Thermal Marangoni convection in fluids with various Prandtl numbers (o[10(-3)]-o[10(3)]) inside a rectangular open boat is investigated experimentally and numerically. In particular, for low-Prandtl-number fluids, e.g., liquid metal, the observation of Marangoni convection is very difficult, because the existence of metal oxides on the free surface disturbs the generation of Marangoni convection. However, development of a double crucible system and control of the atmosphere around the liquid metal have made such experiments possible. In the present experiments, the velocity at the center of the free surface under various conditions is measured. Using the experimental and numerical results, correlation equations are derived using dimensionless numbers, Re, Ma, Pr and A (aspect ratio), These equations are applicable for estimating Re, and for judging which of either Marangoni or buoyancy convections is dominant in the fluid.