Reliability Of The Stability Of Yacht Rolling In Random Sea And Wind

As the booming of domestic economy,yacht is becoming more and more popular among people's quality time.When it comes to the properties of a yacht,stability is an important factor.How to calculate and improve the stability of a yacht is one of the most important issues the yacht designers are facing now.Yacht stability is assumed to be a certain event when dealt with traditional method which is based on hydrostatics.In fact the external forces a yacht suffered such as sea load and wind load are random variables and the ability to resistance capsizing of a yacht will change as the yacht's flotation conditions change.All the factors lead to a conclusion that the stability of a yacht in random seas and wind is a random event.In order to study yacht stability in a more scientific way,reliability theory can be used in the study which takes the random factors in both inner and outside a yacht in to account and uses reliability index and capsizing probability to express yacht stability.Based on the thinking on the issues mentioned above,the rolling random differential equation is founded of yacht subjected to random wind-wave loading and in order to obtain joint probability density and marginal probability density of the differential equation subjected to different waves and winds,random process theory and path integral method are used in the paper.Value and variance of flooding angle are calculated by counting different flooding angles in different flotation conditions of the yacht.Failure evaluation criterion is assumed to be rolling angle exceeding flooding angle and limit state equation is founded.Reliabilities of yacht stability subjected to different wind-wave loading and different sea area are calculated by the JC method.In order to exam the reliability of yacht stability a fluid dynamical simulation software is used and the influence factors of reliability of yacht stability such as the position of flooding angel,the nonlinear damping moment coefficient and the absolute value of 3 times restoring moment are studied by a case-study.In order to carry on a scientific assessment to the safety of yacht,the reliability of yacht stability subjected to different wind-wave loading and different sea area is divided in to different classes.Random process theory and reliability theory are used in the study of the reliability of yacht stability and the suitability and feasibility of the theory is demonstrated.The study shows that increasing sheer height by 0.16 m,increasing beam by 0.1m can increase the reliability of yacht stability by 0.1.One should choose suitable sheer height and beam which meet the requirementof properties and process to improve reliability of yacht stability.The different classes of reliability of yacht stability can be used by designers to evaluate the threat to yacht safety and by the yacht owner when one need to decide whether to sail or not.