Bilge Pumps

Bilge pumps remove water that finds its way into your boat’s bilge via spray, rain, small leaks in plumbing or through-hull fittings, and other sources. » Read More

Bilge Pump Basics

All boats inevitably will end up having water that needs to be removed. Water may enter the boat by ice from a cooler, an overflowing livewell, leaking seals around the plug or lights, rainwater, or other sources. When water is left to accumulate in the boat and bilge area this can have unintended and undesirable consequences for one’s boat. For example: destabilizing the boat, formation of osmotic blisters in the gelcoat of a fiberglass boat, or even the boat eventually sinking.

Bilge pumps exist to remove these extra smaller amounts of nuisance water from the boat. In most boats, a bilge pump will not be sufficient to save a boat from hull damage caused by a collision or swamping of the boat caused by waves. However, this is not what bilge pumps are designed to handle. They are typically used to remove smaller amounts of water taken on and do not provide sufficient capacity to keep a truly compromised boat from sinking.

The American Boat and Yacht Council (ABYC) specifies that bilge pumps are meant for “controlling spray, rainwater, and the normal accumulation of water due to seepage and spillage.” With that in mind, never rely solely on automatic bilge pumps to keep your boat afloat when aware of damage or leaks.

If damaged, you should always remove the boat from the water and repair the issue rather than trust a bilge pump to sufficiently handle the water leak. Next, we will discuss important factors when choosing a suitable pump for your type and size of boat: 

Bilge Pump Types

Manual and Electric Pumps

Electrical bilge pumps are easy to install, cost-effective, and have a high water capacity compared to manual bilge pumps. Electrically operated pumps rely on power provided by the batteries on the boat, and if the power were to fail because of water entering the boat the pump would no longer function. To alleviate this risk, it is recommended that you carry an additional hand-operated manual pump.

While electrical pumps are much more convenient and efficient, manual pumps are also capable of moving sufficient water to evacuate unwanted water from a boat. However, a manual pump will be physically demanding and may be awkward to operate in a restricted or inaccessible bilge area.

Diaphragm Pumps

A diaphragm pump is commonly used in boats and operates a flexible diaphragm that moves back and forth to create a pumping action drawing water in and then expelling it from the pump. The diaphragm is typically made of rubber or other flexible material and as the diaphragm moves, it creates a vacuum that draws water into the pump and valve. When the diaphragm moves in the opposite direction, it compresses the water and forces it through the outlet valve.

These types of pumps can be either manual or electrically powered (automatic or manual switches). Manual pumps require users to manually move the diaphragm, and electric pumps are equipped with an electric motor that automates the pumping process. Electric diaphragm bilge pumps are preferred because of their convenience and ease of use; however, manual pumps are also carried as a backup if an extra pump is needed, or power failure occurs.

Diaphragm pumps have an advantage in their ability to expel debris and small solids in addition to the water without clogging or damaging the pump. However, a strainer is needed at the end of the system to avoid debris clogging the outlet valve. They are also self-priming, meaning that they can remove the air from the system and pump water without manual priming. These pumps are considered reliable, easy to install, and can easily remove small amounts of water from a boat. 

Centrifugal Pumps

Centrifugal pumps are the most common type of bilge pump. They are almost always electrical and used in small to medium-sized boats. These pumps consist of an impeller, casing, inlet valve, and outlet valve. In a centrifugal pump, the impeller is made of anything from metal to plastic and has curved blades or vanes designed to create a centrifugal force as the impeller spins.

This force propels the water away from the center of the rotation and out of the outlet hose of the boat. The casing of the impeller is designed to provide a pathway from the water to flow and maximize the pump’s efficiency. 

This type of pump is not self-priming and must be submersed into the water to pump. They are usually placed in the sump area of the boat’s bilge where the water is designed to collect and have a screen or filter on the inlet to avoid clogging or damaging the impeller.

This filter can be easily removed for cleaning and removal of debris. Centrifugal pumps are desirable because of their ability to remove large amounts of water quickly and their relatively high flow rates.

Considerations for Capacity & Flow Rate

1. Size and use of the boat: When determining the bilge pump you will install for your boat, you should first consider the size of your boat and how or where your boat will be used. Smaller boats typically hold much less water than their larger counterparts and will sink quicker from small leaks. So, a smaller boat that is used in rougher conditions will need a higher pump capacity than a larger boat used on a calm river or lake. Furthermore, installing a larger pump capacity is never a bad idea. When provided with the option, always go with the largest pump that your plumbing and hoses will allow to ensure your boat can remove unwanted water.

2. Identify water sources: Choosing a bilge pump is also dependent on the expected sources of unwanted water. Some potential sources can be rainwater, through-hull fitting leaks, wave splashes, or other smaller leaks. Consider the largest possible source of water and how much water could be taken on by these sources at one time.

3. Backup pumps: Boaters are typically advised to have at least one backup to the automated or electrical pumps that are installed onboard. This can help in case of failure or if the boat is overwhelmed by water and needs to have more water evacuated than a single pump could reasonably handle.

How Much Water Can a Bilge Pump Move?

Voltage Drop

Most bilge pumps are rated at power levels higher than a boat electrical system will deliver, and the gallons per hour rating is based on that higher power rating. Batteries can deliver optimal power levels when they are being actively charged through shore power or when the engine is running.

However, when not actively charging, the voltage delivered by the battery can decrease significantly. Furthermore, corroded or undersized wires can limit the voltage given to the bilge pump, decreasing the capacity even more. 

Vertical Climb

Bilge pumps are also rated when pumping water horizontally. Bilge pumps are located in the bilge, the lowest part of the boat below the waterline. Water will be pumped to reach the outlet above the waterline. All bilge pumps will have to lift the water up some vertical climb, and this significantly impacts the efficiency and capacity of the bilge pump.

Typically, this will cut down the capacity of the pump by 20-40%. If you’re looking to reach a capacity mark, be sure to up the size of the pump to make up for the loss in efficiency.

Recommended Minimal Bilge Pump Size by Boat Length

Float Switch Types

When using an electrical bilge pump, the two most common types of bilge switches to enable electrical pumps to function automatically are the built-in automatic switch or a separate float switch:

Built-in Automatic Switch

Some pumps come with a float switch integrated into the housing and wiring of the pump itself. This helps to simplify the installation and makes it easier to install into tight bilge areas. The built-in float switch determines the water level based on the switch height when water is present.

The switch will then allow the pump to operate until the water level lowers to an acceptable level. Often, these switch and pump types do not remove all the water from the bilge and leave less than an inch of water in the sump area.

Separate Float Switch

Another popular type of switch is the separate float switch. These separate float switches can be used to convert a manual electrically operated pump into an automatic system. This switch operates similarly to the built-in automatic switches, but they are not integrated into the casing or wiring of the pump and are installed separately.

Replacing an Existing Pump

When replacing an existing pump, it is easiest to match the hose diameter, wiring requirements, and thru-hull fitting sizes to ensure the easiest replacement and installation. This will minimize the need for modifications to the boat or hull.

If you would like to install a higher capacity pump, then you will likely need to update your wiring size, thru-hull fitting, and hoses to match the new requirements of the larger pump size. For an existing installation, it is recommended to find the largest pump that will still be compatible with the existing system.

Before removing the current pump and buying new parts, ensure that you have the correct size. Like with any project, a 30-minute plan can quickly turn into a multi-day event if proper understanding and planning of the requirement are not done beforehand.

Plumbing and Installation Considerations

Hose Length

  • A longer hose will decrease the efficiency and lower the overall performance ability of the bilge pump. Use the shortest length of hose that is possible for the bilge area.

Smooth Hoses

  • Corrugated or ribbed hoses may be cheaper overall, but they will also negatively impact the performance of the bilge pump installed. They will also be more likely to catch any debris that enters the system, more likely to kink, and likely have a much shorter usable life. Always use smooth hoses, when possible, to avoid these issues.

Vented Loop

  • If a boat has a bilge outlet that is lower than the water line, it is possible that the bilge would begin to suck water into the boat rather than expel it. To ensure this doesn’t happen, it is best to include a one-way valve in the bilge pump system to ensure that water cannot be sucked into the bilge.
  • However, another option is to create a U-shape in the hose designed to trap air in the upper portion of the loop forming a barrier that prevents water from reversing into the boat. This is why you often see bilge pump hose thru-hulls high on the side of boats.

Waterproof Connectors

  • As with any electrical project or connections on a boat, always use waterproof connectors. Moisture in electrical connections can have severe consequences like failure of the circuit or electrical components, or even fire. The waterproof connectors will also safeguard against corrosion and last much longer in the marine environment. This is especially true in the bilge area where water is designed to gather from the boat.

Causes of Bilge Pump Failure and Maintenance

  1. Flow Restrictions: Bilge pumps can fail when flow is restricted in the hoses, or at the intake. This can be caused by trash, dirt, or other objects obstructing the normal operation of the pump. Be sure to check for dirt, debris, or trash in the bilge area of your boat and regularly clean it out.
  2. Siphoning: If water starts to flow backward into the boat, it may overwhelm the bilge pump and cause the pump to fail and fill the boat with water.
  3. Plugged Intake: Bilge pumps should have strainers or filters installed to prevent debris from clogging hoses, impellers, and diaphragms of the pumps. Again, regular cleaning of the bilge area is key to maintaining a bilge pump that functions properly.
  4. Wiring or Battery Issues: Saltwater, fuel, and other chemicals can have detrimental effects on wiring. Always use marine-grade tinned wire and waterproof heat shrink terminals and connectors. A pump also won’t function if the battery is drained. This is an especially important consideration if you leave your boat in a slip or dock unattended.

Final Considerations

Bilge pumps are crucial for maintaining a safe and dry bilge area on boats. They remove the unwanted and sometimes unexpected water that can lead to issues with the function of the boat or even swamping and sinking. As discussed, the most important factors when choosing a bilge pump are the boat’s size, common use cases, and bilge area and swamp capacity.

Proper consideration should be given to the plumbing and installation of bilge pumps and opting to install without understanding the possible implications can be catastrophic to the reliability of the system, electronics, and life of the boat.

By understanding the limitations, proper use, and maintenance requirements of bilge pumps and boats, you can maintain a reliable system to help keep your boat safe, clean, and free of unwanted water.

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Commonly Asked Questions about Bilge Pumps

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