Added mass values for plates partially submerged in water are obtained from a series of experiments. Both solid and perforated plates are tested. Each plate is circular and is constrained to move as a piston in a horizontal cylindrical container. Water flow past the perimeter of a plate is controlled by the narrow gap between the container and the edges of the plate. The perforated plate has an equilateral triangular pitch pattern of circular holes. The plate vibrates as a rigid mass on a flexible suspension system. The plate excitation is produced with an impact hammer, although shaker excitation is used to confirm natural frequencies. The natural frequency of the plate is measured for seven water levels in the container ranging from empty to full. For each water level, added mass is computed using the ratio of the natural frequency in air to the natural frequency in water. The added mass for fully submerged plates is compared to published experimental and theoretical results.