PORTSMOUTH, N.H. — The trick is this: How do you get something that’s 689 feet long, 103 feet wide and almost 200 feet tall through a slot 200 feet wide and 200 feet tall?

It would be hard enough on dry land, but that is the maneuver pilots of huge tankers must do when they pass under the Sara Mildred Long Bridge on the Piscataqua River. Sara Long, as it is commonly known, is the middle bridge.

Oh, and there is one other thing.

Because of a design flaw, ships must come in on an angle in order to get through. Acting port authority Director Tom Orfe says the bad angle cuts down the space between the trusses of the bridge to about 150 feet.

With ships coming in at night, sometimes when the wind is high, it’s no wonder that crew of the tankers think the pilot deserves his money for doing that one bit of navigation.

Safety on the waterways periodically becomes an issue. Most recently, a tugboat caught fire off the Rhode Island coast, causing an oil barge to break apart on a sandbar and resulting in one of the worst oil spills in that state’s history.

That barge, according to New Hampshire Port Authority member Bob Snover, had traveled up and down the Piscataqua many times. The question then became, can such a disaster happen in New Hampshire?

Orfe, from the port authority, said he believes that the system in place for dealing with river traffic precludes such a disaster from happening as much as humanly possible.

This is the story of how those tankers we see docked so serenely along the various ports on the Piscataqua get there in the first place:

The trip up river involves a combination of state and federal agencies, as well as the participation of Portsmouth Pilots and Moran Co. tugboats.

One recent trip started at 8:15 a.m. on a Saturday.

While waiting for a favorable current, the Shell Oil tanker Isocardia stays anchored out at the federal anchorage for a day or so before heading up the Piscataqua. This is the ship that’s 689 feet long.

While it’s anchored, the Coast Guard makes an inspection of the ship to make sure everything is up to code. Coast Guard Lt. Jerry Shatinksy goes over the firefighting and lifesaving equipment, the cargo piping and other critical operations.

Shatinksy says if any of this equipment is not up to code, the ship can be kept out of port. The Isocardia inspection takes about three hours, and Shatinksy says everything is up to snuff.

A Moran tug leaves its berth and heads out for the Shell tanker, which is carrying 8,700 tons of propane gas from Venezuela for delivery in Newington. The ship isn’t fully loaded with propane. If it were full, the huge ship couldn’t navigate the relatively shallow Piscataqua.

The tug is carrying Dick Holt, the pilot who will navigate from the anchorage point near the mouth of the river to the terminal. Holt and his son, Dick Jr., are the owners of Portsmouth Pilots Inc., the company appointed by the port authority to bring ships up the Piscataqua River.

According to Orfe, it would be impossible to ask the captains of the tankers, which come in all sizes and from all over the world, to have an intimate knowledge of the river and its hazards. Having the ship’s captain relinquish the navigation to Holt is a safety precaution, says Orfe.

Holt has been doing this work since 1961, but the mood of the water can never be taken for granted.

“I learn every trip,” he says while looking out of the bridge of the tug. “I may know half of it when I’m done.”

The bridge of the tug is small, almost cozy, and decidedly low-tech. The steering wheel is wooden, with a brass center.

The tug gently eases its way over to the huge hull of the tanker. A gangway is swung over from the Isocardia to the tug, and the pilots hop from the tug to the tanker — not an easy jump if the seas are rough.

The huge bridge of the Isocardia is alight with radar and sonar equipment. A huge bank of automated systems control the very delicate propane cargo.

The captain is Ernest Wilkinson, who is taking his second to last voyage before he retires. After 39 years on the sea, he’s taking a job with a consulting firm in England.

“Light wind,” Holt says to Wilkinson.