Can a Bilge Pump be Completely Submerged?

ByKern Campbell

If you do any amount of boating, you’ve probably heard of a bilge pump. You may also know there are a few different options when it comes to pumping bilge water out, and you may even wonder about the safety of submerging it.

Can a Bilge Pump be Completely Submerged?

Can a Bilge Pump be Completely Submerged? A bilge pump can be completely submerged as long as you bought and installed a submersible bilge pump. There are a few other options for getting bilge water out but they go by different names and cannot be completely submerged. A submersible bilge pump is the simplest option.

For such a small part on a boat, the bilge pump plays a very big role. There’s quite a bit to understand when it comes to a submersible bilge pump and what it entails. We’ll talk about different submersible pumps as well as some of the basics of the bilge.

Can a Bilge Pump be Completely Submerged?

bilge pump

A bilge pump can be completely submerged, assuming you have a submersible bilge pump. There are a couple of options for pumping bilge water out of the bilge wells on your boat. Take a look:

  • Submersible centrifugal pump
  • Diaphragm pump
  • Manual pump

Here’s a little more about each of these pumps and how they operate.

Submersible Centrifugal Pump

As you can probably see from the name of this bilge pump, a submersible centrifugal pump can be completely submerged. In fact, these pumps are actually designed to operate underwater. Submersible centrifugal pumps are the most common types of bilge pumps used today.

There are a number of benefits to this type of pump. They can pump out quite a bit of water; they’re reliable, inexpensive, small, and fairly easy to install.

Of course, as with any piece of equipment, there are downsides as well. Submersible pumps can get clogged with debris, and it can be difficult to waterproof the wiring needed to run the pump since everything is underwater.

Rating Submersible Pumps

Submersible bilge pumps are rated by the number of gallons per hour (gph) they can pump. Generally, a submersible pump with a 300 to 500 gph rating is enough for boats up to 18-feet. Boats up to 23-feet will need a pump with a rating of 500 to 700 gph, and pumps with a rating of 700 to 1,600 gph will work for boats up to 26-feet.

Diaphragm Pump

A diaphragm pump accomplishes the same thing as a submersible bilge pump, but it goes about it a bit differently. Diaphragm pumps are much less common than submersible pumps.

Diaphragm pumps are mounted somewhere on your boat other than in the bilge. They have a pickup hose that goes down to the bilge and then a discharge that runs overboard.

The advantage of a diaphragm pump is that it is located above the bilge water, which makes it much easier to maintain the wiring and plumbing. On the flip side, they’re fairly expensive, clog easily, have low pumping capacity, and have a lot more moving parts than a submersible bilge pump. This leaves a lot of points for failure.

Manual Pump

You probably don’t want to rely solely on a manual bilge pump, but it’s always a good idea to have one handy, whether you have a submersible bilge pump or a diaphragm pump.

A manual pump at its simplest has a pick-up at one end that goes into the bilge water and a discharge hose at the other end, which pumps the water overboard. You operate the manual pump by hand.

One thing to mention, while most boats need an actual pump, a manual pump is a good idea for smaller boats where it might not be possible to install one of the other pump options.

Background on All Things Bilge

When you’re talking about a submersible bilge pump, you can’t focus on just the pump. That’s why we’ve got to go back one step and explain all things bilge; that includes the bilge well, bilge water, and the bilge pump.

The Bilge Well

The bilge is the lowest space on your boat, where two sides of the boat meet. You can easily find this area if you go to the corner of the lowest level of the ship’s engine room and look under the floor plates.

Once you find the bilge, along with it, you’ll see a nasty, oily black liquid. That liquid is bilge water and the compartment is a bilge well.

The number of bilge wells on your boat, and the depth of those wells depends on the size of your boat, its capacity, and the amount of bilge water your boat generates. The bilge well is an incredibly important collection tank in your engine room because if any water with oil in it gets into the open sea, you’re in big trouble.

Bilge Water

The bilge water that collects in the bilge wells is not just water but rather a collection of a whole lot of stuff. Take a look at some of the substances that can compose bilge water:

  • Freshwater
  • Seawater
  • Oil
  • Sludge
  • Chemicals

Where Does Bilge Water Come From?

Naturally, you may be wondering how all this stuff gets into your boat and the bilge wells. How the substance gets to the well really depends on the substance. Sea and freshwater can get there through leakage in pipelines, a leaky pump, machinery, overflowing tanks, or accidental spills.

Oil generally gets into the bilge well from oil purifiers, leakage in a fuel line, or an oil spill.

The bilge wells are a great area for all of this bilge water, but they only have so much space. If the well overflows, it can be a threat to your engine room because if the level of water rises up to or above the floor plates, it can cause an accident or emergency. That’s where a bilge pump comes into play.

The Bilge Pump

A bilge pump is responsible for pumping water out of the bilge well, so the level remains as low as it needs to be for safety. Of course, as I mentioned before, you can’t just pump this potentially oil-filled seepage out into the open water, so it goes through a bit of a process first.

  • The bilge water passes through an oily water separator.
  • In the separator, the level of oil particles in the water is reduced to a level that is can be dispensed into the ocean. Even then, though, the water can only be thrown overboard when the ship is moving.
  • Another feature of a bilge well is a strainer. The strainer prevents solid particles from getting into the pump. These particles could clog the oily water separator or the bilge pump.

Now that you know all things bilge, it’s time to move onto whether a bilge pump can be completely submerged.

The Overboard Discharge

After all this talk of bilge pumps, and whether a bilge pump can be completely submerged, it’s important to note that the overboard discharge cannot be submerged. The overboard discharge is the place where the bilge water is pumped out.

The overboard discharge needs to be placed well above the waterline but low enough that the bilge pump still operates efficiently. Don’t just think about where the waterline will be sitting when your boat is level. You need to consider where it will be when the boat is fully loaded, tilted, or “squatting” under power.

If the bilge water is pumped out underwater, you risk water back-siphoning through the fitting into the boat. You don’t want that nasty water coming back into your boat because the purpose of the bilge pump would be negated. Installing the overboard discharge in the proper place is an easy job, but one that many people fail to think about.

As you can see, you have a few different options when it comes to pumping the bilge water out of your boat. If you want a bilge pump that can be completely submerged, just make sure you buy that specific kind of pump.