(1) Where the tonnage of a ship has been ascertained in accordance with the Tonnage Regulations and such tonnage has been assigned to the ship, that tonnage shall be registered and thenceforth be deemed to be the tonnage of that ship.
(2) The tonnage of a ship as ascertained under subsection (1) shall be entered in every subsequent registration of the ship unless—
(a) an alteration is made in the form or capacity of the ship; or
(b) it is discovered that the tonnage of the ship has been erroneously computed,
and in either of those cases the ship shall be remeasured and the tonnage of the ship ascertained and registered according to the tonnage regulations.