Section 4 Structure design and construction requirements

4.1.1 Podded unit struts and pod bodies may be of cast, forged or fabricated construction or a combination of these construction methods.

4.1.2 Means are to be provided to enable the propeller shaft, bearings and seal arrangements to be examined in accordance with LR's requirements and the manufacturer's recommendations.

4.1.3 When high tensile steel fasteners are used as part of the structural arrangement and there is a risk that these fasteners may come into contact with sea-water, carbon-manganese and low alloy steels with a specified tensile strength of greater than 950 N/mm² are not to be used due to the risk of hydrogen embrittlement.

4.1.4 For steerable pod units, an integral slewing ring is to be arranged at the upper extremity of the strut to provide support for the slewing bearing.

4.1.5 The strut is to have a smooth transition from the upper mounting to the lower hydrodynamic sections.

4.1.6 For fabricated structures, vertical and horizontal plate diaphragms are to be arranged within the strut and, where necessary, secondary stiffening members are to be arranged.

4.1.7 Pod unit structure scantling requirements are shown in Table 4.4.1 Podded propulsion unit - fabricated structure requirements. Where the scantling requirements in Table 4.4.1 Podded propulsion unit - fabricated structure requirements cannot be satisfied, direct calculations carried out in accordance with Vol 2, Pt 4, Ch 4, 4.3 Direct calculations may be considered.

4.1.8 The connection between the strut and the pod body should generally be effected through large radiused fillets in cast pod units or curved plates in fabricated pod units.

4.1.9 The structural response under the most onerous combination of loads is not to exceed the normal operational requirements of the propulsion or steering system components.

4.1.10 For cast pod structures, the elongation of the material on a gauge length of 5.65 is to be not less than 12 per cent where S _o is the actual cross sectional area of the test piece.

4.1.11 In castings, sudden changes of section or possible constriction to the flow of metal during casting are to be avoided. All fillets are to have adequate radii which should, in general, be not less than 75 mm.

4.1.12 Castings are to comply with the requirements of Ch 4 Steel Castings or Ch 7 Iron Castings of the Rules for Materials.

4.2.1 For supporting the main slewing bearing outer faces, a system of primary structural members is to be provided in order to transfer the maximum design loads and moments from the podded propulsion unit into the ship’s hull without undue deflection. Due account is also to be taken of the loads induced by the maximum ship’s motions in the vertical direction resulting from combined heave and pitch motion of the ship. Account is also to be taken of any manoeuvring conditions that are likely to give rise to high mean or vibratory loadings induced by the podded propulsion unit. See Vol 2, Pt 4, Ch 4, 2.2 Documentation required for design review 2.2.1.(d).

4.2.2 The hull support structure in way of the slewing bearing should be sufficiently stiff that the bearing manufacturer’s limits on seating flatness are not exceeded due to hull flexure as a consequence of the loads defined under Vol 2, Pt 4, Ch 4, 4.2 Hull support structure 4.2.1.

4.2.3 Generally, the system of primary members is to comprise a pedestal girder directly supporting the slewing ring and bearing. The pedestal girder is to be integrated with the ship’s structure by means of radial girders and transverses aligned at their outer ends with the ship’s bottom girders and transverses, see Figure 4.4.1 Hull support structure. Proposals to use alternative arrangements that provide an equivalent degree of strength and rigidity may be submitted for appraisal.

4.2.4 The ship’s support structure in way of the podded unit may be of double or single bottom construction. Generally, podded drives should be supported where practical within a double bottom structure; however final acceptance of the supporting arrangements will be dependent upon satisfying the stress criteria set out in Table 4.4.2 Direct calculation maximum permissible stresses for steel fabricated structures, see also Vol 2, Pt 4, Ch 4, 4.3 Direct calculations 4.3.5.

4.2.5 The shell envelope plating and tank top plating in way of the aperture for the podded drive (i.e. over the extent of the radial girders shown in Figure 4.4.1 Hull support structure) is to be increased by 50 per cent over the Rule minimum thickness to provide additional local stiffness and robustness. However, the thickness of this plating is not to be less than the actual fitted thickness of the surrounding shell or tank top plating.

Figure 4.4.1 Hull support structure

4.2.6 The scantlings of the primary support structure in way of the podded drive are to be based upon the limiting design stress criteria specified in Table 4.4.2 Direct calculation maximum permissible stresses for steel fabricated structures, see also Vol 2, Pt 4, Ch 4, 4.3 Direct calculations 4.3.5. Primary member scantlings are, however, not to be less than those required by Vol 1, Pt 6, Ch 3, 7 Single bottom structures and Vol 1, Pt 6, Ch 3, 8 Double bottom structures.

4.2.7 The pedestal girder is to have a thickness not less than the required shell envelope minimum Rule thickness in way. Where abutting plates are of dissimilar thickness, then the taper requirements of Vol 1, Pt 6, Ch 4 Hull Girder Strength are to be complied with.

4.2.8 In general, full penetration welds are to be applied at the pedestal girder boundaries and in way of the end connections between the radial girders and the pedestal girder. Elsewhere, for primary members, double continuous fillet welding is to be applied using a minimum weld factor of 0,34.

4.3.1 Finite element or other direct calculation techniques may be employed in the verification of the structural design. The mesh density used is to be sufficient accurately to demonstrate the response characteristics of the structure and to provide adequate stress and deflection information. A refined mesh density is to be applied to geometry transition areas and those locations where high localised stress or stress gradients are anticipated.

4.3.2 Model boundary constraints are generally to be applied in way of the slewing ring/ship attachment only.

4.3.3 The loads applied to the mathematical model, see Vol 2, Pt 4, Ch 4, 2.4 Global loads 2.4.1, are to include the self weight, dynamic acceleration due to ship motion, hydrodynamic loads, hydrostatic pressure, propeller forces and shaft bearing support forces. In situations where a pod can operate in the flodded conditions or where flooding of a pod adds significant mass to the pod, details are to be included.

4.3.4 Based on the most onerous combination of normal service loading conditions, the stress criteria shown in Table 4.4.2 Direct calculation maximum permissible stresses for steel fabricated structures are not to be exceeded. See also Vol 2, Pt 4, Ch 4, 2.2 Documentation required for design review 2.2.1.(d).

4.3.5 Where the structural design is based on a fatigue assessment and the stress criteria shown in Table 4.4.2 Direct calculation maximum permissible stresses for steel fabricated structures are not applicable, details of cumulative load history and stress range together with the proposed acceptance criteria are to be submitted for consideration.

4.3.6 For cast structures, the localised von Mises stress should not exceed 0,6 times the nominal 0,2 per cent proof or yield stress of the material for the most onerous design condition.