Investigation of Longitudinal Vibrations and Slamming of a Trimaran in Regular Waves
Authors: Tang Haoyun , Ren Huilong , Wan Qian 
Published: July 2017

Slamming and longitudinal vibration are two important factors during the preliminary structural design of trimarans. In order to better understand wave-induced loads of trimarans in regular waves, hydroelastic experiments were carried out to observe the wave slamming and longitudinal vibration using a segmented self-propelled model. Details of the model design and the experimental method are provided. Characteristics of the slamming pressure and the wave-induced response were measured under different wave lengths, wave heights, and sailing speeds. Strain and pressure gauges attached to the model in different locations were employed to investigate the distribution of the wave loads and pressures. The high-frequency and low-frequency components of the measured loads were separated using Fourier filter, and the influence of environment parameters on these load components were analyzed. In the high-frequency load, the effects of whipping and springing were found, and their characteristics were illustrated. Based on the experimental results obtained, some considerations for the structural design of trimarans are discussed.

1. Introduction
Due to the special structure design of trimaran, the slamming and hull vibration that the trimaran is subjected to are always more complicated than the load response of an ordinary ship. Besides the bow slamming, the slamming at cross-bridge and wave load change caused by mutual interference of the main hull and demihull add to the challenges encountered by research on trimaran hull vibration and slamming.
In order to obtain a comprehensive understanding of trimaran vibration and slamming, various model tests have been conducted by many researchers over the years. Hampshire et al. (2004) undertook a series of segmented model tests in QinetiQ Rosyth, and the experiment results provided data support for wave load prediction methodology. Kennell (2004) designed a back-spline structure in segmented model and conducted the model experiment under different speeds to determine the extent to which the sailing speed influenced wave load and slamming. Chao (2008) used a partial trimaran model in water entry test to observe the phenomenon of cross-bridge slamming and studied the influence of air-cushion and water splash effect on the pressure peak. Wang et al. (2010) undertook a model towing experiment in different wave azimuths and speeds, observing the characteristics of trimaran load response under different wave conditions. In a different study, Hu et al. (2010) carried out a set of irregular wave experiments on a trimaran model. The test results were used to determine the design wave load for the trimaran preliminary design. Meanwhile, Peng et al. (2011) studied the cross-bridge slamming by using a small-scale trimaran model water entry test. Through comparative analysis of numerical prediction and experimental results, the relevant parameters of numerical simulation were determined. Yu et al. (2014) observed the hull girder high-frequency vibration caused by slamming in the trimaran model test and validated its importance for the whole hull girder vibration.