Propeller jets can have a damaging effect on bed, bank and quay structures. A block mattress is one of the various types of bed protection which can be applied to prevent the development of scour holes and subsequent undermining of bed, bank or quay structures. However, not much is known about the stability of block mattress in propeller induced loads. One of the design guidelines which can be used is the formula by Pilarzcyk, in combination with an expression for the dispersion of the propeller jet. To assess the validity of this approach laboratory experiments were performed on the stability of a block mattress in a propeller jet. The model setup comprised a non-sloping horizontal bed and a single horizontal propeller at a certain elevation above the bed. In the test section ripples were applied to mimic the typical unevenness of the bed. Fur-thermore, a distinction was made within the test programme between an ‘open edge’, i.e. an outer edge of the protection, and a ‘closed edge’, i.e. a transition between mattresses. According to the test results, the presently available design guidelines for the stability of a block mattress in a propeller jet may under predict the required thickness of an open and closed edge of a mattress. Furthermore, these guidelines may not yet cover all related aspects correctly, such as the influence of the height of the propeller and the distinction between an open or closed edge
