What live fuel-flow data actually is
Most boats report fuel the way cars did fifty years ago: a float in the tank wired to a needle, damped so heavily it can take minutes to admit anything changed. The gauge answers one question, imprecisely — roughly what fraction of the tank is left. It says nothing about how fast you're spending it, and nothing about what a given throttle setting costs you per mile.
Live fuel-flow data is the other half of the picture. It's a reading, typically updated about once a second, of what the engine is consuming right now, in gallons per hour. Hold 4,200 rpm and see 11.4 gph; ease back to 3,800 and watch it fall to 8.9. Pair the flow rate with your speed and you get the number that actually sets your range — nautical miles per gallon at this throttle, this load, this sea state — while it's happening, instead of averaged out of a fuel receipt two weeks later.
That's the case for flow data over gauge-watching in one line: a gauge tells you what's left, badly; a flow meter tells you what you're spending, precisely enough to act on. On a modern boat that data rides a network called NMEA 2000, and the ten-minute version of how it works is worth knowing — because where each number comes from tells you exactly how far to trust it.
How GPH gets onto the NMEA 2000 bus
NMEA 2000 is the marine electronics industry's shared network standard: one backbone cable run the length of the boat, with short drop cables teeing off to each instrument. Every device broadcasts its readings as numbered messages called PGNs, and any listener on the bus can read them — a reading goes out to the whole network, not to any one display. Fuel data mostly rides three: engine parameters including fuel rate (PGN 127489), tank fluid level (PGN 127505), and, on some engines, a running trip-fuel total (PGN 127497). Speed and position ride alongside on their own messages, which matters later, because flow only becomes economy once it's paired with speed.
There are two ways a fuel-rate number gets generated in the first place. The first is the engine itself. A fuel-injected engine's ECU already knows precisely how long it holds each injector open on every cycle, so it can compute the flow it's commanding without any extra sensor touching the fuel. Most outboards and many inboards from roughly the mid-2000s on will broadcast that figure — either natively onto NMEA 2000 or through a small brand-specific gateway that translates the manufacturer's own engine network onto the standard bus.
The second source is an in-line flow sensor, for engines too old or too analog to report anything: a small turbine sits in the fuel line, spins as fuel passes, and the sensor counts pulses — so many pulses per gallon — and broadcasts the result the same way. One honest wrinkle: on a diesel this takes two sensors, because diesels return unburned fuel to the tank. Actual burn is supply minus return, and a single sensor on the supply line reads wildly high — wrong enough to be useless.
Tank level comes from a different device entirely: the tank's own sender, usually a float or pressure sensor, reporting the height of fuel in the tank translated to a percentage. Keep that distinction in mind — flow is measured or computed at the engine; level is inferred at the tank. They fail in different ways, which is what the limits section below is about.
Wi-Fi gateways and SignalK: the bridge to your phone
The bus is wired; your phone isn't on it. Bridging that gap is the job of a Wi-Fi gateway — a device the size of a deck of cards that tees into the backbone like any other instrument and rebroadcasts the bus traffic over the boat's Wi-Fi. Yacht Devices' YDWG series is the best-known of the type; Actisense and others make equivalents. An app on your phone joins the boat's network and reads the same messages the chartplotter sees.
The other common bridge is a SignalK server: open-source software, usually running on a Raspberry Pi wired to the bus, that translates NMEA 2000 (and the older NMEA 0183) into an open, human-readable format any app can subscribe to. It can also log, serve dashboards, and merge other data sources — at the cost of being a small computer you now administer. Either bridge delivers the same fuel data to the same phone; the SignalK route trades plug-and-play simplicity for flexibility.
One property matters more than anything on a spec sheet: everything described here can be done receive-only. A well-behaved app listens to the bus and transmits nothing onto it — which means it has no way to interfere with your engine, your autopilot, or anything else your boat depends on, no matter how badly it misbehaves. That's how Far Enough connects, and it's worth demanding of anything you let aboard: an app that wants to write to your boat's network should have an extraordinarily good reason.
What you can read once you're connected
What's available varies with the engine and the installation, but a typical fuel-instrumented NMEA 2000 boat broadcasts more than most owners realize.
The pair to watch is fuel rate and speed. Divide speed by flow — 22 knots over 11 gph is 2.0 nmpg — and you have live economy: the per-mile cost of the exact throttle you're holding. Nudge the throttle and trim while watching that number and you can map your hull's sweet spot in an afternoon, something no fuel gauge will ever show you. Here's the typical menu:
- Fuel rate, per engine, in gallons per hour — the headline number, updated about once a second.
- Tank level, per tank, as a percentage — and from some senders, the tank's capacity too.
- Engine hours, the ECU's running total — the same figure you log at fill-ups when you calibrate, and that a [ship's log](/blog/how-to-keep-a-ships-log) records at every departure and arrival.
- RPM, which lets you tie a fuel rate to a throttle setting and find it again next weekend.
- Speed, as speed over ground from the GPS and, when a paddlewheel is fitted, speed through the water.
- Position, heading, and often depth and wind, depending on what else shares the bus.
- On some engines, a trip fuel total — gallons burned since reset, which is fill-to-fill arithmetic done for you.
The honest limits
All of the above is real and genuinely useful. Now the caveats, because every number on that list has a way of being wrong, and a fuel plan that trusts electronics uncritically is worse than one that never had them. Three limits deserve their own headings.
Tank senders lie in chop and heel
A tank sender doesn't measure fuel; it measures the height of the fuel's surface, usually with a float on a pivoting arm. On a level boat in a flat calm, height maps to volume tolerably well. Underway, the surface won't sit still: chop sloshes it, heel and trim tilt it, and a long acceleration piles fuel toward the back of the tank. The reading swings with the boat, and the damping that hides the swing also hides the truth.
Tank shape makes it worse. Plenty of tanks are irregular — wider at the top, tapered into the bilge — so equal changes in height aren't equal changes in gallons, and accuracy is poorest near the bottom, exactly where it matters most. Treat tank level as a trend to sanity-check, never a number to plan on. The trustworthy version of “what's aboard” is arithmetic: what you put in, minus what the flow data and your log say you've burned since.
Flow numbers need calibrating against real fills
ECU-reported flow is a calculation, not a measurement. The computer reports the fuel it commanded through the injectors, assuming healthy injectors and nominal fuel pressure — usually accurate to within a few percent, but “usually” is not a plan. In-line turbines measure real flow and bring their own errors instead: a pulses-per-gallon constant that drifts, a minimum flow below which they read nothing (idle and trolling can sit under it), and on diesels the supply-minus-return problem from earlier.
The fix is the discipline you'd use with no electronics at all: check the meter against the pump. Top off to the same automatic click, run normally, top off again, and compare the gallons the pump says you burned to the gallons the meter claims. One fill proves little — a half-hearted starting click or a generous nozzle skews it — so do it across several fills before trusting the meter to a couple of percent. Our guide to how much fuel a boat uses walks through the fill-to-fill method in detail.
This is where live data quietly earns its keep over time, and where software should be skeptical on your behalf. Far Enough feeds auto-captured running intervals through the same fill-to-fill calibration as manual fills, and it deliberately distrusts flattering data: an interval that implies an implausibly low burn — the kind that inflates your range — is dropped rather than averaged in, a higher-than-rated burn is believed immediately, and a lower one only eases the estimate down slowly. For a fuel number, suspicion in the flattering direction is the only correct default.
Never bet the return leg on electronics
Everything in this post hangs off one backbone. A corroded tee, a chafed drop cable, or a tired power supply can drop several instruments at once, and the phone route adds more links — the gateway, the Wi-Fi, the battery in your pocket. None of it fails often. All of it fails sometimes, and offshore, in the rain, after dark is when it will choose to.
So the rule is simple: live data may sharpen the plan; it must never be the plan. Know your measured burn, your usable fuel, and your reserve before you leave the dock, so that if the screen goes dark halfway out you're still running the same conservative plan you left with. The discipline in planning fuel for a boat trip — reserve first, pessimistic numbers, the return leg costed worse than the outbound — doesn't change because the helm got smarter.
It's also how planning software ought to treat the feed. Far Enough ages every live reading out within about ten seconds — a value that stops updating is discarded, not silently reused — and it only uses a live burn rate at all when it arrives alongside a live speed, because an idle-at-the-dock gph anchored at cruise speed would fabricate range out of thin air. When the feed drops, the estimate falls back to the conservative curve calibrated from your fills — or the rated numbers you entered, if calibration hasn't accrued yet — and the go/no-go verdict keys off the pessimistic end of the estimate either way. Electronics as a bonus, never a dependency.
What it costs to add (rough, honest ranges)
If your boat has a chartplotter from the last decade networked to a fuel-injected engine, you may already own most of this. Check whether the plotter can show fuel rate; if it can, the data is already on the bus and the only thing missing is the bridge to your phone.
Rough US street prices as of mid-2026, hardware only — installation is extra, and marine labor varies too much to generalize:
- Wi-Fi gateway (YDWG-style): roughly $200 to $350. If the data is already on the bus, this is the whole project.
- SignalK server on a Raspberry Pi with a CAN interface: roughly $150 to $300 in parts, plus your evenings.
- In-line fuel-flow sender for a gas engine that doesn't report its own: roughly $200 to $400 per engine.
- Diesel differential flow kit (supply and return measured together): roughly $600 to $1,200 and up — the expensive case.
- NMEA 2000 backbone starter kit (power tee, terminators, a few drops) for a boat with no network at all: roughly $75 to $200, before drop cables and the labor of running them.
- The zero-dollar option: skip all of it. A notebook and the fill-to-fill method produce the number that actually matters. Electronics buy immediacy and resolution, not necessity.
