The steering system shall be capable of reliably altering the ship’s heading at a rate appropriate for the navigational hazards that might be expected in normal and abnormal conditions. The steering system shall also be capable of reliably holding or returning the ship’s head to a given course to counteract the effects of wind, currents and waves.

The rudder, steering nozzle or other directional control device shall have sufficient strength to meet the demands of service in both ahead and astern operation, and in normal and emergency situations. Consideration shall be given to peak, fatigue and shock loading.

The rudder, steering nozzle or other directional control device shall be designed and constructed to avoid or reduce the effects of corrosion and erosion.

The steering system shall be deemed to have satisfied the Required Outcomes in subparagraph 9.25.1 to 9.25.3 if it complies with paragraph 9.25.5 to 9.25.9.

• 9.25.5.1Strength of steering gear

  The steering gear shall be designed to withstand maximum helm at maximum ahead and astern speed. The rudderstock, rudder or steering nozzle and tiller arm or quadrant shall comply with paragraphs 9.25.6, 9.25.7, 9.25.8 and 9.25.9 below.

• 9.25.5.2Steering arrangement

  The steering arrangement shall be such that the person at the helm has a clear view ahead while at the normal steering position.

• 9.25.5.3Secondary means of steering

  All ships, except twin screw ships, shall be fitted with 2 independent means of steering unless steering is normally achieved via a hand tiller, in which case a second means of steering need not be provided. The secondary or emergency means of steering shall be capable of being brought speedily into action.

• 9.25.5.4Rudder movement

  Rudder movement shall be no less than 35° to port to 35° to starboard.

  NOTE:

  The formulas specified in paragraphs 9.25.6, 9.25.7 and 9.25.9 are based on helm angles not exceeding 35°. Rudder movement in excess of 35° will require a change in the minimum diameter of the rudderstock and the scantlings of the rudder and tiller arm or quadrant.

• 9.25.5.5Performance

  In ships of 12.5m measured length and over, the steering gear shall be capable of putting the rudder over from 35° on one side to 30° on the other in 30 seconds when the ship is at maximum ahead service speed with the rudder totally submerged. It shall be designed to prevent violent recoil of the steering wheel.

• 9.25.5.6Helm movement direction

  The trailing edge of a rudder blade of a ship shall move in the same direction as the top spokes of the steering wheel. Where any ship is not fitted with a conventional steering wheel, movement of the helm actuator to port or starboard shall cause the ship’s head to move in the same direction.

• 9.25.5.7Rudder position indicator

  A rudder position indicator shall be fitted on all ships of 15m measured length and over which are fitted with power-operated steering gear. The rudder position indicator shall be in full view of the person at the helm while the person is at any steering position. This latter requirement need not apply to a person at the helm at the emergency steering position.

• 9.25.5.8Steering component material

  Components that transmit torque, tensile stresses or shock loads, including the tiller or quadrant, shall not be manufactured from ordinary grades of cast iron or other non-ductile material.

• 9.25.5.9Hydraulic steering systems

  Hydraulic steering systems shall comply with the following requirements—

  • (a)Means shall be provided to facilitate a quick change over from the primary to the secondary steering.
  • (b)a relief valve shall be installed in hydraulic systems that incorporate a power pump. The relief valve shall be set to prevent mechanical damage to the steering gear.
  • (c)Hydraulic hose and piping shall comply with paragraphs 9.28.3 and 9.28.4 and shall be located and arranged to minimise the possibility of mechanical, fire or other damage.

  NOTE:

  Mechanical damage includes chafing, crushing and holing.

• 9.25.5.10Mechanical transmission or actuator shaft bearings Steering transmission or actuator shafts shall be adequately supported in bearings spaced apart not more than 70 times the diameter of the shaft. Bearing spacing adjacent to sprockets or gears shall be such that no undue bending load can be applied to the shaft.

• 9.25.6.1

  Definitions

  For the purposes of this part, the following definitions apply—

  Balanced rudder means a rudder having blade area forward of the rudder stock or pintles (see Figure 14 and Figure 15).

  Unbalanced rudder means a rudder having no blade area forward of the rudder stock or pintles (see Figure 16).

  

  Figure 14 — Balanced spade rudder of FRP construction

  Figure 15 — Balanced rudder of single plate construction

• 9.25.6.2Manufacture

  Rudders shall be manufactured in accordance with one of the following methods, or by a means that can be demonstrated to be equivalent—

  • (a)The rudder blade shall be fabricated or cast, and shall incorporate either an integral flange that is secured to a flanged rudder stock with fitted bolts or shall be secured by means of a taper, key and retaining nut.
  • (b)The rudder blade shall be fabricated with an integral rudder stock.

  NOTE:

  Special attention should be given to the attachment of the rudder stiffeners, pintles and rudder coupling to the main-piece of the rudder.

  Figure 16 — Unbalanced rudder of double plate construction

• 9.25.6.3Component materials

  Unless otherwise permitted in this Chapter, the following shall apply—

  • (a)Rudder stocks, couplings, coupling bolts, pintles and similar parts subject to dynamic stress shall be manufactured from materials having minimum mechanical properties as specified for shaft materials in subparagraph 9.14.1. Other materials may be used where equivalence can be demonstrated.
  • (b)Cast rudder blades shall be manufactured from ductile material.

  NOTE:

  Most of the formulae for rudder components contained within this Chapter are based on — “minimum required” rather than “as fitted” diameters in order to give minimum scantlings. Designing to the minimum required diameter may limit future flexibility in regard to rudder modification or alterations to the ship which result in an increase in speed.

• 9.25.6.4Rudder stock and main-piece-unbalanced rudders
  • 9.25.6.4.1Arrangement of bearings

    For the purposes of this regulation , unbalanced rudders are assumed to have at least one pintle (at the heel) with a neck bearing, or additional pintles, or both a neck bearing and additional pintles.

  • 9.25.6.4.2Upper stock size

    A rudder stock at the tiller or quadrant for an unbalanced rudder shall not be less than that obtained from the following formula—

    

    Where—

    • d _U = minimum diameter of upper stock, in millimetres;
    • R = distance from the centre line of stock to the centre of area of the rudder (see Figure 16), in metres;
    • A = area of rudder, in square metres (m²);
    • V = speed of ship in knots with—
      • (a)a minimum of 8 in ships less than 30m in measured length; and
      • (b)a minimum of 9 in ships of 30m in measured length and over.
    • UTS _stock = ultimate tensile strength of stock material, in megapascals (MPa);
    • C = the coefficient obtained from Table 6;
    • fc = rudder cross-section coefficient —
      • (a)one for normal cross-section rudders;
      • (b)1.08 for hollow cross section rudders; eg whale-tail; and
      • (c)1.19 for high lift rudders with active trailing edge.
    • f _N = nozzle coefficient—
      • (a)one for rudders behind an open propeller; and
      • (b)1.09 for rudders behind a propeller in fixed nozzle.

    Table 6 — Values of coefficient C for varying ships speed V

    V (knots)	8	9	10	11	12	13	14	15 and over
    C (Measured length of ships less than 30m)	21.66	21.25	20.84	20.43	20.02	19.61	19.20	19.20
    C (Measured length of ship 30m and over)	N/A	21.66	21.25	20.84	20.43	20.02	19.61	19.20

    NOTE:

    Values of coefficient C for ship speeds between those shown in the table may be obtained by applying the formulas C = 24.94 − 0.41V for ships of measured length less than 30m (and speed up to 14 knots) and C = 25.35 − 0.41V for ships of measured length 30m and over.

  • 9.25.6.4.3Lower stock size

    The minimum required diameter of the lower stock d_l above the top pintle or in way of the neck bearing of an unbalanced rudder shall be the same as the minimum required diameter of the upper stock d_u.

  • 9.25.6.4.4Mainpiece size

    The mainpiece of an unbalanced rudder may be gradually reduced from the minimum required diameter of the lower stock at the top of the rudder blade d₁ (as calculated in subparagraph 9.25.6.5.2) to 0.75 d_l at the heel pintle.

• 9.25.6.5Rudder stock and mainpiece — balanced rudders
  • 9.25.6.5.1Upper stock size

    A rudder stock above the neck bearing for a balanced rudder shall not be less than that obtained from the following formula—

    

    Where—

    • d _U = minimum diameter of upper stock, in millimetres;
    • b = horizontal distance, in metres, from the centre of the lower stock to the centre of area of total rudder area (see Figure 14 or Figure 15);
    • A = area of the rudder, in square metres (m²); and
    • V = speed of ship in knots with—
      • (a)a minimum of 8 knots in ship less than 30m in measured length; and
      • (b)a minimum of 9 knots in ships of 30m in measured length and over;
    • UTS _stock = ultimate utensile strength of stock material, in megapascals (MPa);
    • C = the coeffiecient obtained from Table 6;
    • f _C = rudder cross section coefficient—
      • (a)one for normal cross-section rudders;
      • (b)1.08 for hollow cross-section rudders, eg whale-tail; and
      • (c)1.19 for high lift rudders with active trailing edge.
    • f _N = nozzle coefficient—
      • (a)one for rudders behind and an open propeller; and
      • (b)1.09 for rudders behind a propeller in a fixed nozzle.
  • 9.25.6.5.2Lower stock size

    The stock in way of and below the neck bearing for a balanced rudder shall have a diameter not less than that determined from the following formula—

    

    Where—

    
    • d _l = minimum diameter of lower stock, in millimetres;
    • R = [0.25a + √a² + 16b²] for rudders lined with neck and heel pintle bearings; or

      a + √a² + b² for rudders not fitted with a heel pintle bearing where (from Figure 14 and Figure 15)—

      • a = vertical distance, in metres, from the bottom of the neck bearing to the centre of area of total rudder area; and
      • b = horizontal distance, in metres, from the centre of the lower stock to the centre of area of total rudder area.
    • A = total area of rudder is square metres (m²;
    • V = speed of ship in knots with—
      • (a)a minimum of 8 in ships less than 30m in measured length; and
      • (b)a minimum of 9 in ships of 30m in measured length and over;
    • UTSstock = ultimate tensile strength of stock material, in megapascals (MPa);
    • C = the coefficient determined in accordance with either Item (a) or (b) as follows—
      • (a)where the speed of the ship is not greater than 4√ LWL (LWL being the measured length of the ship, in metres, at the designed waterline), the value of C shall be obtained from Table 6;
      • (b)where the speed of the ship is greater than 4√ LWL(LWL being the measured length of the ship, in metres, at the designed waterline), the value of C shall be 16;
    • f _C = rudder cross-section coefficient—
      • (a)one for normal cross-section rudders;
      • (b)1.08 for hollow cross-section rudders eg whale-tail; and
      • (c)1.19 for high lift rudders with active trailing edge.
    • f _N = nozzle coefficient—
      • (a)one for rudders behind an open propeller; and
      • (b)1.09 for rudders behind a propeller in a fixed nozzle.
  • 9.25.6.5.3Main piece size (with heel pintle bearing)

    The main piece of a balanced rudder having neck and heel pintle bearings (see Figure 15) shall be the full diameter of the lower stock for at least two-thirds of the distance from the neck bearing to the heel pintle bearing. The diameter may be gradually reduced below this point to 0.75d_l at the heel pintle.

  • 9.25.6.5.4Stock and main piece size (no heel pintle bearing)

    The stock and main piece of a balanced spade rudder that has no heel pintle bearing (Figure 14) shall be the required diameter of the lower stock d_l from the neck bearing to the underside of the top rudder arm if a single plate rudder, or to the top of the rudder if a built-up rudder. The diameter of the main piece may be gradually reduced below this point until it is 0.5d_l. The length of main piece in way of the rudder shall not be less than two-thirds of the depth of the rudder at the centre line of the stock. The stock above the neck bearing may be gradually reduced from the required diameter of the lower stock d₁ to the required diameter of the upper stock d _uf at a point just below the upper bearing.

• 9.25.6.6Non-circular sections

  The width, depth, section modulus and torsional modulus of a main piece or stock of non-circular section shall not be less than those required for a circular main piece or stock. When calculating the section modulus of the main piece, the effective width of plating that may be included on each side of a web forming the main piece shall not be greater than the thickness of the rudder at the centre line of the stock. Where the material of the mainpiece differs from that of the stock, the section modulus of the mainpiece shall not be less than that determined from the following formula—

  

  Where—

  • Z _mpiece = minimum section modulus of mainpiece at the top of the rudder, in cubic centimetres (cm³);
  • d _l = required diameter of the mainpiece, in millimetres, as determined in subparagraph 9.25.6.4.4, 9.25.6.5.3 or 9.25.6.5.4;
  • UTS _stock = ultimate tensile strength of stock material, in megapascals (MPa); and
  • UTS _mpiece = ultimate tensile strength of mainpiece material, in megapascals (MPa).
• 9.25.6.7Rudder bearings, pintles, gland and stops
  • 9.25.6.7.1Rudder support
    • 9.25.6.7.1.1Rudder bearings shall be adequately supported, and their housings shall be rigidly attached to the ship’s structure.
    • 9.25.6.7.1.2The weight of a rudder shall be supported at a pintle bearing (normally the heel pintle) or a carrier bearing. The structure in way of the pintle or carrier bearing shall be strengthened for that purpose.
  • 9.25.6.7.2Sole-pieces
    • 9.25.6.7.2.1Ratio of width to depth

      For the purposes of the formulas contained in subparagraph 9.25.6.7.2.2 to 9.25.6.7.2.4, the width to depth ratio of a sole-piece shall not be greater than 2.3 to one nor less than 1.8 to 1.

    • 9.25.6.7.2.2Section modulus

      The section modulus of the sole-piece about the vertical axis at a distance l_S from the centreline of the rudder stock shall not be less than that determined from the following formula—

      

      Where—

      • Z _S = required section modulus of the sole- piece about the vertical axis, in cubic centimetres (cm³);
      • C _S = a coefficient varying with speed obtained from Table 7;
      • A = total area of rudder, in square metres (m²);
      • V = maximum speed of ship, in knots;
      • l _S = horizontal distance from the centreline of rudder stock to the particular section of the sole-piece, in metres;
      • UTS _S = ultimate tensile strength of sole-piece material, in megapascals (MPa);
      • f _C = rudder cross-section coefficient—
        • (a)one for normal cross-section rudders;
        • (b)1.08 for hollow cross-section rudders, eg whale-tail;
        • (c)1.19 for high lift rudders with active trailing edge.

      Table 7 — Values of coefficient C _S for varying ship speed V

    V (knots)	10	11	12	13	14	15	16 and over
    C S for ships without an outer post	2.054	1.811	1.617	1.464	1.339	1.235	1.138
    C S for ships with an outer post	1.707	1.540	1.394	1.283	1.179	1.096	1.026

    NOTE:

    Values of coefficient C_S for ships speeds between those shown in the table may be obtained by linear interpolation.

    • 9.25.6.7.2.3Stiffness

      Where the sole-piece is a material other than carbon steel, the moment of inertia about the vertical axis at a distance l _S from the centreline of the rudder stock shall not be less than that determined from the following formula—

      

      Where—

      • I _S = required moment of inertia of the sole-piece about the vertical axis, in cm⁴;
      • Z _S = required section modulus of the sole-piece about the vertical axis as calculated in subsection 9.25.6.7.2.2, in cubic centimetres (cm³); and
      • E _S = modulus of elasticity of the sole-piece material, in gigapascals (GPa).
    • 9.25.6.7.2.4Area

      Where the sole-piece is a material other than carbon steel, the cross-sectional area of the sole piece at a distance l _S from the centreline of the rudder stock shall not be less than that determined from the following formula—

      

      Where—

      • A _S = required area of the sole-piece, in square centimetres (cm²);
      • Z _S = required section modulus of the sole-piece about the vertical axis as calculated in subsection 9.25.6.7.2.2, in cubic centimetres (cm³); and
      • UTS _S = ultimate tensile strength of sole-piece material, in megapascals (MPa).
  • 9.25.6.7.3Rudder stock neck bearing

    Neck bearings for rudders shall incorporate bushes and shall be fitted as shown in Figure 15. The bush shall have a length not less than that determined from the following formula:

    l_n = k_n d_l

    Where—

    • l _n = required length of neck bearing, in millimetres;
    • k _n = 4 for spade rudders without an upper bearing, or 1.5 for all other balanced rudders; and
    • d _l = minimum required diameter of lower stock, in millimetres.
  • 9.25.6.7.4Spade rudder bearing pressure
    • 9.25.6.7.4.1High bearing loads are likely on the neck and upper bearings of a spade rudder (see Figure 14). Calculations shall be made to ensure that the pressure on the neck and upper bearings does not exceed that specified by the manufacturer of the bearings. Where allowable bearing pressure information is not available, then the maximum nominal bearing pressure shall not exceed 3.9 MPa.

      NOTE:

      For the purposes of this calculation, a neck bearing of a rudder having no upper bearing as in Figure 14 may be modelled as 2 bearings (a neck bearing and an upper bearing) with a gap between the bearings of at least 1.0 times the required diameter of the lower stock d _l.

    • 9.25.6.7.4.2The nominal bearing pressure may be determined by first determining the rudder force from the following formula—

      F_p = 196AV²

      Where—

      • F _p = rudder force in newtons (N);
      • A = area of rudder, in square metres (m²); and
      • V = speed of ship, in knots,
    • 9.25.6.7.4.3The nominal bearing pressure is then determined as follows —

      

      Where—

      • P _B = nominal bearing pressure, in megapascals (MPa);
      • F _p = rudder force from the above formula, in newtons (N);
      • d = actual diameter of rudder stock in way of the bearing, in millimetres; and
      • l _B = length of bearing, in millimetres.
  • 9.25.6.7.5Rudder stock upper bearing
    • 9.25.6.7.5.1Upper rudder stock bearings, where fitted, shall have a length not less than the required upper stock diameter d _uf in way of the bearing. For spade rudders of the type shown in Figure 14 the upper bearing (not depicted in the figure) should have a length not less than that determined from the following formula—

      

      Where—

      • l _u = length of upper bearing in millimetres;
      • d _uf = fitted diameter of upper stock in way of upper bearing, in millimetres;
      • h _u = height of centre of upper bearing above centre of rudder area, in millimetres;
      • l _n = required length of neck bearing, in millimetres;
      • d _l = required diameter of lower stock in way of neck bearing, in millimetres; and
      • h _n = height of centre of neck bearing above centre of rudder area in millimetres.
    • 9.25.6.7.5.2For the purposes of this calculation, the bottom of the upper bearing should be located no less than d_l from the top of the lower bearing.
  • 9.25.6.7.6Distance from tiller or quadrant boss to nearest bearing

    The distance from the tiller or quadrant boss to the nearest upper or neck bearing; gland; or other support should not exceed 2.5 times the fitted diameter of the rudder stock in way of the boss.

  • 9.25.6.7.7Rudder pintle diameter

    Where a single heel pintle (Figure 15), or multiple equidistant pintles (Figure 16) are fitted, the diameter of pintles shall not be less than that determined from the following formula—

    

    Where—

    • d _pi = required diameter of heel or intermediate pintle, in millimetres;
    • d _l = minimum required diameter of rudder lower stock, in millimetres;
    • N = number of pintles supporting the rudder inclusive of the heel pintle; and
    • K _p = 0 for rudders having a neck bearing, or one for rudders with no neck bearing.

    NOTE:

    Rudders with only a single pintle at the heel are required to have a neck bearing.

    • UTS _stock = ultimate tensile strength of stock material, in megapascals (MPa); and
    • UTS _pintle = ultimate tensile strength of pintle material, in megapascals (MPa).
  • 9.25.6.7.8Rudder pintle bearings

    Pintle bearings, if fitted, shall incorporate bushes. The length of pintle bearings shall not be less than that determined from the following formula—

    

    Where—

    • l _p = required length of pintle bearing, in millimetres;
    • k _p = a factor of 0.93 for balanced rudders having a bottom pintle bearing, or one for other rudders; and
    • d _pi = required diameter of pintle calculated in accordance with subparagraph 9.25.6.7.7, in millimetres.
  • 9.25.6.7.9Rudder stops

    Rudders shall incorporate stops at the full over position to prevent the rudder coming into contact with the propeller or hull. Vertical movement shall also be limited by stops or jumping collars.

  • 9.25.6.7.10Rudder trunk and gland
    • 9.25.6.7.10.1The rudder trunk shall be of a thickness sufficient to support any rudder stock bearings carried within the trunk. For materials subject to corrosion, the thickness shall incorporate a 25% allowance for corrosion. The thickness of the rudder trunk shall not be less than that of the hull shell thickness to which it is attached.

      NOTE:

      The thickness of the rudder trunk is typically 25% greater than that of the hull shell thickness to allow for boring, support of bearings, welding and/or integration into the structure.

    • 9.25.6.7.10.2The rudder trunk enclosing the rudderstock and neck bearing should extend above the fully loaded waterline. A gland shall be fitted to seal the rudder trunk if the trunk terminates below the level of the deck.
• 9.25.6.8Rudder couplings

  Figure 17 — Flange couplings of fabricated construction

  • 9.25.6.8.1Coupling types

    Rudder couplings shall be one of the following types—

    • (a)Flange couplings of fabricated construction, which have been stress relieved subsequent to welding (see Figure 17).
    • (b)Flange couplings formed by upsetting the end of the stock, provided that there is no necking or narrowing of the stock.
    • (c)Taper couplings, keyed and held in place by a nut. The taper coupling may be arranged to secure the boss of a flanged coupling or alternatively, to secure the stock directly into the mainpiece without the need of a flange coupling.
  • 9.25.6.8.2Flange coupling dimensions and bolting arrangements
    • 9.25.6.8.2.1The dimensions and bolting arrangements of rudder flange couplings shall be as follows—
      • (a)The minimum thickness of a coupling flange shall be the greater of those calculated in accordance with the following formulae—

        

        Where—

        • t _f = minimum flange thickness, in millimetres;
        • k = 0.25 for a rudder with one or more pintles, or 0.32 for a spade rudder;
        • d _l = required diameter of the rudder stock in way of the coupling, in millimetres;
        • d _b = required diameter of the coupling bolts, in millimetres, calculated in accordance with subparagraph 9.25.6.8.2 (e)
        • UTS _stock = ultimate tensile strength of rudder stock material, in megapascals (MPa);
        • UTS _coup = ultimate tensile strength of coupling flange material, in megapascals (MPa); and
        • UTS _bolt = ultimate tensile strength of coupling bolts of diameter calculated in accordance with subparagraph 9.25.6.8.2 (f) below, in millimetres.
      • (b)The fillet radius at the base of the flange shall not be less than 0.125 times the actual diameter of the stock in way of the coupling.
      • (c)The ligament thickness outside the coupling bolt holes shall not be less than 0.6 times the required diameter of the coupling bolt.
      • (d)The pitch circle radius of bolts for couplings of the forged or welded flange type shall not be less than the required diameter of the rudder stock in way of the coupling, and for couplings keyed to the stock, shall be not less than 1.25 times the required diameter of the rudder stock.
      • (e)Where a rudderstock is 150mm or more in diameter in way of the coupling, at least 6 bolts shall be used in each coupling flange. Where the diameter is less than 150mm, at least 4 bolts shall be used in each coupling flange.
      • (f)The total area of bolts shall not be less than that determined from the following formula—

        

        Where—

        • A = total bolt area at root of threads, in square millimetres (mm²);
        • d = required diameter of stock in way of coupling, in millimetres, calculated in accordance with subparagraph 9.25.6.4 or 9.25.6.5 as appropriate;
        • R = pitch circle radius of bolts, in millimetres;
        • UTS _stock = ultimate tensile strength of stock material, in megapascals (MPa); and
        • UTS _bolt = ultimate tensile strength of bolt material, in megapascals (MPa).
    • 9.25.6.8.2.2Rudder coupling bolts shall be machine finished, neat fitting and the nuts shall be locked to prevent any possibility of backing off while in service. Rudder coupling bolts need not be neat fitting on small rudders not being spade rudders and having a lower stock diameter of less than 75mm, provided a key of dimensions complying with paragraph 9.15 is incorporated into the flange coupling.
  • 9.25.6.8.3Tapered couplings

    The dimensions of tapers and taper retaining nuts for tapered couplings shall be in accordance with the requirements for shafting given in regulation 9.14.11 and 9.16.4, except that a taper as steep as one in 8 may by used. Keys for taper couplings shall comply with the relevant requirements of regulation 9.15 and shall be sized on the required upper stock diameter. The boss thickness of flange couplings fitted on a taper shall not be less than 1.5 times the required thickness of the key, and the boss length shall not be less than 1.6 times the required diameter of the rudder stock in way of the coupling.

• 9.25.7.1Single plate rudders Refer to Figure 15.
  • 9.25.7.1.1Plate thickness

    The minimum plate thickness for single plate rudders shall be the greater of those calculated in accordance with the following 2 formulas—

    

    Where—

    • t = thickness of plating, in millimetres;
    • V = maximum service speed, in knots, that the ship is designed to maintain in a fully loaded condition;
    • h = vertical distance between the centres of stiffeners, in millimetres; and
    • UTS _plate = ultimate tensile strength of plating material, in megapascals (MPa).
  • 9.25.7.1.2Distance between stiffeners

    The distance between centres of single plate rudder stiffeners shall not exceed 1000mm.

  • 9.25.7.1.3Section modulus of stiffeners

    The section modulus of the stiffeners immediately forward and aft of the stock shall not be less than that determined from the following formula—

    

    Where—

    • Z = section modulus of stiffeners, in cubic metres (cm³);
    • V = maximum service speed, in knots, that the ship is designed to maintain in a fully loaded condition;
    • l = horizontal distance from the aft edge of the rudder to the centre of the rudder stock, in metres;
    • h = distance between centres of stiffeners, in millimetres;
    • UTS _stiffr = ultimate tensile strength of stiffener material, in megapascals (MPa).
  • 9.25.7.1.4Tapering of stiffeners

    The width of the stiffeners may be tapered forward and aft of the maximum widths required to satisfy the above section modulus. The minimum stiffener section modulus at the leading and trailing edges of the rudder shall not be less than that determined from the following formula—

    

    Where—

    • Z _t = section modulus stiffeners at the leading and trailing edges of the rudder, in cubic centimetres (cm³);
    • Z= section modulus of stiffeners immediately forward and aft of the stock, in cubic centimetres (cm³) (see subparagraph 9.25.7.1.3); and
    • UTS _stiffr = ultimate tensile strength of stiffener material, in megapascals (MPa).
  • 9.25.7.1.5Attachment

    The blade of a single plate rudder shall be attached to the mainpiece by a full penetration continuous weld. Stiffeners shall be attached to the mainpiece and blade by a double continuous fillet weld.

• 9.25.7.2Double plate rudders

  Refer to Figure 16

  • 9.25.7.2.1Arrangement and testing

    Double plate rudders shall have horizontal internal webs. They shall be watertight and tested to a head of water of 2.5m or equivalent. A means for draining shall be incorporated in the rudder.

  • 9.25.7.2.2Plating thickness-equivalent carbon steel rudder upper stock diameter less than 75mm
    • 9.25.7.2.2.1The thickness of carbon steel plating for a double plate rudder having a required equivalent carbon steel rudder upper stock less than 75mm diameter shall be as specified in Table 8. Horizontal and vertical webs in double plate rudders not replacing the mainpiece shall have the same thickness as the side plates. Plates forming the top and bottom of the rudders shall not be less than the thickness given in Table 8 for webs spaced at 600mm.

      NOTE:

      The equivalent carbon steel rudder upper stock diameter is determined by the following formula—

      Where—

      • d _ue = equivalent carbon steel upper stock diameter, in millimetres;
      • d _u = required upper stock diameter for the actual stock material, calculated in accordance with subparagraph 9.25.6.4.2 or 9.25.6.5.1, in millimetres; and
      • UTS _stock = ultimate tensile strength of rudder stock material, in megapascals (MPa).

      Table 8 — Carbon steel plate thickness for rudders — equivalent carbon steel rudder stock less than 75mm diameter

      Required equivalent carbon steel diameter of upper stock, in millimetres, calculated in accordance with subparagraph 9.25.6.4.2 or 9.25.6.5.1 as appropriate	Carbon steel plate thickness (mm)
      	Webs spaced 300mm or less	Webs spaced 450mm	Webs spaced 600mm
      Less than 40	4.5	4.5	6.5
      40 and over but less than 45	4.5	6.5	6.5
      45 and over but less than 60	4.5	6.5	8.0
      60 and over but less than 65	6.5	6.5	8.0
      65 and over but less than 75	6.5	8.0	9.5

    • 9.25.7.2.2.2For plating material other than carbon steel, the required thickness of plating shall be determined by multiplying the tabular value by—

      

      Where—

      • UTS _plate = ultimate tensile strength of the plating material, in megapascals (MPa).
  • 9.25.7.2.3Plating thickness-equivalent carbon steel rudder upper stock diameter 75mm and over;
    • 9.25.7.2.3.1Where the required equivalent carbon steel rudder upper stock diameter is 75mm or over (see note to subparagraph 9.25.7.2.2), the thickness of the rudder side plating and webs shall not be less than that determined as follows—
    • 9.25.7.2.3.2The thickness of rudder side plating and webs t_p shall be determined from a reference thickness t adjusted for the variation between the actual spacing of web centres and a reference spacing of web centres S_p.
    • 9.25.7.2.3.3The reference thickness shall be determined from the following formula—

      

      Where—

      • t _r = reference plate thickness, in millimetres;
      • V = speed of ship in knots with—
      • (a)a minimum of 8 knots in ships less than 30m in measured length; and
      • (b)a minimum of 9 knots in ships of 30m in measured length and over;
      • A = total area of rudder, in square metres (m²); and
      • UTS _plate = ultimate tensile strength of plating material, in megapascals (MPa).
    • 9.25.7.2.3.4The thickness of the rudder side plating tp shall be determined from the following formula—

      

      Where—

      • t _p = required minimum thickness of rudder side plating, in millimetres;
      • t _r = reference plate thickness, in millimetres;
      • UTS _plate = ultimate tensile strength of plating material, in megapascals (MPa);
      • S _a = actual spacing of web centres, in millimetres;
      • S _p = reference spacing of web centres, in millimetres calculated in accordance with the following formula—

      

      Where—

      • V = speed of ship in knots with—
        • (a)a minimum of 8 knots in ships less than 30m in measured length; and
        • (b)a minimum of 9 knots in ships of 30m in measured length and over
      • A = total area of rudder, in square metres (m²).
    • 9.25.7.2.3.5The minimum thickness of plates forming the top and bottom of the rudder shall be the greater of—
      • (a)the thickness of the rudder side plating t_p, calculated for the actual spacing of web centres; and
      • (b)the reference thickness t_r.
  • 9.25.7.2.4Attachment of stiffeners

    Horizontal and vertical webs in double plate rudders shall be attached to the main-piece by continuous double fillet welds and to the plating by fillet welds consisting of 75mm lengths, spaced 150mm between their centres. Where the interior of the rudder is inaccessible for welding, the stiffeners shall be fitted with flat bars and the plating connected to these flat bars by continuous or slot welds.

• 9.25.7.3Spade Rudders

  Acceptable forms of spade rudder are as follows—

  • 9.25.7.3.1Fabricated or cast rudder blade with integral flange secured to a rudder stock with flange by fitted bolts. (With this type of rudder the sizes of the couplings and bolts are to be based on the required diameter of the lower rudder stock, but due regard shall also be given to the bending and tensile stresses to which they maybe subject arising from the forces on the rudder).
  • 9.25.7.3.2Fabricated or cast rudder blade attached to rudder stock by means of a taper with key and securing nut. (The length of the taper is not to be less than 1.5 times the required diameter on the lower rudder stock. The taper is to be between the limits of one in 12 and one in 16 on diameter, but should preferably be in one in 12).
  • 9.25.7.3.3Fabricated rudder blade integral with rudder stock.
  • 9.25.7.3.4Fabricated or cast rudder blade shrunk on to a parallel rudder stock and additionally secured with dowells.
  • 9.25.7.3.5Rudders with the blade cast on to the rudderstock are subject to the approval of the Chief Executive Officer. The approval will be dependent upon the design and construction method and may also be dependent on the result of proof load testing.
  • 9.25.7.3.6Cast rudder blades shall be of ductile material.
  • 9.25.7.3.7The pressure on the rudder bearings should, in general, not exceed 3.9 MPa. For bearings with efficient lubrication a pressure of 5.9 MPa may be accepted.
    • 9.25.7.3.7.1For the determination of the pressure on the bearings the rudder force may be calculated from the following formula—

      F _p =196AV ²

      Where—

      • A = area of rudder in m²;
      • V = speed of ship in knots; and
      • F _p = rudder force in N
• 9.25.7.4Fibre-reinforced plastic (FRP) rudders

  [Refer to Figure 14]

  • 9.25.7.4.1Construction

    FRP rudders shall incorporate a substantial spider, formed by plate arms approximately half the rudder width in length and welded to the rudder mainpiece. The spider arms shall be perforated or otherwise arranged to facilitate a rigid connection between the mainpiece and the FRP blade. The mainpiece should be continuous through the rudder wherever possible or alternative arrangements should be made to ensure continuity of strength and alignment. The blade shall be manufactured from reinforced epoxy or polyester resins. The rudder should be filled with a suitable material such as a resin/glass dough, timber or a micro-balloon mixture.

    NOTE:

    The formulae given in subparagraph 9.25.7.2 for double-plate rudders are not applicable to FRP rudders constructed with a spider and solid core. Typically the skins of FRP rudders have a minimum mass of reinforcement of 3000 g/m². Lighter laminates down to 2300 g/ m² may suffice on small sailing ships or where advanced composite materials are used.

• 9.25.7.5Wooden rudders

  The Chief Executive Officer may consider the use of other materials for the construction of rudders and special consideration is given for the construction and scantlings of such rudders.

Refer to Figure 18

• 9.25.8.1Testing

  Steering nozzles shall be watertight and tested to a head of water of 2.5m or equivalent. A means for draining shall be incorporated in the nozzle.

• 9.25.8.2Shroud plating in way of propeller blade tips

  The shroud plating in way of the propeller blade tips shall extend forward and aft of this position for a distance appropriate for the limits of rotation of the nozzle. Shroud plating may be carbon or stainless steel. The thickness of this shroud plating shall be determined from the following formulae—

  • (a)If P × D is less than or equal to 6300—
  • (b)If P × D is greater than 6300—

    Where—

    • t _s = thickness of shroud plating in way of propeller tips, in millimetres;
    • P = power transmitted to the propeller, in kilowatts;
    • D = propeller diameter, in metres; and
    • UTS _S = ultimate tensile strength (UTS) of the shroud plating in way of propeller tips, in megapascals (MPa).

    Figure 18 — Steering nozzles details and dimension

• 9.25.8.3Flare plating, cone plating and shroud plating clear of propeller tips

  The thickness of flare plating, cone plating and shroud plating clear of propeller blade tips (see Figure 18), which shall not be less than—

  

  shall be determined in accordance with the following formula—

  

  Where—

  • t _p = thickness of shroud plating clear of propeller tips, flare and cone plating, in millimetres;
  • t _s = required thickness of shroud plating in way of the propeller tips, in millimetres;
  • UTS _p = ultimate tensile strength of the shroud plating clear of propeller tips (assumed to be the same material as the web plating), in megapascals (MPa); and
  • UTS _S = ultimate tensile strength of the shroud plating in way of propeller tips, in megapascals (MPa).
• 9.25.8.4Fore and aft webs

  Fore and aft webs, which shall not be less than the thickness of shroud plating clear of propeller tips, shall be fitted between the inner and outer skins of the nozzle. Fore and aft webs in way of the head box and pintle support structure shall have their thickness increased in accordance with the following formula—

  

  Where—

  • t _w = thickness of webs in way of head box and pintle support, in millimetres;
  • t _p = thickness of shroud plating other than that in way of the propeller tips, in millimetres; and
  • UTS _p = ultimate tensile strength, in megapascals (MPa), of the shroud plating clear of propeller tips (assumed to be the same material as the web plating).
• 9.25.8.5Ring webs
  • 9.25.8.5.1Ring webs, which shall not be less than the thickness of the shroud plating clear of the propeller blade tips, shall be fitted to maintain the transverse strength of the nozzle. A minimum of 2 such webs should be fitted.
  • 9.25.8.5.2The thickness of ring webs in way of the head box and pintle support shall be increased in accordance with the formula given in subparagraph 9.25.8.4, and this increased thickness shall be maintained to the adjacent fore and aft web.
• 9.25.8.6Leading and trailing edge members

  The wall thickness of leading and trailing edge members shall not be less than the required thickness of shroud plating clear of propeller blade tips.

• 9.25.8.7Stiffening
  • 9.25.8.7.1Local stiffening shall be fitted in way of the top and bottom supports, which shall in turn be integrated with the fore and aft webs and the ring webs.
  • 9.25.8.7.2Continuity of bending strength shall be maintained in areas where stiffening is fitted.
• 9.25.8.8Fins

  Fabricated fins should be adequately reinforced. The plating thickness of double plate fins should not be less than that of the plating clear of propeller tips.

• 9.25.8.9Nozzle stock, heel pintle etc
  • 9.25.8.9.1Scantlings
    • 9.25.8.9.1.1The diameter of the upper and lower nozzle stock shall be calculated in accordance with subparagraph 9.25.6.5 for balanced rudders, assuming the steering nozzle has the geometric properties given in subparagraph 9.25.8.9.2 to 9.25.8.9.4 below (refer to Figure 18).
    • 9.25.8.9.1.2Scantlings for the heel pintle, keys, coupling bolts etc shall be determined from the required stock diameter as per the relevant paragraphs for rudders.
  • 9.25.8.9.2Equivalent area

    The equivalent area of the nozzle and fin shall be determined from the following formula—

    A = 2D_n l_n + 0.85 h_f l_f

    Where—

    • A = equivalent area of nozzle and fin, in square metres (m²);
    • L _n = nozzle length, in metres;
    • D _n = inner diameter of the nozzle, in metres;
    • h _f = mean height of fin, in metres; and
    • l _f = length of fin, in metres.
  • 9.25.8.9.3Equivalent horizontal lever arm

    The equivalent horizontal lever arm b shall be calculated as the greater of the absolute values of the following 2 formula—

    b=	lfhf(lx = 0.21f – 1.5 Dnlnla	or b =	lfhf(lx + lf) + 1.5 Dnlnlh	[	0.45V + 2	]	2
    	A		A		V + 2

    Where—

    • b = equivalent horizontal distance from centre of lower stock to the centre of area of total rudder area, in metres;
    • A = equivalent area of nozzle and fin, in square metres (m²);
    • D _n = inner diameter of the nozzle, in metres;
    • l _n = nozzle length, in metres;
    • l _a = distance from nozzle leading edge to stock axis, in metres;
    • l _b = distance from nozzle trailing edge to stock axis, in metres;
    • l _x = distance between stock axis and fin, in metres;
    • h _f = mean height of fin, in metres;
    • l _f = length of fin, in metres; and
    • V = speed of ship in knots with—
      • (a)a minimum of 8 knots in ships less than 30m in measured length; and
      • (b)a minimum of 9 knots in ships of 30m in measured length and over.

    NOTE:

    Refer to Figure 18 for details of dimensions.

  • 9.25.8.9.4Equivalent vertical lever arm

    The equivalent vertical lever arm a, in metres, shall be the vertical distance from the nozzle axis to the bottom of the nozzle stock neck bearing.

• 9.25.9.1Section modulus of tiller arms or quadrant clear of boss
  • 9.25.9.1.1The section modulus of a tiller arm just clear of the boss, or quadrant just clear of the boss, shall not be less than that determined from the following formula—

    Where—

    • Z = required section modulus of quadrant or tiller about the vertical axis, in cubic centimetres (cm³);
    • d _u = required diameter of the upper rudder stock, in millimetres, calculated in accordance with subparagraph 9.25.6.4.2 or 9.25.6.5.1 as appropriate;
    • a = distance from the point of application of the steering force on the tiller or quadrant to the centre of the rudder stock, in millimetres;
    • b = distance between the section of tiller or quadrant just clear of the boss and centre of the rudder stock, in millimetres;
    • UTS _stock = ultimate tensile strength of stock material, in megapascals (MPa); and
    • UTS _Arm = ultimate tensile strength of tiller arm or quadrant material, in megapascals (MPa).
  • 9.25.9.1.2The section modulus of tiller arm or quadrant just clear of the boss about the horizontal axis shall not be less than one-third times the required value of Z determined above.
• 9.25.9.2Section modulus at point of application of load

  The section modulus at the point of application of the load shall not be less than one-third times the required value of Z calculated in subparagraph 9.25.9.1.1.

• 9.25.9.3Thickness of tiller boss or quadrant boss

  The thickness of the tiller boss or quadrant boss should not be less than 0.4 times the required upper rudder stock diameter. The depth of the boss shall not be less than the key length determined in accordance with subparagraph 9.25.9.4.

• 9.25.9.4Securing of tiller or quadrant boss on the rudderstock

  The tiller or quadrant boss shall be securely affixed to the rudderstock by means of a key or other equivalent means. Where a key is fitted, the size of the key shall be determined in accordance with subparagraph 9.15 using the required diameter of the upper stock du in place of the shaft diameter d.

  NOTE:

  Methods similar to those applied to attaching shaft couplings to shafting may provide a suitable means for securing the tiller or quadrant boss to the rudder stock. See subparagraph 9.16.2.