Fig 6: Deckhouse and cabin ventilation is protected from sea water by a combined capsize or anti-flooding air intake valve developed by the RNLI design office. When all is well, air coming in passes through a perforated box, B, past the open valve flap, C, and through A into the boat's ventilation trunking. Should the boat be rolled over, however, a heavy ball, D, responds to gravity and closes the valve flap. Should green water come aboard while the boat is upright, then a lightweight float, E, will rise up on the incoming water and close the valve..