How do I know if a ship is just out of range?

Switch on Receiver coverage shading in the map's Settings. It shades the areas where receivers heard ships in the last hour. Water outside the shading is out of range of every receiver, so a ship there is not seen. That is not the same as no ship being there. About explains the setting.

Why did a ship jump across the map?

One reason is that she was not heard for a while and then reappeared somewhere else. Shiptrail accepts a ship's first position as it is, and any position that comes more than 6 hours after her last one. Inside that time, it holds back a position that is more than 10 nautical miles from the last one and further than a ship at 80 knots could have gone, and draws it only if a second position agrees with it 30 seconds or more later. Two transmitters sharing one MMSI can still hop: What AIS is has that.

Why a ship disappears from the map, and what stays

The short answer

A ship leaves the map for one of three reasons. She has sailed out of range of every receiver that reports to Shiptrail, or her transponder is off, or her last report is now older than Shiptrail will draw: 3 hours if she was moving and 6 if she was stopped. From a missing dot you cannot tell which.

She is not gone from Shiptrail, though. Her page stays and says when she was last heard, and the moment a receiver hears her again she is back on the map. What AIS is says why a ship can go quiet, and How to track a ship is where to start if you are looking for one.

The 3 and 6 hour rule

Shiptrail draws a ship for 3 hours after her last report if she was moving at that report, and for 6 hours if she was stopped. Moving means 1 knot or faster. Then she comes off the map.

The rule is shorter for a ship under way because her last position stops being true as she sails on. At 15 knots, a position that is 3 to 6 hours old is 45 to 90 nautical miles from where she really is, so drawing it would put her in the wrong place and look sure of itself. A stopped ship's old position is still where she is, so she keeps the longer time.

What the map does while she is quiet

The map shows a ship going quiet well before she leaves. A mark is at full strength for five minutes after her last report and then fades: to three quarters at 20 minutes, and to about a third at an hour. A pale mark is an old position.

Between reports the map also carries a moving ship ahead along her course, so she glides instead of jumping from fix to fix. It does that for at most two and a half minutes, and only while her last report is under 3 minutes old. It never carries her across the shore. After that she is drawn exactly where she was heard, so a ship that has gone quiet does not carry on sailing across the map, over a headland or into a town. She stays where she was last heard and fades.

What stays when she goes

A ship that has left the map keeps her page, for a year after she was last heard. It says she is not currently on the live map and gives the day she was last heard, and where, if her last position is known. The search box lists her under "Not on the map now", with the day she was last heard, and choosing her opens her page.

If she had just left a port, she stays on that port's Departed list for the whole 48 hours, with her last-heard time under her name. A ship that has only sailed out of range does not vanish from the port she left. How ports are counted has the rules.

What brings her back

A receiver hearing her again. That is all it takes: the next report that reaches Shiptrail puts her back on the map, wherever she is by then. A ship crossing an ocean can be missing for days and then appear off the far coast.

When a ship that is there is not seen

Some ships are on the water and never on the map. There are rivers and stretches of coast with no receiver within about 20 to 40 nautical miles, and the open ocean, where no receiver on land reaches. (Norway's feed carries some satellite reports for Norwegian waters.) And a small boat with no transponder is not on any AIS map at all.

So a ship missing from the map is not necessarily missing from the sea, and a ship on the map is only where her last report put her. Positions are not for navigation.

Questions people ask

How do I know if a ship is just out of range?: Switch on Receiver coverage shading in the map's Settings. It shades the areas where receivers heard ships in the last hour. Water outside the shading is out of range of every receiver, so a ship there is not seen. That is not the same as no ship being there. About explains the setting.
Why did a ship jump across the map?: One reason is that she was not heard for a while and then reappeared somewhere else. Shiptrail accepts a ship's first position as it is, and any position that comes more than 6 hours after her last one. Inside that time, it holds back a position that is more than 10 nautical miles from the last one and further than a ship at 80 knots could have gone, and draws it only if a second position agrees with it 30 seconds or more later. Two transmitters sharing one MMSI can still hop: What AIS is has that.