In this study, the natural convection heat transfer from tubes placed in a ship bunker tank filled with heavy fuel oil (HFO) was investigated numerically at Rayleigh numbers 6.81x106 ≤ RaD ≤ 2.47x107. The effects of the Rayleigh number and different boundary conditions on the natural convection heat transfer were investigated. The horizontal walls of the tank were maintained at a low temperature (TC), while the tube was maintained at a high temperature (TH). Periodic boundary conditions were applied on the vertical walls of the tank. The finite volume method was used to solve the continuity, Navier–Stokes, and energy equations. The variations of the heat transfer coefficient on the tube, bottom and top walls are presented for different Rayleigh numbers. The results showed that the time-averaged heat transfer coefficient on the tube increased with an increase in the Rayleigh number. However, the heat transfer coefficient on the tube was not affected by temperature changes at the bottom and top walls. It is concluded that the Rayleigh number and, the bottom and top wall thermal boundary conditions have a significant effect on the final temperature of the fluid in the tank.