Getting the water out

Apr 4, 2014

(page 2 of 3)

Positive displacement

This looks like a good setup, but note the sponge right next to the strainerless manual pump hose, it would easily pull the sponge into the hose, causing a clog. Also note the hard 90° where the hose comes out of the centrifugal pump.

The next most common type of electric bilge pump is the diaphragm pump. These pumps are positive displacement pumps, meaning they pump a set volume of water with each cycle. These pumps work by pushing and pulling a flexible piece of rubber mounted on top of a chamber. As the rubber is pulled up, it creates low pressure in the chamber which pulls water in. When the rubber is pushed back down, it pushes the water back out. There are normally two openings in the chamber with check valves at each opening allowing the water to only flow in one direction. Diaphragm pumps are self-priming, allowing them to be mounted up out of the bilge. This has an advantage in that they are more accessible for service. Because diaphragm pumps are positive displacement, they are less affected by head pressure and inline restrictions. They can be a bit more tolerant of trash, but it should be noted that both excessive back pressure and bits of trash can damage or block the check valves reducing efficiency or even rendering the pump inoperable.

A couple of distinct advantages of a diaphragm pump is its ability to pump the bilge almost completely dry as the internal check valves stop water in the discharge line from flowing back into the bilge. This can be important with small bilge sumps to prevent pump cycling when used with an automatic switch. A disadvantage of diaphragm pumps is they do not move as much water as quickly as a centrifugal pump can. Many centrifugal pumps are rated at 1,500 to 2,000 GPH while few diaphragm pumps exceed 700 GPH.

There are other types of pumps available as well for removing bilge water. These are generally referred to as utility pumps and come in different sizes and types. Most of these are either impeller pumps or pressure water pumps utilizing three or more small diaphragms operating in a small chamber. These pumps are not generally recommended for removing bilge water as they do not tolerate any trash or debris normally found in bilges. The impeller pumps in particular are a poor choice since if they are run dry the impeller can be damaged.

Along with electric pumps, every boat should have one or more manual pumps. Should the boat lose power, manual pumps are the only choice. Almost all manual pumps are diaphragm pumps consisting of two basic types, single diaphragm and double diaphragm. The single only has one pump chamber and will pull water in on one stroke and push it out on the other. The double, as the name implies, has two chambers and will pull water into one chamber and push it out of the second chamber on a single stroke. This will increase the pump capacity and improve pumping efficiency. The drawback is a double diaphragm pump requires more energy to operate, increasing operator fatigue.

Often these pumps are mounted so that they can be operated by the helmsman while steering. This is good and bad as the helmsman may not always be able to operate the boat and pump at the same time. For this reason, it’s wise to have at least one additional manual pump. Having a second pump inside the cabin could be an advantage as the operator would be out of any weather.

No matter where the pump is located, the operator should be able to operate the pump for long periods of time under varying heel angles and sea conditions. “Avoid the use of handles that are too long for manual pumps,” said Will Keene of Edson International, a long-time manufacturer of marine pumps. “Although a longer handle will make pumping easier, it can easily damage the pump.” A sitting position with the operator’s legs braced against a solid surface is best.

Although manual pumps may be the last option, they have a major weakness: the “man” in manual. Most people cannot pump for endless hours and if there is a leak it needs to be fixed, taking at least one person away from pump duty. In addition to diaphragm pumps, a good strong bucket can be important. One should never underestimate just how much water can be moved with a bucket. A bucket may be a last resort as they are only really practical when the water is above bilge level. Every boat should have at least one really sturdy bucket.

Crash pumps
The last line of defense is what many call “crash pumps.” These are pumps set up only to be used in emergencies. Like normal bilge pumps, crash pumps come in many types and sizes. A crash pump is usually a high-volume pump often powered by the main engine. Some will recommend having a tee in the engine raw water intake with a line running to the bilge so the engine pump can help remove water from the bilge. I do not think this is a good idea. In an emergency it is important to keep the main engine running to keep the batteries charged and to assist in maneuvering the boat. If the pump should clog, run dry or pick up debris it could be damaged. This would result in the loss of the engine just when it is most needed.

An alternative is a separate belt-driven pump powered by the engine. Yet another interesting option is the Ericson safety pump. This is a large-capacity centrifugal pump that bolts to the propeller shaft just behind the transmission. The propeller shaft turns the impeller inside the housing. If the water should get as deep as the propeller shaft, it will enter the housing through the space between the shaft and housing. This pump is really the Hail Mary pass of pumps as in most cases the water would have to be pretty deep to allow the pump to operate.

No matter what type of pump is being employed, the installation is critical to its efficient operation. For all pumps the plumbing and hose runs are important to the pump’s efficiency. Restrictions in any of the plumbing need to be avoided. Don’t use cheap corrugated hose as this adds resistance inside the hose. Pick a hose with a smooth inside surface and one that will not kink easily. I like to use the white sanitation type hose. For the suction hose, make sure the hose will not collapse when clogged. Avoid sharp turns or the use of elbows or other fittings. Every connection in the hose will add resistance.

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