Naval Architecture and Marine Engineering

Summary

The process of designing a ship involves two complementary disciplines: naval architecture and marine engineering. Naval architecture has two subdivisions: hydrodynamics and ship structures. Hydrodynamics is concerned with the interaction between the moving ship hull and the water in which it floats. Ship structures is concerned with building a hull that has the strength needed to withstand the forces to which it is subject. Marine engineering is concerned with all the machinery that goes into the ship. The machinery must perform the following tasks: propulsion and steering, electric power generation and distribution, and cargo handling.

Definition and Basic Principles

Modern ships are very complex vessels. Some ships must be able to load and unload themselves, while others depend on facilities in ports of call. A ship must propel itself from place to place and control its direction of motion. It must produce its own electricity and freshwater. In addition to the space devoted to cargo, a ship must provide space for fuel, freshwater, living accommodations for crew and perhaps passengers, propulsion, and related machinery. A ship must float, and it must remain upright in all sorts of sea conditions. There are many different kinds of ships: aircraft carriers, submarines, containerships, tankers, passenger ships, ferries, and ships that transport liquefied natural gas, to name a few. The size of a ship is chosen based on cargo-carrying capacity, but it is limited by channel depth, pier length, and other characteristics of the ports it must enter. Most ships are part of a transportation business. To maximize profit, the costs of construction and operation must be minimized, while the income from transporting cargo must be maximized. Working together, naval architects and marine engineers design each ship to satisfy the requirements mentioned above. A design team is assembled that represents the needed areas of expertise. The team may work for a year or more to produce the complete design. When the design work is finished, one or more shipyards may be invited to submit bids for construction.

Background and History

One of the earliest well-documented ships is the funeral ship of Pharaoh Cheops that was built in about 2600 BCE The Greeks and the Romans built large oar-powered ships called biremes and triremes. Bartholomew Diaz sailed three caravels around the tip of Africa in 1488, and Christopher Columbus reached the New World in 1492. All these ships were built by artisans who knew no theory of ship design. Scientific ship design began in France and Spain in the late 1600s. Steam was first used to propel watercraft in the late 1700s, and the first successful steam powered vessel, Charlotte Dundas, made its first voyage on the River Clyde in Scotland in 1802. By 1816, steam-powered passenger ships sailed regularly between Brighton, England, and Le Havre, France. William H. Webb designed and built clipper ships along the East River in New York City in the years before the Civil War. Webb and his contemporaries and predecessors called themselves shipbuilders, but they designed their ships too. During the twentieth century, the functions of designing ships and building them became separate.

How It Works

The two principal issues in designing the hull of a ship are its structural strength and the force required to push it through the water at the desired speed. Other issues include stability and the way the ship moves up and down and side to side in waves.

Designing the Hull. Forces on a ship's hull may be divided into static (constant) forces and dynamic (variable) forces. There are two static forces: the force of gravity pulling down on the ship and its contents and the upward force exerted by the water on the hull.

Wave action is responsible for the dynamic forces, but some of these forces are direct results of the waves and some are indirect. When the ship's bow meets a wave, the wave exerts an upward force, lifting the bow. As the ship passes through the wave, the upward force moves along the ship from bow to stern. When the bow reaches the low point between two wave crests, the bow drops and the stern rises. Wave action causes the ship to flex, like a board that is supported at its ends does when someone jumps up and down at the middle.

Wave action also causes the ship to roll from side to side. As the ship rolls, liquids in its tanks slosh from side to side. This causes dynamic forces on the walls of the tanks. Waves may slap against the sides of the ship, and sometimes waves put water onto the decks.

Resistance and Stability. As a ship moves through the water, it must push aside the water ahead of it. This water moves away from the ship as waves. Water flowing along the sides of the ship exerts a friction force on the hull. The combination of these forces is called the resistance. The propulsion force provided by a ship's propeller must overcome the resistance. Naval architects can accurately predict the friction resistance, but the resistance associated with pushing the water aside is usually determined by testing a scale model.

A ship must be stable. This means it must float upright in still water, and it must return to an upright position after waves cause it to tilt. A naval architect must perform detailed calculations to ensure that the ship being designed meets requirements for stability.

Propulsion and Auxiliary Machinery. It is the responsibility of the marine engineer to select the machinery that will provide the required power. There are ships powered by steam turbines, gas turbines, and diesel engines. The propeller may be driven directly by the engine, it may be driven through a speed-reducing gear, or the engine may drive an electric generator that provides power to a motor that drives the propeller.

The marine engineer must also select pumps, piping, oil purifiers, speed-reducing gears, heat exchangers, and many other pieces of auxiliary machinery. The marine engineer designs the systems that connect all of the components and allow them to work together. A ship must generate its own electricity and produce freshwater from seawater. Machinery must be provided to control the direction of motion of the ship.

Applications and Products

Naval architects and marine engineers are called on to design many different types of ships. Each ship type has its own unique design requirements. The designers must consider many factors, including how deep the channels are in the intended ports of call, what type of cargo will be carried, and how time sensitive it is. Several examples are discussed below.

Passenger Ships. Passenger ships may be liners or cruise ships. Liners are used to transport people from one place to another. Before air travel became common, this is how people traveled across oceans. In the early twenty-first century, most passenger ships are cruise ships. They embark passengers at a port, take them to visit interesting places, and return them to the same port where they embarked. Cruise ships have extensive passenger-entertainment facilities, which may range from rock-climbing walls to casinos. Most cruise ships are propelled by diesel engines. These engines may drive the propellers directly, or they may drive electric generators. In the latter case, the propellers are driven by electric motors.

Container ships. Shipping of cargo in standard rectangular containers has revolutionized the shipping business. At specialized container ship ports, a ship can be loaded or unloaded in twelve hours or less by large container cranes mounted on the dock. The largest container ships can carry more than 10,000 twenty-foot- long containers. Containers can hold all manner of cargo from cameras and flat-screen TVs to food and cut flowers. Container ships are typically the fastest category of merchant ships. Speeds between 20 and 25 knots are common because these ships carry time-sensitive cargo.

Tankers.Tankers range from ultra-large crude-oil carriers that carry more than 250,000 long tons of crude to small coastal tankers that carry 10,000 long tons or less. Ultra-large crude-oil ships transport large amounts of crude oil from sources in the Persian Gulf to refineries in Europe and North America. There are many other categories of tankers. The most specialized are chemical tankers, which are equipped to carry corrosive cargo such as sulfuric acid and highly flammable cargo such as gasoline.

LNG Ships. These tankers are designed to carry liquefied natural gas (LNG). Natural gas, which is mainly methane, changes from gas to liquid at −160 degrees Celsius. Although the tanks are heavily insulated, some heat does leak into the LNG. This causes a small amount of LNG to vaporize. One of the major decisions in designing an LNG ship is how to handle this gas. Many LNG ships are propelled by steam turbines, and the gas that boils off from the tanks is burned in the boilers. Other LNG ships are propelled by diesel engines. In some cases, the boil-off gas (BOG) is burned in the engines. On other diesel-powered ships the BOG is condensed and returned to the tanks. Many LNG ships operate at about 19 knots. Depending on their size, they may require as much as 50,000 horsepower for propulsion.

Naval Surface Ships. Naval surface ships range from huge aircraft carriers, which may weigh 100,000 long tons or more, to much smaller destroyers and frigates, which may weigh 4,000 long tons. Modern aircraft carriers and cruisers are often nuclear powered. Destroyers and frigates are often powered by gas turbines. On an aircraft carrier the main weapons are the aircraft. Other warships are armed with guns, missiles, and torpedoes. The ships must have the capability to locate and track enemy targets and to launch weapons at them. They must also be able to defend themselves against weapons launched by enemy ships and aircraft.

Submarines. Submarines range from one-person research vessels powered by batteries to the US Navy's large, nuclear-powered missile submarines. Two things make submarines different from other ships: First, they must be able to operate without access to the atmosphere, and second, their hulls must withstand the pressure of the sea at the depths where they operate. Navy submarines are powered by nuclear reactors so that they do not need air for the combustion of fuel. The oxygen required to support human life aboard a submarine is produced by using electricity to split water molecules into hydrogen and oxygen, a process called electrolysis.

There are many other types of ships: ferries, roll-on/roll-off ships, heavy lift ships, fishing vessels, dredges, tugboats, and yachts of all shapes and sizes. Two very interesting specialized ships are hovercraft and hydrofoils. Hovercraft ride on a cushion of air that is trapped between the hull and the surface of the sea. Hydrofoils are supported by “wings” that are submerged in the water. Both hovercraft and hydrofoils are capable of much higher speeds than conventional ships.

Careers and Course Work

A small number of colleges and universities in the United States offer degrees in naval architecture, marine engineering, or a combination of the two. Similar programs are offered in Canada, Europe, and Asia. Students of naval architecture take courses in strength of materials, ship structures, hydrodynamics, ship resistance, and propeller design. Marine engineering programs include thermodynamics, heat transfer, and machine design. Advanced mathematics is a part of both programs. A bachelor of science degree is the minimum requirement, and many working professionals in these fields have master of science degrees.

Most ship design is performed by companies that produce only the plans and specifications for a ship. The would-be shipowner takes these documents to one or more shipyards and invites bids for the actual construction. Shipyards also employ naval architects and marine engineers who deal with design issues that arise during the construction of a ship.

Social Context and Future Prospects

When cargo must be transported across large bodies of water, such as the Atlantic and Pacific oceans, there are only two possible ways to do it—ships and airplanes. Lightweight, high-value, time-sensitive cargo goes by air, but more mundane cargo goes by ship. Ships are slow, but in many cases, time is not of the essence. Crude oil and natural gas are abundant in a part of the world where demand for these materials is low. A steady stream of large tankers carries crude oil nearly halfway around the world. LNG ships do the same. It is hard to imagine an alternative. Although it is possible to transport cargo from France to Algeria by road, it is not practical to do so.

Transportation by ship is far less expensive than by air. Among other factors, the fuel consumed to move a given amount of cargo a given distance is much less. Shipping products in standard rectangular containers has drastically reduced the cost and time required for handling cargo at both ends of its travel by sea.

It appears that cargo transportation by ship will remain an important business for the foreseeable future.

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