'Greece to Turkey'. Given how spread out Greece is across various islands, it would help if you were a bit more specific on ports.

What is this 14-18 hour trip? From where to where?

Information from this year (I travelled with a car in April/May).

There are 4 car ferry routes:
Lesbos - Ayvalik
Chios - Cesme
Samos - Kusadasi
Kos - Bordum.

Of the four routes, only the top two are year round I think, and you don't state when you wish to travel.

There are no ferries from mainland Greece to Turkey.

'If the ferry is worth it'? Do you mean worth the saving in cost? Time? Or if the experience of using the ferry in itself 'worth it'?

The international ferries are not cheap, a lot more than the local ferries. Again, you don't mention where you mean by the south of Greece - if southern mainland, I think driving to Turkey would be cheaper and easier. If you are in Crete or the Dodecanese Islands, taking a ferry to Kos and on to Bordum might be cheaper - but that only works in summer.

Unlike the internal ferries, the information online on the international ferries is not very clear and for vehicle travel I think you need to book in person.

I travelled Turkey to Greece - I think you'll need to have insurance (Green card with Turkey valid) before you reach Turkey. In Bodrum cruise port for example, I didn't see an insurance agent.

EO

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EurasiaOverland a memoir of one quarter of a million kilometres by road through all of the Former USSR, Western and Southern Asia.