The 44-year-old man was found guilty of kidnapping and extortion charges over the May 2010 attack off Oman on the chemical tanker Marida Marguerite which had 22 crew aboard.
The hostages were only released in December of that year after a USD 5 million ransom was paid by the German shipping company that owns the Marshall Islands-flagged vessel.
The pirate commander was arrested in May 2013 when he applied for refugee status in Germany, using false papers. He was identified by his fingerprints, and later by witnesses flown in by the court from India.
During the hostage ordeal, the mostly Indian crew suffered "cruel treatment, torture and mock executions," said a statement by the court in northwestern city of Osnabrueck.
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The captain was threatened with a bullet fired next to his head, the chief engineer was handcuffed and suspended by a pipe, and some crew had their genitals tied with cable binders, the court said.
Prosecutors could not prove that the defendant was head of the entire operation, but they were able to convince the court that he had at least tolerated the torture of the hostages.