Suck It Up: Fuel flow meters - which one's right for your boat?
That tournament-winning fish could strike at any moment, but you’re worried you’ll run the fuel tanks dry if you troll any longer. So you pull the lines, cruise home, and hope for better luck next time as you bypass the weight ins. Back at the dock, the fuel gauge shows an eighth of a tank. Dang—you had at least an hour’s worth of troll-time left, and if you’d known it, you might have ended up a winner. Too bad you didn’t install fuel flow meters in your boat.
Having a fuel flow system is a big perk; you can constantly and accurately monitor your fuel burn, keep the throttles set at the most efficient cruise, know how much you’ve burned on any given trip, and exactly how much fuel remains in your tank. But there are several fuel flow systems on the market, and naturally each manufacturer claims theirs is the best. How are you supposed to know which one to buy? And, do you have to spend big bucks on the most expensive system to get the best accuracy? To answer these questions, I installed multiple fuel flow meters on a 19’ Twin Vee powered with a 140-hp Suzuki four-stroke outboard.
A 140-hp four-stroke has a pretty low fuel burn, which makes it a challenging motor to meter. Why? To understand, first you need to be aware of some specifics about flow meters themselves. Most meters available to the public, including all mentioned in this article, measure fuel flow with a simple mechanical paddlewheel. The faster the flow of liquid, the faster the paddlewheel turns. The paddlewheel, contained in a transducer of some type, then sends a signal to a monitor mounted in the dash, and translates that signal into a number—your per-hour fuel burn.
This paddlewheel design is inexpensive and efficient, but the paddlewheel has to be sized properly for the amount of flow. A tiny wheel, for example, would be good for low fuel flows like one GPH (gallon per hour,) but that same wheel wouldn’t be able to turn fast enough to keep up with a 20 GPH flow—for that you’d need a larger paddlewheel. And on a boat, fuel flow ranges can be a lot wider than this. The solution? Use a mid-sized paddlewheel which can handle the larger flows and still turns at low flow. Finding this middle-ground, however, means compromising. Mid-sized paddlewheels may be able to handle both the lower and upper ranges, but they’re very inaccurate at low flow and may give erratic readings or none at all. So a motor like the 140 four-stroke, which has very low fuel flow at low RPM and a modest flow at cruise, is the biggest challenge for most paddle wheel type fuel flow meters on the market.
What about all-electronic fuel flow systems like Mercury’s SmartCraft, E-Tec’s ICommand, and Caterpillar’s digital fuel flow? These are dead-on accurate. Instead of using a paddlewheel they have computer brains to calculate how many times per second the injectors fire, what the air temperature and density is, the resulting fuel/air mix, and a zillion other variables. So long as the brain’s in good shape these systems are bound to be far more accurate than any mechanical paddlewheel system can ever hope to be. But there’s one huge drawback: if your boat didn’t come with a powerplant that’s already wired for service, it can’t be retrofitted. So for the purposes of this article, electronic systems are not an option.
To create a solid baseline from which to judge the meters, I first measured fuel flow with a graduated cylinder. (Read: idiot-proof accuracy.) The cylinder, divided into tenths of a gallon, was mounted on a base with a valve that allowed me to pull fuel from the tanks until a specific RPM range was set, then flip a switch and pull fuel from the cylinder, only. I used a stopwatch to record how long it took for the motor to pull a tenth of a gallon from the cylinder, then did the simple math to get a GPH burn. At each RPM range I repeated the process at least three times, then averaged the numbers together to be as accurate as possible and account for the fraction of a second of slop-time caused by my thumb and the stopwatch.
Here’s the 140-hp outboard’s fuel burn in increments of 500 RPM, as measured by flowing fuel through the graduated cylinder:
With a baseline established, I then tested the Yamaha Fuel Management System, one of the most prevalent flow systems on the market by virtue of the fact that it’s manufactured by Yamaha and installed with the most popular large-horsepower outboards found on modern boats today. If you own an older motor, another brand motor, or want to add the Fuel Management System don’t despair; you can order the parts from a Yamaha dealer (including a gauge for the dash) for a little over $500. If, that is, you like these results:
20000.7 – 1.4
25001.8 – 2.4
The next contender was a Northstar 210 Fuel Computer, a unit which was previously marketed under the Navman nameplate but physically remains the same. This is an inexpensive paddlewheel unit, which costs about $150 per meter. If you have Northstar electronics at your helm you can interface the flow transducer with your display units, eliminating the need for a separate gauge. Northstar states up-front that this unit has a minimum accurate flow rate of 1.3-gph. Here are the numbers this unit gathered:
10000.7 – 0.9
15000.7 – 0.9
20001.5 – 1.9
25002.4 – 2.6
The third unit came from FloScan. In the world of fuel flow monitors, FloScan is the often considered the gold standard. They offer a wide range of meters and gauges for both single and twin engine installations, and specific meters for specific flow ranges. For this test, we used their meter rated for 0 – 20 GPH, which goes for about $275. FloScan also offers meters for engines burning up to 50 GPH, units which cost a hair over $500. Their TwinScan units, which indicate separate fuel burn figures for port and starboard powerplants plus speed, MPG, total burn, and other details, can cost as much as $550. FloScan is also the only company that offers custom-made systems for boats with even larger powerplants and fuel burns. Here’s what the FloScan told us:
10000.3 – 0.6
15000.7 – 0.8
20001.1 – 1.3
Gauging the Results
At very low flow rates, the FloScan may be imperfect but it’s also the most accurate of the group. From 1000 to 2500 rpm, the Yamaha either failed to register a reading at all or jumped around erratically. Yes, the FloScan did also jump around a bit at minimal flows, but not as much as either of the other units and it maintained pretty good accuracy. According to FloScan, this superior low-flow performance is at least partially due to the fact that they use jeweled bearings in their units, while other manufacturers use lower quality bearings.
From 3500 to 5500 rpm, however, the Yamaha had excellent accuracy. And when it comes to the Yamaha, experience has shown that their paddlewheel transducers show very little variability between units. Over time these systems have proven reliable and unreliable in the same ways from boat to boat, paddlewheel to paddlewheel, with very few systems acting differently than the one tested for this article. Other FloScans also show similar performance to that of our test unit, from boat to boat and unit to unit, as long as they’re properly mounted and calibrated. They do tend to be a bit more temperamental when it comes to sharp bends in the fuel line (which can create turbulence that throws the paddlewheel off pace,) but once dialed in, they generally provide accuracy to within two or three percent.
Wait a sec—if all of these units commonly do better than five percent accuracy, why didn’t they show readings closer to the graduated cylinder during this test? Remember that the readings will smooth out a bit over time. If one reads a hair low then a hair high, for example, by the time you’ve burned through 100 gallons of fuel, the numbers average out some.
So, which aftermarket system is best for your boat? If you’re concerned with low-flow accuracy (fishermen who troll with relatively small motors for extended periods of time would be a good example) then the FloScan looks like a winner. For cruisers who spend most of their drive-time at high RPM, the Yamaha is a good bet. If expense is a concern the Northstar is a winner. It’s accuracy at both low and high flow volumes is pretty darn good, and it costs about half as much as the next least expensive option. Whichever of these units you may decide to go with, one thing is for sure: the next time you’re trolling with your eye on the prize, fuel level fears won’t cause you to quit fishing one moment too soon.
Installing a Northstar in-line flowmeter transducer - it's a simple job.
With the fuel flow meter installed, you'll know exactly how much fuel remains in the tanks at any time.
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