Saturday, August 8, 2009

Ship rudder

Every cruise ship and cargo vessel needs at least one rudder. This is usually installed behind a propeller which is located at the stern of the ship or boat. Although we cannot see it, the functions of rudder are vital for the maneuverability of the ship. A rudder, according to page 87 of Ship Design and Performance for masters and mates, is needed
1. To keep the ship on a straight line, from Port 'A' to Port 'B.' In other words, to have good course keeping properties.
2. To turn the ship in a small turning circle diameter. In other words, to have good turning characteristics, say in confined waters.
The dimensions of a rudder are determined by the coefficient K according to the type of ship, the length between perpendiculars, and the loaded draft of the ship.


The following formula is used by naval architects to calculate the rudder area of ship. AR = K x LBP x d (m2)
If a ship is a fast one, generally, its K value is lower. So, the rudder of high speed marine vehicles is smaller than conventional speed marine vehicles with the same displacement. Below is the coefficient K for several merchant ships. The above formula can also be used for ships with twin screw propellers.
We have to remember that rudder is not the only equipment that is available for ship's maneuvering. Most cruise ships and cargo vessels have bow thrusters to help them turning to the right or to the left.

Usually the height of a rudder is 1.75 its length. Please, see the following illustration as a guidance for naval architects when designing ship's rudder.

It is important to note that the rudder size should not be too large. If it is larger than appropriate, it will be an additional resistance for the ship itself as well as decreasing the course keeping properties of the ship.Rudder is turned by hydraulic mechanism that has been installed above it inside the stern part of the ship's hull. by Charles Roring Also read: Are cruise ships safe for the marine environment?