Currently, most ship bottom paints contain cuprous oxide as an antifouling agent, but when they come into contact with metal parts, they do cause electrolytic corrosion. There are also ship bottom paints that do not contain cuprous oxide (UNAGI rainbow), but these have less zinc reduction, but their antifouling properties are slightly lower than those of copper oxide-based paints. Besides, it can be said that zinc protects against electrolytic corrosion, so it is natural that zinc decreases.