The permeability of a breakwater is of great importance for the stability of the armour layer. The influence of the structural permeability on the stability of rock armour units was already researched by Van der Meer using the 'notional' permeabiltiy. However, for single layer interlocking armour units the influence core permeability is rather unfamiliar. The goal of this research is to extend the knowledge on the failure mechanism of the armour layer for different structural permeability. To achieve this goal, model tests are conducted in the permeameter of the Technical University of Delft and in the wave flume of Delta Marine Consultants, Utrecht. The tests show that the stability of the armour layer decreases with increasing and decreasing core permeability.@en