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ObjectivesThe hull plate of a trimaran cross-deck bottom is often subjected to slamming loads under high-level sea conditions, potentially causing severe structural damage. The objective of this study is to analyze the influence of the air cushion effect on the impact load of a trimaran cross-deck plate and propose a new type of hull plate for the cross-deck bottom in order to reduce the slamming load.MethodsThe two-phase flow problem of a trimaran ship section is simulated using the computational fluid dynamics (CFD) method. First, the accuracy of the numerical model is verified by comparing the predicted results with the experimental measurements. The influence of the air cushion on the slamming load of the trimaran cross-deck structure is then obtained by analyzing the evolution of the flow field. Finally, the influence of the air cushion is analyzed and the effects of different shapes and sizes of the proposed hull plate are evaluated.ResultsThe numerical results show two slamming peaks that appear during the process of a trimaran entering the water. The rectangular shaped plate reduces the slamming load by around 20% compared with the flat plate bottom.ConclusionsThe air cushion has a significant effect on the slamming load. A rectangular shaped hull plate can be used to cover the trimaran cross-deck bottom.

Objectives The motion attitude (sinkage and trim) of a semi-planing ship (at Froude numberFr > 0.4) have significant effects on its resistance performance during voyage. The sinkage and trim effects on the resistance components (frictional resistance and pressure resistance) is discussed.Methods A CFD study is carried out on the resistance performance of trimaran in calm water, and the k-ε turbulent model and overlapping grid are adopted. The free model and constrained model are computed. The computed results are compared to analyze the effects of sinkage and trim on the resistance components.Results The results show that the change of total resistance caused by sinkage and trim is more than 20% in semi-planing conditions. The sinkage and trim have a significant effect on the pressure resistance, up to nearly 50%; and a relatively small effect on frictional resistance, less than 8.5%. More than 90% of the resistance increase caused by motion attitude comes from the change of pressure resistance.Conclusions Predicting the motion attitudes and resulting pressure resistance changes accurately is extremely important for semi-planing trimaran.

Objectives Reducing the weight of the cross-deck is an important method for reducing the weight and center of gravity of a trimaran. Structural strength analysis is used as the premise for judging whether a weight reduction method is feasible.Methods Based on the loading characteristics of a trimaran, a new dispersed cross-deck structure is proposed in which most of the unnecessary panels are removed. The finite element (FE) direct calculation method is adopted to analyze the structural strength of the dispersed cross-deck, and its strength is then compared with that of the traditional integral cross-deck.Results The results show that using the dispersed cross-deck can reduce the structural weight and center of gravity of a trimaran by 6.75% and 5.54% respectively on the premise of meeting the structural strength requirements.Conclusions Using the dispersed cross-deck can greatly reduce the structural weight and vertical location of the center of gravity, which provides new approaches for the structural design of trimarans.

[Objectives] As a high-tech ship, the trimaran is particularly prominent in structural strength. Traditional numerical methods are analysed by deterministic methods. However,such variables as the loads acting on the hull,the uncertainty of the calculating model and the performance of the material are random. Therefore,reliability measures are taken to account for the influence of these factors on hull strength.[Methods] The three-dimensional potential flow theory and direct calculation of strength for the hull structure are used in this paper to obtain the short-term time-period distribution of the von Mises stress of special structural elements,then the distributions of long-term von Mises stress and sequence statistics of extreme value are obtained. The limit state equation is listed in combination with the structural capacity and load requirements of the units to obtain the reliability indexes. The failure probability and safety factor of typical structures are calculated,and certain random components are analysed on impact.[Results] The research shows that the structural strength of a trimaran is concentrated at the connecting bridge,the safety factor is 0.86 and the deck safety factor is 2.4. Due to the high neutral axis,the material uncertainty has a greater influence on the reliability indexes.[Conclusions] The method of yield strength reliability research for trimaran proposed in the study can provide good data-support for the optimization of trimarans.

[Objectives]To study the influence of side-hull layout of trimaran on its motion performance,[Methods]A new method which is based on Monte Carlo method and potential flow theory is presented to help side-hull layout of trimaran optimize its motion and multi-objective optimization models and calculation techniques are provided. Firstly,we use Monte Carlo method to select the solutions and use AQWA software(potential flow theory) to calculate the motion responses of these solutions. Then by comparing the rolling,pitching and heaving motion responses of these solutions,we select the optimization solution in which these three indicators are all smaller than those of the original ship.[Results] The results show that in the regular wave,the motion responses of the trimaran are reduced when side-hull is located before the mid-ship of the main-hull. In irregular wave,the roll motion of the trimaran is decreased when side-hull is located before the mid-ship of the main-hull;Also in the regular wave,the roll motion and pitch motion are decreased when we increase the lateral distance between the side-hull and the main-hull.[Conclusions]To study the influence of side-hull layout on the motion of trimaranon, using Monte Carlo method and potential flow theory can increase calculation efficiency and guarantee high calculation accuracy.

According to the problem of a trimaran cross-deck structure's fatigue strength, taking one trimaran as an example, the responses of ship motion and hydrodynamic pressure on the ship's surface in regular waves are calculated on the basis of the 3D linear potential flow theory. Next, the stress responses of hot-spots in regular waves with different wave angles are evaluated by the finite element analysis of the global trimaran structure. On the basis of linear cumulative damage theories, the fatigue damage is calculated according to the direct calculate method using spectral analysis. Finally, the effect of different sea areas and heading angles' time factors are discussed. The results show that the fatigue damage is greater when using an inshore sea area and considering the time factors of heading angles. The result can offer a reference for the design and development of trimarans.