The settlement of marine organisms on a ship's hull can significantly affect its operational costs. The growth of these fouling organisms leads to the spread of invasive species, an increased drag caused by water friction, higher fuel consumption and a corresponding increase in greenhouse gas emissions. Hence, it is crucial to either prevent or remove biofouling. Traditional cleaning methods which are performed in situ underwater have some ecological and regulatory drawbacks. These negative consequences of fouling form the framework for the development of a laser-based underwater ship cleaning system, whose interaction between laser radiation and its lethal effect on biofouling is investigated in this work. Biofouling samples from the North Sea were irradiated underwater in a functional demonstrator. The post-treatment detachment behaviour of the biofouling was investigated for different coating systems at laboratory scale using a hydrodynamic flow cell, which simulates ship movement at speeds of up to 9.3 knots. Keywords: blue Laser, under water, shipp hull, cleaning, Biofouling, water flow