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 is necessary for the main engines, boilers, etc., to provide a vibration-resistant solid platform capable of supporting the concentrated loads. On 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 per cent.

The method adopted.

A cellular void space within a ships structure is called a coffer dam. Like a bulkhead separates two spaces or divides a space into two, a coffer dam does the same with a larger degree of integrity since it incorporates a void which would contain any breach of either of the boundaries. This would contain the leakage and prevent it spreading into other areas. A cofferdam can be defined as an empty space separating compartments to prevent the contents of one compartment from entering another in case of leakage.

The thrust block is positioned close to the propulsion machinery (usually just aft of the main engine). Reasons are as follows:

• The axial thrust generated by the propeller could cause deformation and misalignment of the shafting system if the thrust block were positioned far from the engine. Placing it close to the engine minimizes the length of shafting subject to this thrust, thus reducing potential for misalignment. The strong double bottom structure directly beneath provides necessary support to mitigate any deformation.

• Differential expansion between the shaft and the hull due to temperature variations is a potential source of misalignment. Positioning the thrust block close to the engine helps to minimize the effect of this differential expansion.

• The weight of the propeller and the dynamic forces it creates can lead to whirling of the tailshaft and misalignment if not properly managed. Positioning the thrust block near the engine helps to stabilize the shafting system and reduce the risk of these issues.

• The substantial double-bottom structure under the main machinery provides an ideal, inherently strong foundation for the thrust block. This minimises the need for extensive additional reinforcement to support the thrust loads.