The protection of breakwater structures is in this research considered. The design of rubble mound concrete armour layers for breakwaters can be divided in a single layer system and a double layer system. The single layer system is nowadays mostly applied because of the high interlocking, high stability of the structure, reduction of concrete use and a decrease of the construction period. There are several types of artificial units which can be placed in a single layer armour. At this moment the development of the new breakwater armour unit Crablock is going on. Small scale physical flume tests have been performed with focus on hydraulic stability. The influence of wave attack on the interlocking properties was determined by conducting some pull tests after wave exposure and in dry conditions. The performance of the Crablock armour layer is based on damage patterns for corresponding wave heights. Damage criteria are defined for displacement of units, individual movement and rocking. The results show that the placement pattern did not influence the hydraulic stability. For long waves, damage was observed in an earlier stage than short waves so there is a certain influence from the wave steepness. The higher crest level showed less displacements but larger settlements than the normal crest level. The lowest point where displacements were observed was for all tests applied with 0.66/Dn2 and 0.69/Dn2 around stability number 4.6. The design value of the stability number was in this stage assumed as 2.8, comparable to other units. This leads to a large safety factor of 1.6. The pull tests consisted of unit extractions from three different levels on the slope. The interlocking degree is defined as the ratio between the force needed to extract a single unit and the own weight. For the locations around and below SWL, the ratio between the interlocking degrees with and without wave exposure is in the order of 2 to 3. So for that locations the settlement caused a considerable higher interlocking degree. For the highest extraction above SWL, the difference is negligible. It seemed that the interlocking degree was dependent on packing density but also on the extraction level on the slope. An increase in packing density led to a higher interlocking degree. This increase was higher at the location below SWL because the additional weight of the units above became important.