation of the longitudinal centre of gravity exceeding 0.25% of L is found or anticipated.

1.2.4.2 A report of each weighing, inclining or lightweight survey carried out in accordance with 1.1.5.1 and of the calculation therefrom of the lightweight condition particulars should be submitted to the Administration for approval, together with a copy for their retention. The approved report should be placed on board the craft by the owner in the custody of the master and should incorporate such additions and amendments as the Administration may in any particular case require. The amended lightweight condition particulars so obtained from time to time should be used by the master in substitution for such previously approved particulars when calculating the craft's stability.

1.2.4.3 Following any weighing, inclining or lightweight survey, the master should be supplied with amended stability information if the Administration so requires. The information so supplied should be submitted to the Administration for approval, together with a copy thereof for their retention, and should incorporate such additions and amendments as the Administration may in any particular case require.

1.3 Provisions for cargo craft

 1.3.1 Buoyancy and stability in the displacement mode following damage

Following any of the postulated damage detailed in 1.1.4.5 to 1.1.4.7, the craft, in still water, should have sufficient buoyancy and positive stability to simultaneously ensure that:

• .1 after flooding has ceased and a state of equilibrium has been reached, the final waterline 150 mm below the level of any opening referred to in 1.1.3.6.6;

• .2 the angle of inclination of the craft from horizontal does not normally exceed 15° in any direction. However, where this is clearly impractical, angles of inclination up to 20° immediately after damage but reducing to 15° within 15 min may be permitted provided that efficient non-slip deck surface and suitable holding points, e.g. holes and bars, are provided;

• .3 there is a positive freeboard from the damage waterline to survival craft embarkation positions;

• .4 essential emergency equipment, emergency radios, power supplies and public address systems needed for organizing the evacuation remain accessible and operational; and

• .5 the residual stability complies with the appropriate criteria as laid out in annex 5.