6. In addition to their power to demand, take and recover ship, passenger and goods dues under section 26 of the Harbours Act 1964, the Authority may demand, take and recover in respect of any floating manufactured article (not being a ship within the meaning of the said Act) entering, using or leaving the harbour such dues as they think fit; and the provisions of sections 30 and 31 of the said Act shall, with any necessary modifications, apply to the dues authorised by this article as they apply to ship, passenger and goods dues.