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Everything you need to know about bilge pumps

What do you do if your boat's taking water and you're far away from land? "Man the pumps!" of course, but bilge pumps aren't just for emergencies: they're often used to pump out water from unmanned boats in the harbour - and that's exactly what an automatic pump should do.

Find out everything you need to know about bilge pumps in our guide. We also tested 10 pumps and compared the results for you to read in our bilge pump test. Find the right pump for your on-board use now.

Why do I need a bilge pump on board?

It doesn't matter where it enters your boat, water will eventually find its way to the deepest part, under the floor boards. That's where the bilge is. A little water in the bilge is quite normal on almost all boats

Water ingress is a regular occurrence, especially on wooden boats. In fact, most sailors are as used to pumping out water as they are to setting sail. During a long winter break on land, the wood dries out so much it starts to contract. Only after it is wet again does it swell and make wooden yachts completely waterproof.

So, before you put your boat into the water, make sure there is an automatic bilge pumpin place to keep the water level low, especially if the boat is unmanned for a while.

With other construction materials, water in the bilge is less likely to occur, and if so, it doesn't always mean there's a critical leak. Water can collect due to spray or rainwater on deck that can get inside through openings in windows, hatches or fittings. Water can also often find its way into the ship through the mast on sailing yachts. On colder days, water in the bilge can even result from condensation on the inside walls of the boat, or from droplets on heat exchangers of coolers or air conditioning systems. There's no need to sweat if a puddle forms in your bilge. Most of the time it evaporates on its own, but if it gets too much, a bilge pump is needed.

Sinking ship

NOTE: If your boat starts taking water while sailing, this is a critical emergency! Even if your pumps keep the water level low, a report must be made immediately to the rescue team at sea or the marine police. Don't forget, lifeboats carry powerful mobile pumps and your pumps may suddenly fail. If ingress is due to hull damage, the leak may also suddenly get worse if a wave hits or the water level rises. If your onboard electrics then fail, so will your pumps and radio system, making it much more difficult to raise an alarm and get rescue.

Types of bilge pump

Electric or manual bilge pump - what do I need?

Emergency bilge pumps can be lifesavers. They must be appropriate to the size of your boat if they are to perform as best they can. Bilge pumps are operated either as electric or manual bilge pumps. What is the difference between an electric and a manual bilge pump?

Many bilge pumps have an electric motor and need power to function. The advantage of this kind of pump is that in an emergency, it can be left to do its job on its own, leaving the crew free to search for the leak and fix it. However, if the power fails, which could happen if seawater causes a short circuit or if the battery is submerged under water, a manual pump will be your last chance to reduce the water level.

These usually have a large rubber diaphragm and are operated by hand with a lever or handle. For safety reasons, it is advisable to have at least one high-capacity manual pump and multiple electric pumps on board.

Are all electric bilge pumps also automatic bilge pumps?

Some electric bilge pumps have a built-in sensor that automatically starts pumping when the level of water in the bilge rises.

These are particularly reassuring to have on board, especially if your boat takes water while it is berthed at the jetty, and there's no-one on board to notice it. An automatic bilge pump will also remove water that could enter through a hatch left open during a downpour. A switch can even be installed independently of the pump. So, any electric pump can be converted into an automatic bilge pump with the right accessory.

TIP: If possible, switch off the automatic mode on the pump while sailing, or at least connect it to an alarm device. Many pump manufacturers offer pumps with a panel switch as an accessory for their bilge pumps. When these pumps switch on, an audible and visual alarm triggers. If there is no such alarm, the automatic system could be activated by an overflowing toilet or burst cooling water hose, which can fill the bilge with water and the pump will carry on pumping until the batteries run out. This is why bilge alarms are often installed, which work completely independently of the pump and alert as soon as there is water in the bilge. You can then activate your non-automatic pumps as soon as possible and not when you start to see water coming up through the floorboards.

When are portable pumps better and when are fixed pumps more suitable?

It pays to keep a powerful mobile bilge pump handy and easily accessible for real emergencies. If you have a stable 240 V mains supply on board, it's a good idea to have a high-performance submersible pump on board. Caution: These pumps usually draw enormous short-term current when starting that is significantly higher than the current consumed during operation. So, try your pump without shore power and when batteries are not fully charged to see if your inverter has any problems.

Manual pumps can also be permanently installed on board. This has several advantages: Fixed bilge pumps should be installed in the deepest part of the bilge. Water only collects there when the boat is horizontal. It will take quite a large amount of water that gets into the boat through spray and waves to reach a permanently installed pump, because as the boat heels, the water sloshes around from side to side. This is all the more so if the boat starts to sink over the bow after a collision. – It will likely stay dry amidship and aft for quite a while. If space is limited for carrying a mobile pump on board, it’s worth making the intake hose as long as possible, so that your manual bilge pump can also be used to pump water out from the side of the bilge. Attention: The hose extension should only be as long as is absolutely necessary, making sure that hose diameter is not impeded when connected, thus reducing output capacity.

How does a bilge pump work?

Bilge pumps on boats are suction-pressure pumps. They can be operated by a motor or manually. A rubber diaphragm pulls water in through an intake valve, then pumps it out through an output valve. When the diaphragm retracts, a vacuum is created, which sucks water from the bilge into the pump through a valve. When the pump is then compressed, overpressure is created, the valve in the pump inlet closes and another valve opens outboard. This forces the water out of the diaphragm. The amount of water expelled on each pump depends on the maximum volume of water that can enter the diaphragm. This is why manual bilge pumps are generally larger than electric bilge pumps: With an electric motor, you can easily make hundreds of strokes in the time it takes for one stroke of a manual pump.

What kinds of switches are available for bilge pumps?

If your boat is left unmanned on the jetty for a while, an automatic bilge pump can be useful. You'll sleep much better not worrying about your boat sinking in the harbour, something which actually happens there more often than at sea. The reason for this is that leaking seacocks or cracked hoses are usually quickly noticed when you're sailing, less so when the boat is berthed in the marina. If the boat then starts to fill with water, the pump will trigger and even without a loud alarm, the constant humming and splashing is usually enough to attract the attention of neighbours and harbour staff. But an automatic bilge pump is also good to have on board when you are sleeping on the boat. Emergency pumps like this can be activated in different ways, depending on the model: most common types are float switches, which are triggered when the water rises and lifts a float on a lever to a certain point. Other variants also exist, the simplest version is a hollow ball that is pushed upwards and presses a button when the water rises, triggering the pump. Contact switches for bilge pumps work in a different way: they use the conductive properties of water. If both contacts come into contact with water at the same time, the water sensor reacts and activates the emergency bilge pump. Without moving parts, the emergency pump release mechanism cannot jam or be blocked by dirt in the bilge water. Float switches are not particularly suitable for use in swell, when the water sloshes back and forth. Above all, a contact switch prevents major disasters in the port! After all, you certainly don't want diesel (because this can also collect in the bilge under certain circumstances) in the bilge, but certainly not in the harbour.

TIP: The better choice for an automatic pump is therefore a contact switch.

If a fuel line ruptures, fuel will also collect in the bilge. Since diesel and petrol are poor conductors, an electric contact switch won't trigger and activate the automatic bilge pump, avoiding it pumping oil and diesel into the harbour. Therefore, it is worthwhile using a contact switch for automatic bilge pumps. Automatic contact switches or float switches can later be fitted to all pumps as an upgrade.

How much power does a bilge pump use in automatic mode?

The downside of pumps with contact switches is that the sensor will always consume a small amount of power, even when switched off. On the other hand, versions with actual float switches only require electricity when they are physically pumping. Nevertheless, this shouldn’t be a problem in the harbour, however, because the boat is usually connected to shore power there.

What should I do if water ingress occurs during a boat trip?

  1. Activate all pumps if they do not start automatically.
  2. If possible, assign a crew member to the manual pump.
  3. If there is a large amount of water and the cause is not immediately clear, make an emergency call to be on the safe side.
  4. Find the cause and try to close or reduce the size of the leak.

Choosing the right bilge pump

Which bilge pump is right for my boat?

After a collision, most crew members will be most concerned about water getting into the boat. In the unlikely event of a collision, the bilge pumps installed on your boat will quickly be pushed to their limits. The amount of water that can enter the ship through a hole, even with a diameter of only 20 centimetres will be too much for any pump to handle. That said, most ships, however, do not sink due to damage caused by a collision, but due to smaller leaks, for example from faulty hoses, broken seacocks, leaking toilets, etc. The minimum pumping capacity of an automatic bilge pump should therefore be matched to the largest through-hull through which water could penetrate unnoticed at the berth. Example: With a usual 3/4 inch through-hull outlet for the WC, half a metre below the water surface, around 60 litres of water per minute flow. Without a pump, this would mean that around 600 kilograms of additional weight would enter the ship every 10 minutes.

NOTE: Part of the safety briefing with the crew should include the right way to inform others of a problem! Instead of casually remarking, "Oh, my bag’s getting wet on the floor there", water on the boat should immediately be communicated loudly and clearly to everyone on board. Encourage crew members to give precise messages such as: water on the bow, ankle high! Doing so will help to quickly estimate the size of the leak. After all, if one moment ‘only’ your ankles are wet and 20 seconds later it’s up to your calf, that’s pretty important.

Attention: How much water can different bilge pumps expel? We pitted a number of bilge pumps against each other and compared the results. View the video here: "Bilge pump test

How many bilge pumps are necessary?

Depending on the size of your boat, emergency bilge pumps should be strategically placed throughout the vessel. This is particularly recommended if your bilge is divided into several individual sections. In this case, a pump for each of these parts makes perfect sense. On an average 35-foot yacht with a fairly wide and shallow hull, bilges also tend to be rather low. Three electric pumps work well: one in the bow and one aft of centre on each side of the bilge. The bow pump is only used when there is heavy water ingress. The two pumps in the deepest part of the bilge are more important for everyday use. In addition, there should always be at least one hand pump on every boat. Its suction point is ideally a large-volume fixed hose with some play in the middle of the ship that can also be moved sideways in the bilge if necessary.

How much power does a bilge pump consume?

Pump motors use a lot of energy to move water through the hose and out of the boat. This means a comparatively high power requirement. Practical 12-volt pumps with a flow rate of more than 60 litres over a height difference of two metres quickly consume eight or more amperes of current. Therefore, in case of water ingress, one should quickly start the engine to produce electricity in order not to strain the batteries excessively. Make sure that shore power is available at the berth.

Installing a bilge pump

How is a bilge pump installed and connected?

A key factor on smaller boats is the height of the bilge, from the lowest point to the floorboards. Pumps with float switches in particular must be mounted so that the switches can float freely. The overall height of the pumps differs depending on the manufacturer, but is usually around ten centimetres. There are also horizontal models, which take up less space. The bilge pump through-hull must always be open, and not submerged even when the boat is at sea. Often the transom is particularly suitable for this. However, there is often no way to completely avoid the outlet being submerged, if only briefly. So, it is advisable to use a gooseneck valve or check valve. The hose connections on the pump and on the on-board outlet should be secured with stainless steel hose clamps so that they do not come loose under the pressure of the water jet. Automatic switches should always be placed near the actual pump so that it is not activated too soon and can run dry. Many pumps also need the water to be pumped out for cooling and to lubricate rubber valves and diaphragms. - Prolonged dry running will damage these models. When installing the pumps, it therefore makes sense to plan well. Above all, the bilge pump should not be able to tip over or slip. Some models are therefore screwed or glued to stringers or other parts in the bilge. If necessary, a bracket could also be fixed to the bilge. Under no circumstances, however, should a pump be screwed directly onto the hull.

Which hose should I use with my bilge pump?

Bilge pumps must be able to use 100% of their power to expel the water. A rigid hose, reinforced with clasps or a spiral, if necessary, is ideal for this. The more rigid the hose, the less likely will kinks occur or objects be able to squash it, reducing the hose diameter. Make sure the inside of the hose is smooth. You don't want the clasps to make indentations on the inside, which will create friction and impede water flow enormously. The same applies to the suction hoses of manual pumps.

A rigid hose is even more important here so that it cannot constrict during suction. In principle, the diameter of the output hose going to the through-hull must not be smaller than the diameter of the opening on the pump, so that the pump can reach its maximum output.

Which fuse is required for a bilge pump?

Depending on the pump, the manufacturer specifies the minimum size of the electrical fuse for a bilge pump. This specification then also determines the cable cross-section of the supply lines. Larger pumps usually draw enormous short-term current when starting that is significantly higher than the current consumed during operation. It is therefore worthwhile to use slow-blow fuses for pumps. If an automatic bilge pump is connected directly (via a fuse) to the battery, the rest of the mains supply can be switched off during longer absences from the boat and the bilge pump will still be ready for use. Spare fuses should be kept in a waterproof container right next to it so that you don't have to search for them in an emergency.

Everything you need for correct connection of an electric bilge pump. The following accessories are required to connect a bilge pump correctly:

  • Suitable seawater-proof hose clamps to connect the hose to the bilge pump
  • A suitable fuse mounted in the positive line
  • Make cable connections and fuse as far above the pump as possible to avoid corrosion
  • Spare fuses
  • Hose line

Bilge pump maintenance

Electric bilge pumps are largely maintenance-free provided the bilge is clean. Failures are usually caused by dirt (especially long hairs) clogging the valves or collecting in the pump diaphragm. Dismantling small inexpensive pumps is not an easy task, as their compact design makes this very fiddly. Therefore, pumps are often mounted directly on a strainer, which is designed to keep the coarse dirt away from the pump. If such protection is missing, it is worthwhile using a simple plastic tea strainer. On models with a separate intake manifold, this must also be protected from penetrating dirt. Manual bilge pumps should always be cleaned after use. The rubber must also be examined for breaks or brittle spots. Well-known manufacturers offer spare parts for all pumps.

What manufacturers make bilge pumps?

Traditional bilge pump manufacturers such as Johnson, Rule, Jabsco and Whale have their roots in commercial shipping and often also manufacture manual pumps with certification for vessels that are legally required to carry pumps. In Germany, vessels used for commercial purposes are legally required. In recent years in particular, electric manufacturers from Asia have also entered the market. These pumps differ only slightly in terms of performance. The traditional manufacturers, however, usually have worldwide service networks through which we can ship spare parts to remote ports and offer suitable accessories for installation.

Author Hinnerk Weiler

Author Hinnerk Weiler

Hinnerk Weiler, sailing journalist, long-distance sailor and sea dog explains about solar on board. As an experienced sailor and expert in boat technology, he knows what he is talking about.

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