Section 4 Design and construction

4.1.1 As an alternative to the following requirements , a fatigue strength analysis of components can be submitted indicating a factor of safety of 1,5 at the design loads, based on a suitable fatigue failure criteria The effects of stress concentrations, material properties and operating environment are to be taken into account.

4.2.1 The diameter, d, of the intermediate shaft is to be not less than:

4.2.2 Beyond a length of 0,2d from the end of a keyway, transverse hole or radial hole and 0,3d from the end of a longitudinal slot, the diameter of the shaft may be gradually reduced to that determined with k = 1,0.

4.2.3 For shafts with design features other than stated as above, the value of k will be specially considered.

4.2.4 The Rule diameter of the intermediate shaft for diesel engines, turbines and electric propelling motors may be reduced by 3,5 per cent for craft classed G1 (Service Group 1), see Pt 1, Ch 2, 3.5 Service area restriction notations.

4.2.5 For shrink fit couplings, k refers to the plain shaft section only. Where shafts may experience vibratory stresses close to the permissible stresses for continuous operation, an increase in diameter to the shrink fit diameter is to be provided, e.g. a diameter increase of 1 to 2 per cent and a blending radius as described in Pt 11, Ch 2, 4.8 Couplings and transitions of diameters.

4.2.6 Keyways are in general not to be used in installations with a barred speed range.

4.2.7 The application of k = 1,20 is limited to shafts with longitudinal slots having a length of less than 0,8d _o and a width greater than 0,15d _o and a diameter of central hole d _i of less than 0,7d _o, see Pt 11, Ch 2, 4.5 Hollow shafts. The end rounding of the slot is not to be less than half the width. An edge rounding should preferably be avoided as this increases the stress concentration slightly. The values of C _K, see Table 1.3.1 C k factors in Pt 13, Ch 1 Torsional Vibration, are valid for 1, 2 and 3 slots, i.e. with slots at 360, 180 and 120 degrees apart respectively.

4.2.8 Where the intermediate shaft is constructed of the same material as the screwshaft and of a material listed in Table 2.4.1 `A’ Value for use in unprotected screwshaft formula , the diameter of the intermediate shaft is not required to be greater than that of the screwshaft.

4.3.1 The diameter at the collars of the thrust shaft transmitting torque or in way of the axial bearing where a roller bearing is used as a thrust bearing is to be not less than that required for the intermediate shaft in accordance with Pt 11, Ch 2, 4.2 Intermediate shafts with a k value of 1,10. Beyond a length equal to the thrust shaft diameter from the collars, the diameter may be tapered down to that required for the intermediate shaft with a k value of 1,0. For the purpose of the foregoing calculations, σ_u is to be taken as the minimum tensile strength of the thrust shaft material, in N/mm².

4.4.1 Screwshafts and tube shafts, (i.e. the shaft which passes through the sterntube, but does not carry the propeller), made from carbon manganese steel are to be protected by a continuous bronze liner, where exposed to sea water. Alternatively, the liner may be omitted provided the shaft is arranged to run in an oil lubricated bush with an approved oil sealing gland at the after end. Lengths of shafting between sterntubes and brackets, which are readily visible when the craft is slipped, may be protected by coatings of an approved type.

4.4.2 Means for the protection of screwshafts and tubeshafts are not required when the shafts are made of corrosion resistant material.

4.4.3 The diameter, d _p of the protected forged steel screwshaft immediately forward of the forward face of the propeller boss or, if applicable, the forward face of the screwshaft flange, is to be not less than:

P and R are as defined in Pt 9 General Requirements for Machinery (losses in gearboxes and bearings are to be disregarded)

4.4.4 The diameter, d _p of the screwshaft determined in accordance with Pt 11, Ch 2, 4.4 Screwshafts and tube shafts 4.4.3 is to extend over a length not less than that to the forward edge of the bearing immediately forward of the propeller or 2,5d _p whichever is the greater.

4.4.5 The diameter of the portion of the screwshaft and tube shaft forward of the length required by Pt 11, Ch 2, 4.4 Screwshafts and tube shafts 4.4.4 to the forward end of the forward sterntube seal is to be determined in accordance with Pt 11, Ch 2, 4.4 Screwshafts and tube shafts 4.4.3 with a k value of 1,15. The change of diameter from that determined with k = 1,22 or 1,26 to that determined with k = 1,15 should be gradual, see Pt 11, Ch 2, 4.8 Couplings and transitions of diameters

4.4.6 Screwshafts which run in sterntubes and tube shafts may have the diameter forward of the forward sterntube seal gradually reduced to the diameter of the intermediate shaft. Abrupt changes in shaft section at the screwshaft/tube shaft to intermediate shaft couplings are to be avoided, see Pt 11, Ch 2, 4.8 Couplings and transitions of diameters.

4.4.7 The diameter of unprotected screwshafts and tube shafts of materials as shown in Table 2.4.1 `A’ Value for use in unprotected screwshaft formula is to be not less than:

where `A' is taken from Table 2.4.1 `A’ Value for use in unprotected screwshaft formula and P and R are as defined in Pt 9 General Requirements for Machinery.

4.4.8 The diameter of unprotected screwshafts of materials as shown in Table 2.4.1 `A’ Value for use in unprotected screwshaft formula forward of the forward sterntube seal is to be determined in accordance with Pt 11, Ch 2, 4.4 Screwshafts and tube shafts 4.4.7 or Pt 11, Ch 2, 4.4 Screwshafts and tube shafts 4.4.3, whichever is less.

4.5.1 Where the thrust, intermediate, tube shafts and screwshafts have central holes having a diameter greater than 0,4 times the outside diameter, the equivalent diameter, d _e, of a solid shaft is not to be less than the Rule size, d, (of a solid shaft), where d _e is given by:

4.5.2 Where the diameter of the central hole does not exceed 0,4 times the outside diameter, the diameter is to be calculated in accordance with the appropriate requirements for a solid shaft.

4.6.1 Cardan shafts, used in installations having more than one propulsion shaftline, are to be of an approved design, suitable for the designed operating conditions including short term high power operation. Consideration will be given to accepting the use of approved cardan shafts in single propulsion unit applications if a complete spare interchangeable end joint is provided on board.

4.6.2 Cardan shaft ends are to be contained within substantial tubular guards that also permit ready access for inspection and maintenance.

4.7.1 Close tolerance fitted bolts transmitting shear are to have a diameter, d _b, at the flange joining faces of the couplings not less than:

P and R are defined in Pt 9 General Requirements for Machinery.

4.7.2 At the joining faces of couplings, other than within the crankshaft and at the thrust shaft/crankshaft coupling, the Rule diameter of the coupling bolts may be reduced by 5,2 per cent for craft classed exclusively for smooth water service.

4.7.3 Where dowels or expansion bolts are fitted to transmit torque in shear they are to comply with the requirements of Pt 11, Ch 2, 4.7 Coupling bolts 4.7.1 . The expansion bolts are to be installed, and the bolt holes in the flanges are to be correctly aligned in accordance with manufacturer's instructions.

4.7.4 The minimum diameter of tap bolts or of bolts in clearance holes at the joining faces of coupling flanges, pretensioned to 70 per cent of the bolt material yield strength value, is not to be less than:

where d _R is taken as the lesser of:

1. Mean of effective (pitch) and minor diameters of the threads.

2. Bolt shank diameter away from threads. (Not for waisted bolts which will be specially considered.)

P and R are defined in Pt 9 General Requirements for Machinery.

4.7.5 Consideration will be given to those arrangements where the bolts are pretensioned to loads other than 70 per cent of the material yield strength.

4.7.6 Where clamp bolts are fitted they are to comply with the requirements of Pt 11, Ch 2, 4.7 Coupling bolts 4.7.4 and are to be installed, and the bolt holes in the flanges correctly aligned, in accordance with manufacturer's instructions.

4.8.1 The minimum thicknesses of the coupling flanges are to be equal to the diameters of the coupling bolts at the face of the couplings as required by Pt 11, Ch 2, 4.7 Coupling bolts 4.7.1, and for this purpose the minimum tensile strength of the bolts is to be taken as equivalent to that of the shafts. For intermediate, thrust shafts, and the inboard end of the screwshaft, the thickness of the coupling flange is in no case to be less than 0,20 of the diameter of the intermediate shaft as required by Pt 11, Ch 2, 4.2 Intermediate shafts.

4.8.2 The fillet radius at the base of the coupling flange, integral with the shaft, is to be not less than 0,08 of the diameter of the shaft at the coupling. The fillets are to have a smooth finish and are not to be recessed in way of nut and bolt heads.

4.8.3 Where the propeller is attached by means of a flange, the thickness of the flange is to be not less than 0,25 of the actual diameter of the adjacent part of the screwshaft. The fillet radius at the base of the coupling flange is to be not less than 0,125 of the diameter of the shaft at the coupling.

4.8.4 All couplings which transmit torque are to be of approved dimensions.

4.8.5 Where couplings are separate from the shafts, provision is to be made to resist the astern pull.

4.8.6 Where a coupling is shrunk on to the parallel portion of a shaft or is mounted on a slight taper, e.g. by means of the oil pressure injection method, the assembly is to meet the requirements of Pt 11, Ch 2, 4.11 Interference fit assemblies.

4.8.7 Transitions of diameters are to be designed with either a smooth taper or a blending radius. In general, a blending radius equal to the change in diameter is recommended.

4.9.1 The contact stress, S _c, at the flanks of mating teeth of a gear coupling is not to exceed that given in Table 2.4.2 Allowable S _c values , where

P and R are defined in Pt 9 General Requirements for Machinery.

4.9.2 Where experience has shown that under similar operating and alignment conditions, a higher tooth loading can be accommodated full details are to be submitted for consideration.

4.10.1 Details of flexible couplings are to be submitted together with the manufacturer's rating capacity, for the designed operating conditions including short term high power operation. Verification of coupling characteristics will be required.

4.10.2 In determining the allowable mean, maximum and vibratory torque ratings consideration of the mechanical properties of the selected elastic element type in compression, shear and fatigue loading together with heat absorption/generation is to be given.

4.10.3 In determining the allowable torque ratings of the steel spring couplings, consideration of the material mechanical properties to withstand fatigue loading and overheating is to be given.

4.11.1 The interference fit assembly is to have a capacity to transmit a torque of S.T _max without slippage.

Where C is to be taken from Table 2.4.3 `C' values for guidance purposes

4.11.2 The effect of any axial load acting on the assembly is to be considered.

4.11.3 The resulting equivalent von Mises stress in the assembly is not to be greater than the yield strength of the component material.

4.11.4 Reference marks are to be provided on the adjacent surfaces of parts secured by shrinkage alone.

4.12.1 Round ended or sled-runner ended keys are to be used, and the keyways in the propeller boss and cone of the screwshaft are to be provided with a smooth fillet at the bottom of the keyways. The radius of the fillet is to be at least 0,0125 of the diameter of the screwshaft at the top of the cone. The sharp edges at the top of the keyways are to be removed.

4.12.2 Two screwed pins are to be provided for securing the key in the keyway, and the forward pin is to be placed at least one-third of the length of the key from the end. The depth of the tapped holes for the screwed pins is not to exceed the pin diameter, and the edges of the holes are to be slightly bevelled. The omission of pins for keys for small diameter shafts will be specially considered.

4.12.3 The distance between the top of the cone and the forward end of the keyway is to be not less than 0,2 of the diameter of the screwshaft at the top of the cone.

4.12.4 The effective sectional area of the key in shear, is to be not less than:

4.12.5 The effective area in crushing of key, shaft or boss is to be not less than:

4.13.1 Round ended keys are to be used and the keyways are to be provided with a smooth fillet at the bottom of the keyways. The radius of the fillet is to be at least 0,0125 of the diameter of the shaft at the coupling. The sharp edges at the top of the keyways are to be removed.

4.13.2 The effective area of the key in shear, A, is to be not less than:

Alternatively, consideration will be given to keys conforming to the design requirements of a recognised National Standard.

4.14.1 Liners may be bronze, gunmetal, stainless steel or other approved alloy.

4.14.2 The thickness, t, of liners fitted on screwshafts or on tube shafts, in way of the bushes, is to be not less, when new, than given by the following formula:

4.14.3 The thickness of a continuous liner between the bushes is to be not less than 0,75t.

4.14.4 Continuous liners are to be fabricated or cast in one piece.

4.14.5 Where liners consist of two or more lengths, these are to be butt welded together. In general, the lead content of the gunmetal of each length forming a butt welded liner is not to exceed 0,5 per cent. The composition of the electrodes or filler rods is to be substantially lead-free.

4.14.6 The circumferential butt welds are to be of multi-run, full penetration type. Provision is to be made for contraction of the weld by arranging for a suitable length of the liner containing the weld, if possible about three times the shaft diameter, to be free of the shaft. To prevent damage to the surface of the shaft during welding, a strip of heat resisting material covered by a copper strip should be inserted between the shaft and the liner in way of the joint. Other methods for welding this joint may be accepted if approved. The welding is to be carried out by an approved method and to the Surveyor's satisfaction.

4.14.7 Each continuous liner or length of liner is to be tested by hydraulic pressure to 2,0 bar after rough machining.

4.14.8 Liners are to be carefully shrunk onto the shafts by hydraulic pressure. Pins are not to be used to secure the liners.

4.14.9 Effective means are to be provided for preventing water from reaching the shaft at the part between the after end of the liner and the propeller boss.

4.15.1 Long unsupported lengths of shafting are to be avoided by the fitting of steady bearings at suitable positions, see Pt 13 Shaft Vibration and Alignment.

4.16.1 Where the sterntube or sternbushes are to be installed using a resin, of an approved type, the following requirements are to be met:

1. Pouring and venting holes are to be provided at opposite ends with the vent hole at the highest point.

2. The minimum radial gap occupied by the resin is to be not less than 6 mm at any one point with a nominal resin thickness of 12 mm.

3. In the case of oil lubricated sterntube bearings, the arrangement of the oil grooves is to be such as to promote a positive circulation of oil in the bearing.

4. Provision is to be made for the remote measurement of the temperature at the aft end of the aft bearing, with indication and alarms at the control stations.

4.16.2 The length of the bearing in the sternbush next to and supporting the propeller is to be as follows:

1. For water lubricated bearings which are lined with lignum vitae, rubber composition or staves of synthetic material, the length is to be not less than 4,0 times the rule diameter of the shaft in way of the bearing.

2. For water lubricated bearings lined with two or more circumferentially spaced sectors, of synthetic material, without axial grooves in the lower half, the length of the bearing is to be such that the nominal bearing pressure will not exceed 0,55 MPa. The length of the bearing is to be not less than 2,0 times the rule diameter of the shaft in way of the bearing.

3. For oil lubricated bearings of synthetic material, the length of the bearing is, in general, to be not less than 2,0 times the rule diameter of the shaft in way of the bearing. The nominal bearing pressure is not to exceed the maximum for which the synthetic material has been approved.

4. For bearings which are white-metal lined, oil lubricated and provided with an approved type of oil sealing gland, the length of the bearing is to be approximately 2,0 times the rule diameter of the shaft in way of the bearing and is to be such that the nominal bearing pressure will not exceed 0,8 MPa. The length of the bearing is to be not less than 1,5 times its diameter.

5. For bearings of cast iron and bronze which are oil lubricated and fitted with an approved oil sealing gland, the length of the bearing is, in general, to be not less than 4,0 times the rule diameter of the shaft in way of the bearing.

6. For bearings which are grease lubricated, the length of the bearing is to be not less than 4,0 times the rule diameter of the shaft in way of the bearing. Other lengths may be considered upon application, subject to the provision of suitable supporting in-service or testing evidence at relevant shaft pressures and velocities.

4.16.3 Synthetic materials for application as stern tube bearings are to be approved in accordance with Rules for the Manufacture, Testing and Certification of Materials, July 2022, Ch 14, 2.13 Sterntube bearings.

4.16.4 Sternbushes are to be adequately secured in housings.

4.16.5 Forced water lubrication is to be provided for all bearings lined with rubber or synthetic material. The supply of water may come from a circulating pump or other pressure source. Flow indicators are to be provided for the water service to the bearings. The water grooves in the bearings are to be of ample section and of a shape which will be little affected by weardown, particularly for bearings of the synthetic material.

4.16.6 The shut-off valve or cock controlling the supply of water is to be fitted directly to the after peak bulkhead, or to the sterntube where the water supply enters the sterntube forward of the bulkhead.

4.16.7 Oil sealing glands must be capable of accommodating the effects of differential expansion between hull and line of shafting for all sea temperatures in the proposed area of operation. This requirement applies particularly to those glands which span the gap and maintain oiltightness between the sterntube and the propeller boss.

4.16.8 Where a tank supplying lubricating oil to the sternbush is fitted, it is to be located above the load waterline and is to be provided with a low level alarm device in the machinery space, see also Pt 11, Ch 2, 5.1 Unattended machinery 5.1.1.

4.16.9 For oil lubricated bearings of synthetic material, the flow of lubricant is to be such that overheating, under normal operating conditions, cannot occur.

4.16.10 Where sternbush bearings are oil lubricated, provision is to be made for cooling the oil by maintaining water in the after peak tank above the level of the sterntube or by other approved means.

4.16.11 Means for ascertaining the temperature of the sternbush bearings are to be provided, e.g. monitoring of the temperature of the oil in the sterntube.

4.16.12 Where an *IWS (In-Water Survey) notation is to be assigned, for water lubricated bearings, means are to be provided for ascertaining the clearance in the sternbush with the craft afloat.

4.17.1 For the requirements for torsional, axial and lateral vibration, and for alignment of the shafting, see Pt 13 Shaft Vibration and Alignment.