At 5:18 p.m. on a recent Friday, this American-owned, Canadian-built tanker - captained by an Italian, crewed by Filipinos and registered in Monrovia, Liberia - nosed through the harbour reefs off San Nicolas, Aruba, and headed seaward under the warm, Caribbean sun.

She was bound for New York on the most routine of voyages, ferrying fuel oil from Exxon's Lago Refinery on the island of Aruba off the coast of Venezuela to the energy-hungry boilers and furnaces of American's most populous city. The 38,000 tons of oil - about 10.4 million gallons - tucked away in her 10 cargo tanks would scarcely warm New York for a single winter day.

Only six months old, the Saint Petersburg bears the unmistakable profile of modern oil ships: a long, low, black hull forward, topped only with longitudinal green pipes and a slight rise at the bow, while the bridge, engine rooms, living quarters and storage areas rose aft in a white, five-deck structure resembling a flat-faced tugboat wheelhouse with bridge jutting out port and starboard.

One deck below the bridge and just aft of her funnel, she carries a small swimming pool capable of submerging a medium-sized Fiat.

She is 628 feet, 6 inches long, 90 feet wide and has a draft of 36 feet, 10 and 21/32 inches. Though this makes her roughly comparable in size to a small aircraft carrier and more than twice the size of the largest tankers of World War II, the Saint Petersburg is a relative guppy in the supertanker age.

By last September, the average displacement of the 156 ships in the Exxon fleet alone (measured in cargo capacity) had risen to nearly 100,000 tons, more than 2 1/2 times the size of the Saint Petersburg.

The Esso Japan, another tanker that the Saint Petersburg passed at anchor off Aruba as she headed a seaward, is, for example, more than four times larger than the average. It is a vast, floating tub of oil nearly as long as the Empire State Building is tall, more than 1 1/2 times in width than a football field and deeper in draft (73 feet) than any seagoing vessel previously built.

Ships like the Esso Japan, which is by no means the world's largest tanker, are referred to by oil men as VLCC's (Very Large Crude Carriers). They dwarf even nuclear supercarriers like the USS Nimitz and Enterprise, which carry crews of some 6,000 men. The crew of a VLCC, however, rarely numbers more than 40.

The Esso Saint Petersburg, which tankermen refer to affectionately as a "handy size" vessel, carried a crew of 29. They busied themselves stowing gear and plotting course as the ship cruised steadily throught the indigo water and swung northwestward toward Jamaica.

On the bridge, Capt. Giuseppe Neri, 46, a slight, taciturn man given to reading and reflection, paced back and forth eyeing the receding Aruban shore. He was dressed in brown Ban-lon sport shirt and looked more like a surgeon on vacation than the captain of a ship.

Sailing has been delayed by cargo loading problems and paperwork and Neri was impatient to get to sea. In New York he would board a flight to Rome to start a two-month vacation. He had been at sea away from his family for five months.

As the evening breeze passed through the open bridge doors of the Saint Petersburg, it swept over an orderly maze of instrumentation that marks a modern ship: gypro compasses, heading indicators, and two independent radar systems to spot oncoming ships, and shores 40 miles away. There were radio sets and receivers, three separate electronic navigation systems to find the ship's position throught intersecting radio beams, and a depth finder to charter the channels.

When night fell, third Mate Napoleon Manso, 26, a mustachioed Filipino in a T-shirt and blue jeans; went out on the bridge wings with his sextant and shot the stars, backstopping technology with seamanship as old as Magellan.

In a season of foundering tankers and oil spills, anonymous ship-owners and foreign flags, the concept of a Liberian tanker usually conjures up an image of a rusting tram ship of uncertain age with jury-rigged equipment, a crew of gutter-swept misfits and some Anthony Quinn captain scratching himself and cursing on the bridge.

While that image has some validity - as records of ships involved in recent oil spills and accidents like the ill-fated Argo Merchants Shnow - the vast majority of Liberian flag ships now rank among the world's newest. Most are creatures of companies like Exxon that use foreign "falgs of convenience" as a sort of maritime "right-to-work" law to dodge high-cost union restrictions and acquire cut-rate foreign crews.

Critics of flags of convenience charge that loose regulations and lax enforcement in countries like Liberia permit on the seas tankers and crews that create peril for both themselves and the environment.

Exxon, which, according to its executives, supports both stricter regulations and stricter enforcement for foreign flag vessels, claims its foreign crews are as well or better trained than American crews of more than twice the price.

The question lingers, unanswered, as the Saint Petersburg steams north-westward through the night into a freshening breeze.

Saturday morning. The ship has been rolling some during the night and the rising sun shows why. The ocean is covered with whitecaps and seas are running seven to 10 feet.

The weather is fair, but the Saint Petersburg is taking white water over the deck with some regularity. Her course is northwest and the wind swells are running almost abeam. Occasionally they slap the starboard side sending spray two decks high on the control structure aft, and a rush of water forward across the low main deck under the mazee of values and piping.

Chief Mate Sergio Ischiale, a sleepy-eyed 36-year-old Sicillian with a cigarette habitually dangling from his lips, has two men with push brooms up forward scouring oil drippings from under the pipes. They stroll casually on the deck in their orange life jackets and white hard hats, laughing and dodging the waves.

Down in the engine room, Chief Engineer Sebastiano Grassi and his assistants are building a scaffolding under the ship's main steam line before welding a leaking flange. Grassi himself was checking over the inventory of 1,000 spare parts ranging from thumb-sized electrical switches to a spare Diesel piston the height and diameter of three tall man.

Grassi, a small, liquid-eyed man of 39 who pops pills for a host of minor ailments, presides over the ship's 12,000 horsepower Hitachi Sulzer Diesel engine, which roars from the depths of a three-story room cobwebbed with grating walkways and ladders of light green steel.

He and his assistants also maintain and repair the labrinthine maze of pumps, switches, pipes, wiring, generators and motors through which the Saint Petersburg conditions its air, does its laundry, preserves and cooks its food, distills its water, treats its sewage, handles its cargo and propels itself through the water.

"A new ship like this is always work. " he said, wiping oil from his hands and white coveralls with a handy rag. "It's like a new car - always bugs to find."

The bugs, he says, involve thins like valves with incorrect name plates, pumps installed backwards, leaking pipes, loose wiring and the like - all of which must be found and corrected during the year that a new ship is under warrantee.Usually, Grassi said, the process takes about three months.

Every ship has its individual quirks and personality, Grassi said, which engineers love to analyze and tinker with.

The Saint Petersburg, he siad, is a good simple ship of manageable size and temperament. He has never served on a much bigger VLCC.

"What if you work on bow of VLCC," he mused in the wardroom, waving a bread stick over a lunch of rigatoni, shrimp tempura, caramel custard and beer, "and you have wrong size wrench? (It) is almost quarter mile from bow back to bridge, take elevator down to engine room, walk long way to tool box, take elevator up and walk back to bow. Takes 15 minutes. You too exhausted to fix."

Chief Mate Ischiale assured him it was no big problem. On VLCC decks, he said, they use bicycles to get around.

Ischiale was wearing a pair of decaying blue jeans, an unbuttoned white shirt with the tails tied at the waist, pilot's dark glass and bedroom slippers. He resembled a postcard peddler on the Via Veneto in Rome.

As he moved around the ship after the meal, Ischiale somehow conveyed the bearing of commands, quietly noting decking to be painted, hallways to be cleaned, and the bridge watch to be supervised.

Tankers are thought of as dirty ships, but the modern breed is almost antiseptically clean, due in part to the sergegation of the cargo and living areas.

With their vinyl tile hallways and spacious staterooms, they seem at times like office buildings. On the Esso Japan the feeling was heightened by the massive bulk of the ship and its insulation from the element on which its rides, a visitor could be deep in some underground bunker or somewhere in space.

On the Saint Petersburg, however, the sea is never far away. Cobalt under the Caribbean sun, it rippled and flower past the ship's sun-baked hull as she headed nortnwest toward Jamica.