Marine transport of natural gas, predominately in the form of Liquefied Natural Gas (LNG), is of growing importance in the global energy markets. LNG is transported with vessels having cryogenic tanks without any means of external refrigeration. Hence, a significant fraction of the cargo LNG volume evaporates during voyage, which is usually called Boil-Off Gas (BOG). According to the type of the energy system, BOG can be utilized as fuel, reliquefied or burned in an oxidizer. The handling of the boil-off gas during the LNG vessel operation and the assessment of its thermodynamic properties are key issues in the technical and economic assessment of the complete marine energy systems of LNG vessels. A detailed dynamic boil-off model has been developed, which accounts for the variation of BOG mass flow, composition and thermodynamic properties during voyage. The model employs coupled non-linear vapor – liquid phase equilibrium thermodynamic equations and differential conservation equations describing the evolution of LNG quantity and composition with time. Various solution and time discretization schemes have been tested to assess the solution accuracy and stability. The model has been used to investigate the variation of the LNG and BOG quantity, composition and thermodynamic properties during typical voyage profiles of a case study LNG vessel. Variations of the thermodynamic properties and of the quantity of BOG in the range of 6 to 10% during voyage have been found, having a significant impact in operation modes and fuel consumption of the energy system. Results have been also compared with the traditional approaches used in the marine technical practice.