High-density polyethylene (HDPE), a representative thermoplastic, has garnered attention as a next generation eco-friendly material for small craft due to its excellent recyclability and impact resistance. However, its feasibility as a structural material for ship requires through evaluation due to its thermal expansion characteristics and heat distortion temperature, comparable to hull temperatures during summer. In this study, mechanical and thermal properties of HDPE were derived within temperature range above room temperature. Furthermore, heat transfer analysis was conducted to simulate the operating conditions during the summer, which covers intense solar radiation and elevated temperatures in the engine room, assessing temperature, stress and displacement distributions of structural components. Results indicated that HDPE craft subjected to ambient temperature and solar radiation during summer exhibits localized deformations and stresses due to thermal expansion, necessitating careful consideration during design and review phases. This paper emphasizes that variations in material properties are to be considered when the countermeasures for hull deformation and establishing structural safety evaluation standards.