How do I choose the right bilge pump for my boat?

Choose a bilge pump by the water it will discharge through your boat's hose and outlet, rather than the capacity printed on the box. For a cruising boat, combine automatic electric pumping with an independent manual pump and a high-water alarm, because blocked strainers, failed switches and flat batteries can disable electric pumping.

Measure the bilge and discharge

Measure the available space, the vertical rise from the pump to the discharge outlet, and the hose's internal diameter and length. These determine which pump fits and how much water it can move once installed.

Find out where the water comes from. Rainwater, stern-gland drips and calorifier relief discharge need different attention from a leaking seacock or stern-drive bellows. A larger pump does not repair a leak.

• Bilge shape: a shallow sump suits a low-profile submersible pump; a deep, narrow keel sump may suit a remotely mounted diaphragm pump with a suction hose.
• Separate compartments: check whether water can reach the pump through clear limber holes. Isolated compartments may need their own pumps.
• Access: leave room to remove the strainer and operate or replace the float switch without dismantling joinery.
• Supply: match the pump to the boat's 12V or 24V system.

Compare output at pumping height

Most small electric pumps advertise an open-flow rating with little or no discharge head. Lifting water, hose friction, bends and fittings reduce that output, sometimes substantially. Compare manufacturers' performance curves at your measured lift, allowing for hose losses.

Check the units: ratings commonly use US gallons per hour. A nominal 1,100 US GPH is about 69 litres/minute; 2,000 US GPH is about 126 litres/minute. Neither figure guarantees that flow through your boat's plumbing.

For a typical 30 to 35ft cruising yacht, the 1,100 to 2,000 US GPH class is a useful starting point for comparing electric pumps, rather than a universal sizing rule. Bilge layout, cruising grounds, discharge height and available battery capacity matter as much as length.

Some Hallberg-Rassy installations show why pump roles matter: their listed electric pumps deliver 16.3 litres/minute, while manual pumps are rated at 90 to 105 litres/minute and separate emergency pumps at 252 litres/minute. These are manufacturer capacities for particular installations, not targets for every yacht. Do not assume an ordinary bilge system can control serious hull damage.

Choose pumps and backup

A centrifugal submersible pump is compact and moves plenty of water for its size, but generally cannot self-prime and leaves some water behind. It needs a clean inlet and a reliable automatic switch.