By Tom Burden, Last updated: 08/10/2016
Virtually all boats end up with unwanted water in the bilge, which must be removed. Water gets in from a propeller shaft packing gland, a leaky portlight, leaky seams in a wood boat, melting ice from the icebox and a multitude of other sources.
Leaving large amounts of water in the bilge can have several undesirable effects on your boat, including destabilizing it, lifting spilled fuel and distributing it throughout the bilge (where it can attack, among other things, the insulation on electrical wiring) and promoting the development of osmotic blisters in fiberglass hulls. And did we mention having the boat sink? Getting this water (we call it nuisance water) out of your boat is the primary function of a bilge pump.
In some cases the leak may be of a catastrophic nature caused by a hole in the hull from a collision or a wave taken on board in a storm. These situations call for very large pumping capacity. In many cases the largest pumps will only buy you a little time to effect repairs to the leak, limp back to shore or make preparations to abandon ship. For emergency situations like this, it is advisable to have multiple electric pumps and at least one high-capacity manual diaphragm pump.
Therefore, we’d like to reinforce the following words of wisdom regarding bilge pumps so you’re not disappointed in the future:
A small racing keelboat that sees lots of rough water action will need a better pump system than a large powerboat that is mainly operated on a calm lake. Here are some suggestions:
Electric bilge pumps are rated by their capacity, which is measured in gallons per hour, or gallons per minute, under open flow conditions with no restrictions to the discharge. The rule of thumb is to get the largest model that is practical for the boat. Your selection will depend on size constraints and power limitations, but choose a 1,000gph model or larger, if possible. Submersible pumps generally have great open flow capacity, which rapidly decreases depending on how high the water is lifted.
Remember that small boats need good size pumps because:
Match existing hose sizes and wire capacity, unless you want to upgrade the wiring and the thru-hull size too. Buy the largest capacity pump that uses the same hose diameters. For a few dollars more you can increase pump capacity significantly and still keep the same hose diameter. For a new installation, use the largest practical pump size.
High-capacity centrifugal pumps are relatively inexpensive and the easiest pumps to install, but they are rendered ineffective if your boat’s electrical system fails, and this is a likely occurrence if your boat takes on a lot of water. For this reason we recommend that, in addition to any electric pumps, you have at least one high-capacity manual diaphragm pump.
Manual pumps can move substantial amounts of water (up to 30 gallons per minute or so). With the exception of the higher-end pumps, most are very tiring to use, so be careful in your selection. Each gallon of water weighs over 8lb. and pumping it 15' or so is a challenging task for both the pump and the pumper. This is especially true if the pump’s location requires you to be on “all fours” or in some awkward position while pumping. An appropriate installation location and an effective pump is critical. When selecting a manual pump, it is most important to consider the gallons per stroke and the ergonomics of the pump.
Automatic operation requires either an automatic pump or a separate water-sensing switch. One disadvantage of using an automatic pump is that you may not be aware of your pump’s operation if you develop a steadily increasing leak. For example, your boat may have a brass gate valve that has become damaged from its loss of zinc through electrolysis.
If an automatic pump cycles on and off without your knowledge, you could be unaware of the leak before it becomes catastrophic and beyond the pump’s capacity. There are two solutions to this problem: Use a cycle counter to record how often your pump turns on and off. Log it. Alternatively, have a light or buzzer that turns on when your pump is energized to show you when it’s in operation.
Boats that have segmented bilges need modifications to the pump system since a single intake point may not get all the water. For a diaphragm pump: a flexible intake hose that can be manually relocated to different parts of the boat, or an intake hose manifold with several hoses connecting at a common point upstream of the pump. Another option would be to use several centrifugal pumps.
Integral automatic switches: many pumps are available with a float switch pre-wired to the side of the pump. This makes it simpler to install, especially in tight vertical bilges. Another way to accomplish this is with the Rule Automatic Bilge Pumps, which spin their motors every few minutes and use the current draw from the motor to detect whether there is water present. This has the minor disadvantage of running the motor whether there’s water present or not.
Electronic switches use a pair of Mirus™ detector cells that sense the presence of water through the plastic housing using a low-impedance electrical field. Switches like the Johnson Ultima Electronic Switch have no moving parts to wear out and do not sense petroleum products, so you won’t accidentally pump fuel overboard if it is spilled in the bilge.
Centrifugal pumps are submersible and non-self-priming, so they must be sitting in the water in order to pump it, and can usually remove all but the last inch of water. They work the best when the bilge has a small sump where water collects. Centrifugal pumps use whirling vanes to draw fluid into the center of the pump and then push it outward from the center through an outlet port. They have a built-in strainer in their base that can be removed quickly for cleaning, which is important because the small impeller can get clogged with debris.
Diaphragm pumps are self-priming, which means they can lift water up an intake hose and expel it outside the hull. They use a membrane to increase and decrease the volume of a pumping chamber, drawing fluid in and pushing fluid out through a set of one-way check valves. Diaphragm pumps require an external strainer at the end of the intake hose, since a small amount of hair or bilge debris can cause the valves to clog.
Flow restrictions: diaphragm pumps try to move the same volume continuously. If something restricts the flow, they work harder and harder until they stop pumping, at which point something usually fails. Centrifugal pumps respond to increased head pressure by moving less and less fluid until they stop pumping. The motor will not burn up, but the pump will stop moving any fluid.
The challenge of getting water overboard: the effect of siphoning occurs when a hose is led to a body of water above the starting point. If this water ever starts to flow backwards it produces suction, that draws water into the boat. This is one reason why bilge pump discharges frequently exit high on the hull, well above the waterline. A vented loop can also be used to prevent back-siphoning.
Keeping the intake clean: a strainer or strum box is used on the intake end of the hose, to prevent debris from clogging the pump or hose. Bilges are full of junk more often than not, so bilge pumps must be easy to clear and clean. The valves in some diaphragm bilge pumps are designed to pass objects as big as a marble, but hair and fiberglass strands are very difficult to defend against.
A complete installation of electric bilge pumps requires several components:
The sizing of wire depends on amp draw and pump capacity. Use the following chart as a guide. It assumes a wire run of about 10 feet. Longer runs need larger wire sizes.
|Pump Capacity||Wire Size|
|< 800 gph||16 ga|
|800 –2000 gph||14 ga|
|2000-3500 gph||12 ga|
|4000 gph||10 ga|