Vessels moving on parallel courses on the border of two mediums interact together. Ship to ship interactions are caused by irregular pressure distribution around the hull. Higher velocity of the fluid between hulls causes formation of the decreased pressure amidships. This is the base of suction forces and moments generation. Analytical computations and computer based CFD simulations were carried out on the basis of 3DOF models of two training ships - VLCC and Passenger Car Ferry. The results of both methods are comparable. Therefore the analytical semi-empirical equations can be implemented in a future control system, which requires ship to ship interactions estimations to ensure proper working conditions.