On small passenger vessels, which material must not be used in a valve or fitting for a hull penetration?

Correct Answer: Option A — Cast iron

46 CFR 182.420 prohibits the use of cast iron for valves or fittings associated with hull penetrations. Cast iron is brittle and prone to failure under impact or vibration, which could lead to catastrophic flooding. Materials like bronze or stainless steel are required for their superior strength and corrosion resistance.