Finding solutions for fully automated performing emersion and immersion manoeuvers of submerged hydrokinetic devices with which to harness renewable energies requires the study of simple and reliable dynamic models. This paper presents a simple dynamic model for an approximately cylindrical body when it performs open loop emersion motions. It includes free surface motion with a single degree of freedom. The proposed method allows a dynamic model to be obtained that can be used to both evaluate the dynamics of a submerged/semi-submerged body and design control systems for closed loop depth control purposes. This model is fully parameterized and requires a minimal computational effort. It is based on the computation of a hydrodynamic and a hydrostatic effect, namely: i) the added mass of the body in motion and ii) the buoyancy force. A simple new interpolated method is developed to compute the variation of the added mass when the body is partially submerged and the device velocities with regard to fluid are not constant. The inherent instability of any submerged compressible body is proved by using Lyapunov/Chetaev functions. The appropriateness of the proposed model has been validated by comparing simulation results with real-time experimental-laboratory prototype measured signals, resulting in excellent agreement. (C) 2016 Elsevier Ltd. All rights reserved.