Rudder Angle Onboard Ship Limited to 35°

The maximum angle upto which the rudder of a ship can turn is limited to 35 degrees. The discussion throws light on the reasons behind it.

Before starting the discussion on this topic, would like to tell that many students have got a different perspective to this question. Though there is only one reason behind it, and that too there happens to be a technical explanation of it.So let's start.

First we need to understand a few technical terms :

1. Angle of attack : It is the angle made by the direction of incoming fluid and the plane of the rudder. See this figure for an easy explanation.

2. Critical angle of attack : Critical angle of attack is the angle of attack at which the fluid flow across the rudder SEPERATES. i.e. at critical angle the fluid flow is such that the formation of eddies behind the face of rudder increase to such an extent that instead of guiding the fluid flow, the rudder tends to stop the fluid flow. See this image for a better understanding.

The above two terms are important to understand the following concept. Give your time and rethink about angle of attack and critical angle before moving ahead.

Moving furhter, this critical angle is the core point of this topic. Depending upon the drag, viscosity, blade shape and flow characteristic of the fluid, the critical angle varies for different rudder and fluid combination.
For marine rudder and water, the critical angle happens to be around 35° . From the going, it is well understood that as the rudder reaches it's critical angle of 35°, then instead of changing direction of the vessel, it will slow down the vessel, which is undesirable.
At the end of discussion, make a note that critical angle is synonymous to stall angle. Thus if someone says that rudder reaches stall angle, it means that rudder reaches its critical angle.
Above all this discussed it has to be noted that though steering gear of ship is designed to move the rudder upto 35 degrees, mechanical stops are provided at 38 degree to provide a fail safe system in case of emergency. Also, to let the rudder angle indicator move unhindered, the calibration is given upto 40 degrees in the rudder angle indicator on deck. 

Hope the explanation would have cleared your doubts.

If left with any doubts, feel free to post your queries in reply section.