Theory of Rudder on Ships

The rudder is used to steer the ship. The turning action is largely dependent on the area of the rudder. The required area of the rudder varies with different type of vessels since desired maneuvering ability differs considerably and the general ship design may imposed restriction.

In practice the rudder area is usually relative to the area of the immersed metal plane. The ratio of the depth to width of a rudder  is known as the aspect ratio and its value is generally 2. High aspect ratio is used in large vessels, where depth is not a constraint. Higher aspect ratio reduces the astern torque considerably. 

Aspect Ratio = (Depth of Rudder / Width of Rudder)

The force on the rudder depend on:

1. Area of the rudder
2. The form of rudder
3. The speed of the ship
4. The angle of helm

Rudder may be hinged on the pintles and gudgeons, or the may turn about an axle which passes down through the rudder. The weight of rudder may be taken by bearing pintles, or by a bearing at the rudder head (rudder carrier), or by a combination of both.

Types of Rudder

Balanced rudder

• When 20% to 37% of the area is forward of the turning axis there is no torque on the rudder stock at certain angles.
• At some angle of rudder, it is balanced. i.e., torque is zero, to keep rudder at that angle.
• Axis of rotation lies between 0.2 L and 0.37 L.

Semi-balanced rudder

• A rudder with a small part of its area, less than 20%, forward of the turning axis.
• At no angle rudder is balanced.
• Axis of rotation lies less than 0.2 L.

Unbalanced rudder

• A rudder with all of its area aft of the turning axis.
• At no angle rudder is balanced.
• Axis of rotation is the leading edge.

Construction of Rudder

• Modern rudders are of stream lined form and are fabricated from steel plate, the plate size being stiffen by internal webs. Where the rudder is fully fabricated, one side plate is prepared and the vertical and horizontal stiffening webs are welded to this plate.
• The other plate often called the closing plate is then welded to the internal webs from the exterior only. This may be achieved by welding, flap bars to the webs prior to fitting the closing plate, and then slot welding the plate.
• The upper face is formed into a usually horizontal flat palm, which acts as the coupling point for the rudder stock.
• A lifting hole is provided in the rudder to enable a vertical inline lift of a rudder when it is being fitted or removed. This lifting hole takes the form of a short piece of tube welded through the rudder with doubling at the side and closing plate.
• A drain hole is provided at the bottom of the rudder to check for water entry when the ship is examined in dry dock. 
• To prevent internal corrosion the interior surfaces are suitably coated, and in some cases the rudder may be filled with inert plastic foam.
• The rudder is tested when complete under a head of water 2.45 M above the top of the rudder.

Rudder Carrier Bearing on Ships

Most of the rudders are supported within the hull. The rudder carrier carries the full weight of the rudder. A rudder carrier may incorporate the watertight gland fitted at upper end of the rudder trunk. 

Why Rudder Angle Limited to 35 Degrees ?

• Beyond 35 degree rudder efficiency is reduced due to formation of eddies on the back of rudder as the flow is no longer streamlined. This is called stalled condition.
• The manoeuvrability does not increase beyond 35 degree, but rudder torque increases and ship’s turning circle increases.

Why Steering Test Rudder angle 35 degree to 30 degree ?

• So that the point at which it is reached can be exactly judged as it crosses 30 degree.
• As hunting gear puts pump stroke to zero, the rudder movement slows down progressively as it approaches 35 degree.

Why Astern Turning Moment much less than Ahead ?

• The propeller thrust adds to the force on the rudder when going ahead, but in astern that thrust is lost.
• The pivoting point (point about which ship turns) shifts aft to 1/3 rd the length from aft. This reduces turning moment greatly.

What is the Pivoting Point for Ships ?

• The ship turns about a point called pivoting point. This is situated about 1/3 rd to 1/6 th of the ship length from forward, depending on the ship design.

Why is Torque on Rudder Stock more on going Astern ?

• While moving astern, trailing edge of rudder becomes leading edge. Center of pressure from turning axis increases.
• Flow of water to rudder is unobstructed causing point of action of force to go closer to the leading edge, 0.31 times the width from leading edge.

Why Rudder is situated Aft of the Ship ?

• To make use of propeller wash for thrust.
• The pivoting point of ship is 1/6 to 1/3 rd of length of ship from bow, the greater the perpendicular distance between point of action of force and pivoting point, the better rudder movement.
• Better protected at astern from damage.
• Drag is reduced if rudder is situated aft.

Why Full Astern Power is usually Less than Full Ahead Power ?

• Propeller blade section is designed for maximum efficiency in ahead.
• In astern direction, angle of attack is high on back of blade.
• Propeller will absorb very little available power, severe eddying occurs on face. Therefore, efficiency is very low.
• Hence, if 80% of full ahead power is available for astern, then boosting it to 100% will have minimal return in thrust from propeller.

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Most ships are driven by the engine-shaft-propeller arrangement shown in Figure 1. The stern tube is a metal tube welded to the hull of the ship connecting the engine chamber and the outside of the ship. The shaft driving the propeller and later transmitting its thrust to the hull goes through the stern tube. A couple of journal bearings are placed within the stern tube, carrying the weight of the shaft and the propeller while allowing rotation of the shaft. To decrease the frictional torque on the bearings the stern tube is flooded with lubricant so the bearings operate while fully immersed in oil. Finally, to ensure the lubricant stays within the stern tube, two sets of rotary lip seals are installed at each end of the tube, namely stern tube seals. The stern tube seal is one of the largest rotary lip seals, along with the seal used in hydropower turbines and wind turbines.

Figure 1. Disposition of the stern tube oil tanks in a ship.

The function of the stern tube seals is to prevent water entering the stern tube as well as to minimize the lubricant spillage to the marine environment and engine chamber. To increase the reliability of the system, a few sealing rings are mounted in line at both ends of the stern tube conforming the aft and forward stern tube seals packages shown in Figure 1. This special type of sealing rings constitutes the only barrier between the stern tube lubricant and the environment.

The propeller of a ship is located below the sea water level, hydrostatically pressurizing the outermost sealing ring. Note that the draught of the ship varies between the loaded and unloaded situations impacting the operating conditions of the seal. Furthermore, the hydrostatic pressure at seal #1 oscillates with the sea waves [1]. To counteract the head of sea water on the outermost seal, the spaces between the stern tube seals are independently pressurized by a set of oil tanks, as shown in Figure 2. By filling each tank to a particular oil height the hydrostatic pressure at each space between seals can be set. The pressure difference over each seal differs from seal to seal according to its position (#1, #2, #3, #4 and #5 in Figure 1). The disposition of the oil tanks, together with the working pressures within the stern tube, is of relevance for the performance of the stern tube system.

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