Bilge Pump Guide: How to Choose the Best Bilge Pump for Your Boat

A bilge pump is not only an important safety tool which can save your life or your boat, but it’s critical for keeping your boat healthy. A good installation keeps your boat dry inside, and prevents slow creeping damage to your hull, wiring and electrical systems.

What is a bilge pump?

The bilge is the lowest point in any boat, and all water and other liquids drain into. Bilge pumps empty that space, and their primary service is keeping a boat dry.

Few pleasure craft have bilge pumps capable of keeping a vessel afloat if it gets holed. A bilge pump can slow sinking, but a 2.5 cm hole one metre below the waterline can let over 150 litres of water per minute into your boat, or 9,000 litres per hour. It takes a powerful pump to keep ahead of the flow from a single broken through hull fitting.

That doesn’t mean they aren’t important safety tools. A high-capacity pump can keep a damaged vessel afloat for hours, buying crucial time to stop or slow the leak, get the boat to safety, or for search and rescue to find you before it sinks.

Standing water left in your bilge can lead to unpleasant smells, electrical problems, mould, and even hull osmosis. Most boats accumulate water from condensation, rain, small leaks, spray and waves, and dripping shaft seals. Your bilge pump is your first line of defence from accumulated water.

Types of bilge pumps

There are many categories of pumps, but two styles are most suitable for permanent bilge pumps – centrifugal and diaphragm pumps. Other styles of pumps may be damaged by running dry, can not self prime, or are in other ways less suited.

Diaphragm vs. Centrifugal

Centrifugal pumps install in the bilge, diaphragm pumps can be installed above the bilge and draw water up a hose to the pump. There are advantages to both, and which is better for your boat depends on its size, what fits, and how you will install your pump. Both types can run dry for some time without damage.

Centrifugal pumps can not prime a feed hose and self prime, they must be submerged to work so you have to put them deep in the bilge. High volume centrifugal pumps are excellent solutions for emergency use and are not too expensive. They need electrical connections in the bilge, so wiring splices may be in the water if you aren’t careful.

Diaphragm pumps self prime and the body can be installed out of the bilge, so wiring for electric diaphragm pumps needn’t get near the water. Head pressure and hose resistance do not affect diaphragm pumps as much as they do centrifugal pumps.

Manual versus electric – which to choose

Every boat bigger than a dinghy should have a manual pump, though it need not be a permanent installation in small boats. Larger vessels should have both, with a manual pump to back up the electric one. It’s not an either or choice.

An electric pump is only as good as your electrical system. A running pump will drain batteries over time, and if wiring and batteries get wet, the system can short and fail. A manual pump won’t run out of batteries and will run as long as your crew can.

Electric pumps with float switches keep the boat dry when it’s unattended. Even a small electric pump can make a big difference removing incidental water when you’re not on the boat.

Should I have more than one bilge pump?

Besides a manual pump, there are reasons to consider additional bilge pumps. If your boat has multiple separate bilge compartments, each needs a pump.

Many larger boats will have a small, low volume submersible pump at the very bottom of the bilge, with a larger emergency pump installed in a higher position. The small pump keeps dries out routine water build up, and the big pump is for emergencies.

For emergency use, you can’t have too many pumps, and several units can work together.

How to choose the right bilge pump for your boat

State laws in Australia tell you the minimum requirements for your type of boat, but that’s just a start. And they rarely specify sizes.

Selecting the right size (gph)

Ratings in Gallons Per Hour (GPH) or Litres Per Hour (LPH) provide how much water moves from the outflow port with no resistance – the open flow rating with no hose attached. It’s for ideal conditions, which never happen.

Real-World Conditions

The pump must lift water from the bilge to the outlet. The vertical distance of lift is the head height and creates head pressure against the pump. Moving water through a hose also causes resistance, as do turns in the run, corrugated hose interiors, and plumbing hardware.

Smaller hoses have higher resistance, and the length of the hose multiplies it. Centrifugal pumps are very sensitive to head pressure and resistance, and small products with poor installations may not function at all.

A one metre lift can drop capacity by 33%, two metres can cut performance by 60%. Coupled with a long, thin hose, this could lead to a fraction of the rated output being delivered, or a pump moving no water.

Bilge pump manufacturers provide information on real-world flow rates too, so review the documentation on their website for a table of flow rates for different head heights and discharge hose diameters. Most ratings are for a full battery charge (13.6V), but pump performance drops with lower voltage.

Limited Range of Choices

Unless you’re willing to replace a skin fitting, the size of your existing discharge outlets limits your choices from a product range. Get the highest flow emergency pump you can within this limit. A more powerful pump may require an upgrade to existing wiring, so check your wiring too. Undersized wires will drop performance and can be dangerous.

The capacity of the small, primary pump is less important, but it needs enough power to overcome the smaller hose size and the head height in your boat.

Matching Pumps to Your Vessel

Different boats have different forms and needs. A small open fishing runabout will take on water in different ways than an offshore sailing yacht, and may even need a larger pump because of the risks of swamping.

A few basic guidelines about boat size can get you started, but every boat is different. It is always better to have a pump that is too big than too small.

Service Your Bilge Pump

Bilge pumps don’t take a lot of maintenance, but you should check them often.

• Run the pump until water comes out. A blocked pump will make noise, but it’s not working, so just flipping it on and listening isn’t a good test.
• Look for worn and corroded wires, in particular at electrical connections near the bilge water level.
• Check hoses for wear.
• Test float switches for free movement and function.
• Check for detritus around the strainer, which may block the pickup, and open any removable bases and strainers and clean them out.

Am I required by law to have a bilge pump on my boat?

Every state in Australia has laws about the pumps required on your vessel. Check your state for guidelines.

Tasmania – all vessels must have a bailer or bilge pump.

Queensland – requires “pumping or bailing equipment” on all vessels. Boats over 5m require 45L/min capacity, and over 8m need 70L/min.

New South Wales – vessels with covered bilges or enclosed compartments must have an electric or manual pump able to drain each compartment, and protected by a strainer.

Victoria – vessels with covered bilges or enclosed compartments must have an electric or manual system.

South Australia – Vessels under 8 metres must have a bailer OR a bilge pump; vessels over 8 metres must have two bailers AND a bilge pump.

Western Australia – bilge pumps mandatory on vessels 7 metres or longer. Automatic switches must display an indicator when on.
Northern Territory – a bilge pump is required for all vessels with covered bilges.

Fitting, Installation and Choices

As you research and shop, you’ll come across a wide range of products and loads of information. Not every pump will fit in your vessel, as a bilge can be a tight space to work and fit gear, especially on yachts with narrow bilges.

You’ll need access to strainers to clean them, and many larger pumps require external float switches. Remember, your pump needs to protect your boat from water intrusion and that’s its primary role. But do not overlook safety, so install as much pump capacity as you can.