Turning Circle of a Ship: Parts, IMO Limits and Key Factors

The turning circle of a ship is the path its midship point traces through a full 360-degree turn with the rudder hard over. This guide explains each part of the turning circle, how it is measured at sea trials, and the IMO limits that apply to it.

It is written for deck cadets, maritime students, and junior officers preparing for shiphandling exams or bridge watchkeeping. The same figures appear on every merchant ship’s wheelhouse poster and pilot card.

What Is the Turning Circle of a Ship?

A ship’s turning circle is the track of its midship point during a full 360-degree turn at maximum rudder angle. It shows how much sea room the ship needs to turn, to port and to starboard.

For a typical merchant ship, the tactical diameter of this turn is about 3 to 4 times the ship’s length. A 200 m bulk carrier therefore needs roughly 600–800 m of clear water to reverse its course under full helm.

The turning circle is not traced by the pivot point of the ship, a common textbook confusion. The pivot point is the point about which the ship appears to rotate. In a steady forward turn it lies roughly one-quarter to one-third of the length from the bow.

Three forces shape the turning circle: hydrodynamic resistance of the hull, the ship’s inertia, and propeller thrust acting across the rudder. Length, beam, draught, displacement, hull form, rudder area, and propeller arrangement all change its size.

What Are the Parts of a Ship’s Turning Circle?

The main parts of a turning circle are advance, transfer, tactical diameter, and final diameter. Drift angle, heel, and speed loss describe the ship’s behaviour during the turn. All are measured from the rudder order.

Part	Measured at	Direction of measurement
Advance	90° heading change	Along the original course
Transfer	90° heading change	Perpendicular to the original course
Tactical diameter	180° heading change	Perpendicular to the original course
Final diameter	Steady turn (after about 180°–360°)	Diameter of the steady circle
Drift angle	Any point in the turn	Angle between heading and direction of travel

Advance

Advance is the distance the midship point travels along the original course, from the rudder order until the heading has changed by 90 degrees. It shows how far ahead the ship carries on before the turn takes effect.

Advance matters most when turning away from a danger directly ahead. A ship with a long advance may reach the hazard before its bow has swung clear. Even so, turning is usually quicker than stopping, because a ship’s stopping distance is several times her advance.

Transfer

Transfer is the sideways distance the midship point moves, measured perpendicular to the original course, at the moment the heading has changed by 90 degrees. Advance and transfer together fix the ship’s position at the 90-degree point.

Transfer is always smaller than advance in a hard-over turn from full ahead. Navigators use it to judge whether a ship can turn into a parallel channel or away from a vessel on the beam.

Tactical Diameter

Tactical diameter is the transfer at a 180-degree change of heading, measured perpendicular to the original course. It is the sideways distance between the ship’s track at the rudder order and its track once heading the opposite way.

Tactical diameter is the headline figure for turning ability, and it is the one IMO limits. Smaller values mean the ship can reverse course in less sea room, which is what owners, pilots, and designers aim for.

Final Diameter

Final diameter, or steady turning diameter, is the diameter of the circle the ship settles into. This happens once its rate of turn and speed stop changing. It is usually slightly smaller than the tactical diameter.

Drift Angle

Drift angle is the angle between the ship’s fore-and-aft line and the direction in which its midship point is actually moving. In a turn, the bow points inside the track and the stern swings outside it.

This is why the stern sweeps a wider path than the bow. Officers turning close to a berth, buoy, or another ship must allow for the stern’s outward swing, not just the bow’s position.

Angle of Heel

A turning ship first heels slightly inwards, because the rudder force acts low on the hull. As the turn develops, centrifugal force acting at the centre of gravity heels the ship outwards, away from the centre of the turn.

Outward heel increases with speed and with a smaller turning radius. It is also larger on a ship with a small metacentric height, so container ships and car carriers need particular care in hard-over turns.

Speed Loss in the Turn

A ship loses speed throughout a hard-over turn, because the drifting hull and the deflected rudder both add resistance. Full-form ships such as tankers and bulk carriers can lose up to around half of their approach speed.

What Are the Three Phases of a Ship’s Turn?

Every turn passes through three phases: initial, transition, and steady. The rudder starts the turn, the drifting hull builds the rate of turn, and the ship then settles into a constant circle.

• Phase 1 — initial (kick): the rudder force pushes the stern away from the intended turn. The ship moves slightly sideways to the outside before its heading starts to change, and heels a little inwards.
• Phase 2 — transition: a drift angle develops and the hull itself starts generating turning force. The rate of turn increases, speed falls, and the heel shifts from inwards to outwards.
• Phase 3 — steady turn: rate of turn, drift angle, speed, and heel all stabilise. The ship follows a circle of constant final diameter for as long as the rudder is held.

The rudder only starts the turn; the hull at a drift angle keeps it going. For the hydrodynamics behind phase 1, see how a ship’s rudder generates turning force.

How Is the Turning Circle Trial Conducted?

The turning circle trial is run during sea trials at the end of the shipbuilding process. The ship approaches at test speed and the rudder is put to 35 degrees. The full turn is recorded to starboard and port.

IMO defines test speed as at least 90% of the ship’s speed at 85% of maximum engine output. If 35 degrees is not permissible at that speed, the maximum permissible rudder angle is used instead.

The IMO explanatory notes (MSC/Circ.1053) set standard trial conditions so that results from different ships are comparable:

• Water depth more than 4 times the mean draught
• Wind not exceeding Beaufort 5
• Waves not exceeding sea state 4
• Uniform current only
• Full load (summer load line draught) and even keel, as the reference condition

The ship’s position is logged continuously by satellite positioning throughout the turn. From that track, the trial team derives advance, transfer, tactical diameter, time to 90 and 180 degrees, rate of turn, and speed loss.

What Are the IMO Standards for a Ship’s Turning Ability?

Under IMO Resolution MSC.137(76), advance must not exceed 4.5 ship lengths and tactical diameter must not exceed 5 ship lengths in the turning circle test. The limits apply to both port and starboard turns.

The IMO Standards for Ship Manoeuvrability, Resolution MSC.137(76), were adopted in December 2002. They apply to ships of 100 m in length and over constructed on or after 1 January 2004. Chemical tankers and gas carriers are covered regardless of length.

Criterion	IMO limit (L = ship length)	Example: 183 m tanker
Advance	≤ 4.5 L	≤ 824 m
Tactical diameter	≤ 5 L	≤ 915 m
Typical tactical diameter in practice	3–4 L	about 550–730 m
Track reach, full astern stopping test	≤ 15 L (up to 20 L if the Administration allows)	≤ 2,745 m (about 1.5 nm)

The standards are an IMO recommendation applied through flag Administrations, not a SOLAS regulation. If a ship fails the full-scale trials, the resolution states that the Administration should take remedial action as appropriate.

What Factors Affect a Ship’s Turning Circle?

The main factors are rudder angle and area, hull form, trim, loading condition, water depth, propeller arrangement, and wind or current. Speed changes how quickly the turn happens more than the size of the circle.

• Rudder angle and area: more rudder angle, up to the usual maximum of 35 degrees, and a larger rudder area give a tighter turn.
• Hull form: full-bodied ships with a high block coefficient generally turn tighter relative to their length. They also tend to have poorer course-keeping.
• Length-to-beam ratio: long, slender hulls resist turning and need a wider circle.
• Trim: trim by the stern increases the turning circle; trim by the head reduces it but makes steering erratic.
• Water depth: the turning circle grows sharply in shallow water.
• Propeller: a single right-handed propeller turns the ship tighter to port than to starboard.
• Wind and current: both distort the circle, and wind matters most for high-sided ships in ballast.

Does Speed Change the Size of the Turning Circle?

In deep water and at a constant rudder angle, a conventional displacement ship’s turning circle is nearly the same size at any speed. What changes is time: at higher speed the ship covers the same circle faster, and heel is greater.

The exception is a turn started from low speed with a burst of engine power, called a kick ahead. The propeller wash hits the rudder before the ship gathers way, so the ship turns in less space than its trial figures suggest.

How Does Shallow Water Affect the Turning Circle?

With little water under the keel, flow around the hull is restricted and the ship resists turning. In very shallow water the turning diameter can reach about double its deep-water value.

This is why trial data, measured in water more than four times the draught, cannot be applied directly in a dredged channel. The same shallow-water flow also causes squat, which reduces under-keel clearance further.

How Do Loaded and Ballast Conditions Affect the Turn?

A loaded ship carries more mass and momentum, so it responds more slowly to the rudder and usually has a longer advance. The rudder and propeller are fully immersed, though, so steering force is reliable.

A ship in ballast is lighter and starts turning sooner. However, it is often trimmed by the stern with the rudder or propeller partly out of the water, and its high freeboard catches the wind. Both effects can widen the turn well beyond the loaded trial figures.

Why Does a Right-Handed Propeller Turn Tighter to Port?

A right-handed propeller turning ahead produces transverse thrust that pushes the stern to starboard. That swings the bow to port, so a port turn is assisted and a starboard turn is resisted.

The difference is small at full ahead but noticeable at low speed. Twin-screw ships can cancel it, or use opposed engines to turn in far less space than their single-screw equivalents.

Is There a Formula for a Ship’s Turning Circle?

No simple exact formula gives tactical diameter; it depends on hull, rudder, and propeller interaction. It is predicted by model tests and confirmed at sea trials. A steady turn’s radius can be calculated from speed and rate of turn.

For a ship in a steady turn, the relationship between speed, rate of turn, and radius is:

R (nm) = 0.955 × V (knots) ÷ ROT (degrees per minute)

At 12 knots and a rate of turn of 30 degrees per minute, the turning radius is about 0.38 nm, or roughly 710 m. The same relationship is used to plan the wheel-over point and turning radius for course alterations in confined waters.

What Other Maneuvering Characteristics Does IMO Measure?

Besides turning ability, MSC.137(76) measures initial turning ability, yaw-checking ability, course-keeping ability, and stopping ability. Together they describe how the ship starts a turn, stops one, holds a course, and comes to rest.

Initial Turning Ability

Initial turning ability measures how quickly a moderate rudder angle changes the heading. With 10 degrees of rudder applied, the ship must not travel more than 2.5 ship lengths before its heading has changed by 10 degrees.

Yaw-Checking Ability

Yaw-checking ability is the ship’s response to counter-rudder once it is already turning. It is measured by the overshoot angle in a zig-zag test: how far the heading keeps swinging after the rudder has been reversed.

In the 10°/10° zig-zag test, the rudder is put 10 degrees to one side and reversed each time the heading has changed by 10 degrees. The permitted first overshoot ranges from 10 to 20 degrees, depending on the ship’s length-to-speed ratio (L/V).

Zig-zag test	L/V below 10 s	L/V 10–30 s	L/V 30 s or more
10°/10° first overshoot	≤ 10°	≤ 5 + ½(L/V) degrees	≤ 20°
10°/10° second overshoot	≤ 25°	≤ 17.5 + 0.75(L/V) degrees	≤ 40°
20°/20° first overshoot	≤ 25° for all ships

Course-Keeping Ability

Course-keeping ability is how well the ship holds a straight course without large rudder or heading oscillations. A ship with slight inherent instability can still be steered acceptably. IMO therefore judges it through the second overshoot angle rather than demanding perfect stability.

Stopping Ability

Stopping ability is measured by a full astern crash stop from a steady approach at test speed. The track reach must not exceed 15 ship lengths, which the Administration may extend to a maximum of 20 lengths for ships of large displacement.

• Track reach: distance along the ship’s actual path from the full astern order until the ship stops in the water.
• Head reach: distance covered in the direction of the original course.
• Side reach (lateral deviation): distance the ship ends up to one side of the original course line.
• Time to dead in water: time from the full astern order until the ship has no way on.

Why Must a Navigator Understand the Turning Circle?

A navigator needs turning circle data to know whether there is enough sea room to avoid a danger by altering course. Without it, an officer cannot judge whether a turn will clear a ship, shoal, or berth in time.

The requirement is written into the collision regulations. COLREGs Rule 6 on safe speed requires the vessel’s manoeuvrability “with special reference to stopping distance and turning ability” to be taken into account. Rule 8 expects avoiding action to be substantial and taken in good time.

Turning circle and stopping data are used day to day for:

• Turning away from an obstacle such as a landmass, reef, ice, or another vessel
• Circling a fixed point, such as during a search or a man-overboard recovery
• Planning course alterations and wheel-over positions in channels and fairways
• Judging stopping distance when slowing down, stopping, or coming alongside
• Holding a planned track without large corrections
• Swerving clear of a sudden hazard without losing excessive time

Under IMO Resolution A.601(15), this information is displayed on the wheelhouse poster and pilot card, and set out in full in the maneuvering booklet. A master or pilot should check it against the ship’s actual draught, trim, and water depth before relying on it.

Frequently Asked Questions