Manoeuvring characteristics describe how a ship responds to the helm and engine orders given from the bridge. Every ship is different, so the characteristics are measured in standard sea-trial manoeuvres and then displayed on board for the officer of the watch and the pilot. This article covers how the ship turns. Stopping distance (head reach and track reach) is a separate subject, covered in its own article. For each term, the article also explains what it is used for, so you know why it matters on the bridge.
1. Pivot Point
The pivot point is the point about which the ship moves inward or outward when she turns.
If the ship is moving ahead, the pivot point is generally ¼ of L from the bow.
If the ship is moving astern, it is ¼ of L from the stern.
At anchor, it is at the hawse pipe.
When the vessel is stopped, it lies on the midship.
Why the Pivot Point Matters
Knowing where the pivot point is lets you:
Predict the swing: the bow and the stern move in opposite directions about the pivot point, so you can tell how much room each end needs.
Place tug or thruster force well: a push is most effective far from the pivot point, as the example below shows.
Judge the ship’s response: with the pivot point well forward, the stern answers the helm and engines more quickly than the bow does, and at anchor the ship swings about the hawse pipe.
Plan berthing: a stopped ship pivots about the midship, so two equal pushes at the ends turn her without moving her bodily ahead or astern.
Because the ship turns about the pivot point, the stern swings out to the opposite side of the turn. Always allow room for the stern when turning near other ships, jetties or hazards.
The pivot point also decides where a push is most effective. A turning moment is the force multiplied by its distance from the pivot point. For a ship 200 m long moving ahead, the pivot point is 50 m from the bow. A tug pushing at the stern is 150 m from the pivot point, while one pushing at the bow is only 50 m away, so the stern push gives three times the turning moment for the same force.
2. Advance
Advance is the distance travelled along the original course, measured between the point where the helm is given and the point where the ship has changed her heading by 90° from her initial course.
Why Advance Matters
Advance tells you how far ahead the ship travels before she has turned 90°. It is used to:
Decide where to put the wheel over for a 90° alteration, so that the ship ends up on the new leg of the passage.
Check there is room ahead before a danger, a bend, a buoy or another ship, because the ship keeps moving ahead while she turns.
Judge avoiding action: a turn does not move the ship away at once, and advance shows how long she keeps closing the danger ahead.
For a 90° alteration at a waypoint, put the wheel over a distance equal to the advance before the waypoint. When her heading has changed by 90°, the ship will be on the new leg, a distance equal to the transfer past the waypoint. For other angles, use the wheel-over point calculation instead.
3. Transfer
Transfer is the transverse distance, measured at 90° to the initial course, from the original course to the point where the heading has changed by 90°.
Why Transfer Matters
Transfer tells you how far sideways the ship moves from her original track by the time she has turned 90°. It is used to:
Check clearance on the side of the turn, for example from the edge of a channel, a bank or a passing vessel.
Judge how quickly the ship leaves her original track when altering course to avoid a close-quarters situation.
Fix where she joins the new track, together with the advance, as in the waypoint example above.
4. Tactical Diameter
Tactical diameter is the distance measured transversely from the initial course to the point where the ship’s heading has changed by 180°.
Why Tactical Diameter Matters
Tactical diameter tells you how much water the ship needs to turn right round. It is used to:
Check that a channel, fairway or turning basin is wide enough to turn the ship. For example, a ship with a tactical diameter of 1000 m cannot turn round in a channel 800 m wide.
Plan a turn in confined waters or an anchorage, and work out where the ship will be when she has turned through 180°.
Compare the ship with the IMO standard, which limits it to 5 ship lengths, and compare one ship with another.
5. Drift Angle
Drift angle is the angle contained between the centreline and the tangent drawn through the turning circle.
Why Drift Angle Matters
In a turn the ship does not point along her path. Her bow points inside the turning circle and her stern swings outside it, because she is moving sideways a little as well as ahead. Knowing the drift angle is used to:
Keep the stern clear: a larger drift angle means the stern sweeps further outside the track, which matters near a jetty, a bank or a vessel on the outside of the turn.
Work out the width of water the ship really needs, because a drifting ship takes up a strip wider than her beam (see below).
Separate heading from track: the heading line on the radar or ECDIS does not show where the ship is going in a turn, so use the predicted track when checking clearance from dangers.
What Changes a Ship’s Manoeuvring Characteristics
Loading and draught: a deeper, heavier ship turns less readily.
Trim: a ship trimmed by the stern is steadier on course but turns less readily.
Shallow water: with little water under the keel the ship responds more slowly, and her turning circle increases.
Rudder angle: more rudder gives a tighter turn, up to the angle at which the rudder stalls, usually about 35°.
Wind and current: they push the ship sideways and change the track actually followed.
Propeller effects: with a fixed-pitch right-handed propeller going astern, the stern tends to swing to port.
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