Section 6 Double bottom structure

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

6.1.2 The thickness of double bottom structural members is in no case to be less than the appropriate minimum requirement given in Pt 6, Ch 4, 2 Minimum thickness requirements.

6.2.1 The scantlings of plate and bar keels are to comply with the requirements of Pt 6, Ch 4, 5.2 Keel.

6.3.1 A centre girder is to be fitted throughout the length of the craft. The web thickness, t _w, is not to be less than that required by:

where k _s and L _R are as defined in Pt 6, Ch 4, 1.5 Symbols and definitions 1.5.1.

6.3.2 The geometric properties of the girder section are to be in accordance with Pt 6, Ch 3, 1.18 Geometric properties and proportions of stiffener sections.

6.3.3 The overall web depth, d _w, of the centre girder is to be taken as not 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 6, 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 floor breadth does not exceed 4,0 m, side girders are not required. Vertical stiffeners are to be fitted to the floors on each side, the number and positions of these stiffeners being dependent on the arrangement of the double bottom structure.

6.4.2 Where the breadth of floor is greater than 4,0 m, additional side girders having the same thickness as the floors are to be fitted. The number of side girders is to be such that the distance between the side girders and centre girder and margin plate, or between the side girders themselves, does not exceed 2,0 m.

6.4.3 Side girders, where fitted, are to extend as far forward and aft as practicable and are in general to to be scarfed into the bottom structure forward and aft of the supporting bulkheads, deep floors or other primary transverse structure

6.4.4 Where additional side girders are fitted in way of main machinery seatings, they are to be integrated into the structure of the craft and extended forward and aft as far as practicable.

6.4.5 Under the main engine, girders extending from the bottom shell to the top plate of the engine seating are to be fitted. The height of the girders is to be not 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 and/or hanging brackets are to be fitted.

6.4.6 The geometric properties of the girder section are to be in accordance with Pt 6, Ch 3, 1.18 Geometric properties and proportions of stiffener sections.

6.4.7 Additionally, the requirements of Pt 6, Ch 4, 4.3 Bottom outboard longitudinal primary stiffeners for bottom inboard longitudinal primary stiffeners are to be complied with.

6.5.1 The web thickness, t _w, of non-watertight plate floors is to be not less than:

where k _s and L _R are as defined in Pt 6, Ch 4, 1.5 Symbols and definitions 1.5.1.

6.5.2 The geometric properties of the floor section are to be in accordance with Pt 6, Ch 3, 1.18 Geometric properties and proportions of stiffener sections.

6.5.3 Additionally, the requirements of Pt 6, Ch 4, 4.6 Bottom outboard transverse web frames for bottom inboard transverse web frames are to be complied with.

6.5.4 Plate floors are, in general, to be continuous between the centre girder and the margin plate.

6.5.5 In longitudinally framed craft, plate floors are to be fitted in the following positions:

1. At every half frame in way of the main engines, thrust bearings, and bottom of the craft forward.

2. Outboard of the engine seatings, at every frame within the engineroom.

3. Underneath pillars and bulkheads.

4. Outside of the engine room at a spacing not exceeding 2,0 m.

6.5.6 Vertical flat bar stiffeners are to be fitted to all plate floors at each longitudinal. Each stiffener is to have a depth of not less than 10t _w and a thickness of not less than t _w, where t _w is thickness of the plate floor as calculated in Pt 6, Ch 4, 6.5 Plate floors 6.5.1.

6.5.7 In transversely framed craft, plate floors are to be fitted at every frame in the engineroom, under bulkheads, in way of change in depth of double bottom and elsewhere at a spacing not exceeding 2,0 m.

6.6.1 Watertight floors, or floors forming boundaries of tank spaces, are also to comply with the requirements for watertight bulkheads or deeptanks as detailed in Pt 6, Ch 3, 7.2 Watertight bulkhead plating or Pt 6, Ch 3, 7.4 Deep tank plating respectively.