This thesis is about the influence of the specific weight on the stability of single layer interlocking armour units on rubble mound breakwaters. The hydraulic stability of the Xbloc armour unit is expressed by the stability number which is based on the assumption of dominance of lift, drag and gravity forces. If other forces have significant influence on the stability the power of one for the relative density in the stability number might change. The objective of this research was to determine the influence of the concrete density on the hydraulic stability of Xbloc armour unit and to determine if the stability can be described as a function of the stability number. 2-D Physical model tests have been done to determine the influence of the specific weight using concrete densities of 2102, 2465 and 2915 kg/m3 and slope angles of 3:4, 2:3 and 1:2. The size of the model block was held constant for all concrete densities. It is concluded from the model tests that the influence of the specific weight on the hydraulic stability of Xbloc armour layers is not correctly described by the stability number. The stability number underestimates the effect of the specific weight on Xbloc armour units for a slope of 2:3 and steeper. The underestimation increases for steeper slope angles. For a slope of 1:2 the stability number tends to overestimate the effect of the heavy concrete element, where as the normal and light concrete elements are in close resembles with each other and the expected start of damage and failure. A new stability formula for Xbloc armour units is suggested in which the power of the relative density in the stability number is a function of the slope angle and the influence of the slope angle on the hydraulic stability is taken into account.