A numerical study is carried out to investigate the natural convection in a ship cargo tank that is filled with high viscosity fluid such as heavy fuel oil. The effects of Rayleigh numbers and turbulence on the flow field and average Nusselt numbers are studied. Governing equations are solved by using the finite volume method which is incorporated in Ansys-Fluent computational fluid dynamics program. The average Nusselt numbers and the distributions of the local Nusselt number on the isothermal walls are obtained. The scaling laws between the Nusselt number and Rayleigh number are presented for 10(3) <= Ra <= 10(9). The turbulence effects on the Nusselt number is found to be high when the Ra number is higher than 10(8). It has been observed that the local Nusselt number values along the hot wall decrease quickly. Furthermore, the effect of Rayleigh number on heat transfer is found to be significant.