3.1 The Rudder
The rudder is a critical component of a merchant ship, acting as its underwater steering mechanism.
Located at the stern, behind the propeller(s), the rudder functions by deflecting the water flow, generating a lateral force that causes the ship to turn.
Positioning and Support:
The rudder is hinged to the transom, the flat vertical plate forming the ship's aft end. This hinge point is often located on a horizontal platform called the steering flat, which provides structural support for the rudder stock. The rudder stock is a vertical shaft that transmits the turning force from the ship's steering gear to the rudder blade.
Function and Shape:
The primary function of a rudder is to generate a turning moment, also known as rudder force. This force is created when the rudder blade, typically a flat, airfoil-shaped structure, is angled to one side. The angled blade disrupts the water flow passing around the hull, pushing water in the opposite direction of the turn. This creates an unbalanced force that causes the ship to rotate about its vertical axis.
Balancing the Force:
The shape and cross-section of the rudder blade play a crucial role in its effectiveness. Traditionally, rudders were un-balanced, meaning the entire blade area was positioned behind the rudder stock. This design required significant force from the steering gear to overcome the water pressure acting on the entire blade.
Modern merchant ships predominantly use balanced rudder designs, such as the spade rudder. In a spade rudder, part of the blade area is positioned forward of the rudder stock. This reduces the water pressure acting against the turning motion, making the rudder more responsive and requiring less steering effort.
Rudder Force and Propeller Diameter:
The effectiveness of the rudder force is also influenced by the propeller diameter. A larger propeller creates a stronger flow of water, which the rudder can then deflect to generate a more substantial turning force. This is a critical factor when designing rudders for larger merchant ships.
Special Rudder Designs:
While the spade rudder is the most common, some merchant ships utilize specialized rudder designs based on operational needs. Here are a few examples:
Flap Rudder: This design incorporates a movable flap on the trailing edge of the rudder blade. Adjusting the flap angle can improve maneuverability at low speeds.
Mariner Rudder: This high-performance rudder features a leading-edge bulb and a streamlined design for enhanced efficiency and reduced drag.
Fishtail Rudder: Primarily used on some fishing vessels, this rudder has a split blade resembling a fishtail, offering improved maneuverability at low speeds.