Effective maneuvering of a ship and navigating through the vast blue ocean is one of the prime objectives while sailing on a ship. That’s when rudders come into picture. A rudder basically is a sheet of material (mostly metal) which is attached to the stern of the ship, in a way that it can be moved so as to redirect the water flowing through it and ultimately making the ship turn.
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What is a Rudder?
To put it as simply as possible, Rudder is a device which is the part of a ship steering apparatus that is fixed on the aft of the hull, or the stern right behind the propeller. The movement of the rudder across the flow of the water is what creates a pressure difference on either side of the rudder, essentially making the ship turn in the desired direction.
Working mechanism of a Rudder
A rudder essentially works on the principle of unequal water pressures. Let us understand this with the help of an exemplary situation. Consider a situation where we want to turn the Ship towards the port side. So, we make the Rudder turn towards the port side. This would cause more water to be deflected towards the port side and hence create a high pressure on port side and a low pressure on the starboard side. This difference in pressure would cause the stern of the ship to move in the direction having the lower pressure i.e., starboard side. This in turn would make the head of the ship to turn towards the direction of port side, ultimately causing the ship to move in the port direction.
Positioning and Design
A rudder is most popularly positioned in the stern part of the ship, right behind the propeller, for the most efficient maneuvering of the vessel. Rudder is useless if there is no water to flow around it. Hence positioning it right behind the propeller ensures that there is flow of water whenever it is required. It is ideal for a ship to have a rudder behind every propeller; therefore, a ship might have as many rudders as there are propellers.
Based on the various functionalities a rudder might serve on a ship, there are various designs of rudders which we will discuss.
Types of Rudders
Depending on the usability and the size of the ship, different shapes and arrangements of rudders are to be employed in various types of ships.
a) Balanced Rudder
In this type, the rudder plate is attached with the rudder stock from the top only. About 40% of the Rudder plate is on the forward and the rest is at the aft side. This type of rudder is mostly used in smaller vessels, ferries etc. because the torque generated by this type of rudder arrangement is not enough for a heavy vessel.
b) Unbalanced Rudder
In this type of arrangement, the rudder stock runs along the chord length of the rudder. The rudder plate is completely on the aft side and is hung on the stern post. This type of rudder arrangement is typically seen in most heavy ships. But nowadays these are replaced by Semi-balanced rudders for their wide range of capabilities.
c) Semi-Balanced Rudder
As the name suggests, this type of arrangement makes use of both balanced and semi balanced types of rudders. The top portion of the rudder is again hung to the stern post. The lower half of the rudder however, is attached in the same manner as the balanced rudder, i.e., 40% of the plate sticks to the forward side and the rest is on the aft side.
Apart from the above mentioned basic and/or traditional forms of rudders, the following are the more advanced and modern forms of rudders which are more efficient in maneuvering and navigating the ship.
d) Schilling/Fishtail Rudder
Now the pivotal factor about this type of rudder, is not in how it operates, but in the design of the rudder plate itself. The rudder is designed in a way that the tip of the rudder is of the shape of a fishtail. This shape helps in accelerating the water more and thereby improving maneuverability.
e) Flap Rudder
These special types of rudders have a unique type of design in which there is a trailing edge flap at the very tip of the rudder which can be mechanically operated. This provides a variable flap angle, thereby increasing the maximum lift and thereby making the ship more maneuverable at slower speeds.
f) Pleuger Rudder
Arguably the most unique and ingenious type of rudder systems that are used in the marine industry these days are Pleuger Rudder. As it is pretty evident by now, the navigation of a ship is majorly dependent upon the speed of water flowing across the rudder blade. Hence, the traditional rudders work best when the ship is moving at higher speeds. But it is not always feasible to operate the ship at those kinds of speeds. Therefore, Pleuger rudders employ a smaller auxiliary rotor/impeller, housed in the apparatus of the rudder, in order to increase the speed of water flow around the rudders and ultimately providing with the better steering, even at slower speeds.
The process of designing a rudder for every particular ship is an elaborate and tedious task. Ship designers have to keep all the intricate details like size, purpose etc. of the ship in order to come up with the most ideal and efficient type of rudder design for every ship. From making preliminary designs of the ship, to actually doing several test runs.