Hull pollution due to accumulation of marine plant and sea animal organisms upon ship’s surface of hull and propulsive mechanisms (i.e. propellers and shafts) is a phenomenon that is responsible for an increase of fuel oil consumption. This is due to the increase of total resistance of the ship, resulting gradually in the deterioration of the ship’s performance, according to relevant studies upon the biofouling impact on vessel performance for merchant types of ships (containerships, general cargo, VLCC). In the current study, the examined ships are of naval types (frigates, gunboats, patrol boats, landing ships, general support vessels). For these types of naval ships, the examined period was five years, beginning from the undocking of the vessel after a full hull cleaning and the renewal of their hull coating paints performed. Taking into account the average speeds, hours travelled and excessive fuel consumptions, for three conditions of hull fouling (zero, soft and hard) and comparing them with the corresponding values of ship’s designed characteristics, the result showed an extra fuel consumption for each type of ship. This excess of fuel consumed, is then calculated in respect of each type’s operating profile and the outcome was the associated yearly cost of the biofouling impact for the examined period. For this economic impact, a risk assessment is attempted, in order to evaluate the potential of an excessive fuel cost, with the use of a Risk Matrix. The assessment of the results underlines the importance of the continuous monitoring of ship’s hull condition and the profit of an optimised hull maintenance schedule, in order to manage the associated costs that derive from operating ships with a fouled hull.