Pelican cases are molded plastic containers that seal with an airtight and watertight gasket. Each bulkhead could be sealed by watertight doors. They were divided into 10 main compartments by transverse watertight bulkheads. The hull was subdivided by 20 transverse watertight bulkheads. The hull was subdivided by 18 transverse watertight bulkheads. Though not entirely watertight, these longitudinal bulkheads trapped water between them. The hulls were divided into eighteen watertight compartments. The White Star vessels were divided by transverse watertight bulkheads. The hull was constructed of iron and divided into eleven watertight compartments. Each float was divided into four watertight compartments. Subdivision of a ship's hull into watertight compartments is called compartmentation. Five of the ship's watertight compartments were breached. Transversely the ship was divided into 14 watertight compartments. Frederick Stonehouse also held that the lack of watertight bulkheads caused "Edmund Fitzgerald" to sink. The bottom part of the reactor is enclosed in a watertight compartment.