A replica of the 17th century sailing ship Godspeed left its moorings on the River Thames today and set sail for Virginia, to retrace the 6,000-mile voyage that brought the first successful settlers to America in 1607.

As Virginia Gov. Charles S. Robb and Britain's Prince Philip watched, the crew of 14 Virginians set two of the 68-foot ship's topsails, and the Godspeed began its 10-week journey across the Atlantic to Jamestown.

Prince Philip, husband of Queen Elizabeth II, boarded the Godspeed at its mooring off east London's Isle of Dogs and spent 15 minutes asking questions about the vessel.

"Are you sure you wouldn't like to engage another passenger?" Philip asked. The skipper, George Salley, 52, a U.S. space scientist, replied: "I'd be happy to have you sail with us."

The prince spoke with each crewman and then said: "Good luck. Have a happy voyage."

The three-masted pine vessel is a replica of the original ship that sailed up Virginia's James River in 1607, 13 years before the Mayflower came to Plymouth Rock in Massachusetts.

The Godspeed was built in America at a cost of $600,000, paid by Virginia. It arrived in England aboard a container ship on March 12.

The state flag of Virginia fluttered from the Godspeed's aftmost mast.

The prince asked what the flag depicts, and Robb's wife, Lynda, unabashedly explained that it shows a female warrior with her foot on the neck of a prostrate enemy and with a Latin motto that translates: "Thus always to tyrants."

"Whom is it referring to?" the prince asked the governor with obvious delight.

Robb responded that it was George III, the British king during the American Revolution.

"Is it Queen Elizabeth II?" the prince asked, not letting the governor off so easily.

Robb assured Philip that it was not the current monarch. Relations between Britain and Virginia have much improved since the colonies rebelled two centuries ago, the governor said.

As the Godspeed left, whistles shrieked from riverboats crowded with onlookers and guns boomed at the Royal Naval College, across the Thames in Greenwich. At least 500 people, including English schoolchildren and a party of visiting Virginians, lined the north bank of the river to watch the departure.

"It was an ideal send-off," said Thomas Mahone, supervisor of the Jamestown committee that is coordinating the trip. "I think they're going to have some hard days ahead for the first week until they hit summer weather. Things dry up and the crew gets revitalized."

"It's been a marvelous week," said Virginia state Sen. Joseph V. Gartlan of Fairfax County. "Our board of trustees got this whole thing started and it was great to see it carried out in person."

The Godspeed's voyage will be monitored in the United States by the Jamestown-Yorktown Foundation with the help of satellite tracking systems maintained by the National Aeronautics and Space Administration. The trip will include stops at Tenerife in the Canary Islands and Martinique and St. Thomas.

The craft is scheduled to arrive in Jamestown July 20. President Reagan has been invited to head the welcoming committee.