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The Different Types of Propulsion Systems Used in Ships

Updated on July 22, 2018
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Merchant Navy Engineering Officer on passengers ship since 2008

A ship, like any body moving in a fluid, is subjected to a force which opposes its movement; to overcome this "resistance to advancement", it must therefore be provided with some energy. This energy can come from the outside; wind on the sails or another towing vehicle or pusher, but it is most often provided by a motor specific to the ship.

Using propulsion forces, ships are able to maneuver themselves in the water. Initially, there had been a limited number of ship propulsion systems, but currently there are many progressions. Today, the propulsion of ships is not only the means to move the ship, but also takes into account the protection of the marine environment.

Some of the different types of propulsion structures used in ships can be listed as follows:

Diesel Propulsion

Motorized propulsion is the most widespread. It consists of operating a thruster with a motor. The engine is responsible for converting the primary energy of the fuel into a mechanical and / or an electrical energy. It is coupled to one or more propellers. The system advances perpendicular to the rotation of the propellers thanks to the blades oriented at an optimized angle which are supported on the water.

Diesel Propulsion device is the commonly used maritime propulsion among the motorized propulsions. Diesel propulsion systems are specially used in nearly all kinds of vessels in conjunction with small boats and leisure vessels.

The semi-rapid diesel engines (from 400 to 500 rpm) with moderate reduction ratio reducers are widely used when a small height requirement is necessary or interesting: ferry boats, cargo ships with horizontal loading, container ships, etc. . High powers are obtained by coupling several motors on the same reducer with clutches.
Fast diesel engines (more than 1,000 rpm) are widely used for the propulsion of small military or civilian vessels (coasters, fishing vessels, service vessels).

Wind propulsion

It consists in the using of the wind forces to propel a craft thanks to the sails. The operating principle of a sail depends on the pace of the ship, that is to say the angle between the direction of the ship and that of the wind.
In order to improve the performance of the boats, the naval architects play on the shape of the boat and sails, and on the types of components to use. They also take into account the phenomenon of heeling: the force of the wind on the sail has a perpendicular component to the axis of the ship which can tip the latter to the side. Among other things, this problem is solved with pins or counterweights; when the direction of the ship is close to that of the wind, it pushes through the sails the ship.

Wind Propulsion System
Wind Propulsion System

Nuclear Propulsion System

Nuclear propulsion of ships requires a nuclear reactor that supplies steam to the turbine. The mechanical energy of the turbine rotated by steam is then used to rotate the propeller shaft of the ship, either directly through a gearbox (this is called turbo-mechanical propulsion), or through the intermediate electricity (it's called turbo-electric propulsion). In addition, the steam produced by the reactor provides electricity and fresh water from the auxiliary machines.

Nuclear Propulsion System
Nuclear Propulsion System
Propulsion Gas Turbine
Propulsion Gas Turbine

Fuel Cell Propulsion

Fuel cells are an electrochemical devices that convert chemical energy directly into electrical energy, and this latter is used to propel the ship. In contrast to internal combustion engines, there is no intermediate conversion into thermal energy.

This year, the European Maritime Safety Agency (EMSA) published a comparative study on the use of different types of fuel cells on board ships based on the results of several European projects.
Various projects have proliferated to test and demonstrate the ability of fuel cells to meet the new environmental and technological constraints imposed on shipowners and shipbuilders.
Today, the results of various experiments and the short-term data on the fuel cell market have led experts to say that fuel cells are "no longer a technology of the future" but a technology ready for industrialization and marketing.

Fuel Cell Principe
Fuel Cell Principe

Gas Turbine Propulsion

Gas turbine propulsion is used for naval in addition to non-naval ships. In its simplest and most widespread form, a gas turbine (also called a combustion turbine) is composed of three elements:

  • The compressor, centrifugal or more generally axial, which has the role of compressing ambient air at a pressure currently between 10 and 30 bar approximately;

  • The combustion chamber, in which a gaseous or liquid fuel is injected under pressure, then burned with compressed air, with a large excess of air to limit the temperature of the exhaust gas;

  • The turbine, generally axial, in which are released gases leaving the combustion chamber.

In this form, the gas turbine is a continuous flow internal combustion engine. Note that the term gas turbine comes from the state of the working fluid, which remains gaseous, and not the fuel used, which can be both gaseous and liquid (gas turbines generally use natural gas or light distillates). There are also closed-cycle gas turbines used for particular applications.

Solar Propulsion System

Solar propulsion for ships appeared for the primary time within the year 2008. It is capable to generate a capacitance as excessive as 40 kilowatts (kW).

A German shipyard in Kiel built the solar-powered TÛRANOR multihull in September 2010 under the Swiss flag. The main objectives of PlanetSolar are to demonstrate the potential of renewable and photovoltaic energy, by participating in the technological development of renewable energies.

Solar Propulsion System
Solar Propulsion System

Steam Turbine Propulsion

A turbine is a rotating device intended to use the force of a fluid (water, steam, air, combustion gas), the torque of which is transmitted by means of a shaft. The fluid energy, characterized by its speed and enthalpy, is partially converted into mechanical energy to drive an alternator, a pump or just a vessel propulsion propeller.

The steam turbine is an external combustion engine operating on the so-called Clausius-Rankine thermodynamic cycle. This cycle is distinguished by the change of state affecting the working fluid which is usually water vapor, but which can be another hot gas. With respect to a steam turbine, this cycle comprises at least the following steps:

  • The liquid water is propelled by a pump (TPA) and sent to the boiler;

  • The water is heated, vaporized and overheated;

  • The steam expands in the turbine providing mechanical energy;

    The expanded steam is condensed in contact with a cold source under partial vacuum.

    Steam turbines are classified into two broad categories often combined in the same machine:
  • Action turbines in which the expansion is done only in the fixed blades. They are well suited to high pressure stages and are better suited for flow control. Their construction is more expensive and reserves their use on the first stages of the turbine.
  • Jet turbines in which the expansion is distributed between the fixed and mobile vanes. The degree of reaction is defined by the distribution of the relaxation between the blades. They lend themselves better to low pressure stages and their cost is lower.


Steam turbine Propulsion System
Steam turbine Propulsion System

Diesel-Electric Propulsion

This propulsion principle aims to reduce the fuel consumption of ships working with different operating regimes. The idea is to optimize the overall propulsion efficiency in order to consume less energy for the same result and thus reduce pollution. Electrical energy is generally produced by four diesel generators sized to the demands of the ship.

Todaty, submarines and merchant ships contain the diesel-electric propulsion gadget to propel themselves.


Gas / Fuel Propulsion System

LNG fuel is now utilised to be burnt in the Main Engine after adopting some amendment inside the propulsion engine to lessen emission from the deliver. It is known as tri gasoline due to the fact it could burn gasoline fuel, diesel and heavy gas. The numerous varieties of propulsion systems offer their very own precise benefits to a vessel. Depending on the necessity and the requirement, the pleasant sort of ship propulsion system needs to be outfitted. Only then the vessel can be capable of provide its top of the line carrier capacitance.

Conclusion

There are several types of propulsion systems for ships: internal combustion engines, electric motors combined with diesel engines (hybrid), gas turbines combined with diesel engines...etc.

Each system operates differently and has its advantages and disadvantages depending on the type of cargo or ship to equip and especially depending on the type of missions (offshore crossings, cabotage, transhipment, etc.).

Kites, perfectly ecological and economical systems, are still in development but are an interesting alternative to traditional systems, and can be good backup solutions in case of failure.

© 2018 Freedom of the sea

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