The Coast Guard is investigating how a tour ferry got too close to Alcatraz Island and struck submerged rocks, denting its hull.

The 90-foot Royal Prince scraped the rocks at about 1500 on Dec. 6, 2007. The vessel, operated by the Red and White Fleet, was giving a tour of Alcatraz to about 15 guests, the San Francisco company said.

The tour boat, on one of its regular Golden Gate Bay Cruises, departed Pier 43 1/2 at San Francisco’s Fisherman’s Wharf and began to circle Alcatraz, in San Francisco Bay. The boat navigated too close to the island.

“The vessel did touch bottom west of Alcatraz,” Coast Guard spokeswoman Petty Officer Mariana O’Leary said. “The hull was damaged (but) not punctured.”

The ferry’s draft is 7 feet. Its capacity is 400 passengers, but it was almost empty for the voyage because of unpleasant weather. No one was injured, and some of the passengers were amused by the accident, said Stefan Merkl, the Red and White Fleet’s marketing director.

Royal Prince was eastbound, southwest of the island, when it hit the rocks. The Coast Guard said the forward port side of the boat was damaged.

“In the process of going around Alcatraz, it occurred,” Merkl said. “It’s like a 180° loop around it. It was a cold and rainy day. There was a total of only 15 guests on the boat. We actually had some guests who thought that was the highlight of their trip.”

The Coast Guard sent a small boat and a helicopter out to meet the vessel. The operator sounded the fuel tanks; no pollution was released. The hull was dented but not pierced, Merkl said.

Royal Prince was taken out of service and sent to an unspecified shipyard to have the dents repaired. Merkl said the vessel was scheduled to return to service in February.

The three-deck vessel was built in 1962 at Stockton, Calif. It is 34 feet wide and was remodeled last year, Red and White Fleet said. The refurbishment included two new Cummins engines. It’s the company’s largest vessel.

The Coast Guard said the crew’s alcohol tests were negative.