The ability of a ship to turn tightly and come exactly on the new intended track after altering course is an important issue in passage planning because of safety considerations.
Normally it takes some time for a ship to respond when the wheel is turned, because of the ship’s inertia. As a result, the wheel needs to be spun at a point before where the ship needs to turn. This point is referred to as “the wheel over the position.
The wheel-over position is therefore the location at which a ship needs to commence a turn in order to come on the desired new track safely. It is essential for a vessel to determine her wheel-over position which is dependent on her turning radius before making a turn in order to avoid getting off her intended new track.
A large vessel is unable to turn around at a single point. Therefore, taking into consideration her ability to turn within a constrained space is something that should not be overlooked. It is essential for a vessel to determine its wheel-over position before making a turn. This article will explain what the wheel over position is, how it is affected by the ships turning radius, and its impact on a ship’s maneuverability.
How To Determine The Wheel-Over Point.
The wheel-over point is the location at which a ship needs to commence a turn in order to come on the desired new track safely. In narrow channels or confined waters determining the wheel over point becomes a critical part of the passage plan if tolerance for cross-track error is minimum.
There are a few different factors that go into determining the wheel-over point, such as the size – deadweight – of the ship, the depth of water, and the speed of the ship. The speed of the ship is probably the most important factor, as it determines how much inertia has to be overcome before the ship begins to turn.
- Current course: 090 deg T
- Next course: 045 deg T
- Speed Over the Ground (SOG): 12 knots
- Turning Radius ( TR = SOG/ROT) = 1.0 nm
- Length Over-All (LOA): 235 meters
- Point-of-overcoming the inertia (POI): 1.5 x LOA = 352.5 meters or 0.2 meters. Also known as the execution point wherein the ship starts to turn. ( Such info is posted in the wheelhouse)
- Lay off the 2 courses on the chart with two (2) parallel index lines drawn inside the turn at a distance of 1’ ( The 1’ corresponds to the turning radius. (TR = SOG/ROT) = 1.0 nm).
- Using a pair of compasses on the datum, draw an arc to serve as a turn onto your course with a radius of 1’ such as illustrated below.
- Applying the point of overcoming the inertia backward, the wheel-over position or point is approximately 0.2 nm from where the turn is tangent with the initial course.
By drawing a line parallel to the final course through the wheel over the position a wheel over the line is created. This line indicates the points of wheel-over when the vessel is not exactly on her course line during the turn or alteration of course. (figure below)
What is The Turning Radius of a Ship?
When a vessel turns under a continuous full helm through 360 degrees, its pivot point will follow a roughly circular track called a turning circle. The radius of this circle is the turning radius of the ship. It is affected by the length of the ship, the speed at which the ship is moving, and the depth of the water.
The general rule is that the turning circle will be larger when the ship is longer. The final diameter will be determined by the kind of rudder and the steering effect that it produces, with the distance between the rudder and the hull having a significant impact on the decision. The turning action is more efficient when there is a smaller clearance between the rudder and the hull.
The following factors will thus affect the rate of turn and therefore the size of the turning circle and turning radius:
- Structural design and length of the ship.
- Draught and trim of the ship.
- Size and motive power of the main machinery.
- Distribution and stowage of cargo.
- Even keel or carrying a list.
- Position of turning in relation to the available depth of water.
- Amount of rudder angle required to complete the turn.
Why is The Wheel Over Point and Turning Radius of ships Important to Consider?
The wheel over point (WOP) and turning radius of a ship are important to consider because they affect the ship’s maneuverability and therefore how the ship handles different situations.
For example, a ship with a large turning radius will have difficulty maneuvering in tight spaces. Conversely, a ship with a small turning radius will be more agile but may be less stable in rough seas.
The wheel over points when calculated and drawn on the charts enable the watchkeeper to know at each waypoint the value of the parameters he/she needs to maintain in order to properly negotiate the course change.
For example, because the officer on watch (OOW) knows the Rate Of Turn (ROT) equals the speed of the ship divided by the radius of turn he/she will know what parameters to use in order to negotiate the turn so as to stay on the correct arc.
With an ECDIS it will then be possible to input those parameters and even if there is a fixed object near the wheel over the position from which radar bearing/range or the visual bearing can be taken, enable the wheel over position to be defined as indicated in the accompanying figures.
Under each waypoint, enter the value of “Turn Rad” and “speed” to create the arc for the alteration of course for each leg in the route.
In order to avoid a hazard or danger during the course of a voyage, especially when transiting a traffic lane, following a traffic separation lane, or passing a shoreline or rock, the ship needs to be kept on the track planned. The cross-track distance should either be zero or kept to a minimum because there is less available sea room.
Determination of the wheel over position coupled with the use of the ship’s turning radius is a technique used as a means to ensure the ship stays on the desired track during and after the course alteration so as to minimize the cross-track distance and keep the vessel at a safe distance from prospective hazards.
FAQs about Wheel Over Point And Turning Radius
What is the wheel-over position of a ship and why is it important?
The wheel-over position is the location at which a ship needs to commence a turn in order to come on the desired new track safely. It is important because it ensures that the ship can navigate the turn without deviating from its intended course and helps avoid getting off the intended track, thus ensuring safety during passage planning.
How can the wheel-over point be determined?
The wheel-over point can be determined by taking into consideration several factors such as the size of the ship, the depth of water, and the speed of the ship. The speed of the ship is particularly important as it determines the amount of inertia that needs to be overcome before the ship can start turning. By applying specific calculations and measurements, such as the ship’s turning radius and the point of overcoming the inertia, the wheel-over position can be determined accurately.
What is the turning radius of a ship and what factors affect it?
The turning radius of a ship refers to the radius of the circular track that the ship’s pivot point follows when it turns under a continuous full helm through 360 degrees. The turning radius is influenced by factors such as the length of the ship, its speed, and the depth of the water. Other factors that affect the rate of turn and the size of the turning circle include the ship’s structural design, draught and trim, main machinery power, cargo distribution, keel position, and the available depth of water.
Why are the wheel-over point and turning radius important for a ship’s maneuverability?
The wheel-over point and turning radius of a ship are crucial for its maneuverability as they determine how the ship can navigate turns and handle different situations. A ship with a larger turning radius will have more difficulty maneuvering in confined spaces, while a ship with a smaller turning radius will be more agile but potentially less stable in rough seas. By considering the wheel-over point and turning radius, ship operators can plan and execute course changes more effectively, ensuring the vessel stays on the desired track and minimizing the risk of accidents or hazards.
The wheel-over point and turning radius play a significant role in course alterations and navigation. By accurately determining the wheel-over point and considering the ship’s turning radius, the watchkeeper or officer on watch can effectively plan and execute course changes. This knowledge helps them maintain the necessary parameters, such as the speed and rate of turn, to negotiate turns and stay on the correct arc. Additionally, modern electronic chart display and information systems (ECDIS) allow for the input of turning radius and speed, facilitating the visualization of the wheel-over position and aiding navigation during course alterations.