The earliest form of a rudder was simply an oar or paddle which was used to propel the stern of a ship in a sideways direction. Later the paddle was fixed to the side of the ship and given a lever at the top of the handle to act as a tiller. It is unknown for sure when the first stern post rudder was invented but was in general use in the twelfth century. An illustration is a Persian manuscript is one of the earliest picture of a sternpost rudder.
The passage of a sailing ship through the water causes water to flow past the rudder. The angle at which the rudder is inclined to the direction of flow is called the angle of attack. The steering action is dependant on the pressure distribution between the two hydrodynamic surfaces of the rudder. When a ship’s rudder is turned, the ship first moves a small direction sideways in the opposite direction to the intended turn and then moves around a circular path until it eventually faces the opposite direction.
During the turn the bow of the the sailing ship lies always inside the turning curve so that a drift angle is formed between the centerline of the ship and the tangent to the turning curve. The tactical diameter is a measure of the rudder to turn the ship and this is very important for a warship.