A vessel entering any port subject to this Act [***] [The words and brackets "(other than a port in Burma)", omitted by the A.O. 1937.] in ballast and not carrying passengers shall be charged with a port-due at a rate to be determined by the [Government] [Substituted by the A.O. 1937 for "Local Government "] and not exceeding three-fourths of the rate with which she would otherwise be chargeable.