Cofferdam (from the German Kofferdamm) is a shipbuilding term used in two meanings.
- A narrow compartment that serves to separate the rooms on the ship. It is used to exclude the leakage of gases from tanks.
- The compartment between the armor belt and the watertight bulkhead. In the design of armored cruisers, it was located at the waterline, between the inclined armored deck and the outer side. It was intended to localize damage to the skin, often filled with cellulose or coconut peelings.
When the vessel's bunker is intended for liquid fuel, then it must be separated from the cargo hold by a narrow impenetrable compartment called a cofferdam; the cofferdam eliminates the danger of oil products entering through the bulkhead leaks into the adjacent room. Cofferdams are also arranged in inter-bottom compartments to separate two compartments adjacent to each other, if they wish to avoid the possibility of mixing liquids in these compartments, such as fuel and fresh water compartments.