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Sifnos is 150 Km from Piraeus 4 hours by ferry, that would cost a fortune for a private boat trip.My guess is 14:55 and 16:30 are the only ferries that day. The first one the cheapest/slowest, in theory gets you there before the 'fast' ferry.
A third ferry earlier is extremely unlikely. Timetables seem to be set for that period.
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You're right when you say "in theory", Ouzo! Got the slow ferry last month and the Aegean Speedlines fast ferry overtook us just before Serifos (the stop before Sifnos), hence it got into Sifnos about 15-20 mins earlier.But I still prefer the slow ferry! Much more relaxing, but take your own picnic on board as the onboard food is pretty crappy.
