It's o-dark-thirty, you're cruising quietly along and suddenly you feel the impact of something hitting the hull. You hear running water. A bilge pump fires. Then a second. The safety of your boat and crew now directly depends on how well those pumps do their job.
Bilge pumps stand as the last line of defense between your boat and the sea if your hull is breached or you develop a leak, and understanding their capabilities is vital. A hole the size of three fingers near the waterline can pour 4,762 gallons of water into the boat in an hour, according to The Yachtsman's Emergency Handbook. A fist-sized hole three feet lower can pull in four times that, meaning the average bilge pump installation can only slow the inevitable.
Ensuring that your pumps are properly installed and maintained will give you time to make repairs, radio for help, deploy the life raft and gather emergency gear. Understanding how these pumps work and how much water they can move – it may be less than you think – will tell you how much time you have.
WHAT'S IN YOUR HULL?
To evaluate your system, start by locating the pumps on your boat, gathering the make and model information and reading the product literature, which is usually available on the manufacturer's web site.
This will tell you how pumps are rated in terms of gallons-per-hour, but the performance of these pumps when installed on your boat may be far different. Pumps are tested under ideal conditions that are seldom replicated on boats, where the realities of marine installation can severely impact performance. Do not be surprised if the pumps on your boat are capable of far less than what the literature says. (For a story on real-world pump performance, see link below).
You will also learn what type of pumps you have. Electric bilge pumps can be divided into three basic categories: centrifugal, impeller and diaphragm.
Paul Esterle
The most common are centrifugal pump, such as Rule pumps, with their trademark red, white and blue housings, and those made by Attwood. Centrifugal pumps work by using an electric motor to spin a disc with curved vanes on its surface. The water is sucked in through a central hole around the hub and is spun out around the periphery of the disc, collected by an outer casing and directed to a discharge port.
These pumps are very quiet and can run dry without damage, though they are susceptible to clogging from debris in the water and are usually equipped with a strainer. Capacities range from 360 to 3,700 gallons per hour. Some pumps are automatic, having the ability to sense water and turn themselves on and off, while others require a separate switch.
Impeller pumps, such as the Jabsco Water Puppy (model 18660), utilize a bladed rubber wheel turning inside an oval housing. As the rubber fins are bent and released, they suck water through the pump. These pumps, which can handle smaller bits of debris and can lift water about four feet, are mounted in a dry, protected space with the intake passed down to the bilge level. They should not be run dry, because it destroys the impeller.
Paul EsterleDiaphragm pumps have one or more chambers fitted with a flexible diaphragm. As this diaphragm is moved into and out of the chamber, intake valves open and close, allowing water to enter the chamber through one valve and be forced out the other. Good examples are the venerable Jabsco 34600 and 36600 series pumps, rated at 648 and 480 gallons per hour, respectively.
One of the best features of diaphragm pumps is that they are self priming to a distance of 10 feet, meaning they can lift water from up to 10 feet below the level of the pump. This allows them to be mounted out of the bilge in a dry, protected area, with the intake hose run down to the bottom of the bilge. These pumps also can run dry without problems and they are relatively good at handling debris in the water. The negatives are that they are larger and noisier than others, and they definitely cost more.
