Map of the Dodecanese: the islands, and the boats to Turkey

Two coastlines

Every island in the group, north to south and measured

Alphabetical lists of these islands are everywhere and none of them tell you anything. Ordered the way the chain actually runs, with the two distances that decide a trip drawn to the same scale, the shape of the group gives itself away. Switch the measure and watch which islands change places.

Two things fall out of that. Nearness to Turkey buys you nothing on its own, because eleven of these fifteen islands look at a coast they have no boat to. And the islands that are awkward to reach from Turkey are usually awkward to reach from Greece too, which is why Kastellorizo, at 7 km from Kaş and 138 km from Rhodes, is the strangest row in the table and the one that explains the group best.

Where the chain actually sits

The group runs down the south-west corner of Turkey for roughly 250 km, from Agathonisi in the north to Kasos in the south, with Kastellorizo hanging out east on its own. It is the eastern edge of the Aegean and the eastern edge of the European Union, and the frontier is drawn tight against the Anatolian shore rather than down the middle of the sea.

That geography is the reason the group behaves the way it does. Greek ferries radiate out of Piraeus, so a chain 400 km from Piraeus and 20 km from Turkey ends up served by its own local network rather than by the national one. It is also why the season lasts longer here than anywhere except Crete, and why picking between Rhodes, Kos and the dozen smaller islands works differently from picking a Cycladic island.

The neighbours are worth fixing in your head, because this is the group people place wrongly most often. The Cyclades, with Santorini and Mykonos, are in the middle of the Aegean, well to the west, and there is a great deal of open water in between. The six island groups laid out across both Greek seas settles it faster than reading about it. Crete is south-west, past Kasos. The islands further up the Turkish coast, Samos, Chios and Lesbos, look like more of the same on a map and belong to a different group with different boats.

The boats to Turkey, island by island

Six crossings, from four islands. This is the table nobody publishes, and the reason it matters is that the water on this map is misleading: the shortest gaps and the busiest boats are not on the same islands. Distances are measured port to port. Times run from the fast catamaran to the conventional ferry, since both are sold on the same quay at very different prices.

What the border actually costs you

Twenty minutes on a boat, and a full external frontier of the European Union at the other end. Four things follow from that, and the third one ruins people's trips every summer.

Everyone on the boat needs a passport. A national identity card will not do it and a driving licence is not close. Entry requirements for Turkey differ by nationality and have changed more than once in recent years, so check yours rather than assuming.

A single-entry Schengen visa is spent the moment the boat leaves Greek water. You will not be allowed back in on it, and finding that out at the passport desk in Bodrum on the last evening of a holiday is a bad way to spend a night. If your visa is single-entry, the day trip is not available to you. This is the one that catches people out, every single season.

The hire car stays in Greece. Rental contracts here do not permit the vehicle to leave the country, and most do not permit it to leave the island either, so the insurance is void the moment it rolls onto a ferry ramp. Leave it at the port and pick it up when you come back. Where you are based on Rhodes decides how far that drive to the port is, which matters more than it sounds when the boat leaves at half past eight.

And the last boat back is earlier than anyone expects. Most return sailings leave the Turkish side in the late afternoon, which makes a day trip a morning and a lunch rather than a day and an evening. Miss it and you are booking a room in Turkey, because there is no later option and no other way across.

The boats that stay inside Greece

The thin blue lines on the map, and the network that carries almost everyone. It splits at Kos: a northern half of short hops on daily catamarans, and a southern half of longer legs on fewer sailings. One crossing joins the two, and how comfortable you are with a four-hour boat decides whether the whole chain is one trip or two.

Routes the map makes obvious

Put a finger on your airport and read outwards. The group rewards short chains built around one gateway, and it punishes anything that tries to cover both ends in a week.

Rhodes, Symi and a morning in Marmaris is the strongest week in the south, and it uses the border rather than ignoring it. Base yourself in Rhodes Town so the ferry quay and the Old Town are both a walk away, which the layout of the island argues for anyway, give Symi two nights so you see it after the day boats leave, and take the catamaran across for one day. Three quite different places, no internal flights, and one passport stamp.

Kos, Kalymnos, Leros and Patmos is the northern equivalent and the easier of the two, because every leg is under an hour and the boats run daily. Fly into Kos, work north, come back for the flight. The airport sits in the middle of the island at Antimachia rather than beside the port, so build the transfer into the first and last day instead of discovering it, and where Kos Town, Kardamena and Kefalos actually fall decides whether you need a car at all. Add Bodrum for a day at the start and the week has a city in it.

What the map argues against is just as useful. Rhodes to Patmos and back in a week is four hours of boat each way for two nights at the far end. Karpathos on a chain does not work: it is two to three sailings a week from Rhodes and often rough, so treat it as a destination you fly to rather than a stop. And Astypalaia looks like a bridge to the Cyclades on the map and is not one, since the seasonal line that links it to Naxos runs a couple of times a week in high summer and nothing else does.

One habit worth keeping whichever route you take. Never put a crossing on the day you fly home, in either country. The wind here is less punishing than the meltemi in the Cyclades, but the small boats on the thin lines still stop sailing, and a cancelled sailing on a border crossing costs you a passport queue as well as a flight.