Steering
Steering a ship is one of the basic skills that you should acquire as a navigating officer. Within the time that you are serving you are required to obtain adequate experience of steering a ship at various stages and develop the skills.
The aim of this blog is to explain the theory of steering a ship and explains the mechanism involved on modern ships. The young cadet shall carry out actual steering practice as an application of this module.
Vessel with headway may be steered in a particular direction by appropriate movements of rudder about the fore aft line of the ship. Big ships have massive rudders, which are turned using hydraulic or Electro -hydraulic power. The ship is steered by turning the rudder at an angle. When the vessel is moving ahead it leaves behind a stream of water.
The rudder creates an obstruction to the stream. The obstruction creates an action of throwing the stream away from the rudder. This action must also have a reaction and this reaction moves the stern in the opposite direction.
This action and reaction has to be very large to swing the ship around. Hence the rudders are provided with a large area to create the thrust.
The sternpost rudder appeared about 1300 AD. Before this invention, ships were steered by large oars near the stern. These steering oars were awkward and often broke during storms. The use of steering oars also limited the size of many ships and how far they could sail from land. The rudder could be used to steer larger ships. It also allowed the ships to sail safely in rough seas. The sternpost rudder, combined with other inventions in shipbuilding and navigation, helped make possible the great voyages that led to the exploration of the New World.
Follow up steering:
Steering system of a road vehicle is a suitable example. Forward tyres follow the position of steering wheel. If the wheel is turned to right, tyres turn to the right. If the wheel is brought back to middle position, the tyres also follow the command & do not remain in the turned direction. Thus in the follow up steering system the rudder faithfully maintains the position to which the steering wheel is turned.
Non follow-up steering:
On some ships two spring-loaded knobs may be provided instead of a wheel. One knob is responsible to turn the rudder clockwise & other one to turn the rudder anticlockwise as seen from top. Unlike the steering wheel the rudder does not return to middle position upon releasing the knobs. If you press green knob or the knob on the right hand side, the rudder is carried to starboard. The moment you release it the rudder stops in that position. To bring the rudder to smaller starboard angle or to middle position, the other knob (Red) is pressed.
Pivot point
Ships rotational moment is about a vertical axis situated along the length of a ship. Invariably steering is affected by the position of the Pivot point. The position of this axis is influenced by:
(a) Shape of ships hull
(b) Direction and velocity of ships motion.
(c) Point of impact and
(d) The magnitude of the forces acting on ship.
Broadly, the axis moves about with a change of ship’s motion — direction and with a change of forces that affect the ship.
Thus, this Rotational Axis (Z axis) changes its position with changes in ships motion and the resistance experienced. This axis if seen from top of the ship is the Pivot Point — not a fixed but a wandering point. (Peripatetic Point).
Let us see how a Pivot Point moves with “circumstances” and its effects on “Turning lever.
Position of Pivot point on a straight course
In the case of a loaded ship stationary in water, on even keel with the center of gravity almost at mid length the Pivot Point is very close to the centre of gravity
Effects of headway or sternway and change in motion.
The following aspects determine the position of the Pivot Point:
Frictional Resistance-
When moving from position of rest in water to ahead or astern, propulsion power overcomes ship’s inertia and overcomes the frictional Resistance. Even though this resistance is felt all along the ship’s sides equally (and though it is taken into account for determining propulsion power), it is not considered for determining the position of Pivot Point.
Residual Resistance
Mainly longitudinal resistance. This forms an important, component in determining the position of Pivot Point and there is relationship between the position of Pivot Point and the ratio of the longitudinal resistance to the propulsion force and the direction of travel.
Lateral Resistance
Though this does not come into play for straight-line travel. As the component of ship’s transverse thrust is small and is overcome as soon as ship gathers momentum. It however, affects the position of the Pivot Point when ship starts turning under the action of rudder and propeller.
A ship underway under the effect of the propeller (after inertia has been overcome and before the, longitudinal resistance is felt) has its pivot point pushed in the direction of travel-forward or aft and this new position is temporarily 1/8 L from bow or stern as the case may be depending on the direction of ships movement (or direction of propeller rotation).
This temporary position of pivot point well ahead, gives a good turning lever and is used for kick-start manoeuvres
When ship starts moving through water and as soon as the longitudinal resistance is felt at the fore (or after) part of the ship, the Pivot Point moves in the direction of the force (resistance) i.e aft when ship is moving ahead or ahead when ship is moving astern. At constant speed, pivot point settles about ¼ L from the bow when ship is making a headway or ¼ L from the stern when ship is making sternway.
If at the same time, the ship also has lateral (side ways) moment, this can affect the position at the pivot point due to lateral forces and lateral resistance. This is important particularly when the ship is turning
Position of Pivot Point When Turning
The centre of the rotational motion of turning ship depends upon length to beam ratio of the vessel. It is generally assumed that the pivot point on a ship under headway and turning under rudder lies about 1/3 L from forward due to Lateral pushing back the pivot point further (from 1/4 L at constant speed).
| L/B Ratio | 9 | 8 | 7 | 6 | 5 |
|---|---|---|---|---|---|
|
Pivot Point position from Bow |
0.33L |
0.34L |
0.35L |
0.37L |
0.4L |
Position of pivot point when turning with propeller and rudder.
Lateral Resistance
As the ship starts turning she slides sideways through the water, both initially and during the turn and meets water resistance all along the shipside towards which the stem is turning. This also reduces rudder force. This is the lateral resistance when turning.
Ship starting and continuing turn at constant speed —
Rudder force and lateral resistance achieve balance in a turn at constant RPM. Thus, turning circle areas at slow, half or full RPM are comparable. Only thing that differs is time taken to complete the turn and therefore the rate of turn.
Speed loss during the turn
Speed during a turn always suffers a marked reduction because during a turn, ship is moving ahead and sideways so she experiences resistance on the side, which acts as a brake. Speed reduction may be as much as 30% to 50%. This fact may be used for speed reduction (by rudder cycling or turning full circle) if sufficient manoeuvring area is available.
Case in which a ship is stopped dead in water and commences a turn with engines ahead. (No other environmental factors affect). Ship starts turn with rudder hard over and with engines ahead (either slow, halt or full).
This rudder and engine action will attempt to turn the ship as well as propel it ahead and in doing so:
i) Forward moment is resisted because of inertia.
ii) Pivot point moves ahead about 1/8 L from the bow because of the propeller thrust (force) —
This gives good lever for turning movement to start before the ship gathers forward momentum or just as the ship starts making headway. Note the pivot point is still not ahead.
► As the ship moves ahead after overcoming inertia, the water resistance on the bow eventually balances the forward propulsion force at a steady speed and the pivot point shifts aft to a position 1/4 from the bow.
► At a steady speed, while turning, the lateral resistance (at the bow on the side Mich the ship is turning and the stern in opposite direction) also comes into play pushing the pivot point further aft to about 1/3 L from the bow. Because of this, turning lever is reduced and rudder force becomes less efficient.