material or fluid advantageously a plurality of devices can be interconnected in the form of a chain or caterpillar to provide a breakwater system alternative embodiments of the invention include means to alleviate high side loads on multi hulled vessels and a propulsion device in which the first and second structures incorporate louver valve assemblies application_number US 57665203 A description fig3 shows an example of an embodiment of the invention that could extract energy from a wave this device 12 comprises first 13 and second 14 floating vertical structures arranged substantially parallel one to another structures 13 and 14 are spaced nominally half a wavelength apart the device 12 is oriented in use so that the planes of structures 13 and 14 substantially orthogonal to the general direction of waves structures 13 and 14 are coupled together by a device 15 the device 15 may be an energy absorbing double acting hydraulic pump but in this example it is allowed to move freely in and out without extracting any energy in this configuration an embodiment of the invention is envisaged under conditions whereby tilt stroke and distance measuring devices are incorporated into the device 15 to accurately measure wave height wavelength and wave.