Electric Bilge Pumps

Nearly every manufactured boat comes equipped with a 12 volt electric bilge pump or perhaps two if you have a really big boat.

If you are expecting these electric bilge pumps to to stop your boat from sinking due to anything but a very minor leak then you are going to be disappointed.

Most electric bilge pumps are rated with their pumping capacity in gallons per hour (gph) and the biggest one I can see in my chandlery catalogue is rated at 3700 gph. I would venture that not many boats have one that size fitted.

It is most likely that the pumps rating is with no head, meaning that the discharge is horizontal with the pump not having to push any water uphill. A very unlikely scenario for a bilge pump. The higher the pump has to pump the bilge water to get it to the discharge point the more its pumping capacity diminshes.

Also the rated capacity will be with full 12 volts of battery power available, which is not always the case.  A large capacity pump like the 3700 gph model mentioned draws 20 amps which would require the engine to be running for prolonged operation.

A bilge pump installation necessarily includes a hose to pump the bilge water overboard. The discharge point must well above the water line or have an anti-siphon loop. The pump has to push the water uphill reducing its rated pumping capacity.

The hose itself can also reduce pumping capacity. A lot of bilge pump hose is corrugated to make flexible and hold its shape when bent round corners. Internally the corrugations create resistance to the flow water. Hose with smooth internal walls will create less resistance to the flow, however it must suitably reinforced so that it holds it shape.

Eventually the hose terminates in a skin fitting. The internal diameter of the skin fitting is narrower than that of the hose again restricting the flow of water.

It is estimated that a bilge pumps capacity can be reduced by between 15 and 30% by these factors.

As a comparison a 1 inch diameter hole in your boat three feet below the waterline will let in around 2000 gallons of water in an hour.

In actual fact the purpose of these pumps is to more remove accumulated water from the bilge once you have found and stopped a leak rather than keep up with a major ongoing flood. So from that perspective whatever you have installed is likely to be adequate.

Anyone wanting enough pumping capacity to control serious flooding would be better off installing an engine or generator driven high capacity pump.