Hi Phil,
It took us about an hour to get from Calshot to Poole Harbour entrance (with me driving!!), maintaining 37 knots (being careful about petrol consumption, lots of pot markers and what looked like some kind of exercise with boats, a helicopter and some marker flags).
We met friends in Poole then went out to Old Harry. This part of the journey we did very slowly (12 - 15 knots most of the way) so we could get right in to the shore and have a look at the rock formations (my friend is a Geography teacher!).
We spent about 3 hours in total, cruising from Dolphin Marina (Poole) around the eastern side of Brownsea Island, along Studland Bay to Old Harry, further west just into the far eastern end of Swanage Bay, then back along the coast, into Poole Harbour going round the western side of Brownsea Island and back to Dolphin Marina.
If you fancy company when you go, give us a shout!