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Old 06-02-2002
Don Casey Don Casey is offline
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Devising a Better Bilge Pump

This article was originally published in March 2001 on SailNet.

The best bilge pumps are not only reliable, but also easily accessible.
Sometimes we sailors really need to think outside the box. Let me give you an example: I arrived at the boat on a Saturday morning to discover the house battery completely flat. As I rotated the selector switch through BOTH on the way to checking the starting battery, I heard the hum of the automatic bilge pump. I felt a little stab of panic as I hastily removed the companionway ladder and opened the engine compartment to gain access to the bilge.

There was water in the bilge, but not that much. Had the pump been staying ahead of the leak, and I just happened to arrive at the boat just as the battery died? Or did the battery die for some other reason, and the pump had just been waiting for juice to pump out a week’s worth of drips from the stuffing box? While my brain tried to fit all the pieces together, it suddenly dawned on me that the water level was not receding.

Cutting to the chase, what I discovered was that the shaft that connects the pump motor and the impeller had broken. When the water level rose high enough to activate the float switch, on came the pump. But since it wasn’t actually pumping, it just continued to run until it drained the battery.

The owner of this particular bilge is a believer in the two-pumps-are-bettter-than-one theory.
All of that is just exposition. What happened next is what really merits our attention.

The normal response, I think, is to pull the old pump, take it to the nearest marine supply store, and buy an identical replacement. Or, if the original one seems to have failed due to low quality, maybe buy a different brand, similar in size and capacity. Back at the boat, in goes the new pump, the dead battery gets a charge, and it is on to other things.

The problem, for me, is that this was a relatively expensive pump, and I doubted that it had run all total more than a couple of hours—not including the time it ran after the shaft broke. When I opened it, the cause of the failure was obvious. The shaft had suffered catastrophic corrosion. Water marks on the housing were testimony that the shaft had spent its entire life half-submerged. That there was always water in the bilge had long troubled me because it can find its way into the laminate. However, it had always seemed prudent to equip my boat with the largest pump that would fit the narrow sump. That was one rated at 2,000 gallons per hour. The outlet port measured 1 1/8 inches, and a hose run of nearly 10 feet was necessary to reach the discharge fitting at the stern. Whenever the pump shuts off, the column of water in this hose drains back into the bilge. I did the math, and it worked out to be more than a half gallon—enough to submerge the pump up to the shaft.

Even shallow bilges like the one above require the services of a reliable pump.
I could get the shaft out of the water by raising the pump, but that wouldn’t do anything about the stagnating water in the bilge doing who-knows-what-evil to the hull laminate. Of course I could install a check valve, but a check valve in a bilge discharge line is a bad idea. It restricts the forward flow, reducing the true capacity of the pump, and it introduces the real risk of blockage. Besides, it wouldn’t be long before debris compromised the valve’s seal and the water would again be draining back into the bilge.

I could reduce the size of the discharge line to reduce the amount of water that would drain back, but that was even more self-defeating because it would severely reduce the bilge pump’s capability. The ability to deal with flooding had to take precedence over keeping the bilge dry. Didn’t it?

This was a problem with no solution—until I recognized that it is really two problems. I needed a bilge pump that would leave the bilge nearly dry, and I needed a bilge pump that would handle serious flooding. Who says they have to be the same pump?

I bought the smallest automatic pump I could find—500 gph model—and installed it in the lowest part of the bilge. A pump this size has a 3/4-inch outlet, but since I wasn’t concerned about the capacity of this pump, I used a reducer to connect it to 1/2-inch discharge hose. Ten feet of 1/2-inch hose will hold less than a pint of water.

"I need a bilge pump to keep the sump dry and one to handle serious flooding—but who says they have to be the same pump?"

Next, I glassed in a plywood bridge several inches above the deepest part of the bilge. This would be the mounting location for my new high-capacity pump. Because the bilge is wider at this bridge, I was able to install a pump with twice the capacity of the one that failed. I dispensed with the float switch for this pump, wiring it instead to a clearly labeled switch in the main saloon.

Shouldn't the high-capacity pump also have a float switch you might ask? When I am aboard it clearly does not matter, but let's consider what happens if a hose fails while the boat is unattended. An inch-and-a-half hole six inches below the waterline will admit water at the rate of about 1,900 gallons per hour. The 500 gph automatic pump will have little effect in this situation, and my 30-foot boat will sink in less than six hours.

Connect the high-capacity pump to a float switch and maybe it can keep up with the leak—but for how long? The pump draws 15 amps. It will drain a full group-27 battery in six hours. Then the boat begins to fill.

When the battery dies and the bilge pump stops running, it's a mere matter of time before dire consequences result.
So won't someone notice? If you mean notice the pump running, probably not. Boats all over any marina discharge water from generators and air conditioning units all the time. If you mean notice the boat sinking, someone likely will notice if it happens in the daytime, and if I’m really lucky, they will try to stop it. Unfortunately, by the time another sailor notices my boat settling and calls me, or breaks in, the float-switch operated pump will already have been running for hours and will have drained the battery. If the pump is not on a float switch, the emergency pump can be turned on (assuming the batteries are still above water), arresting the problem and providing time to correct it.

For an unattended boat, an even better configuration is to put the high-capacity pump on a float switch but also wire the switch to an outside alarm horn to make sure the trouble does get someone's attention. Combine that with a plaque mounted next to the companionway hatch providing more than one emergency telephone number, and you give yourself an excellent chance of dealing with the unthinkable without sustaining any serious damage.

These days my bilge stays bone-dry, and my expensive, high-capacity pump sits high and dry, unlikely to ever need replacing. Two pumps optimized for differing objectives is such a clearly superior approach that I am still shaking my head at how obvious this should have been. Maybe reinventing the wheel every now and then isn’t such a bad idea.

Suggested Reading:

Bilge Pumps, the First and Last Lines of Defense by Tom Wood

Battery Charging by Don Casey

Sinking at the Dock by Mark Matthews

SailNet Store Section: Bilge Pumps and Accessories