Swedish scientists say a 17th century warship raised nearly intact from Stockholm's harbor to become one of the country's top tourist attractions is deteriorating faster than expected.

It's been known for years that the Vasa is decaying because of chemical processes begun after it was raised in 1961 and placed in a museum. But scientists are struggling to find ways to stop the corrosion, and a new study shows it's happening quicker than previously thought.

Lars Berglund, a professor at Sweden's Royal Institute of Technology, said Thursday the strength of wood has degraded by at least 50 percent since 1961.

About 1 million visitors a year admire the intricate wood carvings of the Vasa, which sank on its maiden voyage in 1628.