The bilge is the lowest inner part, or bottom point, on a boat, and is designed to collect excess water. On vessels of virtually any size, a pump inside the bilge performs a very important function: to remove accumulated water by creating pressure or suction so that the water can be removed. 

Before getting underway, you should learn how the boat’s bilge system operates, the location of pumps and switches and where manual pumps and buckets are stowed. This basic understanding will go a long way in helping to avoid potentially serious problems caused by unwanted water in the boat. 

Water in the bilge can accumulate gradually during a voyage from propeller shaft leaks, a leaking portlight or hatch, wave spray, melting ice, or a variety of other sources. During a rainstorm, water can very quickly gather in the bilge. If enough bilge water is not removed, it can overwhelm the pumps. Bilge pump systems are designed to remove relatively small amounts of water and usually are not large enough to keep up with a major intrusion of water from hull damage. Relying on a pump to remove water from a known leak to keep the boat afloat is an invitation for danger. 

To remove bilge water, electric pumps wired into the boat’s electrical system are used. A sensor or float switch in the device allows the pump to engage if rising water is detected; the pump can also be activated by a dashboard switch. On boats without electric pumps, mechanical, hand-operated pumps, and/or water scoops and buckets should be on board. 

Electric Bilge Pumps

  • Centrifugal: Draw water into the pump with whirling vanes and push it out through an outlet port. The pump must be submersed in the bilge water to operate and can usually remove all but the last inch or so of water. Centrifugal pumps have a built-in strainer that can be easily removed for cleaning.
  • Diaphragm: Use a membrane to lift water up the intake hose and expel it outside the hull. Diaphragm pumps have an external strainer to keep bilge debris from clogging the valves. 

Bilge on a BoatTips for Bilges and Bilge Pumping 

  • Allowing large amounts of water to remain in the bilge, particularly while underway, can destabilize a vessel and affect its balance, performance, and buoyancy characteristics.   
  • The very first step a boater should take while preparing to get underway is to “blow the bilge.” Most boats are fitted with a fan that vents bilge air that may be saturated with potentially combustible fuel fumes. A rule of thumb is to activate the blower for a solid five minutes well before departing a dock or moorage. 
  • As part of getting underway preparation and before leaving the dock, open the bilge compartment cover to muck out the bilge and clear the pump strainer. Use rags or towels to wipe the bilge interior and the wire strainer covering the water intake hose or casing. Dispose of bilge rags or towels responsibly. 
  • Blowing the bilge is a critical step after taking on fuel at a fuel dock or manually filling the tanks. After refueling and before getting underway, blow the bilge for five minutes. 
  • Besides water, spilled fuel can find a way into the bilge that can erode the pump’s electric wiring and promote fiberglass hull blistering. The best defense against fuel in the bilge is to follow safe fueling techniques when filling the fuel tank.  
  • A hull leak caused by a collision or water shipped over the bow or gunwales during heavy weather requires immediate removal. In many cases, even high-capacity pumps will buy only a little time to keep up with water intrusion, to repair a hole, to get to shore or to prepare to abandon ship. For these and similar emergency situations, an auxiliary electric pump and a high-capacity manual diaphragm pump, as well as water scoops and buckets can serve as back-ups.      
  • Automatic bilge pumps, now common equipment on most boats, are a real convenience, but can create a false sense of security if they cycle on and off without the boat operator’s knowledge. For example, if a sizeable hull leak occurs, it could go undetected and place the boat in danger of sinking. To alleviate this concern, boaters use a cycle counter to determine how often the pump turns on/off, or install a light or buzzer that activates when the pump is on to indicate it is operating. 
  • If the electric pumps are overwhelmed or fail, use a manual (hand-operated) pump; most are designed to move substantial amounts of water (about 20 gallons per minute), but can quickly wear out the operator. Pumping water can be a tiring task for the person doing the pumping, especially if the pump is in an awkward position. Buckets may actually be more effective.
  • Effective pump operation relies on an obstruction-free bilge water outlet, usually a through-hull aperture positioned above the water line in the transom or the aft hull quarter. With the pump operating and before departing and while underway, it is a good idea to periodically check the outlet and be certain it is expelling water. 
  • When checking the bilge, use the nose and eyes to determine if fuel is present. If so, follow safety and environmental guidelines to safely remove the fuel before getting underway. 
  • The sense of taste can also determine a leak source. If the water is salty, the leak is coming from below the boat’s waterline. If the water is not salty (meaning it is freshwater), the leak is above the waterline. Particularly if the leak is more than a few drips per minute, this simple test can quickly identify the leak source and possibly keep the boat from sinking. 
  • Keep in mind that larger sailboats and powerboats with low freeboards are much more susceptible to shipping water. Sailboats often have more than one bilge pump, so if using a sailboat, it is important to know where the pumps are located and to ensure that they are operating properly while underway.  

Though mostly out of sight, a boat’s bilge and accompanying pump system are critical components to check before departing for any length of voyage and while underway. A properly operating bilge pump keeps a boat in balance and maneuverable, and avoids concerns about sinking.