Large horizontal motion of moored large vessels and breaks of mooring ropes are mainly induced by long period waves traveling into harbor. One of the most popular countermeasures is the construction of breakwaters. Meanwhile they say the construction of hard breakwater may disturb the seawater exchange between the outside and inside of a harbor. A permeable breakwater becomes suitable if it becomes effective to reduce the long period wave in the harbor. An experiment studying the wave transmission coefficient in the permeable breakwater for long swells and a numerical simulation estimating that in long period waves is carried out in this paper. The harbor tranquility for long period waves in the target port increases when the long period wave transmission coefficient becomes about 0.1 to 0.4.
