Glass-reinforced nylon Marelon fittings are lighter than bronze and not subject to galvanic corrosion.For use above or below the waterline, Marelon fittings like these Mushroom Head, Threaded Thru-Hull fittings are designed for years of trouble-free use. They are extremely durable, strong and unlike heavier bronze fittings, not subject to galvanic corrosion. For use above or below the waterline.StrongDurableLightweightNot subject to galvanic corrosionFor use above or below the waterline