sank / sunk
The past and the past participle of sink.
The past of sink is sank. The past participle is sunk, after forms of have and forms of be: The crate sank. The crate has sunk. The crate was sunk. Do not use The crate sunk for the simple past.
Examples
Use The barge sank at the dock.
Use The barge has sunk.
Avoid The barge sunk at the dock.