Double Bottom Construction

The bottom shell construction consists of the central keel of the ship, with the flooring structure and side shell plating on either side. Almost all vessels built today are fitted with a double bottom. This is an internal skin fitted about 1 m above the outer shell plating and supported by the flooring structure.

Keel

The keel runs along the centreline of the bottom plating of the ship and for the majority of merchant ships is of a flat plate construction. At right-angles to the flat plate keel, running along the ship's centreline from the forepeak to the aft peak bulkhead is a watertight longitudinal division known as the centre girder or vertical keel. 

    Where a double-bottom construction is employed, the centreline strake of tank top plating results in the formation of an I-section keel. This provides considerable strength to the structure and resistance to bending. The flat plate keel or 'middle line strake of plating' is increased in thickness for strength purposes and for a corrosion allowance, because of the difficulty in maintaining paint protection systems in way of the docking blocks during the vessel's life.

Some double bottoms have a duct keel fitted along the centre line. This is an internal passage of watertight construction running some distance along the length of the ship, often from the forepeak to the forward machinery space bulkhead. Use is made of this passage to carry the pipework along the length of the ship to the various holds or tanks. An entrance is usually provided at the forward end of the machinery space via a watertight manhole. No duct keel is necessary in the machinery space or aft of it, since pipework will run above the engine room double bottom and along the shaft tunnel, where one is fitted.

The construction of the duct keel uses two longitudinal girders spaced not more than 2.0 m apart. This restriction is to ensure that the longitudinal girders rest on the docking blocks when the ship is in drydock. Stiffeners are fitted to shell and bottom plating at alternate frame spaces and are bracketed to the longitudinal girders. The keel plate and the tank top above the duct keel must have their scantlings increased to compensate for the reduced strength of the transverse floor

Double-bottom structure

Where a double bottom or inner shell is fitted it is watertight up to the bilges, thus providing complete watertight integrity should the outer shell be pierced in way of the double bottom. The minimum depth is determined by rille requirements for the size of the vessel but the actual depth is sometimes increased in places to suit double-bottom tank capacities. The double bottom may have a sloping margin leading to the bilge radiused plating or a continuous double bottom extending to the side shell.

The sloping margin construction requires the use of margin plates to connect up with the side framing and provides a collecting bay or well for bilge water.

The continuous tank top or flat margin must have bilge water collecting points or drain 'hats' fitted into it. The flat margin is connected to the side framing by a flanged bracket. The flat margin type of construction is much used in modem construction.

The structure is made up of vertical floors which may be watertight, solid or of bracket construction. The floor structure is continuous from the centre girder to the side shell and supports the inner bottom shell. Side girders are fitted in the longitudinal direction, their number depending on the width of the ship. These side girders have broken either side of the floors and are therefore called intercostal girders.

Watertight or oil-tight floors are fitted beneath the main bulkheads and are also used to subdivide the double-bottom space into tanks for various liquids. Solid plate floors of non-watertight construction, usually lightened by manholes, are positioned in other places as required to stiffen the structure. Between solid plate floors, bracket floors are fitted. Bracket floors consist of plate brackets attached to the centre girder and the side shell with bulb plate stiffeners mnning between. The stiffeners are supported by angle bar struts at intervals and any side girders which are present in the structure.

The arrangement of flooring will be determined by the type of framing system adopted, which may be either transverse or longitudinal.

Transversely framed double bottom

When transversely framed, the double-bottom structure consists of solid plate floors and bracket floors with transverse frames. The bracket floor is fitted between the widely spaced solid floors. It consists of transverse bulb angle sections stiffening the shell and inner bottom plating. Vertical support is provided by brackets at the side shell and centre girder, any side girders and intermediate struts. The number of intercostal side girders fitted is determined by classification society mIes. 

Longitudinally framed double bottom

This is the system favoured as a result of tests and it provides adequate resistance to distortion on ships of 120 m in length or greater. Offset bulb plates are used as longitudinal stiffeners on the shell and inner bottom plating, at intervals of about 1 m. Solid floors provide support at transverse bulkheads and at intervals not exceeding 3.8 m along the length of the ship. Brackets are fitted at the centre girder and side shell at intermediate frame spaces between solid floors. These brackets are flanged at the free edge and extend to the first longitudinal. Channel bar or angle bar struts are provided to give support at intervals of not more than 2.5m where solid floors are widely spaced. Intercostal side girders are again fitted their number

depending upon classification society mIes.

Machinery space double bottom

The construction of the double bottom in the machinery space regardless of the framing system has solid plate floors at every frame space under the main engine. Additional side girders are fitted outboard of the main engine seating, as required. The double-bottom

height is usually increased to provide fuel oil, lubricating oil and fresh water tanks of suitable capacities. Shaft alignment also requires an increase in the double-bottom height or a raised seating, the former method usually being adopted.

Continuity of strength is ensured and maintained by gradually sloping the tank top height and internal structure to the required position. Additional support and stiffening are necessary for the main engines, boilers, etc., to provide a vibration resistant solid platform capable of supporting the concentrated loads. On the slow speed, diesel-engined ships the tank top plating is increased to 40 mm thickness or thereabouts in way of the engine bedplate. This is achieved by using a special insert plate which is the length of the engine including the thrust block in size.

Additional heavy girders are also fitted under this plate and in other positions under heavy machinery as required. Plating and girder material in the machinery spaces is of increased scantlings in the order of 10%.

Double bottom tanks

Access to the double-bottom tanks is usually by manholes cut in the tank top. These manholes are suitably jointed and bolted to be completely watertight when not in use. Docking plugs are fitted in all double-bottom tanks and are a means of completely draining these tanks for inspection in drydock. Air pipes are fitted 10 all double-bottom tanks to release the air when filling. Sounding pipes are also fitted 10enable the tanks 10be sounded and their capacity determined. All double-bottom tanks are tested on completion by the maximum service pressure head of water or an equivalent air test.

Structure to resist pounding

Pounding or slamming results from the ship heaving or pitching, thus causing the forward region to 'slam' down onto the water. Additional structural strength must be provided from the forward perpendicular aft for 25-30% of the ship's length. The shell plating on either side of the keel is increased in thickness, depending upon the ship's minimum draught. The frame spacing is reduced, fun- and half-height intercostal side girders are fitted and solid floors are installed at every frame space.

With longitudinal framing the longitudinal spacing is reduced, intercostal side girders are fitted and transverse floors are installed at alternate frames.

Single-bottom construction

ln older oil tankers particularly, and in some smaller vessels, a single-bottom construction is employed.

The construction of the single bottom in smaller ships is similar to double bottom construction but without the inner skin of plating. The upper edge of all plate floors must therefore be stiffened to improve their rigidity.