Section 6 Double bottom structure

6.1.1 Except as otherwise specified within this Section, the scantlings and arrangements for double bottom structure are to be determined in accordance with the procedures described in, or as required by Pt 8, Ch 3, 6 Double bottom structure for mono-hull craft, using the pressures from Pt 5 Design and Load Criteria appropriate to multi-hull craft.

6.1.2 The minimum thickness requirements detailed in Pt 8, Ch 3, 2.1 General are to be complied with as appropriate.

6.2.1 The breadth and thickness of plate keels are to comply with the requirements of Pt 8, Ch 4, 3.2 Keel plates.

6.3.1 A centreline girder is to be fitted throughout the length of each hull. The web thickness, t _w, of centre girders 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.

6.3.2 The web thickness for a centreline girder of single plate laminate construction is to be two times the thickness as required by Pt 8, Ch 4, 6.3 Centreline girder 6.3.1.

6.3.3 The overall depth of the centreline 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 4, 4.8 Bottom inboard longitudinal primary stiffeners for bottom inboard longitudinal primary stiffeners are to be complied with.

6.4.1 Where the breadth of the floor at the upper edge, within a single hull, does not exceed 4,0 m, side girders are not required. Vertical stiffeners are to be fitted to the floors on each side of the centreline girder, the number and positions of these stiffeners being dependent on the arrangement of the double bottom structure.

6.4.2 Where the breadth of the floor at the upper edge, within a single hull, exceeds 4,0 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 Additionally, the requirements of Pt 8, Ch 4, 4.8 Bottom inboard longitudinal primary stiffeners for bottom inboard longitudinal primary stiffeners are to be complied with.

6.5.1 Plate floors may be of single skin, sandwich skin or `top-hat' type construction, and are to comply with the requirements of Pt 8, Ch 4, 5.5 Floors where applicable.

6.5.2 The web thickness, t _w, of 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.

6.5.3 The web thickness for transverse floors of single plate laminate construction is to be two times the thickness as required by Pt 8, Ch 4, 6.5 Plate floors 6.5.2.

6.5.4 The amount of reinforcement in laminates that form the skins of a sandwich laminate is to comply with the requirements of Pt 8, Ch 4, 2.1 General 2.1.2.

6.5.5 Additionally, the requirements of Pt 8, Ch 4, 4.11 Bottom inboard transverse web frames for bottom inboard transverse web frames are to be complied with.

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