Bilge Pump Basics: What is it, and How Should You Use It?

what is a bilge pump

A bilge pump is a key piece of equipment found on just about any boat, but for many new boaters in particular, they may find themselves asking the basic question: "What is a bilge pump?" 

what is a bilge pump on a boat

The function of the bilge pump is to remove water that collects in the bilge, which is the bottom of the inside of the hull. Most of the time, water that collects in the bilge is incidental:

  • It could be rain water or water from spray collecting in the boat;
  • Wash-down water that collects in the bilge while the boat is being cleaned; 
  • Or water from built-in coolers that drain to the bilge.

On larger boats, built-in coolers, the lips around large hatches, and even drink holders may be drained overboard, but on smaller boats this water is simply routed to the bilge. Water may also collect in the bilge from minor leaks, such as water dripping from the shaft gland on an inboard boat.

Very few boats have a bilge pump large enough to keep the boat from sinking in a catastrophe, such as a large hole in the hull. In that scenario, the bilge pump or pumps may give you time to either repair or mitigate the leak, to call for assistance, or to prepare to abandon the boat in an orderly manner.

Parts of a Boat: Understanding the Anatomy of a Boat


Where are Bilge Pumps Located on a Boat?

At least one pump or bilge pump pickup should be installed at the lowest point in the bilge. Larger boats should have one in each enclosed area that can retain water. Discharge outlets need to be a minimum of eight inches above the waterline.

where is a bilge pump located on a boat

All bilge pumps on the boat should be accessible, so you can inspect the pump and its float, and clear debris from around the pump pickup. If the pump is located in a spot that’s inaccessible or hard to reach, such as below the engine on a sterndrive boat, consider relocating it if possible.

How and When Should You Turn on a Bilge Pump?

The pump may have a float or switch to turn it on automatically when water collects in the bilge. This is especially important if the boat is kept in the water, as you’ll want the pump to activate after a heavy rain storm, for example. The pump may also just have a manual switch at the helm. Often the bilge pump switch has an “auto” and “manual” setting.

Regularly inspect the area around the pump for debris, which could clog the pickup or foul the pump. Surprisingly, this is especially important on new boats, as all kinds of construction debris – sawdust and fiberglass dust, bits of foam and epoxy, even screws—can work its way back to the bilge.

Periodically, check the function of the float switch, which can get crusty or corroded over time.

  • Wiring for the bilge pump should be routed up to keep it out of the damp bilge, and any wiring connections should be water-tight.
  • If the pump is not on frequently, check its function by running a little water into the bilge.
  • This confirms more than the function of the switch, as you’ll actually see that the pump is able to move water.

Packing a Boat Toolkit


Selecting a Bilge Pump for Your Boat

A bilge pump is rated by its ability to move water, expressed in gallon per hour, or GPH. The rating is relative, however, and a good rule of thumb is that the pump will move about 60 percent of its rating. The distance and height it must move the water, and the type of discharge hose, all affect the real-world pumping rate.

If the discharge hose is a corrugated type, replacing it with a smooth-wall hose can increase the flow rate by up to 30 percent. It seems logical that a small boat can make due with a pump with a lower flow rating, but the opposite is true; in an instance of a hole in the hull or another major incident, that water is going to impact a small boat much faster than it will a larger boat.

Experienced boat owners use a two-pump strategy, especially if they head far from shore. A smaller (400-500 GPH) automatic bilge pump is mounted low in the bilge to pump out rain other incidental water, and a high-capacity pump (3,500 GPH) mounted higher to handle a more-serious situation.

Read Next: Basic Boat Maintenance Checklist

You May Also Like: