Understanding a Turning Circle in Ship
Turning Circle is a fundamental performance characteristic of any vessel. It refers to the vessel’s ability to change course or direction, deviating from its intended route. Every vessel must be able to change course when necessary. These requirements include:
- Changing course or direction from time to time.
- Changing course or direction due to weather, adverse sea conditions, or internal vessel factors.
- Maintaining the desired course or route.
- Navigating in winding waterways (such as rivers, canals, and waterways).
- Avoiding obstacles such as land, rocky mountains, coral reefs, marine structures, and other vessels.
- Detouring around berths, docks, or islands due to insufficient berths, tidal conditions, rough sea conditions, weather, or heavy maritime traffic.
Turning Radius
After a vessel is launched, manoeuvring trials are part of sea trials. These trials help evaluate the vessel’s manoeuvrability and performance under various operating conditions. These trials are based on reasonable Turning Circle actions that the vessel must perform under different conditions during its service life.
According to the Turning Circle test guidelines in the International Maritime Organisation (IMO) Code 76 (MSC), all ocean-going vessels exceeding 100 meters in length must undergo these tests. Regardless of length, all liquefied gas and chemical transport vessels must undergo these tests after launch and before delivery to the customer.
All planned high-speed vessels are exempt from Turning Circle tests due to their significantly different hydrodynamic characteristics. Standard Turning Circle tests include turning, zigzagging, spiralling, reverse spiralling, and stopping at full speed in reverse.
As mentioned above, sea trials must meet the following conditions:
- Deep water, unrestricted waters,
- Calm winds, undisturbed seas, and normal sea conditions. Furthermore, there must be no random tidal fluctuations.
- Fully loaded and in a balanced cruising state.
- Constant port entry speed, typically the vessel’s design speed. According to the IMO’s internal guidelines in Annex 6, the vessel under test must maintain approximately 90% of its design speed, and the main engine power or propulsion force must reach at least 85% of its maximum rated power in all practical applications.
- Ideally, the test waters should be open and free of any maritime traffic or other obstructions.
Unlike drag and propulsion tests, which are primarily conducted in pools or tunnels on scaled-down models and do not require full-scale testing, Turning Circle tests are different.
According to IMO guidelines, even if vessels of the above categories have undergone typical scale tests, full-scale tests must be conducted once they are fully ready. Furthermore, the results of typical and full-scale tests must be entirely consistent, with only minor differences allowed within acceptable limits. In other words, for most vessels, typical Turning Circle tests are redundant. However, the International Maritime Committee (ITTC) has developed specific guidelines for model scale tests.
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What is a rotating circle?
Imagine you are driving a car on a flat, open field. First, hold the steering wheel in a specific position, then slowly turn it. The car will rotate in the direction the steering wheel is turned and begin to draw a circle with a particular radius.
Or, simply put, run on a football field or open ground. First, turn to one side. If you don’t turn back to your original position immediately, you’ll tend to keep spinning around the same point. This is a simple law of nature: any finite object that keeps rotating to one side will draw a circle.
However, according to this simple law of nature, the smallest circle any object can draw is directly related to its size. In other words, the smallest radius or diameter of the circle drawn by a rotating object increases with its size because this depends on the center of gravity of the moving object. Obviously, the smallest circle you draw while running on a field will be much smaller than the circle drawn by a constantly spinning SUV!
For ships, the turning radius measures their steering ability, that is, the radius of the smallest circle the ship can draw when a constant torque is applied. Simply put, the turning radius determines how easily and how quickly a ship can avoid or overtake obstacles.
Smaller boats can avoid obstacles much faster than bulk carriers. Technically speaking, a ship’s turning radius refers to the center of the circle drawn by the ship’s pivot point when a specific torque is applied to one side of the boat.
As is well known, this torque is generated by the force applied by the rudder or other rotating mechanisms. Therefore, when the rudder turns a certain angle in a specific direction, a torque is generated, forcing the ship to rotate in the same direction.
What are the stages of rotation?
When the rudder turns at a certain angle, a torque is generated, forcing the ship to rotate in the direction the rudder is turning. The interaction of various hydrodynamic phenomena determines the physical characteristics of rotation. Pressure changes on the hull generate the angular acceleration required for rotation.
After passing a certain point, the ship forms a right angle (90 degrees) with its original trajectory.
All forces, torques, and pressures reach equilibrium, thus cancelling out all unbalanced accelerations and bringing the rotation to a state of equilibrium. At this point, centrifugal force begins to exert its influence at the geometric center of the circle.
The ship begins to move in a circular path of constant radius. If no torque is applied, the ship will continue to maintain equilibrium and rotate.
Factors Affecting Turning Radius
Now, let’s explore the factors affecting the turning radius of a ship under the condition of constant torque.
- Ship Dimensions and Length
- Hoop Shape
- Draft and Inclination
- Available Depth
- Propulsion System and Machinery
- Applied Torque
- Displacement and Load Distribution
- Speed
- External Forces and Wind Conditions
It is well known that larger dimensions generally result in a larger turning radius, and vice versa. However, hull shape also plays a crucial role. The narrower the underwater hull, the larger the turning radius. Therefore, container ships or frigates have a larger turning radius compared to bulk carriers of the same length, speed, torque, and sea conditions.
Water depth and ship draft play a vital role in the final torque generated. In shallow waters, the shorter distance between the hull and the riverbed or seabed affects flow patterns and overall hydrodynamic characteristics. Reduced water depth leads to pressure buildup, resulting in greater drag. Furthermore, bulges appear in the bow and stern areas. And, speed is significantly reduced.
All these factors work together to increase the force coefficient, requiring greater forces to turn, thus increasing the turning radius. Similarly, the deeper the draft, the larger the turning radius. Observations show that when a ship tilts, its turning radius increases significantly; conversely, when a ship pitches, its turning radius decreases.
Basic physics can explain this. Like most ships, due to its hull shape, the pivot point (or geometric center of gravity) is located aft of the ship’s center of gravity. For the bow, the draft at this point is greater than that of a standard bow. It is well known that the turning radius of any ship is directly proportional to its draft.
Displacement and speed also significantly affect a ship’s turning radius. This is based on the fundamental principle of Newton’s law of inertia: the more vigorous the motion and the greater the mass, the stronger the tendency of an object to maintain its original shape, meaning that rotating it requires a greater force, and therefore a larger turning radius.
Finally, as expected, the turning radius or tendency to rotate is also affected by external conditions. In adverse sea and weather conditions, the force required for a ship to rotate increases significantly due to increased hydrodynamic forces, pressure, and wind. For ships with larger superstructures, the increased surface area also increases wind resistance, adversely affecting steering torque. Therefore, the turning radius increases again.
