Engine and Plant Selection
During the construction of a ship the choice of the marine diesel engine is a major decision and it involves a number of factors. Some owners like to use only one type of marine diesel engine or a particular model for reasons like familiarity of the crew, good past experience, common spares kept on sister ships, good service backup and support by the manufacturers. But the others have a wide variety of engine models, manufacturers, and layouts available and have to choose from. The decision of the engine layout and the model to be selected has to be made by the naval architects and the marine engineer on the following factors:
- Low speed engines are normally used in large ocean going ships, while local coastal ferries, tugs, and supply vessels use medium speed engines.
- Past reliability of the engine in the minds of the ship owners.
- Common spare spool that can be provided to sister ships thereby reducing costs. Instead of every ship having a spare propeller and a spare tail shaft, only one can store them and they can be shipped to whichever vessel needs them around the world. This reduces the locking up of capital and is cost effective. Also these spare parts are rarely used.
- Initial cost of the engine and the costs of the spares.
- Costs of the maintenance.
- The complexity of the engine and the skill of the crew required.
- The type of fuel that can be burnt. The cost of the fuel is a major part of the daily expenses of the ship running. An engine that can burn low grades of fuel is more cost effective.
- The total cost of the engine spread over the life time of the ship, say, twenty years.
- The maintenance workload on the engine room personnel. The service interval of the engine is also a vital factor. Nobody likes to work on an engine where they have to do a major overhaul at every port, restricting the shore leave of the engineers in that process.
- The suitability of the engine for UMS operation. UMS stands for Unmanned Machinery Spaces and in these ships the engineers work on 8 to 5 basis and the engine room is put in automatic operation during the night time. Needless to say it is more comfortable to the engineers and fewer crew can be used as watches need not be kept.
- Propulsive efficiency of the engine is another factor. The engine that can be driven at the lowest rpm and drive the largest propeller is the most efficient.
- The size of the machinery is another factor. In ships like Ro-Ro or roll on-roll off, the engine height has to be low as the engine room is under the ro-ro deck.
Main Engine layout
Different Layouts and Configurations
Thus depending on the above factors, a number of layouts and configurations of the marine diesel engines are available that are now discussed.
- Direct coupled low speed reversible engines with fixed pitch propeller.
- Direct coupled low speed reversible engines with controllable pitch propeller.
- Medium speed engine coupled through reduction gears.
- The above installations with a shaft generator.
- Electric propulsion with synchronous motors with a number of generators.
These are just the simple and commonly used layouts used worldwide. A lot more layouts are available and there are a number of factors more that are to be considered while designing a ship. The factors discussed above are just a fraction of what passes through the design engineer’s mind.
Pounder’s Marine Diesel Engines