Bilge pumps: guide to choosing and installing the right one for your boat

Bilge pump: what it is and why you can't do without one

The bilge is the lowest part of the hull, where water inevitably collects: rain, humidity, small leaks from hull fittings, engine condensation. Under normal conditions it's just a few litres. Under extraordinary circumstances — a failing seal, rough seas, a wave shipped over the bow — it can become a serious problem within minutes.

The bilge pump is the device that removes this water, discharging it overboard. It is not a comfort accessory: it is a primary safety element, required by law on recreational vessels above a certain size and recommended on any boat that navigates away from shore. Choosing the right one and installing it correctly makes the difference between a reliable system and one that fails exactly when you need it most.

Types of bilge pump: manual, electric or both

The first distinction to make is between manual and electric pumps. They are not mutually exclusive — in fact, the ideal setup includes both.

Manual pumps — operate independently of the onboard electrical system. This makes them indispensable: if the battery is flat or the electrical system is compromised, the manual pump is the only available solution. Their output is generally lower than electric pumps, but they require zero energy and zero electrical maintenance. Diaphragm or piston models operated by lever are the classic choice for mandatory safety equipment.

Electric pumps — are the main solution for automatic and continuous water removal. They are powered by the 12V onboard system and can work in combination with a float switch that activates them automatically when a certain water level is reached. With outputs from 500 to over 3,500 litres/hour, they are far more effective than manual pumps in a real emergency.

The recommended configuration on a cruising boat is: one electric pump with automatic float switch as the primary system, one manual emergency pump accessible from inside, and ideally a second electric pump as backup on boats over 10 metres.

Choosing the right flow rate

Flow rate is measured in litres per hour (L/h) or gallons per hour (GPH) and indicates how much water the pump can remove per unit of time. It is the most important factor in your choice, but it must be read carefully: figures declared by manufacturers refer to ideal conditions (zero back pressure, minimal pipework). In practice, with the discharge hose rising to the hull side and making bends, the actual flow rate can be reduced by as much as 30–40%.

Practical guidance:

• Boats under 7 metres / tenders / inflatables: 500–1,000 L/h is sufficient for routine bilge management.
• Boats from 7 to 12 metres: 1,500–2,500 L/h as the primary pump. An adequate flow rate allows you to handle even significant infiltrations.
• Boats over 12 metres or with a large bilge: 3,000 L/h and above. On these vessels it is worth considering two pumps in parallel or a system with a high-capacity emergency pump.

A common mistake is overestimating the required flow rate on the assumption that "more is better". An oversized pump draws more current, cycles on and off more frequently and wears out the float switch sooner. A well-sized pump running continuously and efficiently is the better choice.

Submersible or non-submersible: which to choose

Electric pumps fall into two broad construction categories:

Submersible pumps — the motor is sealed and can work fully immersed in water. These are the most common choice for bilge installation: they sit directly on the lowest point of the hull and draw water in from their base. Easy to install, compact and generally quieter.

Non-submersible pumps (self-priming) — the motor runs dry and draws water through an intake hose. They require a more involved installation but can be positioned outside the bilge, in a dry and easily accessible location for maintenance. They are better suited when you want easy access to the pump without opening lockers or squeezing into confined spaces.

For most installations on recreational boats, the submersible pump is the most practical choice. Rule and Seaflo offer a very wide range of submersibles in different outputs and formats; Johnson Pump and Jabsco stand out for build quality across both categories.

The automatic float switch: how it works and when to use it

The float switch is the device that automatically activates the pump when the water level in the bilge exceeds a preset threshold. It is a fundamental component for two reasons: it allows infiltrations to be managed even when no one is on board (boat at the dock, night in harbour) and ensures the pump kicks in without you having to remember to check.

There are three main types:

• Mechanical ball float: the simplest and most reliable. A ball inside a housing opens or closes the electrical contact based on the water level. No electronics, no delicate components. The weak point is that it can jam in the presence of debris or oily residue.
• Electronic float switch (electrode sensor): detects the presence of water via contact sensors. More precise and less prone to mechanical jamming. Requires continuous power even in stand-by mode.
• Float switch integrated into the pump: many submersible pumps — particularly Rule and Seaflo models — have the float switch already built into the pump body. A compact solution, ideal for small bilges.

Whatever type you choose, the float switch should be inspected every season: check that it moves freely, that it is not encrusted, and that the electrical contact works correctly by operating it by hand with the pump connected.

Installation: the critical points you cannot get wrong

A correct installation is what turns a good pump into a reliable system. The main steps are not complicated, but there are details that make all the difference.

1. Pump position
The pump should be installed at the lowest point of the bilge, where water naturally collects. If the bilge has multiple compartments, consider whether to install a pump per compartment or channel everything to a single point with an adequate slope. The pump should not be in direct contact with the fibreglass keel without a support: vibrations can create microcracks over time.

2. Discharge hose
The discharge hose should be as short and direct as possible, with wide curves rather than tight elbows. Every metre of hose and every 90° bend reduces the actual flow rate. The diameter must match the pump outlet — do not use reducing fittings that create restrictions. The discharge must exit above the waterline or be fitted with an anti-siphon valve (sea cock) to prevent seawater from flowing back aboard when the boat is at rest.

3. Electrical wiring
Use only tinned copper marine cable, with a cross-section appropriate to the current drawn by the pump and the length of the run. The circuit must be protected by a correctly rated fuse — generally 15A for pumps up to 2,000 L/h, 20–25A for higher outputs. The fuse should be positioned as close as possible to the battery, within 30 cm of the positive terminal. All connectors must be watertight (Ancor type or similar): never use simple insulating tape joints in a damp environment.

4. Three-position switch
The standard configuration uses a switch with three positions: OFF / AUTO / ON. In AUTO the pump is controlled by the float switch; in ON it runs continuously (useful in an emergency or for testing); in OFF it is completely disabled. This type of switch panel is easy to find and inexpensive — far more practical than a simple on/off switch.

Routine maintenance: what to check every season

A bilge pump works in one of the harshest environments on the boat: damp, dirty, often with oil and fuel residue. Maintenance does not take much time, but it must be done regularly.

• Filter/strainer cleaning: most submersible pumps have a protective screen that prevents debris from entering the impeller. It should be removed and cleaned every season, and whenever the pump seems to be working less effectively than usual.
• Impeller check: on self-priming pumps the rubber impeller is a wear component. Inspect it visually every year and replace it if the vanes show signs of deformation or cracking.
• Function test: before each season, pour water into the bilge and verify that the pump activates automatically (if you have a float switch) and discharges correctly overboard. Do not assume it works just because it worked last year.
• Electrical connections check: verify that connectors are not oxidised and that the wiring shows no signs of abrasion or insulation deterioration.
• Discharge hose inspection: check for cracks, ensure hose clamps are intact and that the anti-siphon valve (if fitted) operates correctly.

Common mistakes to avoid

Having seen dozens of installations, these are the problems that come up most often:

• Pump installed too high: it cannot reach the residual water at the bottom of the bilge. Result: a few centimetres of stagnant water always remain, encouraging corrosion and bad odours.
• Discharge hose with an accidental siphon: a hose bend that dips down and then rises again creates a siphon that can draw seawater back aboard when the pump is idle. The hose run must always rise continuously towards the hull outlet.
• Float switch jammed by dirt: the most frequent problem. The float switch sticks in the ON position (pump runs continuously, battery drains) or in the OFF position (pump fails to activate). Regular cleaning and annual inspection are the prevention.
• Wiring without a fuse or with an oversized fuse: a short circuit in the bilge, where cables are exposed to constant moisture, can start a fire. The correct fuse is not optional.
• Relying on a single pump: redundancy is the fundamental principle of safety at sea. A manual emergency pump costs little and takes an hour to install. Having no backup is an unnecessary risk.

Conclusion: a dry bilge is a safety matter

Choosing and correctly installing a bilge pump is not a minor technical detail: it is one of the most tangible safety measures you can take on your boat. A well-sized pump, properly installed and kept in working order, gives you the confidence that — in the event of a sudden infiltration — the system will do its job without requiring manual intervention.

Whether you are fitting out a new boat, replacing an old pump or adding a second safety backup, at Mareshop you will find models from the leading brands in the marine industry, along with all the components needed for a complete installation.