Two new types of mobile breakwaters consisting of tire and sphere assemblies, respectively, were tested. Tests covered a wide range of wave conditions, water depths, and breakwater dimensions. Experimental measurements were made to determine the wave reflection and wave transmission coefficients, the power dissipated by the breakwaters, and the forces in the mooring lines of the breakwaters. It was found that for these types of floating breakwaters to be effective their length should be on the order of one-half to one wavelength, their depth on the order of half the water depth, and the wave steepness equal to or greater than about 0.04. The effectiveness of these breakwaters can be improved by decreasing their porosity and flexibility; however, this would cause an increase in the mooring forces and the drift, the drift being the result of the difference between the forces in the seaward and shoreward mooring lines.