There are various prevention methods of mooring ropes breaking on board. These methods are Rope Protector, Rubber Protector, Serving, Racking, Splicing on a part of rope near a fair leader and so on. The effect of these methods is not researched well and depends upon seaman's experience. Rubber Protector, Canvas Protector and Splicing are chosen as the subjects of the study. Mooring ropes are trailed on boards and the quays, and then it is feared that mooring ropes are stained with something like sands. The strength of ropes with sands is researched, too. In this paper, the strength of mooring ropes is examined under the repeated load in order to evaluate prevention methods of mooring ropes breaking on board. The obtained results are summarized as follows : (1) Splicing on a part of rope near a fair leader is more effective than Rubber Protector or Canvas Protector in all. (2) Without prevention methods, sands make the strength of ropes weakened. (3) Prevention methods can keep the strength of ropes with sands.