I've done the exact trip on a Sabre 28, and a Catalina 380.
On the Sabre, I believe that we left the dock at 1400 and sailed to Scituate harbor. We picked up a mooring at 2030. The next morning we left Scituate at 0800 and arrived at the canal without favorable current. So, we sailed around the entrance for an hour, and then started through the canal with the current still against us, but it would turn favorable. It was frustrating to be motoring at 5 knots, yet the people strolling along the bike path were passing us... According to my GPS log, our SOG through the canal was between 1 and 3 MPH. After passing through a squall with visibility reduced to 50' (and from which a car carrier emerged
) at the west end of the canal we finally put in to Onset at 2000.
On the return trip, on the same boat, we left Cuttyhunk at 0700, and sailed on a reach the entire way, and put in to Salem at 2000 on the same day.