Marine Fuel Filters vs Fuel-Water Separators 

Fuel and water obviously don't mix well, so separating them and keeping the fuel clean in a boat engine while you're out on the water is of vital importance. 

To do that, your boat probably has two separate components: a fuel-water separator and a fuel filter. And you need to know the difference between these two components. Watch the video above or read on below to learn about fuel filters and fuel-water separators.

How It Works: Fuel-Water Separators and Fuel Filters

If you've got a fuel filter and a fuel-water separator set up on your boat, the first stop on the fuel's journey up to the motor is the fuel-water separator. 

How it works is the fuel pump draws fuel through the fuel line into the fuel-water separator, and there are two individual chambers inside. The first chamber works like a centrifuge, especially on larger diesel boats like sport fishing boats. It pulls from the fuel pump, and that causes the fuel to spin around while the denser water gets slung to the outside chamber. 

That's typically what you'll find on larger sport fishing boats: a diesel system where it has a send and return line, and it has enough flow to create that vortex within the system. On gasoline-powered boats, the setup is a little bit different, so you need to make sure you choose the correct fuel-water separator. The system is usually a little bit slow as far as the amount of fuel that passes through the filter, so it really can't generate that centrifugal force. 

The way they separate it is having an area where the water can drop out, because it’s denser than the fuel. That allows the water to collect at the bottom, and then lets the fuel head to the engine or to the secondary fuel filter. This type of filter looks just like a standard oil filter. However, the filter part goes down to about the middle, and the bottom is completely open so the water can drop out. 

On a fuel-water separator like the one in the middle pictured below, you can easily see the filter section, and then a clear bowl on the bottom that collects the water. 

This type of unit is not meant for an inboard application, as the catch pan down at the bottom needs to be metal and see-through. However, it can be used for an outboard application. This version of a fuel-water separator drops the water out at the bottom either by gravity or centrifugal force. From there, it heads toward the engine, where it's going to run through a secondary fuel filter, usually inside of the VST tank on an outboard application. 

However, on an inboard setup you'll probably find a secondary fuel filter as well as a fuel-water separator, which gives it two lines of defense.

On a smaller outboard, you'll probably only see a filter like the one being held in the picture below, because the tank is removable, and that lessens the likelihood of having condensation built up inside of it. 

However, if you've got a 6 gallon tank and it’s only half full, it's going to let in condensation from the vent on the side of it if it’s not closed off. If that happens, you can see it by simply opening up the tank and looking down at the bottom. 

Choosing the Right Fuel Filter or Fuel-Water Separator

Finding the right fuel filter is pretty straightforward. Just choose your boat engine’s make and model and use the fuel filter’s part number to choose the right one. However, finding the right fuel water separator is a bit different because it's technically not a standard part of the engine's fuel system. 

When shopping for a fuel-water separator, the most important thing to consider is the flow rate of the fuel pump. If the flow rate is lower than the fuel rate on the pump, the fuel-water separator will inhibit the fuel flow and affect the engine's performance.

Do You Really Need a Fuel-Water Separator?

Some people say you don't even need a fuel-water separator if you have removable fuel tanks on your boat. The thinking on that is most of the condensation in the fuel happens when the boat isn't running. 

So if you're removing the tanks and storing them, they should be safe, right? Not exactly. Some fuel has water in it straight out of the pump at the refueling station, so you may want to consider installing a fuel-water separator. And even then you can still get water through a breather vent or a fill cap. A basic fuel-water separator is inexpensive, so you should get one just for good measure.