SHIPS

From the earliest times, ships and boats were propelled by human power and wind power. In the nineteenth century very large and efficient sailing ships (many 70 to 100 m long) were transporting passengers and cargo to ports all over the world. In the Western world most of the cargoes were carried by sailing ships until the 1890s. By the middle of the nineteenth century, with the development of reliable steam engines, the designers of ships began to use coal-burning steam power plants to propel ships, using paddle wheels and then propellers. Other power plants were developed in the late nineteenth century and during the twentieth century, including steam turbines, spark-ignition engines, diesel engines, gas turbines, and nuclear power plants. These ship power plants were developed from their land-based counterparts to operate in the severe ocean environment. A relatively new power source, the fuel cell, is being developed to provide electrical power for underwater vessels and also is being evaluated for future applications in surface ships.

High-speed ships and large-displacement slower ships can have power requirements of 30,000 to 75,000 kW (approximately 40,000 to 100,000 hp) for propulsion. Some nuclear-powered aircraft carriers have power plants that develop 200,000 kW. In addition to the power required to propel the vessel, some energy and power have to be provided for various ship services, such as heating, lighting, and cooling; this is often described as the "hotel load." Power also is required to refrigerate the cargo in some vessels and to pump oil in tankers.

SHIP RESISTANCE

Experiments by William Froude in the nineteenth century indicated that ship resistance has two main components: frictional, and wave-making resistance or drag. Frictional resistance is caused by the movement of the underwater hull (wetted surface area) through the viscous fluid, water. Wave-making drag is the component of resistance resulting from the energy expended on the waves generated by the moving ship. Short, bluff ships have less wetted surface area than long, slender ships for the same displacement and therefore have lower frictional drag. The long, slender ships, however, tend to generate smaller waves and thus have lower wave-making drag at high ship speeds, where wave-making drag predominates. Placing bulbous bows on large commercial ships such as tankers has been shown to reduce wave-making drag. In addition to frictional and wave-making drag, there is form drag induced by the complex fluid flow around the hull near the stern of many ships, and there is also the wind resistance of the structure of the ship above the sea surface.

Information on ship resistance has been determined from large numbers of tests on scale models of ships and from full-size ships, and compilations of these experimental results have been published. For a new and innovative hull form the usual procedure is to construct a scale model of the ship and then to conduct resistance tests in a special test facility (towing tank). Alternatively, analytical methods can provide estimates of ship resistance for a range of different hull shapes. Computer programs have been written based on these theoretical analyses and have been used with success for many ship designs, including racing sailboats.

Methods for reducing wave-making drag are often introduced for ships that operate at high speed. Lifting the hull partly or completely out of the water by using hydrodynamic lift or air pressure has been investigated for high-speed ship designs. Fast but short-range ferries have been built on these principles using planing hulls, air-cushion designs, and hydrofoil configurations. In air cushion and surface-effect ships, 15 to 30 percent of the total power has to be supplied to the fans that provide the air pressure to lift the hulls.

Another concern is the ability of a ship to operate in storms. The pounding of waves on the hull increases the resistance; this can cause structural damage, and the violent ship motions can harm the passengers, cargo, and crew of the ship. Some designers have proposed special configurations that can maintain the ship speed in storms.

PROPULSORS

Propellers are the predominant propulsive devices driving ships, although water jets are now used in some high-speed ships. An experimental installation in a small ship of a magnetohydrodynamic propulsor has been tested, but it achieved rather low propulsive efficiency. Fish-like propulsion also has been examined for possible application to ships and underwater vehicles.

In the middle of the nineteenth century some ship designers began to replace paddle wheels with propellers (or screws). A propeller has a series of identical blades placed around a hub that is driven by the engine. W. J. M. Rankine presented a theoretical model of the ideal propeller in his classic paper "On the Mechanical Principles of the Action of Propellers" (1865). He demonstrated that the efficiency of an ideal frictionless propeller to produce a specified thrust was improved as the propeller diameter was increased. There are, of course, practical limitations to the propeller diameter, depending on the ship geometry and to some extent on the fabrication capabilities of the propeller manufacturers. Designers have developed the shapes of the propeller blades to achieve good efficiency.

Methods for defining the details of the geometry of the propeller and its blading to achieve high efficiency are well established and are continuously being improved. Digital computer programs have been developed that are widely used to provide the design and analysis of propulsors. These programs were originally used to design conventional propellers, and the methods have been extended to apply to propellers in ducts and also to the pumps for waterjets.

To produce thrust it is necessary to apply torque to the propeller; this results in a swirling flow downstream of the propeller from the reaction of this torque acting on the water. The swirling flow is not used to produce thrust and is therefore a source of energy loss. Several methods have been developed to raise the propulsive efficiency by canceling or removing this swirling flow. Stationary blades (stators) upstream or downstream of the propeller have been installed in a few propellers for this purpose, and counterrotating screws have been used. An additional device downstream of the propeller that rotates freely, called a Grim wheel or a vane wheel, has been used in some applications. The inner portion of the vane wheel absorbs the swirling energy from the propeller wake while the outer portion acts as an additional propeller and produces thrust.

In the 1930s ducts or nozzles surrounding highly loaded propellers were introduced. Experiments had shown that propeller efficiency was improved, and many of these devices (often referred to as Kort nozzles) have been used in tugs and fishing boats. Theoretical analysis of ducted propellers has shown that the efficiency can be improved compared to conventional propellers when some of the thrust is produced by the fluid flow around the nozzle.

The designer is expected to develop a propeller design to minimize the power required by the power

plant to propel the ship at the design speed. At this speed the effective power, P_e, is the product of ship resistance and ship speed and would be determined by model tests, correlations of experimental results, or computed using theoretical predictions, discussed earlier. The shaft power developed by the engine, P_s, is the power required to drive the propeller through the transmission and shafting. The propulsive efficiency or propulsive coefficient (PC) is the ratio of effective power to shaft power: PC = P_e /P_s. The propulsive coefficient is usually between 60 and 75 percent for most ship designs.

The propulsor, in a steady situation, has to provide the thrust equivalent to the resistance of the ship. The prediction of this balance between ship resistance and propeller thrust is a complex process because the water flow in the stern region of the ship interacts with the flow through the propeller. The ship resistance may be modified by the action of the propeller, while the propeller efficiency usually is influenced by the disturbed flow regime near the stern of the ship.

The phenomenon of propeller cavitation often occurs at high ship speed and on highly loaded propellers. It provides a very serious limitation to propeller performance because it can damage the propeller and ship components near it. In addition, the efficiency may be reduced and the phenomenon can cause noise and vibration. Cavitation occurs when the local pressure of the water on the blade surface is reduced below the vapor pressure of the water passing over the blade surface. The high velocity of the water passing over regions of the propeller blades creates this reduction in pressure. The water boils when the local pressure becomes very low, and pockets of water vapor are formed at the blade surface. These vapor pockets and their subsequent collapse cause the problems associated with cavitation. Modern methods of propeller design have allowed designers to predict the onset of cavitation and to define the performance limitations.

WATERJETS

Waterjets have been developed for application to high-speed ships. The waterjet has an inlet usually on the side or bottom of the ship in the region of the stern, which allows water to flow into a water pump. The pressure of the water is raised in the pump, and the water is expelled as a jet to produce the desired thrust. The direction of the jet flow can be controlled to provide maneuvering forces, eliminating the need for rudders. The propulsive coefficient of modern waterjets usually is lower than propellers for the same application, but they have been used where severe cavitation would occur in conventional propellers.

SHIP POWER PLANTS

Most ships are powered by thermal power plants, including diesel engines, gas turbines, and nuclear systems. Merchant ships usually have diesel engines, while gas turbines or a combination of diesel engines and gas turbines often power naval vessels. Some of the larger ships in the U.S. Navy have nuclear power plants.

Large, low-speed diesel engines are remarkably efficient and convert about 45 percent of the energy of the fuel-air reaction into power. In addition, the cooling water and engine exhaust can be used to generate steam to provide heat and power for the ship. The steam has been used in some designs to generate power through steam turbines.

Improvements also have been made to the gas turbine for naval applications. An intercooled recuperative (ICR) gas turbine has been designed to improve the fuel consumption of naval power plants. The engine has a recuperator to take the heat that would otherwise be wasted in the exhaust and transfers it to the air entering the combustor. The new engine is expected to save about 30 percent of the fuel consumed, compared to the simple gas turbine. The ICR engine is, however, larger and more expensive than the simple gas turbine.

In the early steam ships the paddle wheels and propellers were directly connected to reciprocating steam engines. The large low-speed diesel engines of today are able to operate efficiently at the low propeller speeds and usually are directly connected by shafts to the propellers. Other ship power plants, such as steam turbines, gas turbines, and medium-speed diesel engines, operate most efficiently at higher rotational speeds than propellers. Speed converters have to be provided to reduce the rotational speed from the high engine speed to the low propeller speed. Mechanical reduction gearboxes are generally used to provide this conversion. Electric drives, consisting of high-speed electric generators driven by steam turbines coupled to low-speed electric motors, were used during World War II, when there was a shortage of gearboxes. A modern version of electric drive sometimes is used in cruise ships and also is being evaluated for the U.S. Navy.

Submarines, submersibles, and other underwater vehicles have a difficult operational problem, namely the absence of atmospheric oxygen, which is necessary for all thermal engines except nuclear power plants. At about the beginning of the twentieth century rechargeable batteries were developed, and these batteries, coupled to electric motors and propellers, were used to propel submarines under the sea. Spark ignition engines and later diesel engines provided the power for operation at the surface and to recharge the batteries. Since 1985, stored oxygen has been used in some thermal power plants and in fuel cells for experimental underwater vehicles. Nuclear power plants, which do not require oxygen for power generation, were introduced in 1954 to power U.S. military submarines. Other nations also have developed nuclear power for naval submarines.

ENVIRONMENTAL PROBLEMS

Two of the main environmental concerns are the atmospheric pollutants generated by ship's engines, and the possibility of damaging oil spills from accidents and ship operations.

Commercial ships have traditionally used the least expensive and therefore the most polluting oil fuels. Large diesel engines can now operate successfully with the lowest grade of fuel oils, and these fuels have a relatively high sulfur content. As a result the harmful SO_x by-products are released to the atmosphere. In addition, the combustion process in diesel engines produces harmful NOX pollutants. International, national, and local restrictions have encouraged engine manufacturers to develop remedial methods to reduce pollution.

The reduction of SO_x is best achieved by reducing the sulfur content of the fuel oil. Ship engineers are required to replace high-sulfur fuel with expensive higher-grade oil fuel as their ships approach coastlines where strict pollution restrictions apply.

Higher-grade fuels also reduce NO_x production in diesel engines, but more stringent methods are required to satisfy pollution regulations in some areas (e.g., California). Experiments conducted on diesel engines indicate that the concentration of NO_x in the exhaust gases increases as the engine power is reduced as ships approach coasts and harbors. Two approaches have been evaluated to improve the situation. First, changes to the combustion process have been tried by such measures as recirculating some of the exhaust and by the addition of water to the fuel to form an emulsion. Tests on engines have shown that about a 25 percent reduction in NO_x can be attained with such methods. The second approach has involved attempts to clean up the exhaust by catalytic reactions. Catalytic converters, as used in automobiles, have not been successful up to now, because diesel engines operate with too much oxygen in the exhaust products. An alternative approach using ammonia vapor as the catalyst has been shown to reduce the NO_x in the exhaust by more than 80 percent. This has been termed the selective catalytic reduction (SCR) method; it relies on the excess oxygen in the exhaust products to react with the ammonia and the NO_x to produce harmless water vapor and nitrogen. Careful measurement of the pollutants in the exhaust gases ahead of the ammonia injector has to be provided so the optimum quantity of ammonia can be injected.

In the last 30 years of the twentieth century, there were very damaging oil spills from the grounding of tankers filled with crude oil. In addition, there have been many smaller accidents, as well as routine operations, that have resulted in significant amounts of oil being released to the environment.

The International Maritime Organization (IMO) has issued a series of rules to tanker designers in an attempt to minimize the outflow of oil after accidental side or bottom damage. New tankers are required to have double hulls or other structural innovations to minimize tanker spills.

When cargo ships and fishing boats are involved in accidents, there are various measures that have to be carried out to limit the flow of fuel oil into the water. Accidents in harbors and close to shore are treated with great care. Oil booms would be placed around the vessel to prevent the spread of oil, and skimmers would be brought to the area to collect the oil released to the environment. In extreme cases the fuel oil cargo may be burned when the oil could not be pumped out or when a stranded vessel could not be refloated.

Routine operations of many ships have resulted in oil pollution. Cleaning up minor spills on deck and in the engine room is now treated very carefully. Fuel-oil and crude-oil tanks are cleaned from time to time. In the past the polluted oil-water mixture was dumped overboard, but now it must be pumped ashore for treatment.

A. Douglas Carmichael Clifford A. Whitecomb

See also: Aerodynamics; Diesel Cycle Engines; Diesel Fuel; Engines; Environmental Problems and Energy Use; Nuclear Fission Fuel; Thermodynamics; Transportation, Evolution of Energy Use and; Turbines, Gas; Turbines, Steam; Waves.

BIBLIOGRAPHY

Giles, D. L. (1997). "Faster Ships for the Future." Scientific American 277(4):126–131.

Rankine, W. J. M. (1865). "On the Mechanical Principles of the Action of Propellers." Trans Institute of Naval Architects 6:13–39.

van Manen, J. D., and van Oossanen, P. (1988). "Propulsion." In Principles of Naval Architecture, Vol. 2, ed. E. V. Lewis. Jersey City, NJ: The Society of Naval Architects and Marine Engineers.

van Manen, J. D., and van Oossanen, P. (1988). "Resistance." In Principles of Naval Architecture, Vol.2, ed. E. V. Lewis. Jersey City, NJ: The Society of Naval Architects and Marine Engineers.