The National Transportation Safety Board ruled yesterday that the Coast Guard cutter Cuyahoga and its skipper were chiefly to blame for the collision in Chesapeake Bay on Oct. 20 that sank the cutter and cost the lives of 11 of her crewmen.

During a hearing at which the official findings were approved, board members were highly critical of the Cuyahoga's commanding officer, Chief Warrant Officer Donald K. Robinson.

In a statement to reporters later, Board Chairman James B. King said, "There are some real questions as to (Robinson's) basic seamanship and skill." King characterized Robinson's behavior as troublesome and said the Cuyahoga skipper seemed to have been in a "fantasy world" just before the collision. The Cuyahoga was struck by the Argentine cargo ship Santa Cruz II an calm seas and under clear skies in one of the worst Coast Guard disasters in recent history.

The Cuyahoga, which had sailed from a Yorktown, Va., base, was struck in the right side near the stern about 9:10 p.m. off Smith Point as the cutter was heading north into the Potomac River for the night. The cutter sank within minutes.

The report cited three probable causes for the collision: The Cuyahoga turned left in violation of the rules of the road as the two vessels headed toward each other in close proximity, her commanding officer failed "to determine the relative motion, course, speed or closest point of approach" of the freighter and the cutter failed to call the Santa Cruz II "by radiotelephone to exchange navigational information."

The board said that lack of emergency lighting aboard the Cuyahoga contributed to the loss of life. The report also noted that the cutter's life jackets were stowed in an upper deck where none of the crew could get to them after the collision.

During the investigation of the collision, Robinson refused to testify, citing advice of his attorneys.

The Coast Guard also came in for criticism in the body of the report, which contained a list of more than a dozen recommendations to the Coast Guard, including a suggested review of its personnel policies on training cruises, its radar equipment and the conditions of its vessels.

But King said later, "I don't think anybody should walk away thinking the Coast Guard is a bunch of incompetents."

Robinson's lawyer, Jerome V. Flanagan of Boston, whose request to address the board was denied under standard procedures, declined to comment. Robinson was unavailable for comment yesterday.

Kieron Quinn, attorney for the Santa Cruz II and its captain, Abelardo Alboniz, said he disagreed with a portion of the report's analysis that said the ships were in a head-to-head position before the collision. Even board members failed to agree on the precise nautical relationship between the ships before they collided, a relationship that would indicate which ship in effect had the right of way and which was responsible for signaling the other.

The report also included a recommendation that harbor pilots more closely observe regulations about signaling other vessels when approaching. John Hamill, a pilot, was guiding the Santa Cruz out of Chesapeake Bay.

Hamill was staunchly defended by board member Francis H. McAdams. "It seems to me he did exactly what he was supposed to do," McAdams said.