Bilge Pumps

Posted on Saturday 6 March 2010

A friend of mine was asking about installing bilge pumps on his boat.  So, I thought I’d write about that today.

My recommendation for a full bilge pump setup has a small “maintenance” bilge pump down in the lowest part of the bilge. The float switch for this pump can be integrated or separate, but it should be mounted as low as possible. This is the pump that takes care of the basic leaks that most boats have—the stuffing box for the prop shaft seal, the water that comes down a keel-stepped mast, etc., which is why I refer to it as the maintenance pump.

This pump should be on a counter of some sort, so you can keep track of how often it is cycling. This will tell you if you have any leaks and if they’re getting worse or not. By writing down the number on the counter each time you leave and return to the boat, you can get an idea of how often it is running. If the number of times it is running per day is going up, and there is no reason for that to happen—like having four really rainy days in a row—you know you have to look for a leak that is getting worse.

The reason to get as small a pump for this as possible is to reduce the cost of replacing it, as it will burn out more often than the other pumps, since it will get the most use, and to keep the back-flushing of hoses to a minimum. Small pumps have smaller hoses, and less water will sit in the hoses and back flush into the bilge when the pump shuts down.

The next pump should be the highest capacity electric bilge pump you can get. Remember, bilge pumps are rated in GPH, not GPM, and even a small hole, 3′ below the waterline lets in a lot of GPM. This is more important on a small boat than a large one, since it takes less water as a whole to sink a small boat. This one should have a float switch mounted an inch to a few inches higher than that of the “maintenance” pump. This is your main bilge pump and should handle things like your water tank bursting, the water that gets down the companionway or hatches when out sailing, etc.

You should also, ideally, have two large manual diaphragm pumps setup. One should be accessible inside the cabin, the other should be mounted where it can be operated from the cockpit. These are the emergency dewatering pumps, in case the hull gets damaged and things like that. The reason you want two of them is simple. First, if you have crew, two pumps means that two people can work on keeping the boat afloat. Second, if you are out in really bad weather singlehanded and have to either hunker down in the cabin or man the cockpit, you can still operate at least one of the pumps.

On a catamaran, you basically have to do this same thing for each hull.  On a trimaran, the setup is a bit different.  Youshould have this type of setup on the main hull. You also need some way of pumping out each ama.  Complicating this further is the fact that most multihulls have the hulls segmented into separate water-tight sections.

Electrical Centrifugal Pumps:

The most common electric bilge pumps found today are the Rule-type centrifugal pumps.  While these pumps are very common, they are not the best bilge pumps in my opinion.  These pumps are usually rated for zero head or lift—which is not realistic in most installations.  This means that their actual working rating is far lower than their nominal rating. These pumps usually must be submersed to function properly, as they can not lift water any significant distance to prime themselves.

Centrifugal pumps have very little in the way of lift capacity, and due to their design really should have a high loop with a vacuum break in the exit hose to prevent siphoning. Centrifugal pumps can run dry with little risk of damage. They can also pass small pieces of debris without much risk of damage. A strum box or screen should be fitted over the intake hose to prevent larger debris from entering the pump.  String or hair is a real killer for these pumps as they can wrap around the impeller shaft and bind it up.

Electrical Diaphragm Pumps:

Another type of bilge pump, though not commonly seen, is the electrical diaphragm pump.  These can run dry without risk and are very good at self-priming.  They also have the ability to generate a lot more pressure than a centrifugal pump and can deal with far more lift. However, they can not pass much in the way of debris and are at risk of puncturing the diaphragm if sharp debris gets into the pump body.  These definitely need to have a strainer in-line to remove debris, which makes them a bit less well-suited as a bilge pump.

Manual Diaphragm Pumps:

Get the highest capacity diaphragm pumps that you can.  I prefer the Whale brand of pumps.  Double action pumps are better than single action pumps, as they pump on both strokes.  While diaphragm pumps are capable of passing small pieces of debris, it is generally a good idea to have a strum box or screen on the intake hose to prevent debris from entering.  Sharp debris can damage the diaphragm, requiring the pump to be re-built. I prefer pumps that have integral handles, but many do not.  For those that do not, having the handle tied to the pump or nearby the pump is critical.  A diaphragm pump with out a handle is useless.

Bilge Pump Hose:

Bilge pump hose should be smooth-walled and wire-reinforced.  The smooth wall interior is essential to maximize pump output. The wire-reinforcement is to prevent the hose from collapsing under pressure. The output hoses should be looped to at least 1′ above the waterline.  While most manufacturers do not require a vacuum break, I generally recommend installing one to prevent the hoses from siphoning water back aboard the boat if the boat is heeled.

There should be no check valves used in bilge pump system.  Check valves can fail and prevent the pump from functioning properly.  One thing that is very common on check valve installations is to have the water filling the hose downstream of the check valve prevent the valve from opening properly.

Float Switches:

You can either have integrated float switches built into the pump or use external switches. Some pumps don’t have a float switch, instead periodically spinning the impeller and if no resistance is felt, shutting down.  For pumps with an external switch, there are generally two choices: electro-mechanical float switches or electronic switches.  The electronic switches generally use capacitance to sense the presence of water.  These are more reliable than their electro-mechanical counterparts, which require a float to move and close the circuit.  However, electronic switches are more expensive than their electro-mechanical counterparts. .

I’ve also built a custom switch that uses a float with a rod and magnetic reed switches to control the pump.  The rod has a magnet attached to it and the rod is placed inside a tube.  The magnet is positioned so that it holds a magnetic reed switch open.  When the float rises, the magnet moves and allows the reed switch to close, turning on the pump. By having the reed switch on the tube, it can be mounted out of the wet portions of the bilge, making maintenance simpler.

2 Comments for 'Bilge Pumps'

    March 7, 2010 | 11:46 pm

    good advice and don t trust blige pump suppliers. Even if they tell you that their pump can operate without water, my marconautic lost its diaphragm last weekend …

    January 13, 2011 | 12:08 am

    […] want to read the post I wrote on Bilge Pump installations…. __________________ Sailingdog Telstar 28 New England You know what the first rule of […]

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