Tonnage tax option remains in force for a fixed term but ceases on specified events, reverting to standard taxation. An option to adopt the tonnage tax regime, once approved, remains in force for ten years from exercise and applies from the assessment year relevant to the previous year in which it is exercised. The option ceases from the assessment year in which the qualifying company ceases to qualify, defaults in complying with chapter conditions, is excluded from the scheme, or files a written declaration to the Assessing Officer, after which profits from qualifying ships are computed under the other provisions of the Act.