Section 6 Double bottom structure

6.1.1 The requirements given in this Section provide for double bottom construction of mono-hull craft in association with either transverse or longitudinal framing.

6.1.2 Where required in accordance with Pt 3, Ch 2, 6 Machinery space arrangements, double bottoms are generally to extend from the collision bulkhead to the after peak bulkhead, as far as this is practicable taking into account the design and proper working of the craft. In addition, the inner bottom is to be continued to the craft's side in such a manner as to protect the bottom to the turn of bilge or chine.

6.1.3 The double bottom structure in way of girders and duct keels is to be sufficient to withstand the forces imposed by dry-docking the craft.

6.1.4 The centreline girder and side girders are to extend as far forward and aft as practicable and care is to be taken to avoid any abrupt discontinuity. Where girders are cut at bulkheads, their longitudinal strength is to be maintained.

6.2.1 The breadth and thickness of plate keels are to comply with the requirements of Pt 8, Ch 3, 3.2 Keel plate.

6.3.1 A centreline girder is to be fitted throughout the length of the craft. The web thickness, t _w, of a centreline girder of `top-hat' type section is to be not less than as required by Pt 8, Ch 3, 1.17 Stiffener proportions, or as determined as follows, whichever is the greater and in no case is t _w to be taken less than 5 mm.

where k _A and L _R are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

6.3.2 The web thickness of a centreline girder of single plate laminate construction is to be two times the thickness as required by Pt 8, Ch 3, 6.3 Centreline girder 6.3.1.

6.3.3 The overall depth of the centre girder, d _DB, is not to be taken as less than 630 mm and is to be sufficient to give adequate access to all parts of the double bottom.

6.3.4 Additionally, the requirements of Pt 8, Ch 3, 4.5 Bottom longitudinal primary stiffeners for bottom longitudinal primary stiffeners are to be complied with.

6.4.1 Where the breadth of the floor at the upper edge does not exceed 6,0 m, side girders are not required.

6.4.2 Where the breadth of the floor at the upper edge exceeds 6,0 m, side girders are to be fitted at each side of the centre girder such that the spacing between the side and centre girders or between the side girders themselves is not greater than 3 m. Side girders, where fitted, are to extend as far forward and aft as practicable and are, in general, to terminate in way of bulkheads, deep floors or other primary transverse structure.

6.4.3 Under the main engine, girders extending from the bottom to the top plate of the engine seating are to be fitted. The height of the girders is not to be less than the height of the floor. Engine holding-down bolts are to be arranged as near as practicable to the girders and floors. Where this cannot be achieved, bracket floors are to be fitted.

6.4.4 Side girders are to have a minimum web thickness, t _w, as required by Pt 8, Ch 3, 1.17 Stiffener proportions, but not less than as determined as follows whichever is the greater and in no case is t _w to be taken less than 5,0 mm:

where k _A and L _R are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1

6.4.5 The face area and face thickness of side girders are to comply with the requirements for plate floors as defined in Pt 8, Ch 3, 5.4 Floors, general 5.4.7 and Pt 8, Ch 3, 5.4 Floors, general 5.4.8 respectively.

6.4.6 Additionally, the requirements of Pt 8, Ch 3, 4.5 Bottom longitudinal primary stiffeners for bottom longitudinal primary stiffeners are to be complied with.

6.5.1 Between plate floors, the shell inner bottom plating is to be supported by bracket floors. The brackets are to have the same thickness as plate floors and, where they are of single skin laminate construction, are to be stiffened on the unsupported edge.

6.5.2 In longitudinally framed craft, the brackets are to extend from the centre or side girder and margin plate to the adjacent longitudinal, but in no case is the breadth of the bracket to be taken less than 3/4 of the depth of centre girder. Brackets are to be fitted at every web frame at the margin plate, and those at the centre girder are to be spaced not more than 1,0 m apart.

6.5.3 In transversely framed craft, the breadth of the brackets attaching the bottom and inner bottom frames to the centre girder and margin plate is to be not less than 3/4 of the depth of the centre girder.

6.6.1 Plate floors may be of single skin, sandwich skin or `top-hat' type construction.

6.6.2 The web thickness, t _w, for non-watertight plate floors of `top-hat' type section is to be not less than as required by Pt 8, Ch 3, 1.17 Stiffener proportions, or as determined as follows, whichever is the greater and in no case is t _w to be taken less than 5,0 mm.

where k _A and L _R are as defined in Pt 8, Ch 3, 1.5 Symbols and definitions 1.5.1.

6.6.3 The web thickness for transverse floors of single plate laminate construction is to be two times the thickness as required by Pt 8, Ch 3, 6.6 Plate floors 6.6.2.

6.6.4 The amount of reinforcement in laminates that form the skins of a sandwich laminate is to comply with the requirements of Pt 8, Ch 3, 2.5 Minimum skin reinforcement in sandwich laminates 2.5.1.

6.6.5 Additionally, the requirements of Pt 8, Ch 3, 4.8 Bottom transverse web frames for bottom transverse web frames are to be complied with.

6.6.6 Plate floors are generally to be continuous between the centre girder and the margin plate.

6.7.1 The scantlings of watertight floors are to comply with the requirements for plate floors as detailed in Pt 8, Ch 3, 6.6 Plate floors.

6.7.2 Watertight floors, or floors forming boundaries of tank spaces, are also to comply with the requirements for watertight bulkheads or deep tanks as detailed in Pt 8, Ch 3, 7.3 Watertight bulkheads or Pt 8, Ch 3, 7.4 Deep tanks respectively.

6.8.1 The scantlings of tankside brackets are to comply with the requirements for plate floors as detailed in Pt 8, Ch 3, 6.6 Plate floors.

6.9.1 Inner bottom laminates forming boundaries of tank spaces, are also to comply with the requirements for watertight bulkheads or deep tanks as detailed in Pt 8, Ch 3, 7.3 Watertight bulkheads or Pt 8, Ch 3, 7.4 Deep tanks respectively and, where forming vehicle, passenger or other decks the requirements of Pt 8, Ch 3, 8 Deck Structures are to be complied with.

6.9.2 The bending moment assumed to be carried by the inner bottom plating is to be not less than that determined from Pt 8, Ch 3, 1.9 Plate and sandwich laminates 1.9.1, using the design pressure from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement type craft as appropriate. This bending moment is to be applied to laminates of both single skin and sandwich construction in the determination of the panel scantling required by Pt 8, Ch 3, 6.9 Inner bottom laminate 6.9.3 and Pt 8, Ch 3, 6.9 Inner bottom laminate 6.9.5 respectively.

6.9.3 An estimate of the thickness of the inner bottom single skin plating is to be determined from Pt 8, Ch 3, 1.13 Determination of properties and stresses for single skin plate laminates 1.13.1. The tensile and compressive stresses are to be determined for each ply of reinforcement in the proposed laminate using Pt 8, Ch 3, 1.13 Determination of properties and stresses for single skin plate laminates 1.13.3 and Pt 8, Ch 3, 1.13 Determination of properties and stresses for single skin plate laminates 1.13.4, see also LR's Guidance Notes for Calculation Procedures for Composite Construction. The allowable tensile and compressive stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading are to be complied with.

6.9.4 In no case is the minimum thickness of single skin plating to be taken as less than 5 mm.

6.9.5 An estimate of the stiffness E , the thickness of single skin plating for outer and inner skins of the bottom sandwich panel and the thickness of core material is to be determined from Pt 8, Ch 3, 1.14 Mechanical properties sandwich laminates 1.14.2 and Pt 8, Ch 3, 1.14 Mechanical properties sandwich laminates 1.14.9 respectively. The tensile and compressive stresses are to be determined for each ply of reinforcement in the proposed laminate using Pt 8, Ch 3, 1.14 Mechanical properties sandwich laminates 1.14.7 and Pt 8, Ch 3, 1.14 Mechanical properties sandwich laminates 1.14.8. The allowable tensile and compressive stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading are to be complied with.

6.9.6 The amount of reinforcement in laminates that form the skins of a sandwich laminate is to comply with the requirements of Pt 8, Ch 3, 2.5 Minimum skin reinforcement in sandwich laminates 2.5.1, see Pt 8, Ch 3, 2.3 Sandwich skin laminate 2.3.1.

6.9.7 Special consideration may be given to laminate thicknesses lesser than that required by Pt 8, Ch 3, 6.9 Inner bottom laminate 6.9.4 and Pt 8, Ch 3, 6.9 Inner bottom laminate 6.9.6, provided that all of the structural strength requirements of the Rules are complied with, a satisfactory water barrier is provided, see Pt 8, Ch 3, 2.3 Sandwich skin laminate 2.3.1, and the equivalent impact resistance is to be demonstrated as required by Pt 8, Ch 3, 2.8 Impact considerations 2.8.2.

6.10.1 The inner bottom longitudinals are to be supported by inner bottom transverse web frames, floors, bulkheads, or other primary structures, generally spaced not more than 2 m apart.

6.10.2 Inner bottom longitudinals are to be continuous through the supporting structures.

6.10.3 Where it is impracticable to comply with the requirements of Pt 8, Ch 3, 6.10 Inner bottom longitudinals 6.10.2, or where it is desired to terminate the inner bottom longitudinals in way of bulkheads or integral tank boundaries, the longitudinals are to be bracketed in way of their end connections, to maintain the continuity of structural strength. Particular attention is to be taken to ensure accurate alignment of the brackets.

6.10.4 The Rule requirements for bending moment, shear force, shear stress and deflection are to be determined from the general equations given in Pt 8, Ch 3, 1.15 Stiffeners general, using the design pressure from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement type craft as appropriate, and the coefficients Φ_M, Φ_S and Φ_δ as indicated in Table 3.1.10 Shear force, bending moment and deflection coefficients for the load model (b).

6.10.5 The allowable tensile, compressive and shear stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading , and the span/deflection ratios indicated in Table 7.2.1 Limiting span/deflection ratio are to be complied with.

6.11.1 Inner bottom transverse web frames are defined as primary stiffening members which support inner bottom longitudinals. They are to be continuous and substantially bracketed at their end connections to bottom web frames, bottom floors and tankside brackets.

6.11.2 Where it is impracticable to comply with the requirements of Pt 8, Ch 3, 6.11 Inner bottom transverse web framing 6.11.1, or where it is desired to terminate the inner bottom transverse web frames in way of centre or side girders, bulkheads or integral tank boundaries, etc. all web frames are to be bracketed in way of their end connections, to maintain the continuity of structural strength. Particular attention is to be taken to ensure accurate alignment of the brackets. All brackets are to be `soft toed' and are to terminate on suitable supporting structure capable of carrying the transmitted bending moment.

6.11.3 The Rule requirements for bending moment, shear force, shear stress and deflection are to be determined from the general equations given in Pt 8, Ch 3, 1.15 Stiffeners general, using the design pressure from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement type craft as appropriate, and the coefficients Φ_M, Φ_S and Φ_δ as indicated in Table 3.1.10 Shear force, bending moment and deflection coefficients for the load model (a).

6.11.4 The allowable tensile, compressive and shear stress limits indicated in Table 7.3.1 Limiting stress criteria for local loading , and the span/deflection ratios indicated in Table 7.2.1 Limiting span/deflection ratio are to be complied with.

6.12.1 A margin plate, if fitted, is to have a thickness as required for the inner bottom plating.

6.13.1 Small wells constructed in the double bottom are not to extend in depth more than necessary. A well extending to the outer bottom may, however, be permitted at the after end of the shaft tunnel of the craft. Other well arrangements (e.g. for lubricating oil under main engines) may be considered provided they give protection equivalent to that afforded by the double bottom.

6.14.1 In double bottoms under widely spaced pillars, the connections of the floors to the girders, and of the floors and girders to the inner bottom, are to be suitably increased. Where pillars are not directly above the intersection of plate floors and girders, partial floors and intercostals are to be fitted as necessary to support the pillars. Manholes are not to be cut in the floors and girders below the heels of pillars. Where longitudinal framing is adopted in the double bottom, equivalent stiffening under the heels of pillars is to be provided, and where the heels of pillars are carried on a tunnel, suitable arrangements are to be made to support the load.

6.15.1 Suitable arrangements are to be made to provide free passage of air and water from all parts of the tanks to the air pipes and pump suctions.

6.15.2 Particular attention is to be given to the positioning of limbers to ensure adequate drainage and to avoid stress concentrations, see also Pt 8, Ch 3, 5.7 Drainage arrangements.

6.15.3 Openings in the webs of stiffening sections, baffle plates, etc. are to be suitably sealed in accordance with Pt 8, Ch 3, 4.16 Combined framing systems.

6.16.1 Sufficient manholes are to be cut in the inner bottom, floors and side girders to provide adequate access to and ventilation of all parts of the double bottom. The size of the manhole openings in plate laminates is not, in general, to exceed 50 per cent of the double bottom depth unless edge reinforcement is provided. Holes are, in general, not to be cut in the centre girder, except in tanks at the forward and after ends of the craft, and elsewhere where tank widths are reduced unless additional stiffening and/or compensation is fitted to maintain the structural integrity.

6.17.1 Double bottoms are to be tested upon completion with a head of water representing the maximum internal pressure which could be experienced in service, but not less than a head of water equivalent to the level of the upper deck.