Rudder

Rudder's are used on sailboats for steering.

What is a Rudder?
A rudder is a type of hydrofoil used on a sailboat for the purpose of steering the sailboat. When the rudder is rotated out of plan with the sailboat (thus generating an angle of attack to the water) lift is generated perpendicular to the boat, creating a moment about the keel, causing the boat to rotate.

Design Considerations
There are features that a rudder must have to be succesful:
 * low drag
 * high lift
 * rigid
 * controllable
 * mechanically stable

Drag
One boat performance limiter is the boats drag, as this limits the boats max speed and how fast it accelerates. As such, it is imperative that the amount of drag supplied by the rudder is reduced as much as possible.

The following designs will reduce drag
 * high aspect ratio, where aspect ratio is h/c (h - is height of the rudder, c - chord of the rudder)
 * elliptical planform
 * low t/c (t - max thickness)

Lift
The lift produced by the rudder should be 0 when the boat is trying to sail straight and, as such, most rudders tend to be symmetrical. As well, as stall is approached the rudder must be able to generate enough lift to overcome the inertia of the about, thus allowing it to turn rapidly. Higher lift can be created generally by increasing t/c.

Rigidity
The rudder must be flexurally rigid in order to prevent significant deflection (and thus changing the flow field and drag) three different things can be done, either the stiffness (E) of the rudder must be increased, or the second moment of inertia (I) of the rudder must be reduced, or the bending moment (M) must be reduced. E can be adjusted through using different materials, and I can be adjusted by either increasing the cross-sectional area of the rudder. Finally the moment on the rudder can be reduced by decreasing the lift or height of the rudder.

Controllability
The rudder becomes uncontrollable if the moment about the pivot of the rudder is greater than that can be generated by the rudder servo.

Mechanical stability
The rudder must be designed so it doesn't fail, either in static/dynamic loading, or by fatigue.