There are four main types of marine engine: the diesel engine,the steam turbine,the gas turbine and the marine nuclear plant. Each type of engine has its own particular application.
The diesel engine is a form of internal combustion engine similar to that used in a bus. It power is expressed as brake horsepower(bhp). This is the power put out by the engine. Effective horsepower is the power developed by the pistin in the cylinder,butsome of this is lost by friction with in the engine. Thepower out put of a modern marine diesel engine is about 40,000 brake horsepower. This is now expressed in kilowatts. By comparison the engine of a small family car has an uotput of about 80 bhp. Large diesel engines,which have cylinders nearly 3 ft in diameter,turn at the relatively slow speed of about 180 rpm. These are known as slow-speed diesel engines. They can be connected directly to the propeller without geaning. Althoung higher power cuond be produced by higher revolution, this would reduce the efficiency of the propeller, because a propeller is more efficient thelarger it is and the slower it turns. These largeslow running engines era used in the larger merchant ships, particularlyin tankers and bulk carriers. The main reason is their low fuel consuption. More and more of the larger merchant vessels are being powered by medium-speed diesel engines. These operate between 150 and 450 rpm, therefore they are connected to the propeller by gearing. This types of engine was once restricted to smaller cargo ships, but now they are used in fast cargo liners as well as in tankers and bulk carriers. They are cheaper than slow-speed diesel engines, and their smaller size and weight can result in a smaller,cheaper ship.
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