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Ask the Editor: Ship Mechanics

What is pod propulsion?

What is the responsibility of the harbor pilot?

What is considered "heavy seas"?

I keep hearing about problems with "pod propulsion" systems on cruise ships. What is pod propulsion, and why do cruise lines still use it if there are so many difficulties?

The easiest way to understand pod propulsion in big ships is to visualize the differences in small boats between conventional powerboats and those with outboard motors. The conventional arrangement has a propeller or propellers in a fixed position that can apply thrust either directly forward or directly aft. Directional orientation is achieved through either rudder(s), or, optionally, sideways facing thrusters. Outboard-powered boats have virtually no rudders and achieve directional control by rotating the entire propulsion unit.

A ship with pod propulsion effectively is a vessel with the internal power plant of a conventional ship geared to drive what are very much like the business ends of gigantic outboard motors, which pivot -- a full 360 degrees in some cases -- to alter the ship's direction. The advantage is in maneuverability; pod-driven ships can stop figuratively on a dime and, in concert with strong bow thrusters, power sideways rapidly into their berth. Picture, if you will, a car whose wheels can turn perpendicular to its long axis, and how easy it would be using that technology to parallel park in a space not much longer than the car, itself. The disadvantage of pod propulsion is its delicacy and vulnerability to breakdowns. For that reason Carnival opted for its Conquest-class vessels to switch to conventional diesel-electric power, augmented by an inordinately large number of powerful thrusters both bow and stern.

Over the years I have heard numerous accounts of cruise ships grounding when entering or leaving harbors. In each case there must have been a harbor pilot onboard. What is the responsibility of the harbor pilot in accidents like these?

When leaving port, from the point that the ship's docking pilot (usually the officer in command, especially on modern ships with thrusters) has cleared the pier, the ship is placed in the command of the local harbor pilot, who takes on full responsibility for navigation of the vessel. (The master retains full command of all other aspects of the ship). When entering a harbor the process is reversed. In either case, the pilot's authority extends for the time between docking and navigating the "high seas." In the case of port calls where the ship anchors rather than docks the harbor pilot is in charge of the time, place and manner of anchoring as well. During the period that the harbor pilot is in charge, he is empowered to issue steering directions, and to set the course and speed of the ship. All crew and officers must obey any pilot order, except that the captain may take over control from the harbor pilot if the pilot appears to be incompetent or impaired (for instance, intoxicated).

The legal rights and responsibilities of a harbor pilot's actions are well established, and, if collision or grounding is due to pilot negligence, the consequences fall squarely on his shoulders, and may even include personal responsibility for financial damages. For that reason, many state governments within the United States require harbor pilots to post a bond to cover those eventualities.

Officially, what is considered "heavy seas" for a cruise ship?

As far as what's typically experienced, the highest seas normally encountered are on the winter northern transatlantic crossings, which can run about 60 feet in storms. Sailing in these seas is by no means comfortable, but hardly dangerous for large, modern liners. However, note the word "typically."

When a captain sails into heavy seas of a particular height and direction, it is a whole different story than encountering a "rogue wave." Have you ever been on an airliner cruising along at 30-some-odd thousand feet, when all of a sudden you encounter unexpected turbulence? The first thing that happens is that the plane suffers a big jolt or drop, and right away you can hear the pilot ease off on the throttle, because going through turbulence at high speed exacerbates the effects on airframe and passengers alike.

When a ship is in expectedly rough seas, the captain steers as much as possible into the waves, and slows the props down. A rogue wave of 30 feet hitting an unsuspecting ship steaming at max cruise speed broadside can have a far greater effect than waves twice that height that are already prepared for.

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