Current trends indicate that both the size and sailing velocity of ships are increasing, which has led to a greater focus on the importance of vibration and noise. Periodic unbalanced forces in the propulsion system inevitably give rise to vibration, and when the excitation frequency is close to a resonance frequency of a vibrational mode of the ship, significant motion of the structure may occur (Nippon Kaiji Kyokai, 1984). Close to resonance, the displacement and acceleration of the hull

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... may be large, which may lead to serious structural effects for the vessel, as well as comfort issues for the crew and passengers. To avoid this, the structure and propulsion system should be designed to avoid exciting the vibrational modes of the vessel (Okumoto et al., 2009). With the development of power electronics, electric propulsion systems have become increasingly important, especially for niche applications and military vessels. Electric propulsion systems have significant benefits in terms of mobility, reliability, and efficiency, as well as in terms of environmental sustainability. For these reasons, electric propulsion has the potential to become the dominant method of propulsion for ships in the future (ABS, 2006). To avoid damage to the vessel and discomfort to the crew and passengers, it is important for engineers to analyze the response to vibration prior to production. The finite element method (FEM) is an effective method for such an analysis, and is a convenient and efficient means of modeling the response to vibration (Bathe et al., 1996). The objective of this paper was to study the vibrations of the hull of a special-purpose vessel with electric propulsion. We used a three-dimensional (3D) FE model to ana-In vibration analysis of ships, the principle aim is to determine the natural frequencies and excitation frequencies, and use this information to avoid resonances and vibration damage. The simplest method is to prevent resonance conditions, which is effective as long as the natural frequencies and excitation frequencies can be regarded as independent from environmental conditions. For ships that use electric propulsion systems, the sources of vibration are reduced compared with those caused by a diesel engine or other combustion-based propulsion systems. However, the frequency spectrum of these vibrations may be different; therefore, to understand the characteristics of the electric propulsion, we also should investigate how the ship responds to these vibrations. We focused on a 1,000-ton deadweight (DWT) ocean-research vessel using an electric propulsion system and analyzed the response to vibration.