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Main Diesel Marine Engine Selection - A Gigantic Dilemma
In this article we will see the different aspects that need to be taken into consideration when choosing a main engine for a ship. The criteria for selecting a main engine may change from ship to ship and also according to the requirements. The emphasis on each criterion will also differ. But there are few general aspects that remain same for almost all types of ships and we will enumerate them one by one, briefly.
The criteria includes: required horse power, weight, space, capital cost, running costs, requirement for electrical power and heat, reliability and maintainability, maneuvering ability, ease of installation , vibration , noise and availability. As you would notice the title takes into account the hull design as well. This is necessary because of the forces of vibration and weight of the engine would have to be borne by the hull, hence ship hull design is equally important to be taken into consideration.
Let’s learn about each of them.
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How Many Horses?
The requirement of horsepower depends of quite a number of factors. It depends on the number, size, and type of propellers used. It also depends on the number of revolutions a propeller makes.
The decision on the requirement of the horse power is also made on the basis of the type of machinery that is available for the ship and the fuel weight capacity.
Apart from all these factors, the overall power required by the ship and the auxiliary machineries is the fundamental requirement.
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This is not considered an important point while choosing the main engine for most of the ships. This criterion is taken into account only in smaller ships such as ferries where the requirement of speed and exposure to confined waters is more.
It is also an important factor in designing war ships where the requirement of speed is more. In making of such designs the power/weight ratio is of utmost importance.
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Space and weight almost holds equal importance when considering the criteria for main engine. But if a choice has to be made in between weight and space then weight of the machinery is taken into consideration even if it occupies a larger space. But if the ship is designed on volume basis then the ship should be fitted with non bulky machinery even if they have higher weight.
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The cost of main engine is always of prime importance. While calculating the cost of engine, the installation costs of the engine are also taken into consideration. It will also include additional costs of gearing and pumps that needs to be set up.
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The cost is an important factor in determining the main engine. The fuel cost is an integral part of these costs. The annual fuel bill is the most important item of running costs. Now as the fuel prices are volatile in this market scenario the costs would vary according to the market price of the fuel.
There are two ways to reduce these costs. One is by choosing machinery that is fuel efficient and uses less fuel oil or choosing machinery that uses cheap and lower grade of oil but doesn’t compromise on the power and load output.
The running costs will also include the cost of lubricating oils, spares, maintenance and the cost of manning.
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Reliability and maintainability
These aspects should be taken into consideration for all ships. Breakdowns and emergency situations are a part of ship operations and efforts should be made to see that costs incurred due to them are as low as possible. On passenger ships these factors are of high importance as the consequences of a breakdown will incur heavy expenditure and will also mar the overall reputation and thus the future profitability.
On war ships these factors are important to prevent loss due to mechanical breakdown or enemy’s action.
The advantage of using a slow speed engine is that as it will have few cylinders the maintenance cost arising will be comparatively low.
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The maneuvering ability of the ship is an important factor while choosing a main engine. The amount of power required and the frequency of running in confined water will affect the type of machinery required. If the ship is running at slow speed for more time then accordingly an engine needs to be chosen.
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Ease of installation
The ease of installation depends upon the size of the ship. If the size of the ship is small and the machinery is bulky there arises a difficulty to install it. Thus according to the size and power requirement the machinery should be choose which will also support the ease with which it is installed also.
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Any vibration force that arises from the main engine should be properly assessed. Its intensity should be measured and its acceptability with the tolerance level of the ship should be checked. Main engine should also be chosen on the basis of the type of the ship. For example if the ship is a passenger ship then the allowance of vibrations would be very less. Thus special care should be taken that the vibrations thus generated should not affect the personnel and equipments on ship.
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All the main engines produce high levels of noise. Bigger the engine, higher will be the noise level. Keeping this thing in mind it is necessary that the engine is enclosed in an acoustic enclosure. Thus it is imperative to choose and install the main engine in such a way that the noise levels doesn’t create unwanted vibrations and doesn’t affect ship’s passengers.
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The choice of machinery should be made in such a way that in case any part of the machinery breaks down or needs to be replaced, spares are available for the same, easily at all places. It would be a situation of grave concern if the ship is using a high end model of machinery whose spares are not so easily available or a model which is not available any more in the market, needs a replacement of a part.
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Practical ship design by D.G.M Watson