Do you need a chartplotter if you have a phone?

The honest starting point is that a phone running a good chart app is a real navigation tool. It knows where it is, it draws a chart, it plots a route, and it does that on hardware you already own. An argument that opens by pretending otherwise deserves to be ignored.

The differences that matter are unglamorous, and they’re worth naming precisely: how bright the screen gets, what happens when salt water lands on it, where the electricity comes from, and — the one that settles the whole question — what else on the boat the device is talking to.

Can a phone really replace a chartplotter?

On a small open boat in water you know, a phone can be the primary navigation display; on a boat carrying radar, AIS, sonar or an autopilot, it can’t be the only one. The reason isn’t screen quality or app quality. It’s that a chartplotter isn’t really a chart display — it’s the head unit for a network, and a phone isn’t on that network.

Garmin describes the chartplotter as the central control/display “hub” of your boat. Raymarine says the Axiom 2 “unifies navigation, sonar, radar, video and NMEA 2000 networks into one powerful display”. Both are marketing sentences. Both are also accurate descriptions of the job.

What is the NMEA 2000 network, and why does it decide this?

NMEA 2000 is the wiring standard that lets everything aboard share data on one cable, and a phone has no way onto it. The National Marine Electronics Association describes the standard as containing “requirements based on CAN (Controller Area Network) communications to inter-connect marine electronic equipment on vessels” — “a low-cost moderate capacity bi-directional, multi-transmitter/multi-receiver instrument network to interconnect marine electronic devices.” It’s “multi-master and self configuring” with “no central network controller,” and everything shares a single channel. NMEA also states that “All products that read and or transmit NMEA 2000 information must be certified by NMEA.”

That bus is where your depth sounder, engine data, wind instrument, AIS transponder and autopilot live. Garmin’s GPSMAP 9000-series announcement puts it plainly: the units “can also connect to compatible existing Garmin marine systems, including other GPSMAP chartplotters, radars, autopilots, sensors and more via NMEA 2000 and NMEA 0183 networks”.

A phone can receive some of that traffic over Wi-Fi if you buy a gateway. It still isn’t a node on the network, it isn’t a certified source of anything, and nothing else aboard is depending on it.

Can you read a phone screen in direct sunlight?

The published figures say the gap is narrower than boaters assume — and that the figures aren’t measured the same way. Apple publishes, for the iPhone 17 Pro, “1000 nits max brightness (typical); 1600 nits peak brightness (HDR); 3000 nits peak brightness (outdoor)”. Raymarine publishes 1800 nits for the 7-inch Axiom 2 and 1500 nits for the 9-inch.

Apple’s headline number is a peak, quoted next to a typical maximum a third as high; Raymarine’s is a single display-brightness spec. Neither maker states how long a peak is sustained.

The more useful finding is what’s absent. Garmin publishes no brightness figure for its GPSMAP chartplotters — not in the installation specifications for the 7x3/9x3/12x3/16x3 series, not in the 8x10/8x12/8x16 specifications, and not in the 9000-series announcement. Furuno publishes none for the NavNet TZtouchXL either. If nits are your deciding factor, two of the three biggest makers of these things decline to give you the number.

What actually kills a phone on a boat?

Water ingress, salt, and a battery nothing is charging. Garmin’s x3-series chartplotters are rated to “IEC 60529 IPX7” — incidental immersion to 1 m for 30 minutes. Raymarine lists “IPx6, IPx7” for the Axiom 2. Apple rates the iPhone 17 Pro “IP68 (maximum depth of 6 meters up to 30 minutes) under IEC standard 60529”, which on paper beats both.

The rating isn’t the whole story. An IP test is a laboratory immersion of a new sample in fresh water. It says nothing about a decade of salt spray, UV and vibration on an open console — the environment a plotter is designed and warrantied for, and a phone is not.

Power is less arguable. Every chartplotter below runs from the boat: 10 to 32 Vdc for Garmin’s x3 series, 8 to 16 V dc for the Axiom 2, 12 to 24 VDC for Furuno’s TZT13X. A phone runs from a battery you have to remember to charge, in the same device that’s also your camera and your way of calling for help.

How do the published specs compare?

Of the devices below, only Raymarine and Apple publish a brightness figure at all.

Device	Published brightness	IP rating	NMEA 2000?	Runs off ship’s power?
Raymarine Axiom 2, 7 in	1800 nits	IPx6, IPx7	Yes — DeviceNet connector built into the power cable, LEN 1	Yes — 8 to 16 V dc, 12 V dc nominal
Raymarine Axiom 2, 9 in	1500 nits	IPx6, IPx7	Yes — as above	Yes — 8 to 16 V dc
Garmin GPSMAP 9x3	not published	IPX7 (IEC 60529)	Yes — NMEA 2000 LEN 2, 75 mA max draw	Yes — 10 to 32 Vdc
Furuno NavNet TZtouchXL TZT13X	not published	not published	Yes — N2K sensors and cables listed as accessories	Yes — 12 to 24 VDC
Apple iPhone 17 Pro	1000 nits typical; 3000 nits peak (outdoor)	IP68 (IEC 60529)	No	No — internal battery

Two caveats. Brightness isn’t reported on a common basis: Apple gives a typical maximum and a separate outdoor peak, while Raymarine gives one display-brightness number — and Raymarine’s own table rates the 9-inch lower than both the 7-inch and the 12-inch. And Furuno’s US product page lists NMEA 2000 sensors and cables for the TZT13X but doesn’t name NMEA 2000 in the specification table itself.

Should your phone be the position source for your VHF?

No — a phone can’t do that job at all. The US Coast Guard states that “DSC distress alerts include position information, but only if the radio has an internal GPS or is connected to a GPS”, and separately “urges, in the strongest terms possible, that you take the time to interconnect your GPS and DSC-equipped radio”, noting that DSC radios and most GPS receivers share an NMEA 0183 two-wire data protocol.

A phone has no NMEA 0183 wires and no NMEA 2000 drop. Either the radio carries its own GPS or something on the network feeds it. Standard Horizon settles it in the radio: the GX2400GPS is specified with a “built-in integrated 66 channel GPS receiver”, “NMEA2000 and NMEA0183 Compatible” interfaces, and a “Built in AIS Receiver.” That’s why a DSC set with internal GPS is worth more than the price difference. For what those AIS targets mean once they appear, see AIS explained.

Whose charts are you looking at, and how old are they?

In US waters the underlying data is NOAA’s electronic navigational charts, and the paper and raster products they replaced no longer exist. NOAA announced in a 2019 Federal Register notice that it was “undertaking a five-year program to end all raster and paper nautical chart production”. That program finished. The Office of Coast Survey states that “The raster sunset program was completed in December of 2024 with the cancellation of the last of NOAA’s 1,007 paper charts” and that “Electronic navigational charts are now our primary nautical chart product.” The RNC Tile Service, the full-size chart PDFs, BookletCharts and the RNC Viewer went with them.

This cuts against the plotter as often as for it. NOAA uploads new ENC update cells “every weekday evening”, so chart currency is a question about your device’s update habit rather than its price. A phone app that syncs over cellular every time you open it may be carrying fresher data than a plotter whose chart card was last updated two seasons ago.

Paper isn’t quite dead, either. NOAA Custom Chart builds printable charts “directly from the latest NOAA ENC data,” using “the same symbols and colors used on traditional paper nautical charts.” You just print them yourself.

When is a phone enough on its own?

A phone is a reasonable primary on a small open boat in familiar, well-marked water, a poor sole system offshore, and never the position source for a distress call. Layer the kit instead of choosing between it: a plotter at the helm as the network’s display, the phone as a second opinion and a planning tool on the dock, a DSC radio with its own GPS, and one thing that needs no electricity at all. A Ritchie bulkhead compass is the last item for the same reason a hiker carries one — the argument in compass vs GPS. And if the budget is going to a sensor rather than a second screen, radar is the one capability no phone app substitutes for.

The short version

A phone with a good chart app is a genuinely capable navigator, and on a small open boat in water you know, it’s enough. What it can’t do is join the boat’s NMEA 2000 network, which is what a chartplotter is actually for — the display for radar, AIS, sonar and autopilot, running off ship’s power instead of a battery you forgot to charge. Published brightness figures are closer than most people expect, and Garmin and Furuno decline to publish them at all. The one place there’s no argument is the VHF: a DSC distress alert carries your position only if the radio has its own GPS or is wired to one, and a phone can’t be that source.