The plotter, the apps and the week's plan

The chart plotter, and what the zoom level hides

The chart plotter at the helm is the best navigation tool on a charter yacht. It shows the electronic chart with your position, your speed and the line you are moving along, and it does it all day without getting tired. It has one big trap, and it is the zoom.

An electronic chart is drawn differently at different zoom levels. Zoomed out, to fit a whole day's sail on the screen, most plotters leave things off to keep the picture clear: the soundings, small rocks, sometimes whole reefs and shallow patches. The route line you drew looks fine. Zoom in along the leg and the same line may run straight over a rock.

The opposite problem is overzoom. Zoom in beyond the scale the chart was made at and the plotter simply enlarges it: coastlines go blocky, and the chart looks far more precise than the survey behind it. Some plotters warn you with a hatched pattern or an "overzoom" note; many do not.

Plan zoomed out, check zoomed in. Follow every leg on the screen at a scale where the soundings show, and keep a sensible distance off every danger.

How old is the chart, and how good is the survey?

The charts in a charter boat's plotter live on a memory card or in the plotter's own memory, and they are not always kept up to date. At check-in it is worth asking how old the charts are; on many plotters the chart card's date is in the settings or the "about" screen. New marinas, moved buoys and changed harbour works may be missing.

Every chart, new or old, is only as good as the survey behind it, and some areas were last surveyed long before satellite positioning. In parts of the Caribbean, the Seychelles and Thailand, a reef can sit a little away from where the screen puts it. The position from the satellites is accurate; it is the chart under it that can be out.

Position, heading and course over the ground

The plotter shows three different things, and mixing them up is one of the commonest mistakes on a charter.

With no current and no wind pushing you sideways, heading and course over the ground are the same. With a current across your course, or leeway when the wind pushes the boat sideways under sail, they are not. The course over the ground already includes both, so it is the line that shows where you will end up. If it points at a rock, you are going to the rock, whatever the bow says.

Waypoints, routes, tracks and the man overboard button

• A waypoint is a position you store in the plotter: the start of a harbour approach, the point off a headland where you turn, the entrance to a bay. You can tap it onto the chart or type in its latitude and longitude.
• A route joins waypoints into legs. Following a route, the plotter shows the bearing and distance to the next waypoint, the time to get there and how far you are off the line (the cross-track error).
• The track is the trail the plotter leaves behind you. It is the easiest way to get back out of a bay you came into, along the line you know is clear, especially after dark.

The one rule for waypoints: never put one on a buoy, a rock or a wreck. Satellite positions are good enough to take you right onto it, and every other boat aiming at the same mark converges on the same spot. Set the waypoint a little way off, in clear water, on the side the mark tells you is safe. Then zoom in and check each leg of the route clears every danger. If your plotter offers automatic routing, treat its line as a suggestion and check it the same way.

Phones and tablets: a superb second opinion

A charting app on a tablet or phone is the best backup you can carry, and handy for planning in the cockpit or the café. Its limits are practical ones:

• Offline charts. Download the charts for the whole cruising area before you leave Wi-Fi. Mobile signal comes and goes at sea, and roaming data can be expensive.
• Position needs no signal. The phone's satellite receiver works without mobile coverage, but only shows you something useful if the charts are already on it.
• Battery. A bright screen and continuous positioning flatten a phone in a few hours. Keep it on a charger or a power bank.
• Heat. In a Mediterranean or tropical sun, phones overheat and dim or switch off. Keep it in the shade.
• Water. Spray and a drop in the cockpit end most phones. Use a waterproof case on a lanyard.

The plotter, the app and the paper chart should agree. When they do not, slow down and find out why.

AIS: seeing the ships, and what it does not show

AIS (Automatic Identification System) is a radio system in which vessels broadcast their name, position, course and speed. On a plotter with AIS, ships appear as triangles moving across the chart. Big ships and passenger ships, ferries included, must transmit it. Many yachts carry a transmitter too; some charter boats have only a receiver, so you see others but they do not see you.

From both boats' courses and speeds the plotter works out the closest point of approach (CPA) and the time until it (TCPA), assuming nobody alters course. In the ferry example below, a ferry 5 miles away on a bearing of 050°T, doing 18 knots, while you make 6 knots due north, has a CPA of 0.0 miles in about 14 minutes: a steady bearing and a closing range, which means risk of collision.

The VHF radio: channel 16, calling a marina, and a Mayday in outline

The VHF radio is the boat's telephone to the coastguard, the marinas and other boats. Most charter countries and companies expect the skipper or a crew member to hold a radio operator's certificate, such as the Short Range Certificate; check yours before you book. In an emergency anyone on board may use it.

• Channel 16 is the international channel for distress, safety and calling. Keep the radio on and listening to it at sea.
• Marinas usually work on their own channel, given in the pilot book or the marina's website. It is often not 16.
• Low power (usually one watt) is enough for a marina in sight; use high power for long range.

A Mayday, in outline only (the briefing and the card by the radio give the exact form):

1. If the radio has a red distress button (DSC), lift the cover and hold it down until it sends. It transmits your position automatically.
2. On channel 16, high power: Mayday, Mayday, Mayday, this is the boat's name three times.
3. Mayday, the boat's name, then where you are (latitude and longitude from the plotter, or a bearing and distance from a headland).
4. What is wrong, what help you need, and how many people are on board. Any other useful information. Over.
5. Listen. If nobody answers, repeat it.

Planning seven days

A charter week runs Saturday to Saturday, but the sailing happens in six days. Every one of our cruising guides follows the same shape:

Some rules of thumb that make a week work:

• Ten to twenty miles a day is three or four hours at a realistic five or six knots, with time to swim, eat ashore and arrive while berths and buoys are still free.
• A rest day, or a short day, mid-week keeps everyone happy and gives you slack when the weather moves the plan.
• Long and far early. Put the longest crossing and the furthest point early in the week. If the wind turns, you are coming home, not setting out.
• Weather windows. Read the week's forecast at check-in and every morning, and move the plan round the windy days: do the open crossing on the settled day, and spend the windy one in a sheltered harbour.
• A bolt-hole every day. For each passage, know the nearest harbour or bay you could reach if someone is unwell or the wind builds, and which winds it is open to.

Getting back on Friday

All our cruising guides start and end at the base marina, and the last leg is Friday's. Most bases want the boat back by late afternoon or early evening on Friday, refuelled, so they can inspect it before Saturday morning's check-out. The base tells you the exact time at the briefing.

• Thursday night within an easy half day of the base, so a windy Friday morning is an inconvenience, not a crisis.
• Fuel early. Fuel stations near the bases have long queues on Friday afternoon. Refuel on Friday morning or on Thursday.
• If the weather stops you, call the base early: they would far rather collect the boat from a safe harbour than have you sail into a gale to make the deadline.

The Isle of Kerran in the widgets and exercises is fictional; never use its chart to navigate a real boat.