The U.S. agency that investigated the collision of a sulfur tanker with a highway bridge over the James River at Hopewell, Va., has urged the Coast Guard to require upgraded rudder controls and alarm systems aboard ships.

In its report, made public this week, the National Transportation Safety Board said the tanker Marine Florida suffered failure in its steering system "of possible critical . . . consequence" shortly before the ship hit the Benjamin Harrison Bridge last Feb. 24.

"Older vessels such as the Marine Floridian are permitted to continue in service with certain electrical standards . . . now obsolete . . . which involve systems vital to ship control . . ." the reporter said. "This is particularly so in the case of steering systems . . ."

While many safety improvements are required of aging vessels undergoing refitting the report said "steering systems are not specifically mentioned . . ."

The World War II-vintage Marine Floridian was rebuilt in 1969, the report said, but the steering system was not updated.

The 612-foot tanker had just discharged a cargo of molten sulfur at the huge Allied Chemical Corp. complex in Hopewell when it veered to the left while approaching a raised sectio of the drawbridge and struck an adjacent span.

The collision knocked a 240-foot truss span loose from the bridge on to the ship, and another 110-foot section collapsed into the river. No one was injured.

Officials of the Virginia Department of Highways and Transportation estimate it will take 18 months and $5.1 million to repair the bridge and another $3.8 million to provide ancillary transportation services in the meantime.

The transportation board report said preliminary investigation into the accident showed the tanker "experienced a loss of rudder control," although "the precise reason for that loss has not yet been determined."

What is known, the report said, is that power was apparently lost to one of the ship's steering motors, and that when it was, an alarm system failed to activate. In addition, the report said, a manual transfer switch, designed to kick power to a backup motor "suffered a mechanical fault of possible critical, but not yet determined, consequence."

"This casually and others and the potentially catastrophic consequences of steering failures, indicate a need to further upgrade rudder control and alarm systems," the report said.

The specific board recommendations indclude requiring a visual and audible alarm to indicate power loss to steering motors; regular tests of steering backup and motor-switching equipment and the retrofit of modern steering systems on all major ocean-going vessels undergoing rebuilding for extended life.