The three parts to the verb ‘sink’ are sink, sank, sunk. You are using the past tense, sank, correctly, although using ‘sunk’ is a common error.
Remember that in English, there are six basic tenses: the present, the past, the future, the present perfect, the past perfect, and the future perfect. The last three are not used as frequently, and they are the ones that take ‘sunk’ as the verb. You can tell when the ‘perfect’ tenses are being used, because the past participle (in this case ‘sunk’) never stands alone. It is always accompanied by a version of the verb ‘to be.’
Examples: The Titanic sank on its maiden voyage(plain old past tense). The Titanic is sunk every day in the movies. (present perfect tense) The Titanic had been sunk by an iceberg. (past perfect tense) The Titanic will have been sunk for 90 years this year. (future perfect tense). Note that all these sentences are also in the passive voice. Better writing uses the active voice, which is where the present, past, and future tenses are employed.