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Thermosyphon cooling by using neon gas has been studied at temperatures between 25 K and 40 K, since it provides advantage of large capacity of heat transport, less complicated and compact construction. This cooling system seems to suitable for a variety of on-board mechanical system with superconductors such as rotating machines. However, it is necessary to clarify the effect of both pitching and rolling motions on thermosyphon. They are temperature stability and the maximum thermal load to maintain the target temperature. In this study, we introduced the thermosyphon cooling system into the research vessel "Shioji-maru", a 425 ton vessel and studied its working properties. We focus on the heat load capability, which is applied to the evaporator, under the influence of the pitching and rolling of the vessel. The incremental or single step-wise heat load was systematically applied to study the effect of the neon quantity on the cooling performance at sea states. The on-board testing results for cooling with the thermosyphon are comparatively discussed with those obtained on the ground. The finite pitching and rolling of the ship considerably encouraged the heat exchange. The obtained result is caused by enhanced thermal exchange on the effective surface area originating from the ship motions.

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