XblocPlus is a single layer armour unit that is recently developed by BAM Infraconsult. It is the successor of the Xbloc. An important advantage of the XblocPlus is the relatively high construction speed compared to the Xbloc. The increased construction speed is caused by the possibility to place XblocPlus in a uniform pattern and the relatively large surface area that can be covered with a small number of blocks. A disadvantage of XblocPlus is the limited block stability at the top armour rows of a low crested structure. The limited stability is caused by the reduced interlocking capacity of the blocks on the top armour row. In previously performed studies, a back support is placed behind the blocks to stabilise the top armour row on the front slope. The back support of a concrete element significantly improves the stability of these armour blocks. However, at normative wave conditions, the stability of the top armour row on the front slope is still insufficient to meet the target stability (Ns,c = 3.0). The stability of the top row on the rear slope is not investigated in other studies. Consequently, no conclusions can be drawn about the stability of the crest blocks on the rear slope. In this study, physical model tests in a wave flume were performed to assess the effect of crest modifications on the stability of the top armour rows on the front and rear slope. The main objective of this study is to design a stable breakwater crest that has the best hydraulic performance with respect to the economically most feasible breakwater dimensions. After an exploratory study into the meaning of the main objective, it can be concluded that a relative crest width of 1 is desirable. The relative crest width is determined as the ratio between the crest width (B) and the significant wave height Hs. The principle of previous studies is used to set a reference situation. This crest configuration contains a concrete element between the top armour rows that functions as a support. This is a rather wide structure with a relative crest width equal to 2.7. After testing the reference situation, the top row on the front slope turned out to be normative for the crest stability. The found stability at failure coincides with the stability found in previous studies. The blocks on the top row of the rear slope remained stable. Also after reduction of the crest width to a relative crest width of 1.9, the crest blocks on the rear slope remained stable while the crest blocks on the front slope failed. It can be concluded that the blocks on the top row of the front slope are normative for the crest stability. The weight of these blocks is too limited to meet the target stability. By application of modified XblocPlus blocks is attempted to obtain stable blocks on both top rows. The adjusted blocks are much bulkier and consequently heavier than the original XblocPlus blocks. The blocks shape is modified in such a way that the crest blocks on the front and rear slope can be placed back to back without the support of a crest element. As a result, the relative crest width is reduced to 1.1. Several variants of modified crest blocks are tested to acquire information about the stabilising effect of the applied modifications. Modified blocks with an increased permeability, an increased interlocking capacity and a relatively large resistance against rotational instability were tested. Overall, the bulky block shape, the block's permeability, and the interlocking generated by the almost vertical back of a block, appear to be very important for the crest stability. The acquired information of this first test series is used as a foundation for further optimisation of the permeable block. An attempt to optimise the modified block is done by reducing the volume at places that have the least impact on the block stability. Application of measures that streamline the block shape in flow direction and further increase the block’s permeability were applied in this optimisation phase. Finally, a block with a 44% larger volume than the original XblocPlus block is obtained that meets the target stability of Ns = 3.0.