Moving Forward – How does a Ship go Ahead or Astern?


A modern day ship consists of all the luxuries which can be imagined in a top rated hotel, but still the basics of propulsion remain the same, as perhaps they were when human beings started to cross the first small stream of water perhaps in a skin boat or a coracle. Of course a lot has changed since then and nowadays modern ships do not use ores or manual means for propulsion, but depend on huge machineries for this purpose.

We all wonder how such a huge ship is able to move forward. There are different forms of propulsion. Some are by taking power directly from the engine or by means of electrical means or by means of steam turbines. Some ship can even be driven by a nuclear energy powered engine. We will learn about various types of propulsion systems used in other articles on this site.

For the time being let us focus on the topic under discussion and take a look at the forward ship movements….

How a ship moves ahead?

  • We all know that a ship is moving with the help of a propulsion device. The propulsion device may be a fixed pitch propeller, controllable pitch propeller, azimuth propeller, shrouded propeller, or a voith schneider propeller. Now lets consider the case of a simple propeller of fixed pitch.
  • Main engine or whatever may be the propulsor, when it rotates it imparts momentum to the medium (ie) to the water.
  • When a propeller is rotating, it produces thrust on water. Now the thrust from propeller is transferred to the water. Since the amount of water is enormous according to Newtons Third Law, the thrust comes back to the ship and this thrust moves the ship in ahead (or astern) direction.
  • This thrust is acting on the ship as shown in the figure 1 down below.

For fig: 1


Why do we need astern movement?

It may seem a bit strange that one asks why an astern movement is required on any means of land transport, but yes this question certainly has a relevance in context of shipping (as well as aviation). A ship normally would not move in the astern direction but here are a few situations wherein the navigation officers might be required to move the ship in astern direction and these situations can be either of the following

  • Astern movement is given to stop the ship soon. Even when the engine is stopped, due to the momentum, the ship will further move to some extent.
  • At this situation when you give astern movement, this momentum will be stopped as the propeller starts to rotate in other-way-round.
  • This is achieved by means of reversing the engine. There are different methods of reversing. They may be achieved by axial shifting of the cam shaft or by using servomotors or by shifting the cam follower rollers.

Apart from these two motions namely forward and aft, check out this article by a senior marine teacher about ship movements.