Superstructure Effectiveness in the Preliminary Assessment of the Hull Behavior

In a study of hull-superstructure interaction, a parametric investigation was undertaken, which was designed to examine a large number of hull-superstructure arrangements, broadly representative of warship configurations. The study was essentially numerical, based on the finite element method, but was supported by an experimental program used to validate aspects of the modeling. The majority of the numerical work was the analysis of three-dimensional simplified models having either a single superstructure on a standardized hull, or a double superstructure arrangement on an alternative standardized hull. Such models represented simplified structural configurations from which preliminary assessments of the hull-superstructure behaviour could be systematically addressed within an initial design sequence. Superstructure effectiveness is not universally defined or used, in the literature. Since stresses are idiosyncratic, effectiveness in this work is defined in terms of overall structural displacement, as the ratio of the maximum displacement of the plain hull to the maximum displacement with superstructures. A variety of cases was considered, including single and double superstructure arrangements of various sizes and in various positions on the hull. Design curves were formed for the different arrangements showing the variation in the effectiveness. Superstructure effectiveness varies with superstructure length, becoming greater as the superstructure becomes longer. There is afinite limit to the maximum effectiveness which is dependent on the relative cross sectional properties. The position of the superstructure also affects the effectiveness, with the greatest effectiveness occurring at the mid hull position. The change in effectiveness is less when the superstructure is small and changing position than when it is long. Two superstructures having comparable total length to a single superstructure are less effective than the single superstructure. Three superstructures are more effective than two, but still less than one. As the number of superstructures increases for a given total length, the effectiveness tends towards the single superstructure value. Superstructures, even of modest dimensions, add considerably to the cross-sectional properties and should be considered as part of the hull girder when doing the preliminary design.