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Volume 4, Number 2
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Big Boats, Narrow Channels

By Michael W. Fincham

Marylad Pilot's diagram of how big boats pass each other

There's an art to running a narrow channel with a wide ship. And a lot of science. A Bay pilot works with a feel for the ship, the water, the weather. He or she needs excellent depth perception, a sharp sense of relative motion, a good working relationship- with the helmsman and officers on the bridge of the ship. That's all part of the art of shiphandling.

But a Bay pilot also works with the physics of underwater forces. A dredged channel changes the hydrodynamics of ship handling in ways a sea captain seldom experiences.

As a deep-draft ship runs a shipping channel, it sinks lower in the water. The hull squeezes water against the bottom of the channel, forcing a faster flow towards the stern. The ship creates a low-pressure zone under its hull, much like an airplane creates a low-pressure zone above its wing. On an airplane that creates "lift;" on a ship that creates "squat." The faster a ship moves, the deeper it sinks.

Along the banks of the channel, the same physics is happening sideways: The hull squeezes water against the side walls, speeding up water flow on that side of the hull and creating a low pressure zone. The back of the ship is sucked towards the bank of the channel. The faster the ship, the stronger the suction.

At the front of the ship, another force is at work. The bow wave is bouncing off the bank and shoving back at the bow. Bank pressure at the front, bank suction at the back — both forces are trying to turn the ship towards the center of the channel.

In the middle, all these bank forces balance out. A pilot running down the center axis has his ship in perfect equilibrium with the same forces pushing equally on each side of the hull. The center, of course, cannot hold two ships at the same time steaming in opposite directions. That's when life gets complicated for a pilot.

In a classic, port-to-port passing, a pilot begins to turn away from the center towards the side of the channel. He's making room for the oncoming ship, but his move towards the channel wall builds bank pressure at the bow, bank suction at the stern.

Pilots, like artists, have their own styles. Eric Nielsen, president of the Association of Maryland Pilots, likes to start his turn early, a mile away in a wide channel. Other pilots like Randy Bourgeois like to wait. "I hang in there a little bit longer," says Bourgeois, the association vice president. "If you get on your side of the bank too soon, then you get all those forces pushing you back to the middle-." He holds off on his turn until the ships are closing to half a mile. "I start making my move, and the other person starts making his move and you start doing this little dance step around each other."

What about the bow wave from the oncoming ship? In a tight channel passing, both pilots actually curve back towards the center, aiming their bows towards the other ship at the last minute, and letting the collision of bow waves straighten their ships out. In the abstract, all those rudder moves describe an S-shaped sequence: a turn away towards the side, a turn back towards the other ship, then a turn away again. The result: each ship seems to steam straight ahead — sliding safely past the oncoming vessel.

Another trick in a tight channel is slowing down. "All these forces vary by the square of the speed," says Nielsen, "so a small reduction in speed would have a large reduction in forces." A pilot has more control at 9 knots than at 12. And at slower speeds he keeps a power reserve that he can unleash when needed.

That's a lot of dance moves for big ships to make in a narrow channel, even on a clear day. "You do it in zero visibility," says Bourgeois, "Then you do it in high winds, in ice, in snow, when the buoys aren't there, when you can't see." When you have big ships dancing in the dark, you hope you have an artist at the wheel.

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