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Electric Bilge Pump 101

Electric Bilge pumps with built-in float switch. Photo: OceanMedia
Electric Bilge pump with built-in float switch. Photo: OceanMedia

If you have ever been on a boat that is taking water, a lot of water, then you know the value of a good bilge pump.

During my sailing career I have been part of 13-man chain gang passing buckets from hand-to-hand on a sinking Dutch barge offshore in the English Channel. And I once sailed a wooden boat alone across the Atlantic that required 22,000 strokes of the pump handle to keep her afloat. I know the numbers are correct because to keep track of the amount of water coming in, I counted each stroke and made a note in the log. The leak never got any worse and I became very fit during the 3000 mile voyage.

Why am I talking about bilge pumps? Well, recently we bought a 30ft cruise/racer and when I checked the electric bilge pump, it was incorrectly wired. If you are going to install an electric bilge pump then sense dictates that it should work automatically as well as manually.

The pump in our boat was wired directly to an oversized breaker. Not only was this a fire hazard it meant that when the main battery switch was turned off, the bilge pump was turned off with it.

Some electric bilge pumps come with built-in float switches while others require a separate switch. Our pump, a Rule-Mate (RM) 500 with a built-in float switch, has three wires coming out of the top of the unit: black (ground); brown (positive); brown/white (positive). To work in either automatic or manual mode, the positive wires must go to a three–way switch. I chose a Rule 41 Switch with a built-in fuse holder and red warning light. The chandlery had a similar switch for half the price. I opened the packet and deemed it half the quality.

Most switches and bilge pumps come with simple wiring instructions that are easy to follow. The switch does not include a fuse. The manufacturer recommends a 2.5 amp fuse for the RM 500. Larger pumps call for a larger fuse. Do not exceed the size of fuse recommended by the manufacturer.

Something else to bear in mind when fitting an electric pump (or any electrical equipment) is the distance the wires run from the power source. The RM 500 called for 16 gauge wire for a run of 25ft., longer runs call for heavier wire. To size wiring, you need the wire length from the power source to the appliance and back to the power source. Doubling the straight line distance to the battery or electrical panel is not adequate. You must determine the actual length of the wire by measuring along the path it will follow-up, over, and around. It is not unusual for a wire run to be more than twice the straight-line distance. Remember, the smaller the wire, the larger the drop in voltage.

Notes on pumps and pump capacity

  • Pumps should be fitted in the lowest part of the bilge.
  • Wires from the pump should be fastened in such a way that they are kept dry. Don’t let them trail in the bilge water. This is important because most pumps do not empty the bilge; they leave behind a small amount of water.
  • The float switch in the RM 500 only starts the pump when there is about three inches of water in the bilge and it switches itself off before all the water is gone.
  • Leave at least two inches of clearance around the pump. This will help prevent clogging with bilges debris.
  • Pumps are rated by the amount of gallons per hour they can shift (GPH). The figure is rather meaningless because so many factors are involved. A 500 GPH pump, installed and wired correctly, and running from a well-charged battery putting out 13.6 volts, might move 500 GPH when pushing a head of water parallel with the pump, however, bilge pump outlets are set high and this calls for lift. Lift dramatically reduces the amount of water a pump can shift. The performance data given for the RM 500 says a lift of 3.35ft will reduce capacity by 140 GPH. A lift of 6.7ft reduces capacity by 270 GPH, and that is more than half!

Gary E. Brown is the Editorial Director of All At Sea. He is a presenter on Island 92, 91.9 FM, St. Maarten, and the author of the thriller/sailing adventure Caribbean High. For more information, visit: garyebrown.net

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