New interactive method to determine parameters for large motor yachts stability

Abstract

The stability of a ship is the most important safety criteria to be considered during the design process, in fact an error in the design of the hull. An error in the stability evaluation has two different levels of danger, each one related to the other: the loss of stability leads to the loss of the ship, and this loss usually happens suddenly and quickly, causing a serious danger for the passengers. At the same time, in last decades, yachts have been changing significantly, increasing their dimensions from “boat” to “ships”, and the Rules have followed this change setting different criteria to be met. This has led to a different approach to motor yacht design, and to the way of building yachts, considering new materials, manufacturing, etc. For a designer ‘managing the stability of a yacht’ is the capability to have control over the trim and heel when an inclining cause occurs. This capability is strictly related to the hull shape and the weight distribution in three-dimensional space; the criteria of stability for motor yachts have been raising especially in last 2 decades, becoming more and more strict, but this request has to fight with the development of the market and the design that requests always larger superstructures, creating a serious conflict for the naval architects. Considering the importance of stability, undoubtedly considered as mandatory, we can underline the relevance of a method for an immediate evaluation of the parameters involved, especially at the beginning of the project, in order to foresee the stability of a motor yacht and operate in an interactive way with the production department. The suggested method aims at filling a void in the available documentation, referring to a methodology already known and used in Naval Architecture for the evaluation of the residual resistance of the various hulls, with the creation of a systematic database, but this methodology has net yet been applied to stability. The search for the stability parameters is usually done applying a direct weight estimation, considering the weight of each items, like shell plates, girders, single machinery etc. and the position of the center of gravity and then, once, estimated the center of gravity of the ship, verify if the stability criteria are satisfied. But the interactivity concept contained in the proposal is to have already a target to aim, in terms of position of center of gravity G, and constantly adjusting the distribution of each item to come close to that value, similarly to the already mentioned method to aim at values of geometrical parameters regarding the resistance. The present work, in order to provide this method for the stability, will examine a series of various motor yachts, with their inclining experiment and data, in order to create a methodological criteria to foresee the performances in terms of stability and the main design criteria that can be adopted. The method will give values and tendencies of main stability parameters to be aimed before starting the project, without having to follow the complete process of a preliminary calculation, with an important saving of time.