All About Bilge Pumps

5 Dec

Bilge pumps are the critical last line of defense against sinking, and boats sink all the time. Bilge pumps and attendant equipment are fairly cheap, and easy to install and wire, yet often neglected, inoperable, or undersized. Some people think just one is enough, when two is the bare minimum, better three or four.

Bilge pumps are neglected because they sit in the least desirable place on the boat, sometimes wedged under an engine, and are usually covered with crud. They lead hard lives down in the wet and darkness, yet are fairly reliable if installed correctly.

First, the Coast Guard requirements for bilge pumps: There is no Coast Guard requirement for bilge pumps on recreational vessels. So we can just forget about them, right? What’s the worst that could happen? (For any kind of commercial use, racing, or to get an inspection sticker from the Coast Guard Auxiliary you’re required to have some bilge pumps.)

Since there’s no requirement to even have a bilge pump on a recreational boat, much less requirements for capacity or installation, people tend to wing it and install small pumps with no redundancy.

Let’s start with some basic information about submersible, centrifugal pumps, usually the core of our bilge pump systems:

-The capacities listed on all centrifugal bilge pumps are blatant lies. It’s just like politics: you have to lie as much as the next guy or you’re out of the game, so they all make equally ridiculous claims as to how much water their pumps will move. The idea that a pump will move 500, 800, 2000, or 4000 gallons of water per hour is very wishful thinking, unless it’s pumping holy water downhill in a laboratory.

It’s not that they don’t make quality centrifugal pumps – they do — but just ignore the numbers. Instead, buy centrifugal pumps from quality manufacturers (Rule, Jabsco, Johnson, SUREflo, et al) and divide them up as follows:

-Small centrifugal pumps, usually with a ¾” discharge, which claim to move 500-800 GPH. These are fine for runabouts or moving water from one chamber to another, where they essentially pump the water horizontally. But if you’re trying to pump water from deep down in the sump of a sailboat, then up higher than the boat’s heeled waterline, the water might not even make it over the hump:

-Medium centrifugal pumps, usually with a 1-1/8” discharge, which claim to move 1500-2500 GPH. These are the workhorses of the centrifugal pump family:

-Big centrifugal pumps, usually with a 1-1/2”-2” discharge, which claim to move 3700-4000 GPH. These will be our emergency pumps, which might actually keep up with a significant leak:

While centrifugal pumps will be the centerpieces of our bilge pump systems, there will be others:

-Manual pumps. It makes sense that we should be able to pump our bilges even if we have a total electrical failure, thus a manual bilge pump is usually part of the system, and required by most inspection authorities:

Now another concept: The pump that keeps your bilge dry might not be the same pump that keeps your boat from sinking.

The problem is that centrifugal pumps don’t get all the water out of the bilge before they start sucking air. They usually leave 1”-1-1/2” of water, and when the pump finishes its cycle accumulated water floods back down the discharge line and back into the bilge, adding another pint or two. If your boat has a well-designed sump – say a one square foot area where all the boat’s bilge water accumulates – then this is fine. An inch or two of water in the sump all the time doesn’t hurt anything. But 1”-1-1/2” is a lot of water to have sloshing around if you don’t have such a sump, especially in a flat-bottomed boat. So we introduce the next kind of pump:

-The diaphragm pump- The diaphragm pump is mounted remotely and its intake hose goes into the bilge, usually with some kind of strainer/weight on its end. With this arrangement a diaphragm pump can get the water down to just a quarter inch or so. Diaphragm pumps are very sensitive to clogging, so usually have an in-line strainer. The intake for a diaphragm pump has a much smaller footprint than a centrifugal pump, so it can fit into tighter spaces, and deeper down into sumps.

A diaphragm pump:

Intake strainers or Strum boxes:


There’s a fourth kind of pump, either an engine-driven high capacity pump, or another electric pump (impeller or vane) that can move some serious water. We’ll call this the catastrophe pump. This pump would probably be actuated manually in an emergency, and would have the capacity to keep up with a holing or skin fitting failure…we hope. This is the pump that might save you from stepping into the life raft, and anyone venturing offshore should consider some giant pumping beast for emergencies.

This high capacity, engine-driven pump can be engaged with a clutch:

Finally there is the bucket. There’s that old adage about there being no better bilge pump than a scared man with a bucket. This is true, and when we consider the false claims about how much water bilge pumps can really move, a motivated crew member with a five gallon bucket can probably move more water than most of the pumps listed above, but how deep would the water have to get before he could start scooping? Always carry a bucket, but best to leave this as a last resort.

Putting it all together:

First, you should have an emergency electric bilge pump, located higher than the normal levels of bilge water. This should be one of the “3700-4000 GPH” models with a 1-1/2”-2” discharge line. It should have a float switch, be wired directly to a battery, and be attached to an audible alarm. Here’s why:

This pump should be big for obvious reasons. It should have a float switch so it will pump when the water gets this high, whether you’re around to notice or not. It should have an audible alarm so that if you are around you’ll know you’ve got a problem, or if your boat is in a marina some kind soul might hear the alarm and investigate. An alarm buzzer is easy to wire into a bilge switch. Like all bilge pumps, it should be connected directly to a battery, or the battery side of a battery switch, so it will be live even if the battery switches are all turned off. Bilge pump discharges are often installed at the waterline so we don’t have disgusting bilge water stains running down our topsides. Since this pump will only see action in an emergency we don’t care, and its discharge can be a foot or two above the waterline, if this eases installation. Since this pump will sit well above the normal level of bilge water it will remain dry and essentially new – a spare, new bilge pump that happens to be wired up with a high water alarm.

A 3700 GPH bilge pump, down there in the dark, but still well above normal water levels: 

This pump can save your boat. With decent battery capacity and charged batteries it will pump for many hours. If you’re plugged into shore power in a marina it will pump indefinitely. This pump might keep up with a skin fitting failure, a broken hose, a blown-out packing gland, or a rudder stock failure.

And we’re probably going to have that manual pump too. Most inspection authorities stipulate that a manual pump must be accessible and useable with all hatches and compartments closed.

These are the bare minimum: one high capacity electric pump on a float switch and one manual pump, which in this case would be the primary bilge pump, and used regularly.

Beyond the emergency pump and the manual pump it’s all up to personal preference and your boat’s design.

If we were to add a third pump it would either be another, smaller centrifugal pump or the aforementioned diaphragm pump, also on a float switch, with its intake as low as possible. This would be the pump attached to a counter (see Bilge Pump Nirvana) and would let you know about your bilge pump activity over time while away from the boat. 

Let’s go back to the issue of centrifugal pumps leaving water behind and the concept that the pump that keeps your bilge dry might not be the pump that keeps your boat from sinking. Dry bilges are good for a variety of reasons: They keep the internal moisture down so we don’t have mold and mildew problems or corroding equipment, they help with blister/hydrolysis issues, and dry bilges keep things smelling fresh. If we aspire to bone-dry bilges we can employ some other tricks:

A diaphragm pump will get the water level lower than a centrifugal pump, but it will still have that issue with water running back down the discharge line into the bilge when it’s done pumping. Three quarter inch hose holds about a pint of water every five feet, which will all flow back into the bilge when the pump cycle ends. Bigger hose; more water to back flow.

We can install a check valve at the intake of our diaphragm pump, and this will keep any water from running back into the bilge. Check valves are notorious for clogging and sticking, and no doubt this check valve will clog or stick from time to time, but remember, this is just the pump that keeps our bilge dry, not the pump that saves our boat from sinking. If our check valve sticks we’ve still got our emergency pump ready to save the day.

We could also put a check valve, and achieve this same end, on our manual pump intake, and make the check valve easy to remove if we need the manual pump for an emergency (the check valve impedes flow somewhat).

No float switch can trigger at down at ¼” of water, so if it's a diaphragm pump this third pump might have to be manually operated. However, one float switch, the Aqualarm Smart Switch, incorporates a thirty second delay, so you could mount the switch as low as possible, depending on your boat’s design, and the extra thirty seconds might dry the bilge before the pump cuts out.

All bilge pump discharge lines should go way up high – deck level or higher – and employ a siphon break, before discharging. If not, water can siphon back in under the right conditions, even with the siphon break.

Bilges should be clean. All this effort is for naught if your bilges are a soup of oil, old rags, sawdust, zipties, peeled off labels from components, and cocktail napkins from the last dock party. Any of these items can jam or clog a pump or float switch. Bilges should contain only clean sea and rain water that gets in from places we may never identify. A few loads of hot, soapy water, a scrub brush, and a shop vac will not only make the bilges safe for your bilge pumps, but will make the whole boat smell better. Stagnant sea water is one of the worst smelling substances on earth.

Bilge pump wiring is often a rat’s nest, and responsible for more failures than bad pumps or switches. To connect two wires, say the bilge pump negative to ship’s negative, a waterproof heat shrink butt connector is good enough. For pumps with built-in float switches you’ll just connect three wires, in this manner, to three other wires:

With a separate pump and float switch you’ll come across instances where you need to join three wires, like where the manual positive and the float switch positive both attach to the positive side of the pump. The electrical industry has yet to develop a good way to connect three wires together. Cramming three wires into the aforementioned heat shrink butt connector is pretty good, but debatably up to ABYC code, and doesn’t guarantee a watertight seal.

The right way to do it is to put ring connectors on the all the wires with heat shrink tubing, then screw them down on a terminal strip. The terminal strip can be placed in a plastic junction box with a screw-on lid and a gasket, and all wires can enter the box through a watertight connection at the bottom, thus protecting all of the connections. With this arrangement you can change pumps or float switches without cutting any wires. Plastic junction boxes are cheap at hardware stores and should be used all over the place on boats to prevent corroded connections.

This is an unrelated project in an unrelated watertight plastic box, but you get the idea:  


  1. steve worsfold

    My older Gemini cat has no evidence of ever having a bilge pump and does not have enough battery capacity to support a big pump. My solution is a gas powered portable generator and a mobile large capacity 110v sump pump with a roll up discharge hose. This will run for days and be placed where needed. The boat has only one below the waterline penetration (speed paddlewheel) and buoyancy tanks in all 4 corners. Another option for boats with an inboard is the huge capacity shaft driven pumps that are always pumping whether its water or just asting as a bilge blower.

  2. Yuje

    Assuming you can’t get it from your local marina or boat reeailtr, GASKETS of many sizes and shapes are available at most home and hardware stores; you might also want to try an electronics or hobbyist shop. Just make sure to bring the pump’s make and model and any relevant specs with you, along with the damaged gasket. Also, check the web for the manufacturer’s home page; they might be able to tell you where you can find it near you, but, if not, they should be able to order it for you.P.S. The word is “diaphragm”.References :

  3. Kelly

    Thanks for sharing this great information.
    As a new Sailboat owner, I am hungry for any tips I can get.

    I have a Sidelmann 32′ and it has only 3″ depth x 8′ of bildge in the inner cabin. It was driving me crazy trying to find a bilge pump this thin. Your article introduced the ‘Switch’ so I can locate the pump external and keep it dry.

    Thanks so much

  4. mustapha

    Great article, every sailor should read it, I see years of experience in it, just one question is there any reference or international or national standard available for this issue.

Comments are closed.

More from the AIM Marine Group