Installing a fuel flow meter in just about any powerboat is a good idea; it gives you a wealth of data about your burn rate, at a glance. But all flowmeters are not created equally. And how you install them can have a dramatic effect on just how accurately they work. Use these tips when you install one, and an endless stream of reliable data will forever-after be at the helm.
GO digital, whenever possible. Most modern engines are computer controlled, and your ability to monitor fuel flow at the helm may be a simple matter of installing a gauge and/or a data wire. In many cases the engine data is NMEA2000 compliant; if your chartplotter is as well, you may need to do no more than connect a data-in/data-out wire, to see fuel flow numbers on-screen. This data is usually a lot more reliable than that which comes from an in-line paddlewheel transducer, so do some research and find out if this is an option, in your case.
GET your fuel lines as straight as possible, when installing a paddlewheel-style transducer. Where you have to make a bend, make it as gentle and wide as possible, as opposed to abrupt changes in direction. Sharp bends can cause turbulence, which introduces air bubbles into the lines. When those bubbles hit the paddlewheel, they can throw it off. As a result, your flow meter’s readings can be wildly inaccurate.
SUPPORT those lines. Drooping fuel lines will move around as the boat runs, which can also cause internal turbulence. Make sure the fuel lines are secured with cushioned clamps or tie-wraps every foot or so, to prevent this problem.
ONCE the meter is set up you need to calibrate it properly, because these things are rarely accurate right out of the box. How you’ll calibrate it depends on what type of unit you have, but the really tricky part is identifying the exact fuel burn at a specified RPM—without the meter’s input—so you have a baseline for calibration. The best way is to burn a measured amount of fuel, and time it. You can accomplish this by hooking up a portable fuel cell, putting in a measured amount of fuel, and using a stopwatch to time the burn. A tenth of a gallon works well, since it makes for easy math: multiply the number of seconds it takes to use up the fuel by 10, and you’ll know exactly how long it takes to burn a gallon of fuel at that speed.
Here’s an example: if it takes 30 seconds to burn a tenth of a gallon, multiply 30 by 10 for 300. Now, you know you’ll be burning a gallon every 300 seconds (five minutes). Take the total number of seconds in an hour, 3600, and divide this by 300 to get 12. That’s your fuel burn—12 gallons per hour.