The analytical bases for obtaining dynamic similarity between model and prototype for rubble-mound stability studies were reviewed, and the results of available small-scale tests were analyzed to determine whether variations in the specific gravity of the armor units in the model, compared with the corresponding specific gravity of the prototype units, resulted in undesirable scale effects. It was concluded that: (a) considering the Kydland and Sodefjed data, the stability number, as defined in this report, varies with the specific gravity of the armor units relative to the water in which the structure is situated, (b) considering the Waterways Experiment Station data and the relatively small variations in the specific gravities of the armor units, model to prototype, that have occurred in the model studies conducted to determine the design of the proposed Public Service Electric and Gas Company of New Jersey breakwaters, the scale effects due to the specific gravity ratio are negligible.