Rudder Design

Forces On the Rudder

The principal force which turns the ship is set up by the wake against the forward side of the rudder. This can be caused by the motion of the vessel or by water moved to the rudder from the vessel’s propulsion system. If the vessel has no way on and the engines are stopped, then in the absence of current the rudder will have no effect on the vessel’s heading.

Rudder Attachment

On small vessels rudders are hung from the craft using a system of pintles, or projecting lugs, and gudgeons, which are sockets designed to receive the pintles.

On larger vessels the rudder is formed around a stock supported by the rudderpost. The rudderpost is controlled by an internal steering mechanism and extends through the hull of a vessel with the rudder attached to the outside portion.

Rudder Balance

• Unbalanced rudders hang completely astern of the rudderpost. This requires the greatest amount of effort to turn.
• Semi-balanced rudders have a short section of blade forward of the rudderpost.
• Balanced rudders have an equal amount of rudder section forward of the rudderpost as after the rudder post. It requires the least turning effort due to the counterweight effect. It also loses effectiveness at the smallest angle of steering as compared to the other two.

Aspect Ratio

The ratio of the height of a vessel’s rudder to its width is referred to as the aspect ratio. A high aspect ratio rudder is tall and skinny, whereas a low aspect rudder is wider but draws less water.

Contra-Guide Rudders

This style of rudder has a separate upper and lower articulating portion to help improve the flow characteristics of the propeller.

Flanking Rudders

Sometimes called backing rudders or articulating rudders, a flanking is essentially a rudder on a rudder. This allows the rudder to take greater advantage of the effects of the propeller flow on the rudders.