Government safety investigators say little can be done to force ship captains to rest, even as they ruled "acute fatigue" a primary cause of Rhode Island's worst oil spill.

The National Transportation Safety Board said that the captain of the Greek tanker World Prodigy had been on the bridge 35 consecutive hours when the ship ran aground off Newport, R.I., on June 23, 1989.But the agency said there is little the government can do to force ship captains of foreign-flag vessels to rest before reaching a U.S. port.

"Hopefully what we can do is attenuate foreign ship operators that fatigue is a major hazard to transportation safety," said NTSB Chairman James L. Kolstad. "We see it in all modes of transportation. Clearly we saw it in this particular accident."

The board unanimously approved a report detailing how "acute fatigue" led Capt. Iakovos Georgoudis to make a series of dubious orders and miscalculations in the hours leading up to the grounding.

The spill of 289,000 gallons of No. 2 heating oil sent ribbons of pollution toward the Rhode Island coast in slicks as long as five miles. Shell fishing, swimming and pleasure boating were halted in some areas.

The NTSB ruled out intoxication, drug use or crew incompetence as possible causes.

Board staffers described Georgoudis as well-organized and conscientious but said his sense of duty led him to overwork.