The screen that flatters you

A chart plotter is one of the genuinely transformative bits of kit on a small yacht. It gives you a repeatable position, a course to steer and a picture of the seabed in a neat little package on the binnacle. The trouble is that it flatters you. It makes a tricky passage round a headland or into a strange harbour feel like following a satnav to a supermarket car park. The icon is always snugly on the line, the line always ends at the waypoint, and nothing on the screen ever suggests doubt.

But the chart underneath is somebody else's drawing of the seabed, sometimes a very old drawing, and the line is a plan rather than a promise. The skippers who get the most out of electronic charting are the ones who understand what it does not know.

Scale discipline: know when you are just zooming

Vector charts stay sharp at every zoom level, which is exactly why they catch people out. If you zoom in beyond the scale of the original survey, the plotter is not producing new detail — it is magnifying what it already has. The coastline stays beautifully crisp, but the seabed does not become any better known. Depth contours that looked like careful curves at 1:150,000 become even smoother and more convincing at 1:15,000, with nothing in between them.

Get into the habit of checking the source scale on the chart display, and treat anything zoomed in beyond roughly 1:25,000 with respect until you have confirmed it against a harbour plan or a pilot book. Some small-craft plotters will happily draw you a comforting picture of a channel that was surveyed by lead line before your grandparents were born.

Datum settings and the hundred-metre problem

Two different datums matter, and both are worth ten minutes of your attention before the season starts.

The first is positional. Modern charts and GPS both work on WGS84, so in theory everything lines up. In practice, waypoints come from all sorts of places: an old cruising guide, a friend's handwritten list, a harbour authority leaflet, a passage plan lifted from a decades-old almanac. If those numbers were recorded on an older datum, your waypoint can sit 100 to 200 metres from where it should be. That is nothing in open water. It is everything when you are lining up the entrance to Chichester Harbour, threading the Swellies or rounding a headland with a race running.

• Check the datum setting on your plotter and leave it on WGS84.
• When you transfer waypoints from an old source, treat them as approximate and verify them on the chart before you use them.
• Never trust a waypoint you cannot see on the chart with your own eyes.

The second is vertical. Charted depths and drying heights are measured from chart datum, roughly the lowest tide you would expect. Heights of land are measured from a different datum entirely. Confusing the two is a classic way to misjudge a bar or a sill. Tidal predictions in your almanac are also given above chart datum, so the sum is straightforward — as long as you are adding the right numbers.

Draft, safety margin and the height of tide

Most plotters will happily shade shallow water or sound an alarm, but only if you have told them what your boat needs. Set your draft honestly, add a proper safety margin — half a metre is not excessive in a swell — and remember that the figure only tells you about depth below the keel at that instant. It says nothing about what the tide will do in the next hour, or what the boat will do when she rolls.

A depth sounder is still the most honest instrument on board. It reports what is actually under you, not what a surveyor recorded in 1963. When the sounder and the chart disagree, believe the sounder, slow down and work out why.

The paper backup that earns its keep

Carrying a paper chart of the area is not nostalgia. It is redundancy against a flat battery, a failed fuse, a soaked screen or a software glitch, and it forces you to look at the whole passage rather than the next mile. A small-scale paper chart spread on the saloon table does something a five-inch screen cannot: it shows you the shape of the coastline and the relationship between headlands, overfalls and tidal streams at a glance.

Keep the relevant almanac pages, a note of your intended track and a written list of hazards — the overfalls off Portland Bill, the race at Alderney, the banks off the Needles — alongside it. If you have a tablet or phone as a third option, fine, but keep it charged, waterproofed and off the main navigation duty.

Habits that keep you honest

• Plan the passage on paper first, then enter it. The plotter should confirm your thinking, not replace it.
• Brief the crew on the hazards and the bail-out options, not just the route.
• Keep a log with times, positions and depths. It is your defence if the electronics go quiet.
• Cross-check position by eye, by transit and by hand-bearing when you can. A glance at a leading mark beats a confident cursor.
• After every trip, note where the plotter and reality disagreed. That list is the most valuable navigation document you own.

Use the plotter for what it is genuinely brilliant at — quick, repeatable, low-workload position fixing — and keep your own judgement in charge of the decisions. That combination will see you safely round more headlands than any screen on its own.