2356. Weight and KG Margin Analysis of Naval Surface Ships

Paper

Abstract

Effective weight and KG (height of vertical center of gravity above the keel acquisition margins are an essential element of the US Navy Weight Control Program. Acquisition margins are not only an engineering tool for making technical predictions, but impact the fiscal process as well. The need for continued improvement in margin determination was recognized when the weight control program was formulated in 1961. The first improvement came with the establishment of a formal margin policy in 1963. The values, restricted to weight at that time, reflected the best corporate engineering judgment based on scattered and, in many cases, unverified weight growths. Because the shipbuilding process is relatively long (compared to aircraft, land vehicle and missile production), it took fifteen years to accumulate a data base large enough to be considered reasonable for a statistical study of margins. In 1978 this data base was used to update the Weight Margin Policy for Surface Ships and expand it to include a KG margin policy, as well. In 1992, a study was undertaken to update the data base and find an appropriate statistical basis for margins prediction with an associated risk management approach to margin selection. This study verified the results of the 1978 study and supplemented the 1978 study by expanding and updating the Design & Build (D&B), Contract Modification (Con Mod), and Government Furnished Material (GFM) data sets. This paper discusses the statistical results of the data, and includes recommendations for updating the current NAVSEA Weight and KG Margin Policy. In addition, a formal margin selection method is presented which produces margins for each design phase and an associated quantifiable risk of exceeding them. Using this method, a Ship Design Manager working with the weight engineer (mass properties) can select a level of risk appropriate for his (her) design and determine weight and KG margin values associated with this risk.