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I just checked and we didn't actually book ferry in the end, we just rolled up and bought tickets (allowing a buffer of time before departure, I think they make you get there at least 15 mins before sailing) - but that was back in October, on the 7am crossing. Obvs now it's peak season they might be slightly busier.
We bought a return for a specific time and then missed it, but they let us on the next one anyway. YMMV - it perhaps felt like we'd been 'let off' rather than it maybe being official policy...
From memory it was about 100km around the island and we stopped in Ryde and Ventnor for something to eat, so 5-6+ hours would do it if you're not in a rush, a bit less if you're cracking on and stopping less.
Might give this a go at the weekend.
How changeable is the ticket if I book in advance for the ferry?
There is a 7 and 7:40 sailing from Portsmouth
Can't work out how much time to give to get down there and then around the island.