The Rudder

A rudder is used to steer a boat effectively inducing minimal drag. This might sound odd at first but what you must realize is that all rudders are either a complete wing* or form a part of one. This form of steering is so effective that aircraft designers borrowed the idear and the name with it.**

There are two basic types of rudders, spade rudders and hung rudders. The former is a relatively recent development (1960s) and is a favorite in the racing community. With a spade rudder the angle (of attack) of the rudder is altered increasing deflection generated force and causing a low pressure zone to form on the lee side of the rudder. The force generated is very large and can turn even the largest vessels quickly and with minimal drag. There are three downsides to spade rudders, they are much weaker than skeg rudders because they are only attached by the means of bearings. Harder to build and it is easier for a novice to "stall the rudder"***.

The second type of rudders split into three types, skeg hung rudders, keel hung rudders and transom hung rudders although in many cases these are mixed. Rudders connected only to the transom are now virtualy extinct, replaced by skeg-transom rudders with superior strength and hydrodynamics, today almost all boats have spade/skeg/transom-skeg rudders.

Balance is a very important design feature of rudders, the balance is used to describe the tendancy of a rudder to rest in a neutral position. All boats (at least those still floating ;D) have "positive balance", this means that the rudder will lie in a neutral position. All transom and keel hung rudders are 100% positive/aft balance while spade rudders are commonly semi-balanced which reduces the force required to move the rudder.

While for a racer the spade rudder is clearly superior its usefullness is debatable among cruisers, it does result in reduced wetted area but this is offset by the risk of loosing the rudder while underway.

For more information on keel and rudder design here is an article written by Ted Brewer. | Article

* Although the term wing is mostly used for aerodynamic objects it applies equally to hydrodynamic objects.

** The rudder is always used in conjunction with the ailerons and elevator which are also forms of rudders!

*** A stall is when the angle of attack is raised beyond the tolerance of the hydrofoil (rudder curvature vs. length), this causes massive drag and an almost complete loss of steering.