Stemming the flood

Most boat owners think of bilge pumps solely as devices to remove small amounts of rainwater and shaft-seal leakage that collects in the boat. Their function isn’t that simple, however. If your boat takes on large amounts of water, whether from swamping or a hole, the bilge pumps must remove that water. The pump capacity of your boat will determine whether it sinks.

Table 1 shows the flow of water through variously sized holes at different depths under the surface. The flow rates of water into the boat shown are in gallons per minute. Could your bilge pumps keep up with this sort of flooding rate until you could find and stop the leak?

Most of the bilge pumps I see in my marine survey work are smallless than 1,000 gph. The largest pump I routinely see in boats is a submersible, rated at 1,500 gph or approximately 25 gpm. The largest commonly available pump of that type is 3,500 gph (approximately 58 gpm). These are the manufacturers’ rated capacities; the pumps never will deliver that much in actual service. The manufacturers’ ratings are based on free flow from the pump discharge. That means there is no discharge piping connected to the pump, and the pump is not required to raise the water to an overboard discharge. In practice, there is often 10 to 20 feet of discharge hose connected to the pump, which causes frictional losses in the hose. The pump is also required to raise water from the lowest point in the boat to an overboard fitting six to 12 inches above the waterline. The total lift required of a pump is frequently three to four feet.

Also, pumps are usually rated at 14 volts. If your engine is running, maybe the pump gets 14 volts. If the battery is providing the power, the pump is probably on less than 12 volts.

All of these factors reduce the actual capacity of the pump. Depending on the location, depth and length of hose, the actual capacity could be as little as one half the rated capacity. It is obvious, then, that even the largest available 12-volt DC pumps will not keep up with a very big hole in actual conditions. So, what can you do to ensure that your boat remains floating? What are the choices?

There are several options. The simplest and cheapest choice is to install multiple electric pumps. Install two identical pumps and you double your capacity; install three pumps and you triple your capacity.

If you have a generator or inverter, a second alternative is to install a 120-volt AC pump. Such pumps are available in capacities up to 100 gpm (actual capacity in most installations), making them ideal emergency bilge pumps. Their only shortcoming is that they function only as long as AC power is available. If the generator or inverter is flooded, the 120-volt AC pumps are useless. The cost of a 100-gpm, 120-volt, AC pump suitable for use as an emergency bilge pump would be approximately $1,500 to $2,000, including installation costs.

One of the most practical options is the installation of an engine-driven bilge pump with clutch. Such pumps are standard equipment on most military small craft. They are available as optional factory equipment on many marine engines. Capacities range from 25 gpm for small sizes to 100 gpm for large pumps. Cost would depend on complexity of installation but would typically be about $2,000 to $3,000 (installed) for a 100-gpm pump with manual clutch.

Another choice would be to rig the engine cooling water suction such that it can be shifted to the bilge in an emergency. A four-cylinder diesel engine, for example, will usually pump 20 to 40 gpm through its cooling system depending on the pump installed. The shortcoming of this modification is that you run the risk of overheating the engine if the bilge suction becomes clogged. Therefore, a strainer on the end of the suction hose is vital, and even then the chances of its getting clogged with debrisparticularly since a flooded vessel is full of adrift detritusremains an important concern. On the other hand, plumbing installation is simple, and cost would be about $100.

Another option has become available recently. The Ericson Safety Pump is a large centrifugal pump that is mounted on the prop shaft. The impeller is fastened to the shaft and turns any time the shaft turns. The pump housing is fixed in place. Unlike a standard impeller pump, the ESP has more clearance between the metal impeller and the housing so it won’t burn when rotating dry. If the water gets high enough to cover the prop shaft, water enters the pump housing and is pumped overboard. As long as the prop shafts turns, the pump will operate. The pumps were developed for commercial fishing boats but are available in sizes suitable for yachts. Capacities of up to 600 gpm (yes, gallons per minute!) are claimed for the yacht-sized pumps. Cost of the smallest model is about $1,000, plus installation. For more information, contact Philip Chancey at 709-364-5768 or visit the web site:

None of these pumps will keep up with large flooding rates, however, despite their impressive gpm rates. Your pumps are only your first line of defense. If you are aboard, you must also quickly locate the hole and stop the flooding if you are to stay afloat. That’s not as easy as it sounds. If the cabin sole is underwater, then debris will be floating everywhere. Your heart is racing, and the water may be cold. Imagine trying to feel around underwater, maybe in the dark, to find the source of water. Frightening? It should be.

Take the time now to think through where the water might come. Memorize the location and exact number of all the through-hull fittings. Know how to reach the shaft packing. Most importantly, have a means available to plug any of these holes. The need for automation

In fact, most boats sink at the pier with no one aboard. They sink because a hose freezes below the waterline and splits, and then thaws and floods into the boat. Or a drain hose from the sink splits at the through-hull fitting, and waves slowly put enough water into the hose to fill the bilge. They sink because the shaft packing leaks excessively, cycling the automatic bilge pump repeatedly until the battery is completely discharged.

Here in the mid-Atlantic, the rare heavy snow can add enough weight to push scuppers under and flood the boat.

If your automatic pumps cannot keep up with the leak, what will keep the boat afloat? To guard against pier-side sinking, you should install both automatic bilge pumps and a high-level bilge alarm on your boat. Most sailboats we survey don’t have automatic pumps. That doesn’t make sense considering that your boat is most at risk when you’re not there.

Even if you install a high-water alarm, it will do little good if it simply causes a light to come on inside the cabin. If you are not aboard, the boat will sink while the alarm alerts no one. To be truly effective, the alarm must attract the attention of someone in the marina when you are not aboard. This can be done with a siren or horn that sounds in the cockpit.

When you install a relatively high-capacity, 12-volt DC automatic bilge pump, the float switch that activates the pump is installed down low. If the water is coming in so fast that the bilge pump cannot keep up with it, an upper float switch activates the high-level alarm. In normal operation the water would get nowhere near the upper float switch.

Both of these float switches should have covers. It is extremely important that this type of float switch be protected from mechanical jamming by a foreign object. In my survey work, I occasionally see float switches of this type that have been jammed in place by hoses, trash, and other objects.

It is also important that the pump float switch be wired into the positive side of the bilge pump circuit. If the switch is in the negative side of the circuit, the pump itself is always maintained at positive potential. This makes any metallic parts inside the pump subject to stray current corrosion. With the switch in the positive side of the circuit, the pump always is maintained at ground potential, and no possibility of stray current corrosion exists.

Submersible pumps have built-in strainers to prevent the impeller from clogging. For diaphragm-type pumps (both hand operated and electric models), 120-volt AC pumps, cooling water suctions, and engine-driven pumps, a strainer is required on the pump suction. Most of the commercially available strainers do not have enough open area to pass the rated capacity of a pump with the same size suction. Two of the more effective types of strainers are shown in the accompanying figure. Have a sheet metal shop fabricate several of the appropriate design.

Regardless of other pumps that may be installed on board a boat, any larger vessel should have a manual bilge pump. Diaphragm pumps are available, like the Model 117 series from Edson, with a capacity of one gallon per stroke. Whether permanently installed or mounted on a board so that it is portable, a large diaphragm pump would allow you to pump the boat even if you lose all electric power during a flooding casualty.

Recognizing that even the largest 12-volt DC automatic pumps cannot keep up with a major leak, I recommend that my survey clients who intend to sail out the range of immediate assistance rely on a combination of automatic pumps, bilge-level alarms, and emergency pumps to keep their boats floating during a casualty. If a boat is pierside, the automatic pumps, though they may not keep up with the leak, will keep the boat afloat until the bilge-level alarm brings help. When someone investigates the alarm, they might start the high-capacity engine-driven bilge pump or bring an emergency gasoline-powered pump from the marina.

Underway, the bilge-level alarms will provide you more than enough notice to start a 120-volt AC pump or an engine-driven emergency pump.

Accordingly, I advise my survey clients to install large, high-capacity, automatic 12-volt DC pumps (at least two, maybe more depending on the size of the boat) to remove casual water from the boat and provide a first line of defense against flooding. I also recommend high bilge-level alarms to alert the captain or marina personnel to a flooding casualty and at least one emergency pump with a capacity of at least 100 gph. I also recommend that every boat carry at least one large diaphragm pump for emergency bilge pumping (two may be required depending on the size of the boat).

Take another look at Table 1. It is nothing less than chilling to contemplate how much water can enter a boat through a relatively small hole in a few blinks of the eye. An in-depth bilge pump system such as we have proposed will increase your boat’s chances of surviving a flooding casualty.

By Ocean Navigator