Simulation of unmanned underwater vehicles (UUVs) plays an important role in the development of submarine robots. The test of controllers in different simulated scenarios is crucial in order to avoid the lengthy development on the real vehicle. Robot simulators like Gazebo have been extensively used for this purpose, even if off-the-shelf physic engines do not consider one of the main aspects of hydrodynamics: the added mass. This lack of representation does not allow for a realistic test scenario of the underwater environment. This paper proposes a mathematical approach to consider the added mass effect in the simulation of UUVs. The proposed solution can overcome the inability of usual robot simulators to represent the added mass effect without changes to the physic engine.