18. The Government shall not be responsible for any act or default of any conservator, port-officer or harbour-master, of any port subject to this Act, or of any deputy or assistant of any of the authorities aforesaid, or of any person acting under the control or direction of any such authority, deputy or assistant, or for any act or default of any pilot, or for any damage sustained by any vessel in consequence of any defect in any of the moorings, hawsers or other things belonging to the Government which may be used by the vessel: Provided that nothing in this section shall protect the Government from a suit in respect of any act done by or under the express order or sanction of the Government.