Ferry between islands

[lavender]

We are going to St John then on to Cooper Island in the BVIs in April. Still I have not booked our flights but am about to! I wondered if any of you knew anything about the Roadtown Fast Ferry - specifically the dependability - and do the ferries leave pretty much on time according to their posted schedules? We have to travel from Cooper Island to Roadtown - ferry to STT and then taxi to airport and I am just trying to judge what time we can get to a flight on St Thomas.