How did the drunken 'Janet Sparrow' ever get near the passenger boat in the first place. The boat would have been tied up bow and stern with even a bow spring on , how did she get them free. Even if the engines were switched off , it would have taken someone 'in the know' how to start them , she must have been onboard beforehand doing a 'rekkie'.
They do not usually moor boats quayside on the Dart. Normal is Bow and Stern to buoys but in this case, and in her condition, it is more likely that they were alongside. Note that she didn't start the engines? She just drifted doing the damage.