Does filling your boat's fuel tank cause you physical pain? Many boaters cringe these days, each and every time they fill the tanks. In some cases, monthly fuel costs can exceed monthly payments on the boat itself.

But you can reduce your fuel costs – and increase your efficiency, range and environmental sensitivity – if you know exactly when your boat is running at its most efficient speed. And for that, you need a fuel meter.

Few pieces of gear have generated more attention in recent years than fuel monitors, which are increasingly becoming standard equipment on new boats and are a popular piece of add-on gear for older vessels. While systems for gas engines are more prevalent – systems for diesels are more complicated, because they must meter both primary and return lines – online boating forums are awash in conversation about how to install monitors of all kinds.

And why not? Fuel-flow monitors help you constantly and accurately watch your fuel burn, and therefore keep the throttles set at the most efficient cruise. You'll see your gallon-per-hour fuel burn at a glance, know how much you burned on any given trip and know exactly how much fuel remains in your tank.

LENNY RUDOWRunning fuel through a graduated cylinder and timing how long it took to pass through makes it easy to calculate exact fuel burn at specific rpm.Or, so you hope – because no one can tell you just how accurate fuel-flow meters made by different brands are. In fact, if you go shopping for fuel-flow meters, you'll encounter reams of contradictory information. Most of it comes directly from the manufacturers, each of which (naturally) claims its meter is the best. On the forums, everybody has their favorites.

To end this debate, I installed multiple fuel-flow meters on a 19-foot Twin Vee powered with a 140-horsepower Suzuki four-stroke outboard. Why use such a small powerplant for this test? There's a method to this madness, and it requires some background information about flow meters themselves.

HOW THEY WORK

Most meters, including all in this test, measure fuel flow via a simple mechanical paddlewheel plumbed into the lines of the fuel system. The faster the flow of liquid, the faster the paddlewheel turns. The paddlewheel, contained in a transducer, then sends a signal to a monitor mounted in the dash and translates the signal into a per-hour fuel-burn figure.

This is an inexpensive, efficient design. However, the paddlewheel has to be sized properly for the amount of flow; a tiny wheel would be good for low fuel flows like 1 gph, for example, but it wouldn't turn fast enough to keep up with a 20-gph flow. And fuel-flow range can be a lot wider than this. The solution? Use a mid-size paddlewheel, which can handle the larger flows and still turn at low flow. Unfortunately, such paddlewheels may give erratic readings or none at all at low flow. Thus, a low-flow motor, like the 140 four-stroke, poses the biggest challenge for most paddlewheel-type fuel-flow meters.

No conversation about fuel flow would be complete without mentioning all-electronic fuel-flow systems, like Mercury's SmartCraft, E-Tec's ICommand, and Caterpillar's digital fuel flow. Instead of using a paddlewheel to determine the flow rate, they use their 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 many other variables.

Mounting the in-line transducers for all of these flow meters is essentially identical: Make a cut in the fuel line after all fuel-water separators and filters, but before the ball. They have two barbed ends, an "in" and an "out," which get inserted into the fuel lines and secured with hose clamps. After that, it's merely a matter of running their wires to the dash and hooking them to your display. If you're using manufacturer-matched brands (i.e., Northstar with a Northstar chartplotter, or Yamaha with a Yamaha multifunction gauge already at the helm), then the information can be displayed on your existing units. Otherwise, you'll also have to cut a hole in the dash to accommodate a dedicated fuel-flow gauge for the meter.

When I ran fuel-flow numbers on boat tests for Boating magazine for more than a decade, including several comparisons between the magazine's highly accurate oscillating fuel-flow meters (which measure flow by an oscillating disk instead of a paddlewheel), I found that these digital systems were more accurate than any mechanical system available. There's one huge drawback: If your powerplant isn't already wired for service, it can't be retrofitted. So for the purposes of this article, they're not an option.

THE TEST

Putting the meters head to head on the same motor would allow me to compare one against the other, but how would I know which was actually the most accurate? To create a solid baseline from which to judge them, I also measured fuel flow with a graduated cylinder, divided into tenths of a gallon.

I mounted the cylinder 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 just the cylinder. 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 simple math to get a gph burn. At each rpm range, I repeated the process at least three times, then averaged the numbers to be as accurate as possible and account for the fraction of a second of lag time caused by my thumb and the stopwatch.

The table to the right shows the outboard's fuel burn, throughout the rpm range, in increments of 500, as evidenced by the graduated cylinder fuel-flow measurements.

Note that I recorded these numbers to create a baseline for comparison with other fuel-flow devices, not to find the specific best cruising speed for the boat. Therefore, speed and mpg weren't a concern. Remember, however, that mpg is the key figure you'll need when determining your boat's most efficient cruising speed. To find it out, simply divide your miles per hour at any given rpm by your gallons per hour.