The rudder evolved from a steering board that was used in ancient times. The steering board was usually mounted on the right-hand side to suit right-handed sailors.
With the advancement of technology and enhancement in ships’ designs, the steering board moved onto the centre line through a stock passing through the vessel. A tiller was then attached to the stock allowing sailors to control the rudder from the main deck. The basic setup continued right up until modern times.
The above picture shows the rudder of the Olympic, and it is evident from the picture that it carries the same characteristics as the original steering board. It’s a small flat board mounted on a stock passing through the ship. Let’s understand how this works.
In the board type rudder, that is a simple rudder attached to the hull, as shown in the above picture, if we keep the rudder amidships, the water flows evenly around the hull-rudder combination, and this means that there’s no turning force generated, so the hull should move in a straight line.
If this board type rudder is turned to one side, let’s say to port side, it directs the water in a different direction. The water is directed at an angle away from the boat. The extra water increases the pressure exerted on one side and decreases the pressure on other side as shown in above image. This pressure difference pushes the stern in the direction (shown by pink arrow in the above image) inducing the desired turn to port. As the speed of the water increases, the effectiveness of the rudder also increases. This is why the rudder is more responsive at higher speeds.
Now what if we change the shape of the rudder slightly? Let’s make it resemble the wing of an aircraft. Aircraft wings generate lift by forcing air to flow quicker across the curved top surface of the wing. This sucks the aircraft into the air and is far more efficient than just using a flat wing at an angle. If we apply this principle to a rudder, it gives us a new shape called airfoil shape.
Again, when the rudder is amidships there is no deflection in water flow, so the boat will move in a straight line. If the rudder is turned as shown in the above image, again the water will be deflected off to one side, this time however, the shape of the rudder forces the water to run over a curved path. The water on one side (refer above image) now has to flow faster to flow around the rudder. The water on the other side conversely has to flow slightly slower. This speed variation adds to the pressure pressure difference generated by deflection alone that we discussed before. The side with the slower water flow will have a higher pressure than it was for the flat board rudder, similarly the side with the faster water flow will have even lower pressure than it was for the flat board rudder.
All this means that for a given speed of water, the curved airfoil shaped rudder will turn a boat more efficiently than a flat rudder.
What if we modify the shape of the rudder further? This time we are going to add an additional flare at the end, we call this a fishtail or a schilling rudder. They are used on lot of ships to improve manoeuvrability at slower speeds.
Again, when the rudder is amidships there is no deflection in the water flow. This time when we turn the rudder, there will be additional water deflection. The initial deflection will be same as it was with the flat board rudder, there will be flow speed differential same as it was with the airfoil shape rudder and now there will be an additional deflection created by the flare at the tail of the rudder. This reduces the wasted water flow that was previously flowing around the edge of the trailing edge. This combination acts to further increase the efficiency of the rudder. These sort of rudders are more effective at slow speeds, making them particularly useful for slow speed ship handling.
The final type of rudder we are going to talk about is an active rudder. If we go back to the airfoil shape and break it near the tip, and the tip is then linked to the main body of the rudder by mechanical linkage that forces it to turn further than the main rudder. For example, if the rudder is turned to 10 degrees, the mechanical linkage will turn the tip further 10 degrees. This applies throughout the whole range of movement of the rudder. At 35 degrees, the tip would be 35 degrees further which is 70 degrees from the direction of movement, we call this type of rudder a ‘flap rudder.’
When we look at the water flow diagram for a flap rudder, again no deflection force is generated when the rudder is amidships. When we turn the rudder, we have the same change in water flow as before. Much like a schilling rudder, the flap rudder generates that additional increase at the tip. This time however the increase at the tip continues to increase even further, the more you turn the rudder. When the rudder is hard over, the tip is practically directing water sideways, this makes the flap rudder one of the best options for very slow speed ship handling.
With all of the rudders that we have talked about, we saw that the water flow is needed for them to work at all. On a sailing boat the boat needs to be moving for the rudder to have any effect. On a motor boat, or on ships powered by engines, there are two options, either the ship needs to be moving through the water or the propeller needs to be turning, pushing water across the rudder.
If you are trying to manoeuvre at slow speeds, you don’t want to have the engine running all the time as you are going to pick up the speed, you can use small bursts from the engine to generate the same effect. You move the rudder over, give a kick ahead, you will get the same turning effect while minimizing the build-up of speed.
Here is a detailed video explaining this interesting subject: