NMEA-2000 Fuel Flow System

Project date: 2009

This project consists of a NMEA-2000 enabled fuel flow metering system. I had wanted a fuel flow system since I sold my last boat; however, the Crusader EFI engines installed in my present boat - with their return-loop fuel system meant there were few options available in fuel flow systems. A return-loop fuel system is one in which fuel is pumped to the engine, and any excess fuel is returned to the tank; meaning there are two fuel lines for each engine. So, to obtain an accurate fuel flow, there must be a fuel flow sensor on each line, and the return line flow must be subtracted from the forward flow. Contrast this with the typical carbureated engine (as well as some EFI engines), where only a single fuel line is connected to the tank, and a fuel flow sensor directly reads the fuel consumption.

The return-loop system is quite similar to diesel engine fuel systems. After a bit of correspondance with FloScan, I found that they do offer a NMEA-2000 capable system; called FloNet, and can be ordered for either Diesel or Gas engines. That was the good news; the bad news is the system is fairly expensive. But you get what you pay for, and I felt the sensors provided by FloScan were much more durable than the offerings by other manufacturers. A sensor not prone to leaking gasoline is pretty important, I think. After some calculation (as well as convincing my wife), we were able to determine that if we even improved our fuel mileage by 10%, the units would pay for themselves in a couple of seasons. In a boat that struggles to get 1 MPG, the investment will pay for itself quickly.



This is not intended to be an endorsement for any particular product, nor are they step-by-step instructions on installing the system. It's simply a review of what ideas I came up with when I installed the system. Thouroughly read installation manual prior to, and during your installation.




I indicated that the FloScan sensors are built very well, as you can see from the comparison above. Would you rather have a cast aluminum sensor with brass fuel barbs (FloScan) or some cheap plastic or nylon barb sensor on your fuel line? I actually used one of the Navman (Northstar) sensors on my old boat, and I recalled crushing the cheap nylon barb when attempting to tighten the clamp the hose. You simply cannot tighten the nylon barbs with a hose clamp. However, I have to admit that I didn't have any leaks.

In contrast, you can easily double-clamp the brass fuel barbs on the FloScan sensors. Fuel leaks on a boat is something I really don't want, as it can be very dangerous, and its my belief that the superior nature of FloScan's sensors will minimize that danger. FloScan's tech support is also quite good, which is nice as I had many questions during the installation. So, the money I spent for the system was well spent.

With excitement, in the spring of 2009, I layed down my cold-cash to purchase two of FloScan's N20G-231-21 FloNet units. Two identical boxes arrived in a few weeks. Apparently they custom-build each kit, testing each of the sensors, calibrating and and matching them to the NMEA-2000 hubs, so it does take a bit of time for them to be shipped.

What is not included, but you have to purchase at extra expense, are the fuel barbs, fuel filters, and hose clamps. Shockingly, that set me back a couple hundred dollars. Since I am into this project big time, I might as well also purchase some 3/4" starboard for mounting all of this stuff. While the kits come with heavy duty mounting brackets, I didn't have a good way to use them in the boat, so I am using the starboard as a mounting panel.

Laying out all of the sensor parts for each engine. The fuel sensors are calibrated, and marked as to which one is for the forward fuel line, which one for the return line, and which NMEA-2000 hub they are matched for. Also in the photo are two pulsation dampers (the round globes), which smooth out the readings.

The red items are the recommended Flow Ezy ILA-02 in-line fuel filters. These are heavy-duty racing filters, and at 238 microns, they protect the sensors by filtering out large pieces of gunk that might find its way into the fuel system. With their large size, hopefully I won't have to change the filters often, as they will be a bear to get to after installed in the boat. You should be able to identify the remaining components.

I also made custom mounting brackets for the sensors so that they would not be stressed after the attachment of the fuel lines.

I cannot stress enough that you must read and understand the installation instructions, as there are some specific requirements for placement of the components.

One difficult task was to determine which fuel line barb to order. Since FloScan has no idea what size fuel line you have, this is one item the kit doesn't come with. You simply order the correct size depending on the fuel line, right? Well, that is an easy process, if you know what diameter the fuel line is.Unfortunately, the fuel line didn't have its size stamped on it. The boat's manual indicated the fuel line was 3/8" but it has been known to be incorrect, and the one thing I did not want to do is find out the barbs were the wrong size after cutting into the fuel line.

What made matters worse is that I purchased a short length of 3/8" USCG A-1 fuel line in case I needed to make a splice, but it's outer diameter was significantly smaller than what was in the boat. Now I am really worried. Do I spend more money on fuel barbs that I may not need?

Fortunately, there was a brand-name and set of numbers on the fuel line... so, I called the manufacturer directly. While they could not determine the exact fuel line (the numbers were a manufacturer's date - not the part number), they did indicate that a few years ago, at the time my boat was manufacturerd, their fuel line had thicker side walls, so I probably had 3/8" fuel line.

I'll not keep you in suspense; turns out I did have 3/8" fuel line, but these are some of the issues that you have to solve when you go about tearing into your boat.

The fuel barbs I ended up ordering were by Moeller Marine, as they had long stems, which made it easier to double-clamp the hoses.

I used the starboard as a mounting panel, assembled and mounted all of the fuel line components to the panel. Extreme caution should be used here... by reading, and re-reading the installation manual. For example, the manual warns that you must NOT use teflon tape on the fittings. The installation manual stipulates using Locktite PST or equivalent thread locking compound. Fortunately, FloScan supplies sufficient thread locking compound in the kit, and that is what you should use.

This is a complex installation, and as I need to state several times, read the manual. The location of the pulsation dampers, as well as vertical and horizontal lay of the fuel line are dependant on where and what kind of fuel pumps the engine has, and whether there are fuel pumps in the fuel tanks. Therefore, the orientation of the pulsation dampers and location of the fuel sensors are only one possible configuration.

I did find though that the thread locking compound cures in the absense of air, which means the compound will stay wet on the outer surfaces, and consequently, get all over everything. Therefore, try not to touch the fittings after you have assembled the sensors.

The mounting board is secured to the boat, fuel lines cut and attached to the sensors. This was a difficult process... well, actually it was an impossible task, as I am a big guy and I could not easily fit myself into the bilge. Worse, as adept at I am with other boat projects, I hate working on engines, which includes fuel lines.

We all have different skills, and simply, my skills are not in this area. Fortunately, a good boating friend, and a skinny guy helped me with this part of the project. Often, you can trade project help with your boating friends - which is what I did here.

Thanks Marc!

Now on to something I am more comfortable with - electronics. The sensors directly connect to matched interface boxes that were located in the rear berth closet. They also provide the NMEA-2000 interface, and in fact, the entire system is powered from the NMEA-2000 backbone. Again, consult the installation manual for wiring and grounding requirements.

Each system has a NMEA-2000 LEN (Load Equivalency Number) of 6; or in more technical terms, they each require up to 300mA. This current is supplied by your NMEA-2000 network so ensure your network has the capacity to provide the needed current to the units without excessive DC voltage drop.

Before the system is operational, some configuration must be done; for instance, each box'es instance number, US or Metric operation, as well as calibration settings, which are all stated in the installation manual.

If you need a refresher on LENs and instances, read my NMEA-2000 primer, which can be found here: NMEA-2000 Primer

While there are several NMEA2000 displays on the boat, my instrument of choice for the fuel flow data is a Garmin GMI-10. As shown, I have customized two different screens. The screen to the left shows Cruising Range (miles until empty), Fuel Economy in MPG (wow - 1 MPG), Fuel Onboard, and GPH Fuel Flow. The MPG rating is possible as GPS data is available from the RayMarine C-80, and fed into the GMI-10, and it in turn calculates MPG from the FloScan FloNet data. It should also be noted that fuel tank level is not from the FloScan instrument, but rather Lowrance NMEA-2000 sensors attached to the fuel tanks. This is truly a heterogenus network.

As you sequence through the pages on the GMI-10, the page on the right is displayed. Fuel Tank level and Fuel Flow for each engine are being displayed here. These two photos should give you some idea of the different display settings possible with the GMI-10.

I can also read this data with my RayMarine C-80 display as well as my Lowrance LMF-200 and LMF-400 instruments. However, that data is not as comprehensive as the display from the Garmin GMI-10. This again illustrates that not all NMEA-2000 devices are equal.

Our navigation project has now grown to the configuration shown below.

(Click on the drawing for a full sized version)


Flo EZY ILA High Performance Fuel Filters.
Moeller Marine Fuel Barbs

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