Maneuvering is one of the critical aspects of any vessel. It is defined as the capability of a ship to change its course or heading from its previous trajectory. Any ship must be able to turn or change its directional sense as and when required, according to Marine Insight.
Understanding The Requirements
The requirements can be:
- Changing its course or heading from time to time.
- Changing its direction of voyage or route due to weather, uncongenial sea conditions, or internal reasons about the ship itself
- Maintaining a desired course or trajectory.
- Sailing in meandering courses like rivers, channels, canals, etc.
- Avoiding obstacles like landmasses, bergs, reefs, offshore structures, and other vessels
- Circling some point like a port or terminal or an island due to unavailability of berths, tidal conditions, rough sea or weather conditions, or marine traffic.
After the vessel is launched, maneuvering trials take place as a part of the sea trials and help assess the vessel’s maneuvering ability and performance under different modes of operation. These maneuvering trials are based on the plausible maneuvers the ship must undergo during its lifetime under different situations it may encounter.
As per the guidelines for maneuvering trials from the MSC 76 codes of IMO, all sea-going vessels above 100 meters in length are required to undergo these maneuvering trials. Irrespective of length, all gas and chemical tankers must undergo them after launching and before delivery to the client.
All planning and high-speed crafts are exempted from the requirements of maneuvering trials as they have entirely different hydrodynamics of motion. Some common maneuvering trials are turning circle, zig-zag, spiral, reverse-spiral, and full astern stopping tests.
What is a Turning Circle?
Imagine driving your car on empty, flat ground. Slowly start turning the steering wheel and keep it fixated at a certain position. The car turns in the direction where the wheel is turned and starts making a circle of radius.
Or even simpler, start running on a football field or an open ground. Start turning towards a side. If you do not turn forward again, you tend to keep going in circles about the same point, isn’t it? That’s the simple law of nature: any finite object constantly tending to turn towards a particular side makes a circular trajectory!
But from the simpleton laws of nature again, the smallest circle traced by any object or body is directly related to the size of the body. In other words, the minimum radius or diameter of the circle traced by a turning body increases with size because this depends on the locus of the centroid of the moving body. From common sense, the smallest circle traced by you running on a field will be far smaller than a constantly turning SUV!
As obvious, a small boat encountering an obstruction shall be able to evade the same much more quickly than a bulk carrier. In a technical sense, the turning circle of a ship is the locus traced by the vessel’s pivot point while applying a definite turning moment towards a particular side.
This turning moment, as we know, is caused by the application of a rudder force or any other turning mechanism it has. So, when a certain angle turns the rudder to a particular side, it exerts a moment that manifests itself in causing the vessel to turn in that same direction.
Stages Of Turning
What are the stages of turning?
- When the rudder is applied at a particular, it creates a rudder moment which causes the vessel to turn in the direction in which the rudder is applied. The interplay of various hydrodynamic phenomena defines the physics of turning. The altered pressure patterns on the hull cause angular acceleration for turning.
- After a certain point, the vessel makes a 90-degree or right angle with the original heading direction.
- A balance of all forces, moments, and pressures leads to all unbalanced accelerations becoming zero and the turning attaining a steady state. Now, at the geometric center of the circle, it is about to trace a centrifugal force in action.
- The vessel starts moving in a circle of constant radius. The ship will continue to turn in the same steady state if no rudder moment is applied.
Factors Affecting The Turning Circle
Now, let us explore the factors influencing the vessel’s turning circle considering a fixed turning moment.
- Size and extent of the vessel
- Hull-form
- Draft and trim the vessel
- Available depth
- Propulsion and machinery
- Rudder moment applied
- Displacement and cargo distribution
- Speed
- External forces and wind conditions
Once again, as we know, the greater the size, the larger the turning circle and vice-versa. However, the hull form also has a vital role to play. The finer the underwater hull form, the greater the turning circle. So, a container ship or frigate will subtend a larger circle while turning compared to a bulk carrier of the same length at the same speed, rudder moment, and sea conditions.
The depth of the waters and the vessel’s draft also play a crucial role in the resultant turning moment of a ship. Due to the lesser clearance between the bottom of the vessel and the river or seabed for shallow waters, the flow patterns and the entire hydrodynamics are affected. Because of reduced underwater clearance, there is a pressure buildup leading to higher resistance values. Moreover, wave patterns are created at the fore and aft regions. Furthermore, there is a dramatic reduction in speed.
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Source: MarineInsight
