Lesson Eighteen Marine engines Marine engine is an engine that propels a water-born vessel. Even in small craft the marine engine must have the following characteristics: reliability, light weight, compactness, fuel economy, low maintenance, long life, relative simplicity for operating personnel, ability to reverse, and ability to operate steadily at low or cruising speed. The relative importance of these characteristics varies with the service performed by the vessel, but reliability is of prime importance. Steam engines Steam, used to drive the earliest powered vessels, is still a common type of propulsion for large ships. The diesel engine has gained wide acceptance in foreign merchant ships, but in the United States, the majority of seagoing vessels use steam propulsion Reciprocating steam engines Early engines commonly used steam flowing in series through as many as four cylinders whose pistons had the same stroke but were of increasing diameters. This system provided for an expansion or increase in steam volume which accompanied the decrease in pressure to the exhaust under a vacuum. The modern, multicylinder, uniflow marine steam engine, with complete expansion in each cylinder, shows better steam economy. Because it has the same diameter for all cylinders( two to six in number ) it is preferable from a manufacturing viewpoint. Equal power is developed by each cylinder; units of four cylinders or more have good torque and balance characteristics. A steam rate of 10 lb/hp/hr(1.7 kg/M) with 275 psi(1.90 Mpa)at 240@F (116C) super heat is attainted. Uniflow engines as large as 5000 hp(3. 7MW) have been used on shipboard. Normally, steam engines are double-acting; that is, steam acts on each side of the pistons. With superheated steam, piston-cylinder lubrication must be provided. Pure feed-water is required by modern, high-capacity boilers; thus, an effective oil filter is installed where the condensate must be returned to the boiler Steam turbines The marine steam turbine has the advantages of direct rotary motion little or no rubbing contact of pressure-confining surfaces, and ability to use effectively both highly super heated steam and steam at low pressure, that is, at a high vacuum where specific volumes of over 400 ft/b are reached For good efficiency of steam turbines, high rotative speeds are required. This requirement led to the introduction of the reduction geared turbine and turboelectric drive. These system give efficient turbine speeds and efficient propeller rmp. With geared turbine, for example, turbine rotor speeds range from 3000 to 10,000 rm while propeller rmp is reduced to the 80 to 400 range Steam is generally supplied to the turbine at 850 psi(5.9MPa) and 950 F(510 C) by pair of oilfired marine water-tube boiler, and the exhaust from the turbine is usually at 1. 5 in. lig(5. IkPa)absolute. Forced draft fans and other auxiliaries are usually motor-driven, except for the main feed pimps which are usually driven by an auxiliary urbine. Electric power is provided by a separate turbogenerator In low-powered geared turbines, steam completes its expansion in one rotor and casing. Such a design has been used in geared turbines of up to 8000 shaft horsepower(ship) or a shaft power of 6 Mw. However, series flow through two or even three casings is preferable in most steam turbines. This arrangement provides more flexibility in turbine design, allowing for different and optimum revolutions for high-and low-pressure rotors. Also, in a seagoing vessel in case of casualty to one turbine or its high-speed pinion, the vessel usually can make port with the remaining turbine in operation A steam turbine is made up of fixed blades, usually called nazzles, and rotating blades. A stage is generally one stationary row and one moving row. Impulse staging has all the steam pressure drop taking place in the fixed blades. The moving row then absorbs the kinetic energy produced. Reaction staging results when some of the assume drop occurs in the moving blades, the degree of reaction depending on the design Modern marine practice favors impulse staging in the high-pressure end of the turbine because of the steam progresses toward the low-pressure end where the value is much greater the reaction stage is more efficient. The arrangement has a cross compound system with a high-pressure unit of 7 to 10stages and a low-pressure unit of 6to 8 stages, each driving a pinion of a reduction gearLesson Eighteen Marine Engines Marine engine is an engine that propels a water-born vessel. Even in small craft the marine engine must have the following characteristics: reliability, light weight, compactness, fuel economy, low maintenance, long life, relative simplicity for operating personnel, ability to reverse, and ability to operate steadily at low or cruising speed. The relative importance of these characteristics varies with the service performed by the vessel, but reliability is of prime importance. Steam engines Steam, used to drive the earliest powered vessels, is still a common type of propulsion for large ships. The diesel engine has gained wide acceptance in foreign merchant ships, but in the United States, the majority of seagoing vessels use steam propulsion. Reciprocating steam engines Early engines commonly used steam flowing in series through as many as four cylinders whose pistons had the same stroke but were of increasing diameters. This system provided for an expansion or increase in steam volume which accompanied the decrease in pressure to the exhaust under a vacuum. The modern, multicylinder, uniflow marine steam engine, with complete expansion in each cylinder, shows better steam economy. Because it has the same diameter for all cylinders (two to six in number), it is preferable from a manufacturing viewpoint. Equal power is developed by each cylinder; units of four cylinders or more have good torque and balance characteristics. A steam rate of 10 lb/hp/hr (1.7 kg/MJ) with 275 psi (1.90 Mpa) at 240°F （116°C）super heat is attainted. Uniflow engines as large as 5000 hp (3.7MW) have been used on shipboard. Normally, steam engines are double-acting; that is, steam acts on each side of the pistons. With superheated steam, piston-cylinder lubrication must be provided. Pure feed-water is required by modern, high-capacity boilers; thus, an effective oil filter is installed where the condensate must be returned to the boiler. Steam turbines The marine steam turbine has the advantages of direct rotary motion, little or no rubbing contact of pressure-confining surfaces, and ability to use effectively both highly super heated steam and steam at low pressure, that is, at a high vacuum where specific volumes of over 400 ft3 /lb are reached. For good efficiency of steam turbines, high rotative speeds are required. This requirement led to the introduction of the reduction geared turbine and turboelectric drive. These system give efficient turbine speeds and efficient propeller rmp. With geared turbine, for example, turbine rotor speeds range from 3000 to 10,000 rmp, while propeller rmp is reduced to the 80 to 400 range. Steam is generally supplied to the turbine at 850 psi (5.9MPa) and 950°F (510°C) by pair of oilfired marine water-tube boiler, and the exhaust from the turbine is usually at 1.5 in. lig(5.1kPa) absolute. Forced draft fans and other auxiliaries are usually motor-driven, except for the main feed pimps which are usually driven by an auxiliary turbine. Electric power is provided by a separate turbogenerator. In low-powered geared turbines, steam completes its expansion in one rotor and casing. Such a design has been used in geared turbines of up to 8000 shaft horsepower (ship) or a shaft power of 6 MW. However, series flow through two or even three casings is preferable in most steam turbines. This arrangement provides more flexibility in turbine design, allowing for different and optimum revolutions for high-and low-pressure rotors. Also, in a seagoing vessel in case of casualty to one turbine or its high-speed pinion, the vessel usually can make port with the remaining turbine in operation. A steam turbine is made up of fixed blades, usually called nazzles, and rotating blades. A stage is generally one stationary row and one moving row. Impulse staging has all the steam pressure drop taking place in the fixed blades. The moving row then absorbs the kinetic energy produced. Reaction staging results when some of the assume drop occurs in the moving blades, the degree of reaction depending on the design. Modern marine practice favors impulse staging in the high-pressure end of the turbine because of the steam progresses toward the low-pressure end where the value is much greater the reaction stage is more efficient. The arrangement has a cross compound system with a high-pressure unit of 7 to 10stages and a low-pressure unit of 6to 8 stages, each driving a pinion of a reduction gear