Q.Why to put rudder on midship before giving an astern movement?

Indeed a very good question.  To answer this, I would first highlight the fact that putting the rudder on midship when engine is running astern is a practice rather than a rule.

There is no rule that we have to keep the rudder on midship when giving astern movement on the engine.

But why this is considered to be a good practice ? There are few reasons. Let us discuss each one of these.

1. Propeller Cavitation

The ship is designed for moving in forward direction. The shape and curvature of the bow is designed in such a way that it cuts through the water smoothly when moving in forward direction. The water resistance is kept to the minimum possible for the ship's motion in forward direction.

But that is not the case with the stern of the ship. Stern of the ship is usually broad. This means that when the propeller is turning in opposite direction, the water flow gets resistance from the stern of the ship.

Because of this, we experiance vibrations when the ship's engines are running in the astern direction.

This is even more prominent during astern movement with ship still having headway forward.

Now let us say that we keep the rudder to hard to starboard when the engines are running astern.

In this case we are restricting the flow of the water to the propeller. This would create more vibrations and can also cause propeller cavitation. 

Regular instances of propeller cavitation can cause some damage the propeller.

While ship's hull and machinery is designed to withstand the usual vibrations but i It is the duty of the prudent navigator to keep these vibrations to the minimum.

2. Shift of Pivot point

When the ship's engines are running astern, the pivot point of the ship moves to 1/4th of the ship's length from the stern of the vessel.

The turning lever in this case is too small for the rudder to actually turn the vessel.

This makes the rudder of the ship virtually ineffective when the engines are running astern.

Even though small, there is still some turning lever when the ship's engines are running astern.

But considering we are subjecting the ship to more vibrations and the possibility of propeller cavitation for generating a small amount of turning lever, it is prudent to keep the rudder at midship.

3. We need more astern power-quickly 

Finally, why we are giving astern movement ? Most of the times it is because we either want to quickly stop the ship or quickly reduce the ship's speed.

Quickly is the keyword in this.

If we do not keep the rudder on midship, we are restricting the flow of water to the propeller. This reduces the stern thrust generated by the propeller and the reduction of the speed would comparetively be slow.

Considering our main aim of giving the astern movement is to slow down or stop the ship quickly, we need to keep the rudder on midship. 

Do you have any other reasoning for this ? If yes, Please share !!!