What two items are necessary for a pump to discharge below the waterline?

For a pump to discharge below the waterline safely and effectively, it is essential to have a seacock valve and a vented loop in place.

The seacock valve is critical because it controls the water intake from the hull below the waterline. This valve must be closed when the vessel is not in use and can help prevent flooding.

The vented loop is equally important as it prevents negative pressure from forming in the discharge hose, which could lead to back siphoning and potential flooding of the vessel. It allows air to enter the system, maintaining the pressure balance necessary for proper operation, especially when the discharge is located below the waterline.

Together, these components ensure that the pump functions effectively without risk of siphoning water back into the boat when the pump is not actively discharging or when the vessel is at rest. This setup is a fundamental safety mechanism in marine plumbing systems.