Balancing a Destroyer on a Wave for Strength and Stability

Abstract

A wave-balancing approach, where a ship is hydrostatically balanced on a wave with either the crest or the trough at midship, is often used to assess the inherent strength and stability of the vessel in waves. This work examines the differences between the wave types (sinusoidal and trochoidal) as well as the values of wave length, wave height, and position of the crest along the ship, with the goal of looking for the worst-case conditions for both intact stability and longitudinal strength. A notional destroyer is used as a case study to look at the trends in strength and stability in the upright condition and at angles of heel in waves. The notional destroyer is intentionally similar to but not the same as any existing design. The study shows that looking at a wider set of wave conditions and ship states can identify more extreme wave loading and stability degradation, suggesting that this more detailed analysis would be beneficial as a standard practice. 1. Introduction Both stability and longitudinal strength are key factors in a successful ship design. In both cases, it has long been recognized that the hydrostatic and hydrodynamic effects of waves are fundamentally important. Before the advent of computers and modern numerical models and methods, naval architects relied on hydrostatic analysis to determine the stability and strength characteristics of a given vessel. They typically added safety margins to account for other influences including wave dynamics. Some, however, used the method of "balancing a ship on a wave" to partially account for the influence of waves. In this methodology, the vessel was hydrostatically balanced on the crest or in the trough of a wave. The method was used first for finding bending moments, and later some experts used the method for assessing the stability of a ship. Although the method does not account for the dynamics of ship motion or hull loading transients, it is still used as a design tool today and is embedded in standards for strength (Lloyd's Register, NSR 2012) and stability (BV 1030-1 2012). It does give a "snapshot" of the hydrostatic forces and moments for a particular condition.