Submersible Bilge Pumps: Installing One Like the Pros

If you’ve ever taken a close look at your bilge, you’d likely admit that it’s probably the nastiest place on your boat, even if you do your best to keep the place clean. Dirt, sawdust, hair, fish scales, and all sort of other grunge somehow migrate to the bilge. Now imagine trying to make a piece of electrically powered marine gear work down there reliably, season after season.  That, my friends, is the life of a bilge pump.

A bilge pump installation diagram.

A simple submersible bilge pump setup. Photo courtesy of ITT Jabsco

A bilge pump is designed to handle nuisance water primarily, but sometimes it’s also the only thing standing between your boat and the bottom of the sea in an emergency. That’s why it’s important that it works right, all the time. Keeping that in mind, we’re going to discuss what you’ll need to know to install a submersible bilge pump yourself, just like a seasoned marine professional would.

If you’re wondering what type of bilge pump to select for this project, have a look at our blog post on the subject, Bilge Pumps: Selecting One With the Right Stuff, and then come right back here for the skinny on how to put it in.

Mount the Bilge Pump and Float Switch

The first thing you’ll want to do is to remove your new bilge pump from the packaging and give it a big hug. OK, just kidding. On the bottom of most submersible pumps is a removable strainer assembly that not only prevents debris from clogging the pump, but also acts as the mounting bracket for the pump itself.

Pop off that strainer from the pump, and then use it as a template to scout for a suitable installation location in your bilge. Also, if you’re going to be mounting a float switch, make sure the location has room nearby for the switch, too. The pump will need to go in an area at the lowest point of your bilge and preferably as far back in the boat as possible. That’s because bilge water generally runs aft. Once you’ve figured out your spot, scrub the area clean, and then dry it as best as you can.

A photo of a bilge pump base.

This is the strainer base of a submersible bilge pump. Note the screw holes, which you can use as a drilling template when mounting it to your bilge.

Place the strainer assembly where you want the pump to go, and use a felt pen or marker to pinpoint where to pre-drill your pilot holes for the screws. Remove the strainer and then drill your shallow pilot holes, using the correct diameter drill bit for the screws you’re using. Know exactly where you’re drilling! Obviously you don’t want to go through the bottom of the boat, but also be aware of sandwich construction that could be compromised. When in doubt, use a bracket — available at many marine supply shops — and fasten your bilge pump to a stringer or other safe location. Sometimes the pump will come with screws, but you may need to supply your own. If your pump didn’t come with fasteners, use self-tapping pan head screws that will fit the holes in the strainer basket and are about three-quarters of an inch long.

Before you screw in the base, put a generous dollop of polyurethane marine sealant (such as 3M 5200) in the pilot holes to prevent the fiberglass from soaking up water. Next, screw down the strainer base, and then click in the pump assembly. If you’re using a float switch, mount it as close to the bilge pump as possible, using the same “template” method you used for installing the strainer plate. Try to leave the sealant to set up for a day, if you can, but don’t worry too much if it gets wet—5200 cures in the presence of water quite nicely.

A photo of a stainless-steel thru-hull fitting.

A stainless-steel thru-hull with hose barb. You don’t have to use a metal thru-hull on your bilge pump project, but they are much more durable than plastic fittings.

Running the Overboard Discharge

Next, you need to decide where to mount the overboard discharge thru-hull for your bilge pump system, and that requires some careful forethought and planning. You want the discharge to be well above the waterline, but not so high as to reduce the efficiency of the pump. Also, think about where the water will end up when it comes out of that fitting. Is it so high that the water will shoot out onto the dock or make a lot of noise when it splashes into the water?

When figuring out a place to install the thru-hull, take into account whether the location you’re thinking of will ever be underwater—such as when the boat is fully loaded, heeled over, or when it “squats” under power. The discharge for your bilge pump should never go underwater in any situation, ever. Otherwise, you create a condition where water can back-siphon through that fitting into the boat. A place that is a low as possible, but not too low, is what you’re looking for.

The thru-hull fitting you select should have a hose barb the same size as the discharge fitting on the bilge pump. A typical Rule bilge pump, for example, has a discharge fitting that is 1-1/8” outside diameter, which means that the thru-hull would need to have a 1-1/8” barb. Consequently, the inside diameter of your house would need to be 1-1/8”, too. Nylon or fiber-reinforced plastic thru-hull fittings are suitable for this application, but consider a bronze thru-hull, if the budget allows. They’re much more sturdy in the long run.

Once you’ve got the location sorted, you’ll need to drill a hole in the hull the same diameter of the threads for the thru-hull, using a drill-mounted hole saw. You’ll want to start on the outside and work in. Taping off the area where you’ll be drilling helps prevent splintering the fiberglass and chipping the gelcoat, but the key is not to use the hole saw at too low a speed; that’s when its teeth can snag on the gelcoat and chip it. Once you have your hole cut, apply a good-quality marine sealant around the fitting, insert it, and then tighten up the nut behind it. Use a paper towel or rag to clean up any excess. If you’re unsure about which marine sealant to use for the job, take a quick look at this helpful article, Picking the Correct Marine Sealants.

A photograph of a piece of bilge-flex hose.

This is what cheap, corrugated “bilge-flex” hose looks like. Stay away from it.

Before you buy your hose, measure the distance from your bilge pump up along the path to where you mounted the thru-hull discharge, and then add a foot or two. We’ll explain why in just a moment. And don’t skimp when you buy hose for this project. That doesn’t mean you have to buy the most expensive, multi-layered, wire-reinforced raw water hose on the rack, but it does mean you’ll want to stay away from cheap, extruded, thin-wall “bilge flex” hose. Also know that corrugated (ridged) hoses are great for situations where you have to make a sharp bend, but they also reduce the efficiency of your bilge pump’s output. That’s because of the turbulence those ridges inside the hose create. When possible, buy a smooth-walled hose with a decent wall thickness that won’t kink when bent and will stand up to the harsh environment in your bilge.

To install the hose onto the thru-hull, first put a properly sized marine-grade, stainless-steel hose clamp on the hose, push the hose onto the thru-hull’s hose barb, and then tighten down the hose clamp firmly. If you feel as if you could use a little “Hose Clamp 101,” read All Stainless is the Key to Selecting Good Hose Clamps.

Next, create a loop that runs above the thru-hull and then down toward the bilge. This loop will prevent water from siphoning back into the boat in the unlikely event that the fitting ever goes below water (you’ll want to hope it never does). Last, run the hose neatly down to the bilge pump, making sure you use lined clamps at regular intervals to secure the hose. Attach the hose to the bilge pump repeating the same method we used on the thru-hull fitting.

Running Wires

A photo of waterproof heat-shrink butt connectors.

Make sure you use waterproof heat-shrink butt connectors like this when wiring up your float switch and bilge pump.

The way you wire your bilge pump setup will have a profound effect on its longevity, reliability, and efficiency. Since the supply wires and their connections will spend a significant amount of time around—and in—some very wet and sloppy conditions, it is crucial that you connect them using only the best materials and ensure that all of your connections are completely waterproof.

For wire, that means using only marine-grade, tinned copper wire of the proper gauge. The size of that wire depends on how far the pump is wired from the power source and how many amps the pump draws, so consult a wire sizing table (you can do an Internet search for one), or ask your marine supply shop pro to help you spec the correct size wire.

When it comes to the connectors that you’ll use to hook up your bilge pump and float switch to the power leads, that means using adhesive-lined, heat-shrink terminals. In fact, we even recommend applying an additional piece of heat shrink over those connectors themselves, just to be safe. If you’ve never installed a heat-shrink crimp connector before, check out this video, How To Install Heat Shrink Terminals, to find out how to do it right.

A bilge pyump wiring diagram.

A wiring diagram showing the most basic of bilge pump setups. Make sure that you use the diagram that comes with the pump and switch you buy to ensure that you wire it up correctly. Diagram courtesy of BoatU.S.

The best way to wire up a bilge pump is by using dedicated three-way bilge pump switch. This switch will have a light to indicate when it’s operating, but also allows the pump to be set in “Auto,” “Off,” or “Manual” modes. Additionally, a bilge pump should be wired so that it still has power when the battery switch is shut off. That usually means running it right to the battery, or terminal strip supplied by the battery. By all means, ensure that you’ve installed a proper-sized fuse. Again, if you’re unsure, ask one of the pros at your local marine supply shop to help spec out the right one.

Each pump, pump switch, and float switch come with specific wiring diagrams that you should follow to the letter. Some bilge pumps come with brown and black lead wires, while some come with other weird combinations, so make sure you follow the instructions in the box religiously. We’ve included a simple illustration of a basic wiring setup here, but use it only as a guide, making sure you pay attention to the instructions for the gear you’re working with.

Once you have everything installed, it’s time to test your setup. Make sure that the bilge area is sealed (put in the drain plug, if you have one), and then fill it up with water using a hose until the pump switches on automatically. Next, wait for it to empty and turn off on its own. If you wired the pump to a three-way (auto/off/manual) switch at the helm or to a stand-alone bilge pump panel, fill up the bilge again and then toggle the switch to “Manual,” to make sure the pump turns on and pumps the bilge dry. Switch the toggle back to “Auto” when you’re done.

Well, if everything worked out, a cold beverage of your choice is in order. Once you’ve kicked back with a cold one, remember that bilge pumps shouldn’t be ignored once you install them. It’s a good idea to check their operation every trip and also to look down in the bilge to make sure there’s no debris clogging the pump or affecting the float switch. Also, it’s not a horrible idea to install a cycle counter so you can spot any sudden increases in the number of times the pump is switching on. That can alert you to a leak. Happy pumping, campers—see you on the water.

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