Marine Auxiliary Engine: The Complete Guide

Modern day ships are behemoths in both size and the load that they can carry. Take, for example, the Panamax cargo ships that negotiate the Panama Canal every day. These ships are 964ft x 106ft in size, with a total weight capacity of 5,000 Twenty-foot Equivalent Units (TEUs). Modern day cruise ships are about the same – 1000ft long.

Have you ever wondered how a huge floating city like this is powered? Let’s understand more about how the marine auxiliary engine of a ship helps power the ship.

What Is a Marine Auxiliary Engine?

Ships require two kinds of power:

  • Power to move the ship (Prime mover)
  • Power to run the electrical devices and appliances on board (the lights, the air conditioning, etc.)

The marine auxiliary engine is the engine that supplies the power to run the electrical devices on board the ship. 

Also Read : Components of IC Engine With Functions & Images

What Is the Purpose of an Auxiliary Engine on Ships?

As mentioned above, the auxiliary engine is the one responsible for providing power to everything on the ship except for the actual movement of the ship.

It’s essentially the life support system onboard the ship and the main source of power for everything on it. Without an auxiliary engine, the ship is essentially like a moving hunk of metal in the water, which you might not even be able to navigate properly (because the navigation system onboard runs on electricity!)

Ships can also use auxiliary engines for many purposes. For example, they provide the power to haul in the nets on a shipping boat. If the ship is a hauling ship, the auxiliary engine powers the cranes on the ship as well. 

Essentially any other equipment on the ship that’s not connected to propelling the ship is run by marine auxiliary engines.

What Is the Marine Auxiliary Made From?

A marine auxiliary engine is essentially an AC generator that works on the principle of electromagnetic field (EMF). 

Also Read : Why AC is preferred onboard ship than DC

This process is similar to the way any generator works: you will be surprised to know that even the small generator that lights up an electric scooter’s lights works with similar principles! Of course, the difference in dimensions is mind bogglingly huge. 

An electrical generator has several components, depending on the configuration used. Below are some of the common components:

  • Stator: This is a set of electrical conductors wound around an iron core in coils
  • Rotor: This is essentially a large, rotating magnet placed inside the stator
  • Field: This is nothing but coiled conductors that receive a DC input
  • Armature: This is the large coiled wiring which receives the AC output of the engine 
  • Prime Mover: This is what generates the direct current to run the auxiliary engine. The prime mover is the primary engine on a ship, which typically runs on diesel.

Apart from this, the transfer of power from the engine to the power distribution center happens in two ways:

Brushed motor

In this configuration, brushes and slip rings are used to transfer direct current to the moving rotor and take away the alternating current from the rotor.

Brushless Motor

This configuration eliminates the need for slip rings and brushes, and the prime mover is replaced by another smaller alternator instead.

Auxiliary Marine Engine Assembly

Marine engines are among the largest machines known to man, and therefore their assembly and manufacturing require special care. 

Typical Auxiliary engines are designed with very tight dimensions specifications, which is why they are increasingly being made with the help of CNC machining

Auxiliary Marine Engine Maintenance

Marine engines work in very harsh conditions at sea and incur a lot of wear and tear. Typically, ships carry a lot of spare parts to compensate for this wear and tear. Rapid prototype technology (3D printing) is increasingly being seen as a way to design on-site replacement parts, thereby reducing the ship’s load as well.

Also Read : What maintenance you will carry out on an alternator to keep in good working conditions

How Does a Marine Auxiliary Engine Work?

To generate electricity in a marine auxiliary engine, you can use two methods:

  • Either rotate the coiled wiring around a static magnetic field
  • Or else rotate a magnet inside stationary coiled wiring.

Ideally, it shouldn’t matter which of the components is moving, the coiled wiring or the magnet. However, it is more difficult to generate alternating current if the coiled wiring is moving, so the preferred route is to move the magnet instead.

The magnet rotates using a mechanical input (usually, the ship’s diesel powers this rotational movement). When the magnet rotates inside the conductors, it produces an alternating electric current in the conductors, which then powers various devices through the power distribution center of the ship.

Marine auxiliary engines are typically large enough to take up close to 15% of the entire fuel consumption of the ship and work in many configurations. An average ship would need two or three of these auxiliary engines working in a parallel configuration.

What Is the Difference Between the Main Engine and Auxiliary Engine on a Ship?

The below table summarizes the key differences between the main engine and auxiliary engine of a ship. The key difference is the use of the two engines, but there are many other differences as well.

ParticularsMain EngineAuxiliary Engine
UsagePrime Mover (Propeller)Electricity Generator
Engine Type2-Stroke Engine4-stroke Engine
TorqueHighComparatively Lower
Rotational SpeedLowHigh
CostHighComparatively Lower
Maintenance CostLowerHigher


Marine auxiliary engines are the lifeblood of a ship; they power everything except for the actual movement of the ship. We hope this guide has helped you understand how these engines generate power for a huge ship.

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