4.8 Dynamically supported craft (DSC)
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4.8 Dynamically supported craft (DSC)

 As the Code of Safety for Dynamically Supported Craft (resolution A.373(X)) is under current revision, the provisions given below are of an interim nature. In particular, such factors as the increase in the number of passengers carried on board and new types of DSC are expected to be among major changes to be introduced into a new code. When the revision of Intact Stability Code is undertaken, the standards for such craft will be replaced by the provisions of the High Speed Craft (HSC) Code currently under development.

Parent topic: Chapter 4 - Special Criteria for Certain Types of Ships

4.8.1 Application

  4.8.1.1 The provisions given hereunder apply to dynamically supported craft as defined in 1.3.8 which are engaged on voyages between a terminal in one country and a terminal in another country, part or all of which voyages are across areas of water (but not necessarily on routes navigable to ships) through which a ship operating on an international voyage, as defined in regulation I/2(d) of the 1974 SOLAS Convention, as amended, would proceed. In applying the provisions of this chapter, the Administration should determine whether a craft is a dynamically supported craft as defined in 1.3.8, or whether its characteristics are such that the SOLAS and Load Line Conventions can be applied. For novel types of DSC other than defined in 1.3.9 and 1.3.10, the Administration should determine the extent to which the provisions of this chapter are applicable to those novel types. The contents of this chapter should be applied by Administrations through more detailed national regulations based on a comprehensive coverage of the provisions contained therein.

  4.8.1.2 The provisions in this chapter apply to DSC which:

  • .1 carry more than 12 passengers but not more than 450 passengers with all passengers seated;

  • .2 do not proceed in the course of their voyage more than 100 nautical miles from the place of refuge; and

  • .3 may be provided within the limits of subparagraphs .1 and .2 with special category spaces intended to carry motor vehicles with fuel in their tanks.

The provisions given below may be extended to a DSC which is intended to carry passengers and cargo or solely cargo or to a craft which exceeds the limits specified in .1 to .3. In such cases, the Administration should determine the extent to which the provisions of the Code are applicable to these craft and, if necessary, develop additional requirements providing the appropriate safety level for such craft.

  4.8.1.3 The provisions of this chapter do not apply to any DSC the keel of which is laid, or which is subject to repairs, alterations or modifications of a major character, on or after 1 January 1996.

4.8.2 General provisions

  4.8.2.1 A craft should be provided with:

  • .1 stability characteristics and stabilization systems adequate for safety when the craft is operated in the non-displacement mode and during the transient mode; and

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

  4.8.2.2 If a craft operates in zones where ice accretion is likely to occur, the effect of icing should be taken into account in the stability calculations in accordance with section 5.5.

4.8.3 Definitions

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

  • .1 length (L) means length of the rigid hull measured on the design waterline in the displacement mode;

  • .2 breadth (B) means breadth of the broadest part of the rigid hull measured on the design waterline in the displacement mode;

  • .3 design waterline means the waterline corresponding to the loaded displacement of the craft when stationary;

  • .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;

  • .5 skirt means a downwardly-extending, flexible structure used to contain or divide an air cushion;

  • .6 fully submerged foil means a foil having no lift components piercing the surface of the water in the foil-borne mode.

4.8.4 Intact buoyancy

  4.8.4.1 The craft should have a designed reserve of buoyancy when floating in seawater of not less than 100% at the maximum operational weight. The Administration may require a larger reserve of buoyancy to permit the craft to operate in any of its intended modes. The reserve of buoyancy should be calculated by including only those compartments which are:

  • .1 watertight;

  • .2 considered by the Administration to have scantlings and arrangements adequate to maintain their watertight integrity; and

  • .3 situated below a datum, which may be a watertight deck or equivalent structure watertight longitudinally and transversely and from at least part of which the passengers would be disembarked in an emergency.

  4.8.4.2 Means should be provided for checking the watertight integrity of buoyancy compartments. The inspection procedures adopted and the frequency at which they are carried out should be to the satisfaction of the Administration.

  4.8.4.3 Where entry of water into structures above the datum as defined in 4.8.4.1.3 would significantly influence the stability and buoyancy of the craft, such structures should be of adequate strength to maintain the weathertight integrity or be provided with adequate drainage arrangements. A combination of both measures may be adopted to the satisfaction of the Administration. The means of closing of all openings in such structures should be such as to maintain the weathertight integrity.

4.8.5 Intact stability

  4.8.5.1 The stability of a craft in the displacement mode should be such that when in still water conditions, the inclination of the craft from the horizontal would not exceed 8° in any direction under all permitted cases of loading and uncontrolled passenger movements as may occur. A calculation of the dynamic stability should be made with respect to critical design conditions.

  4.8.5.2 For guidance of the Administration, methods relating to the verification of the stability of hydrofoil boats fitted with surface piercing foils and fully submerged foils are outlined in 4.8.7.

4.8.6 Stability of the craft in the non-displacement mode

  4.8.6.1 The Administration should be satisfied that when operating in the non-displacement and transient modes within approved operational limitations, the craft will, after a disturbance causing roll, pitch, heave or any combination thereof, return to the original attitude.

  4.8.6.2 The roll and pitch stability of each craft in the non-displacement mode, should be determined experimentally prior to its entering commercial service and be recorded.

  4.8.6.3 Where craft are fitted with surface piercing structure or appendages, precautions should be taken against dangerous attitudes or inclinations and loss of stability subsequent to a collision with a submerged or floating object.

  4.8.6.4 The Administration should be satisfied that the structures and components provided to sustain operation in the non-displacement mode should, in the event of specified damage or failure, provide adequate residual stability in order that the craft may continue safe operation to the nearest place where the passengers and crew could be placed in safety, provided caution is exercised in handling.

  4.8.6.5 In designs where periodic use of cushion deformation is employed as a means of assisting craft control or periodic use of cushion air exhausting to atmosphere for purposes of craft manoeuvring, the effects upon cushion-borne stability should be determined, and the limitations on the use by virtue of craft speed or attitude should be established.

4.8.7 Methods relating to the intact stability investigation of hydrofoil boats

 The stability of these craft should be considered in the hull-borne, transient and foil-borne modes. The stability investigation should also take into account the effects of external forces. The following procedures are outlined for guidance in dealing with stability.

4.8.7.1 Surface piercing hydrofoils

  • .1 Hull-borne mode

    • .1.1 The stability should be sufficient to satisfy 4.8.5.

    • .1.2 Heeling moment due to turning.

      The heeling moment developed during manoeuvring of the craft in the displacement mode may be derived from the following formula:

      where:
      MR = moment of heeling
      Vo = speed of the craft in the turn (metres per second)
      Δ = displacement (tonnes)
      KG = height of the centre of gravity above keel (metres)
      L = length of the craft on the waterline (metres)

      This formula is applicable when the ratio of the radius of the turning circle to the length of the craft is 2 to 4.

    • .1.3 Relationship between the capsizing moment and heeling moment to satisfy the weather criterion.

      The stability of the hydrofoil boat in the displacement mode can be checked for compliance with the weather criterion K as follows:

      where:
      Mc = minimum capsizing moment as determined when account is taken of rolling;
      Mv = dynamically applied heeling moment due to the wind pressure

    • .1.4 Heeling moment due to wind pressure

      The heeling moment Mv is a product of wind pressure Pv the windage area Av and the lever of windage area Z v.

      • Mv = 0.001 PvAvZv (kN*m)

      The value of the heeling moment is taken as constant during the whole period of heeling.

      The windage area Av is considered to include the projections of the lateral surfaces of the hull, superstructure and various structures above the waterline. The windage area lever Zv is the vertical distance to the centre of windage from the waterline and the position of the centre of windage may be taken as the centre of the area.

      The values of the wind pressure (in Pa) associated with Force 7 Beaufort scale depending on the position of the centre of windage area are given in table 4.8.7.

    • .1.5 Evaluation of the minimum capsizing moment Mc in the displacement mode.

      The minimum capsizing moment is determined from the static and dynamic stability curves taking rolling into account.

      • .1.5.1 When the static stability curve is used, Mc is determined by equating the areas under the curves of the capsizing and righting moments (or levers) taking rolling into account - as indicated by figure 4.8.7-1, where θz is the amplitude of roll and MK is a line drawn parallel to the abscissa axis such that the shaded areas S 1 and S2 are equal.

        Mc = OM, if the scale of ordinates represents moments
        Mc = OM x displacement, if the scale of ordinates represents levers

      • .1.5.2 When the dynamic stability curve is used, first an auxiliary point A should be determined. For this purpose the amplitude of heeling is plotted to the right along the abscissa axis and a point A' is found (see figure 4.8.7-2). A line AA' is drawn parallel to the abscissa axis equal to the double amplitude of heeling (AA' = 2θz) and the required auxiliary point A is found. A tangent AC to the dynamic stability curve is drawn. From the point A the line AB is drawn parallel to the abscissa axis and equal to 1 radian (57.3°). From the point B a perpendicular is drawn to intersect with the tangent in point E. The distance is equal to the capsizing moment if measured along the ordinate axis of the dynamic stability curve. If, however, the dynamic stability levers are plotted along this axis, is then the capsizing lever and in this case the capsizing moment Mc is determined by multiplication of ordinate in metres by the corresponding displacement in tonnes.

      • .1.5.3 The amplitude of rolling θ z is determined by means of model and full-scale tests in irregular seas as a maximum amplitude of rolling of 50 oscillations of a craft travelling at 90° to the wave direction in sea state for the worst design condition. If such data are lacking the amplitude is assumed to be equal to 15°.

      • .1.5.4 The effectiveness of the stability curves should be limited to the angle of flooding.

  • .2 Stability in the transient and foil-borne modes

    • .2.1 The stability should satisfy the provisions of 4.8.6 of this chapter.

      • .2.2.1 The stability in the transient and foil-borne modes should be checked for all cases of loading for the intended service of the craft.

      • .2.2.2 The stability in the transient and foil-borne modes may be determined either by calculation or on the basis of data obtained from model experiments and should be verified by full-scale tests by the imposition of a series of known heeling moments by off-centre ballast weights, and recording the heeling angles produced by these moments. When taken in the hull-borne, take-off, steady foil-borne and settling to hull-borne modes, these results will provide an indication of the values of the stability in the various situations of the craft during the transient condition.

      • .2.2.3 The time to pass from the hull-borne mode to foil-borne mode and vice versa should be established. This period of time should not exceed two minutes.

      • .2.2.4 The angle of heel in the foil-borne mode caused by the concentration of passengers at one side should not exceed 8°. During the transient mode the angle of heel due to the concentration of passengers on one side should not exceed 12°. The concentration of passengers should be determined by the Administration, having regard to the guidance given in 4.8.8.

    • .2.3 One of the possible methods of assessing foil-borne metacentric height (GM) in the design stage for a particular foil configuration is given in figure 4.8.7-3.

      where
      nB = percentage of hydrofoil load borne by front foil
      nH = percentage of hydrofoil load borne by aft foil
      LB = clearance width of front foil
      LH = clearance width of aft foil
      a = clearance between bottom of keel and water
      h = height of centre of gravity above bottom of keel
      1B = angle at which front foil is inclined to horizontal
      1H = angle at which aft foil is inclined to horizontal

4.8.7.2 Fully submerged hydrofoils

  • .1 Hull-borne mode

    • .1.1 The stability in the hull-borne mode should be sufficient to satisfy the requirements given in 4.8.5.

    • .1.2 Paragraphs 4.8.7.1.1.2 to 4.8.7.1.1.5 of this section are appropriate to this type of craft in the full-borne mode.

  • .2 Transient mode

    • .2.1 The stability should be examined by the use of verified computer simulations to evaluate the craft's motions, behaviour and response under the normal conditions and limits of operation, and under the influence of any malfunction.

    • .2.2 The stability conditions resulting from any potential failures in the systems or operational procedures during the transient stage which could prove hazardous to the craft's watertight integrity and stability should be examined.

  • .3 Foil-borne mode

    The stability of the craft in the foil-borne mode should be in compliance with 4.8.6 and 4.8.7.2.2.

  • .4 Paragraphs 4.8.7.1.2.2.1 to 4.8.7.1.2.2.4 should be applied to this type of craft as appropriate and any computer simulations or design calculations should be verified by full-scale tests.

Typical wind pressures for Beaufort scale 7 100 nautical miles from land
Z above waterline (metres) 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
P (Pa) 46 46 50 53 56 58 60 62 64
Note: These values may not be applicable in all areas

 Figure 4.8.7.1

 Figure 4.8.7.2 Dynamic stability curve

 Figure 4.8.7.3

4.8.8 Passenger loading

  4.8.8.1 A mass of 75 kg should be assumed per passenger except that this value may be reduced to not less than 60 kg where this can be justified. In addition, the mass and distribution of the luggage should be to the satisfaction of the Administration.

  4.8.8.2 The height of the centre of gravity for passengers should be assumed equal to:

  • .1 1 m above deck level for passengers standing upright. Account may be taken, if necessary, of camber or sheer of deck.

  • .2 0.30 m above the seat in respect of seated passengers.

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

  4.8.8.4 Passengers should be considered as distributed to produce the most unfavourable combination of passenger heeling moment and/or initial metacentric height which may be obtained in practice. In this connection, it is anticipated that a value higher than four persons per square metre will not be necessary.

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