Getting the water outApr 4, 2014
An example of a good installation. Hose run straight off pump, clean bilge, and wire connections made with heat shrink and secured up high out of bilge.
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A couple of thousand years ago, Archimedes taught us the principles of displacement and the importance of keeping water out of our boats. Knowing this it is often surprising just how little thought most boaters give to their bilge pump systems.
CE rules for European boats require all pleasure vessels be equipped with at least one manually operated bilge pump. The U.S., however, does not require any kind of bilge pump on pleasure vessels at all. We all know keeping water out of the boat is a priority when sailing offshore. Most of the time this is not a problem, but sometimes things can and do go wrong. Having the right equipment to deal with leaks and flooding is your first defense. Leak control and water removal is a multi-part process that can use several different resources depending on the severity of the situation.
The first defense in controlling leaks is knowing you have a leak. ABYC now requires all boats with enclosed cabin spaces to have a high water alarm. A high water alarm is simply a water-activated switch located a few inches above what is considered a normal bilge water level. High water switches can be connected to a simple audio alarm, warning lights and even devices that will send a text message to your cell phone. Audio alarms should be located at or near all helm stations as well as in the cabin.
Finding leaks in calm conditions is a relatively easy matter as the boat is not moving and you can usually trace where the flow of water is coming from to its source. At sea this can be an extremely difficult task with water sloshing around in the bilge space. And the deeper the water gets, the harder it can be to find the leak. Once a leak has fallen below the bilge water level, it is all but impossible to spot visually.
A high water alarm saves precious minutes in locating a leak. A light at the helm indicating when the pump is operating is also a great first warning. If the helmsman knows the pump is running more than once or twice during their watch they can investigate the cause before it becomes a real problem.
A large capacity manual diaphragm pump mounted under floor boards. Note handle secured close to pump for immediate use. Provided there are not restrictions like turns in the piping or significant head pressure, this pump should be able to move large volumes of water.
First line of defense
In any leak situation, the boats primary electric bilge pumps will be the first line of defense to remove water from the boat. The most common type of pump is the submersible centrifugal pump. These pumps work by spinning an impeller at high speed. This spinning creates a pressure differential that will pull water into the center of the impeller housing and then in turn forces the water out of the housing and up the discharge hose.
This type of pump is relatively inexpensive and works well under most conditions. It has the ability to pass small bits of trash, but can become clogged with anything much larger than a pea-sized particle. Because centrifugal pumps are not positive displacement, meaning there is no set volume of water moved with each rotation, they can be affected by restrictions in the discharge hose.
Centrifugal pumps can move large amounts of water quickly, however, the height they have to lift or push the water up can affect their efficiency. This height of lift is called head pressure and is important to keep in mind when designing any system. As these pumps are submersible, they are generally not subject to corrosion and can be located in the bilge.
This type of pump is not self-priming, but this problem is overcome by placing the pump itself underwater. This is why these pumps will not remove every last drop of water from the bilge. As soon as they start to pull air into the pump, they lose prime and stop pumping. As there are no valves in this type of pump, any water left in the downhill side of the discharge line will flow back into the bilge when the pump stops. This can result in cycling when an automatic switch is used.
Centrifugal pumps come in many sizes, but the buyer needs to understand rated flow is not likely to be what they get on their boat. As head pressure and other restrictions in the line can greatly affect the actual flow rate.
For practical, real world results, assume about one-half the rated capacity of the pump for an average installation. If the pump is rated for 2,000 GPH it is more likely to pump closer to 1,000 GPH installed.
It is important to design a system with as few restrictions as possible. Voltage can also affect the efficiency of centrifugal pumps, as the lower the voltage the slower the pump will spin, reducing the amount of water moved.