a turbine is capable of operation in only one direction. In order to provide reverse power, a second turbine is installed on the shaft of the low-pressure ahead turbine. The astern turbine is usually not more than three moving rows of blades, but it may be only two. It produces about 40% of the normal ahead horsepower. Since this unit is urning backward in normal ahead operation, it is located in the low-pressure end of the low-pressure turbine. The steam at this point has a very low density, and hence the astern turbine has a low windage loss The propeller is reversed by closing the steam value to the ahead turbine and opening the value to the astern urbine Gas turbine The gas turbine is a relative newcomer to the marine field. It generally considered of an axial compressor discharging compressed air to a combustion chamber where fuel is burned, adding heat. The produces of combustion at high temperature and pressure then pass through a gas turbine that drives the compressor and load. Generally, the term"gas turbine "is applied to the entire plant. If lower pressure ratios (final pressure leaving the compressor divided by initial pressure entering) are used, a large amount of heat is available in the exhaust gas which may be recovered by heating the compressed air before it enters the combustion chamber. This is done in a regenerator. With higher pressure ratios the expansion through the turbine is so great that the exhaust gas temperature is insufficient to heat the compressed air Two distinct types of gas turbines are appearing in the marine field the aircraft-derived type and the industrial type. The aircraft-derived type uses a jet engine as a gas generator, which discharges to a gas turbine driving the load. This type of plant offers simplicity and light weight but must burn high-quality fuel. The industrial gas turbine is a more rugged machine designed for long life and is capable of using low grades of fuel properly washed. This plant usually uses a regenerator. The gas turbine offers simplicity, ease of control, and efficiency, but requires special fuel or special treatment of the fuel Large amounts of air and exhaust gas are used and as a result uptakes and air supply are a special problem. The aircraft-derived gas turbine seems destined to drive a large number of naval combatant ships. Selections of this type for a new class of naval destroyers has been announced. The industrial type will be used for merchant vessels where its greater weight will be of little disadvantage Both diesel and gasoline internal combustion engines are used in marine applications. Many moderate and lowpower marine installations use automotive or locomotive engines designed for variable load and intermittent service. High-power marine propulsion units normally are called on to operate continuously under load. Therefore, the brake horsepower(bhp)rating of units selected for marine service should be conservative The gasoline engine is the most common power plant for pleasure craft. It is inexpensive to buy and maintain Because of its widespread use in automobiles, most parts are readily available. In most areas gasoline costs slightly more than diesel fuel, but the cost differential is usually insufficient to make up the difference between the cost of gasoline and diesel engines. Gasoline presents an explosion and fire hazard, which is its major Direct-drive diesels For typical commercial freight vessels, direct-drive diesels provide economical service. For good propeller efficiency, the propeller rpm should be under 120+. Such a top limit on engine revolutions results in a large, heavy, bulky, slow-rpm engine. However, the direct-drive diesels have a lower fuel oil consumption than do higher-rpm units, and with suitable fuel treatment they will operate on the better grades of the cheaper fuel oil burned in boilers Slow-speed, direct-drive diesels are favored by many European owners and shipbuilders. Turbo-charged, two-cycle, single-acting diesel engines of 50 000 bhp(37MW of brake power) are now available; such engines weigh more than 100 lb/bhp(60kg/kw of brake power). For high horsepower the total machinery weight fo diesels is more than the weight of geared turbine machinery, including boilers and auxiliaries Moderate-speed diesels Diesel engines of 250-500 rpm are available in two and four cycle, single-acting types, generally with trunk pistons. In some marine applications they are connected directly to the propeller and thus fitted only with reverse gear. However, they are also employed with geared diesel and diesel-electric driveA turbine is capable of operation in only one direction. In order to provide reverse power, a second turbine is installed on the shaft of the low-pressure ahead turbine. The astern turbine is usually not more than three moving rows of blades, but it may be only two. It produces about 40% of the normal ahead horsepower. Since this unit is turning backward in normal ahead operation, it is located in the low-pressure end of the low-pressure turbine. The steam at this point has a very low density, and hence the astern turbine has a low windage loss. The propeller is reversed by closing the steam value to the ahead turbine and opening the value to the astern turbine. Gas turbine The gas turbine is a relative newcomer to the marine field. It generally considered of an axial compressor discharging compressed air to a combustion chamber where fuel is burned, adding heat. The produces of combustion at high temperature and pressure then pass through a gas turbine that drives the compressor and load. Generally, the term “gas turbine” is applied to the entire plant. If lower pressure ratios (final pressure leaving the compressor divided by initial pressure entering) are used, a large amount of heat is available in the exhaust gas which may be recovered by heating the compressed air before it enters the combustion chamber. This is done in a regenerator. With higher pressure ratios the expansion through the turbine is so great that the exhaust gas temperature is insufficient to heat the compressed air. Two distinct types of gas turbines are appearing in the marine field: the aircraft-derived type and the industrial type. The aircraft-derived type uses a jet engine as a gas generator, which discharges to a gas turbine driving the load. This type of plant offers simplicity and light weight but must burn high-quality fuel. The industrial gas turbine is a more rugged machine designed for long life and is capable of using low grades of fuel, properly washed. This plant usually uses a regenerator. The gas turbine offers simplicity, ease of control, and efficiency, but requires special fuel or special treatment of the fuel .Large amounts of air and exhaust gas are used, and as a result uptakes and air supply are a special problem. The aircraft-derived gas turbine seems destined to drive a large number of naval combatant ships. Selections of this type for a new class of naval destroyers has been announced. The industrial type will be used for merchant vessels where its greater weight will be of little disadvantage. Internal combustion engines Both diesel and gasoline internal combustion engines are used in marine applications. Many moderate and lowpower marine installations use automotive or locomotive engines designed for variable load and intermittent service. High-power marine propulsion units normally are called on to operate continuously under load. Therefore, the brake horsepower (bhp) rating of units selected for marine service should be conservative. The gasoline engine is the most common power plant for pleasure craft. It is inexpensive to buy and maintain. Because of its widespread use in automobiles, most parts are readily available. In most areas gasoline costs slightly more than diesel fuel, but the cost differential is usually insufficient to make up the difference between the cost of gasoline and diesel engines. Gasoline presents an explosion and fire hazard, which is its major disadvantage. Direct-drive diesels For typical commercial freight vessels, direct-drive diesels provide economical service. For good propeller efficiency, the propeller rpm should be under 120±. Such a top limit on engine revolutions results in a large, heavy, bulky, slow-rpm engine. However, the direct-drive diesels have a lower fuel oil consumption than do higher-rpm units, and with suitable fuel treatment they will operate on the better grades of the cheaper fuel oil burned in boilers. Slow-speed, direct-drive diesels are favored by many European owners and shipbuilders. Turbo-charged, two-cycle, single-acting diesel engines of 50 000 bhp (37MW of brake power) are now available; such engines weigh more than 100 lb/bhp (60kg/kw of brake power). For high horsepower the total machinery weight for diesels is more than the weight of geared turbine machinery, including boilers and auxiliaries. Moderate-speed diesels Diesel engines of 250-500 rpm are available in two and four cycle, single-acting types, generally with trunk pistons. In some marine applications they are connected directly to the propeller and thus fitted only with reverse gear. However, they are also employed with geared diesel and diesel-electric drive