Section 6 Double bottom structure

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

6.1.2 Double bottoms are generally to be fitted in accordance with Pt 3, Ch 2, 6.6 Double and single bottom structure and where fitted are to extend from the collision bulkhead to the after peak bulkhead, as far as this is practicable within 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.

6.1.5 The scantlings of the double bottom structure are to comply with the appropriate minimum requirements given in Pt 7, Ch 3, 2 Minimum thickness requirements.

6.2.1 The scantlings of bar and plate keels are to comply with the requirements of Pt 7, Ch 3, 5.3 Centre girder.

6.2.2 Duct keels, where arranged, are to have a side plate thickness not less than:

but need not be taken as greater than 90 per cent of the centre girder thickness given in Pt 7, Ch 3, 6.3 Centre girder.

d _DB is the Rule centre girder depth given in Pt 7, Ch 3, 6.3 Centre girder 6.3.3

k _ais as defined in Pt 7, Ch 3, 1.5 Symbols and definitions 1.5.1.

6.2.3 Where a duct keel forms the boundary of a tank, the requirements of Pt 7, Ch 3, 7.4 Deep tank plating and Pt 7, Ch 3, 7.5 Deep tank stiffening for deep tanks are to be complied with.

6.2.4 The duct keel width is in general to be 15 per cent of the beam or 2 m, whichever is the lesser, but in no case is it to be taken as less than 630 mm. The inner bottom and bottom shell within the duct keel are to be suitably stiffened with primary stiffening in the transverse direction, whilst the continuity of the floors is maintained. Access to the duct keel is to be by means of watertight manholes or trunks.

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 _a and L _Rare as defined in Pt 7, Ch 3, 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 7, Ch 3, 1.18 Geometric properties and proportions of stiffener sections.

6.3.3 The overall depth of the centre girder, d _DB, 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 7, Ch 3, 4.3 Bottom longitudinal primary stiffeners for bottom longitudinal primary stiffeners are to be complied with.

6.4.1 Where the floor breadth does not exceed 6,0 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 6,0 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 3,0 m.

6.4.3 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.

6.4.4 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 Additionally, the requirements of Pt 7, Ch 3, 4.3 Bottom longitudinal primary stiffeners for bottom longitudinal primary stiffeners are to be complied with.

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

where k _a and Lare as defined in Pt 7, Ch 3, 1.5 Symbols and definitions 1.5.1.

6.5.2 Additionally, the requirements of Pt 7, Ch 3, 4.6 Bottom transverse web frames for bottom transverse web frames stiffeners are to be complied with.

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

6.5.4 In longitudinally framed craft, plate floors or equivalent structure are in general 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 engine room.

3. Underneath pillars and bulkheads.

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

6.5.5 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 7, Ch 3, 6.5 Plate floors 6.5.1.

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

6.6.1 Between plate floors, the shell and inner bottom plating is to be supported by bracket floors. The brackets are to have the same thickness as plate floors and are to be stiffened on the unsupported edge.

6.6.2 In longitudinally framed craft, the brackets are to extend from the centre girder and margin plate to the adjacent longitudinal, but in no case is the breadth of the bracket to be taken as not less than 75 per cent of the depth of the centre girder. They 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.

6.6.3 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 75 per cent of the depth of the centre girder.

6.7.1 The scantlings of watertight floors are to comply with the requirements for plate floors as given in Pt 7, Ch 3, 6.5 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 7, Ch 3, 7.3 Watertight bulkhead stiffening or Pt 7, Ch 3, 7.5 Deep tank stiffening respectively.

6.8.1 The scantlings of tankside brackets are to comply with the requirements for plate floors given in Pt 7, Ch 3, 6.5 Plate floors.

6.9.1 The thickness of the inner bottom plating is to be determined from the general plating equation given in Pt 7, Ch 3, 1.16 Plating 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 craft as appropriate.

6.9.2 Inner bottom plating forming the boundaries of tank spaces is, in addition, to comply with the requirements for watertight bulkheads or deeptanks as detailed in Pt 7, Ch 3, 7.2 Watertight bulkhead plating or Pt 7, Ch 3, 7.4 Deep tank plating respectively. Where the plating forms vehicle, passenger or other decks the requirements of Pt 7, Ch 3, 8 Deck structures are to be complied with.

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

6.9.4 The inner bottom longitudinals are to be continuous through the supporting structure and are to be satisfactorily stiffened against buckling.

6.9.5 Where it is impracticable to comply with the requirements of Pt 7, Ch 3, 6.9 Inner bottom plating 6.9.4, or where it is desired to terminate the inner bottom longitudinals in way of bulkheads or integral tank boundaries, they are to be bracketed in way of their end connections to maintain the continuity of structural strength. Particular care is to be taken to ensure accurate alignment of the brackets.

6.9.6 The requirements for section modulus, inertia and web area are to be determined from the general equations given in Pt 7, Ch 3, 1.17 Stiffening general, using the design pressures from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement craft as appropriate, and the coefficients Φ_Z, Φ_I, and Φ_A as detailed in Table 3.1.1 Section modulus, inertia and web area coefficients for the load model (b).

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

6.10.2 Where it is impracticable to comply with the requirements of Pt 7, Ch 3, 6.10 Inner bottom transverse web framing 6.10.1, 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. they are to be bracketed in way of their end connections, to maintain the continuity of structural strength. Particular care is to be taken to ensure accurate alignment of the brackets.

6.10.3 The requirements for section modulus, inertia and web area are to be determined from the general equations given in Pt 7, Ch 3, 1.17 Stiffening general, using the design pressures from Pt 5, Ch 3, 3.1 Hull structures or Pt 5, Ch 4, 3.1 Hull structures for non-displacement or displacement craft as appropriate, and the coefficients Φ_Z, Φ_I, and Φ_A as detailed in Table 3.1.1 Section modulus, inertia and web area coefficients for the load model (a).

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

6.12.1 Small wells constructed in the double bottom structure 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.

6.13.1 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.

6.14.1 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 is not, in general, to exceed 50 per cent of the double bottom depth unless edge reinforcement is provided. Holes are 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.

6.15.1 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.

6.16.1 Sufficient limber holes are to be cut in the internal bottom structure to allow for the drainage of water from all parts of the bilge to the pump suctions.

6.16.2 Particular care is to be given to the positioning of limber holes to ensure adequate drainage and to avoid stress concentrations.

6.16.3 Suitable arrangements are to be made to provide free passage of air from all parts of tanks to the air pipes.