…and a phone to steer her by

Mobiles have had a bad press as navigational tools, but if I were forced to choose one single piece of electronics to take to sea it would be my phone. That’s not a popular view among professionals.

Instructors, coastguards and rescue services learn of many cases where boat owners, especially of powerful motor yachts and RIBs, set off for the open sea with nothing beyond a chart app on a mobile phone, and no knowledge of the underlying skills needed to navigate safely. For the Royal Yachting Association, mobiles are well down the list of recommended priorities, because of the risk that they will be used badly. Textbooks give stern warnings that you must not use them for navigation.

But that overlooks the fact that, as a last fallback, what you fundamentally need to know is where you are. So I think mobiles should have a better press as a backup navigation tool at sea.

I have been aware for years that my mobile tracks my Ordnance Survey maps accurately over long upland walks well outside any mobile and wifi signal, relying entirely on satellites. So why wouldn’t it be just as accurate at sea, where there are no obstructions to signals from above and no mountains nearby?

What I confirmed with my own checks was that, away from tall buildings, the two mobiles I have owned recently both show a radius of accuracy on satellite navigation alone of less than 10 metres and most of the time 3 to 6 metres. That accuracy level is reliable over long periods.

I’ve kept my phones on the setting for satellite location only for a year now and have double checked by using Airplane mode. Except among tall buildings or indoors, switching to the high accuracy setting does not improve the fix, even where I know wifi hubs and phone masts are nearby.

At sea, the accuracy levels and reliability of fixes are good, just as they are on upland walks. I tested this in the summer on two 180 mile passages.

For how I logged the satellite accuracy of my two mobiles, one 5 years old and one bought in April 2021, see ‘how to check satellite accuracy on phones.

My tests are not worldwide or professionally rigorous. I do not know how my phones would perform in the Himalayas or among mountainous waves. But I do know that my phone’s satellite navigation is good in the places I frequent.

It is not just the chips and the software that are improving, but also the number of usable satellites. Modern GNSS mobiles have access to the Russian, Chinese and EU satellite systems, so there are far more satellites in view at any one time than when phones, and indeed older chartplotters, had to rely on the American GPS. 

Confusion about accuracy is generated by the phones themselves. My phone is always nagging me to switch to its ‘high accuracy’ setting to allow wifi, bluetooth and phone mast tracking, and it says I won’t get a good service without them.  The language implies you only get an accurate position if these other aids to triangulation are helping your satellite chip. 

In fact, that’s misleading except  in certain obvious places, such as indoors and among tall buildings or other sources of interference and satellite signal reflection. Research is under way to correct even those inaccuracies –  there’s a lecture on the Royal Institute of Navigation website about new satnav signal processing software which is claimed to achieve good accuracy even in cities and which its creators expect to be available on mobiles in the next few years. 

The cynic in me thinks that behind this pressure to switch on the high accuracy setting there is a money-making motive. Wi-fi location tells Google, everyone else in similar information harvesting businesses and their vast armies of advertisers which shop, restaurant or other facility you are in.

Why would I choose to have my mobile on board in the unlikely event that it was my only instrument?  I’m old enough to remember the days when you were never quite sure where you were until you got there. Crossing the English Channel in bad visibility in a boat doing 5 knots with a varying  cross tide of up to 3 knots could leave you uncertain of your landfall within five or even 7 miles if you were unlucky, or were thrown about too much to steer accurately. Radio beacons, the best electronic aid, were difficult to use in a bouncing small boat, and gave inaccurate fixes.

 So it is hard to exaggerate the enormous pleasure and relief that our first proper electronic instrument brought us in the 1980s – a Decca Navigator, which actually told us where we were to 50 metres. That accurate position seemed like a miracle but my phone gives a position at sea an order of magnitude better.

 Everything else we now get from electronic navigation – way points, route planning, tracking, course over ground, electronic charts, overlaid displays  – is secondary to that magical fact of knowing a position. And that’s what a mobile phone can tell you accurately if you have no other electronics with you.

 It goes without saying that you’ll have to have paper charts to use the information in those circumstances, which is why heading out to sea in a powerboat with nothing but a phone loaded with a Navionics chart is a very bad idea indeed, as the RYA and coastguard are absolutely right to insist. Carrying some paper charts, if only as backup to electronic charts, remains important. And of course, I’m not really suggesting I’d go to sea with only a mobile when other electronics are available, because there is so much that is useful in many different ways.

Here is my list of factors to consider in using mobiles for navigation:


  • Accurate position finding, giving a last ditch backup if all else fails.
  • Easily rechargeable for days on end with a few small, cheap booster batteries on board, useful if boat electrics fail.
  • During ordinary navigation, when you are away from the main chart plotter  you can quickly consult a phone for position, course and speed over ground (particularly good for saving the energy of a skipper resting on a bunk!)
  • Many modern mobiles are water resistant, typically for half an hour submerged to 1.5 metres, so can be kept in an oilskin pocket in wet weather and would not be ruined by a scramble into a liferaft.
  • Close inshore: easy access to weather and other information through the phone network, and to emergency services.
  • Offshore: much useful data can be downloaded to the phone and taken with you, including the weather forecast at the time you leave. Tide and current apps can be downloaded with several years worth of data.
  • On shore away from the boat, and in harbour: mobiles with electronic chart apps are useful for route planning – you can switch quickly between charts, weather and tide information to get an overview.
  • Charts on mobiles can be updated whenever you are connected to a phone network.
  • Mobiles can be protected easily – there are plenty of impact resistant cases designed for eg building site work which can make it probably the most physically robust instrument on board.


  • Small screen.
  • Not readable in sunlight.
  • The touch screen needs to be dry to operate.
  • Charts on mobiles do not have many of the useful extra functions available on software for tablets, laptops and dedicated plotters.
  • Unreliability if exposed in extreme cold weather
  • Mobiles sometimes fail unexpectedly (though that applies to much other electronics, and on a yacht all the crew are likely to bring their own mobiles).

There are other factors that can cause reliability problems for electronic navigation (see Pass Your Yachtmaster, which has a brief summary) but as far as I know these apply equally to a chartplotter or tablet.

This is another copy of the link mentioned above: ‘how to check satellite accuracy on phones’.

* GNSS means Global Navigation Satellite System, which embraces all four worldwide systems now available.

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