Dive and Discover : Expedition 3 : Daily Updates

Perhaps you’ve heard the phrase “where the rubber meets the road”? Besides being a slang term relating to performance, that’s where you get a lot of what is known as “traction”. One definition of traction is “the adhesive friction of a body on a surface on which it moves”. That’s why your car tires and your shoe soles are made of rubber; so you get lots of traction and can go where you want to go.

Driving a ship is not like driving a car! A ship is floating in water and so it has a lot less traction than we enjoy on land. That means much more time and distance are required to speed up, slow down, and even turn the ship. This is complicated by the sheer size and bulk of the vehicles involved. Your average family car probably weighs between 2,000 and 4,000 pounds, and it responds quickly because of the traction with a solid surface. RV Melville is a relatively small ship (85 meters long), but even so, she tips the scales at more than 2,700 long tons (that’s more than six million pounds!) on the average trip . So in maneuvering the ship, there is a lot of inertia and momentum to be dealt with. For example, a loaded super tanker, which can weigh well over 250,000 long tons, may travel several miles in the original direction of motion before stabilizing on a new course or speed.

Aboard ship, the Captain and the deck officers (often called Mates) navigate and maneuver the ship to get to the desired destination. Out on the open ocean, the deck officers themselves determine and execute whatever maneuvers are necessary. When the ship is near shore or in hazardous waters, such as channels and harbors, the maneuvering falls to the person with the most experience; usually the Captain. The practice of maneuvering a ship is called “shiphandling”.

The shiphandler must know all about the ship’s characteristics, such as tonnage, propulsion system, rudders, response times, etc. Even the underwater shape of the hull and the size of the superstructure (the part of the ship above deck) may need to be factored into maneuvering decisions. A shiphandler must always be thinking ahead about the time and distance it will take before the ship responds to a change in direction or speed. Unlike a car, there are no brakes that will stop you very quickly! When a ship must come to a new speed or heading, the action to make it happen must occur at some time or distance before it is actually needed. This time or distance can be calculated -- and for precise maneuvers, it is. In most cases, however, a shiphandler relies on experience and an intimate knowledge of the vessel to execute maneuvers in time. No two ships are alike, and even sister ships will handle differently. While a textbook knowledge of the techniques and forces involved is important, a shiphandler gets good at it only through actual hands-on experience. A seasoned shiphandler will also employ a good measure of “seaman’s eye” in judging speed, time, and distance.

Besides the ship-generated forces, such as from propellers and rudders, there are other forces that the shiphandler must contend with. Wind can push a ship in an unintended direction, and currents in the water can do it even faster. In shallow waters and channels, or near structures like piers and wharves, other forces with names like bow cushion, stern suction, and squat come into play. Any one of these forces can lead to disaster if they are not taken into account. By thinking ahead and planning, the shiphandler can actually use these adverse adverse forces to help, rather than hinder, the maneuver.

Many ships use a fixed propeller on a horizontal shaft to push themselves through the water. A rudder is mounted behind the propeller to steer the ship. RV Melville is among a growing group of vessels using “azimuthing (directional) propellers” or “thrusters” to drive themselves. These vessels have no rudders. The propellers can be swiveled on a vertical shaft to get thrust in any direction. RV Melville has two such thrusters mounted side by side at the stern of the ship, one port and one starboard. The propellers are about nine feet in diameter and are mounted inside a nozzle (see today’s slide show). RV Melville carries a third thruster in the bow of the ship. This bow thruster, or B/T for short, is used in conjunction with the stern thrusters to give the ship a very high degree of precise maneuverability. It is used when holding a position out at sea or when maneuvering alongside a dock in a harbor. When not in use, it is retracted into the hull.

Each thruster has its own controller on the Bridge so the shiphandler can manually set azimuth (direction) and RPM (revolutions per minute of the propeller). RV Melville also has a dynamic positioning system (DPS). When we are using the DPS, the control of all three thrusters, or any combination of them, is turned over to a single joystick. The combination of thrusters and joystick control allows the shiphandler to do some pretty fancy maneuvers, including turning the ship on a dime and moving sideways through the water! The ship responds to the thrusters very quickly, which greatly reduces the lead in time and distance that we talked of earlier. For certain scientific operations, it is important to hold the ship in position for hours, sometimes days. The DPS can hold the ship within a 10-meter radius, even in 40 knots of wind, 20-30 foot waves, and several knots of surface current.

Shiphandling is a very technical subject and I’ve just given you the basics here. There are lots of excellent books out there. One of my favorites is Crenshaw’s Naval Shiphandling published by the Naval Institute Press. Another noteworthy publisher of technical nautical books is Cornell Maritime Press.