STOCKHOLM, April 14 -- A Swedish museum has launched a massive four-year project to preserve the sagging hull of the Vasa, a majestic warship that sank nearly 400 years ago and is now one of Sweden's most popular tourist attractions.

Experts have begun putting in place a complex metal structure to support the hull, which more than 60 years after its salvage has begun to sag in the Stockholm museum custom-built for it.

"Today we put in a part of the new support structure, one cradle, and it's needed because the ship needs better support, because the old one from 1961 doesn't cut it anymore," said project leader Peter Rydebjork, showing off the new structure around the 17th century warship.

Due for completion in 2028, in time to mark the...