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Rudder and Steering Systems

The rudder is the principal directional-control surface of a ship, a vertical plate or aerofoil deflected by the steering system to generate a side force that turns the vessel. Rudder design (semi-balanced, fully balanced, spade, flap, twisted leading edge), rudder area (typically 1.5 to 2.5% of $L \times T$, where $L$ is length and $T$ is draught), rudder profile (NACA, IFS, modified profile), and steering system architecture (electrohydraulic, electric, redundancy provisions) are governed by SOLAS Chapter II-1 Regulation 29, the IMO Standards for Ship Manoeuvrability (Resolution MSC.137(76), in force January 2004) and the classification society rules of DNV, Lloyd’s Register, ABS, BV, NK, KR, RINA, CCS and others. The steering system must satisfy stringent reliability and redundancy requirements (typically two independent steering gear systems with separate hydraulic circuits and separate electrical power supplies) because loss of steering on a loaded merchant vessel is a catastrophic safety event that has triggered many of the worst marine casualties in history (notably the Torrey Canyon 1967, Amoco Cadiz 1978, Exxon Valdez 1989). Modern rudder developments include the twisted leading edge rudder (Becker Schilling, Becker Twisted Fin) developed as part of the energy-saving devices family, with typical 1 to 3% main-engine fuel saving, and the flap rudder for high-manoeuvrability applications. The IMO Standards for Ship Manoeuvrability define quantitative tests (turning circle, zigzag manoeuvres, stopping distance) that all newbuilds must satisfy. ShipCalculators.com hosts the principal computational tools: the rudder area calculator, the rudder force and torque calculator, the turning radius calculator, the advance and transfer calculator, the crash stop distance calculator, the twisted rudder savings calculator, the rudder stock diameter calculator and the steering gear power calculator. A full listing is available in the calculator catalogue.

Contents

Background

Function of the rudder

The rudder serves three principal functions:

  • Directional control: deflecting the rudder to port or starboard generates a side force that creates a yaw moment about the vessel’s centre of pivot, turning the vessel.
  • Heading hold: small rudder corrections (typically 1 to 5 degrees) maintain the vessel on a desired heading despite environmental disturbances (wind, current, propeller torque reaction).
  • Manoeuvring: large rudder deflections (typically up to 35 degrees per side) for course changes, harbour manoeuvring, collision avoidance, and emergency response.

The rudder is conventionally located directly aft of the propeller, where the propeller race provides accelerated water flow that increases the rudder’s effectiveness, particularly at low ship speeds when the freestream flow is weak.

Rudder force generation

The rudder generates a side force through lift, in the same way as an aircraft wing. The lift force is approximately:

$$ L_R = \frac{1}{2} \rho V^2 A_R C_L(\alpha) $$

where:

  • $\rho$ is the water density.
  • $V$ is the flow velocity over the rudder (the higher of the ship speed or the propeller race velocity).
  • $A_R$ is the rudder area.
  • $C_L(\alpha)$ is the lift coefficient as a function of rudder angle $\alpha$.

The lift coefficient rises approximately linearly with rudder angle up to the stall angle (typically 30 to 35 degrees for conventional rudders), beyond which the flow separates and the lift drops sharply. The rudder is therefore designed to operate at angles up to but not beyond the stall.

The maximum useful rudder deflection is conventionally 35 degrees on each side, with a hard mechanical stop at 37 to 38 degrees. The rudder must reach 35 degrees within 28 seconds from the time the steering wheel is moved (SOLAS requirement).

Rudder types

Spade rudder

The spade rudder is supported only at the upper edge by the rudder stock; the lower part of the rudder is unsupported (cantilevered from the stock). Spade rudders are common on small to medium vessels (cruise ships, ferries, naval vessels, smaller container ships).

Advantages:

  • Compact, no rudder horn structure required.
  • Good hydrodynamic efficiency (clean flow at lower edge).
  • Low manufacturing cost.

Disadvantages:

  • High bending moment on the rudder stock.
  • Limited size (cantilever moment becomes excessive for very large rudders).
  • Less resilient to ice impact.

Semi-balanced rudder

The semi-balanced rudder (also horn rudder) is supported at the upper edge by the rudder stock and at the lower edge by the rudder horn (a rigid structure extending below the hull). The rudder is partially balanced (a portion of the rudder area is forward of the stock) to reduce the steering torque.

Semi-balanced rudders are common on medium to large vessels (large container ships, bulk carriers, tankers).

Advantages:

  • Lower stock bending moment than spade.
  • Larger size feasible.
  • Better ice resilience.

Disadvantages:

  • More complex structure.
  • Slightly higher hydrodynamic drag.
  • Heavier.

Fully balanced rudder

The fully balanced rudder has approximately 25 to 30% of the rudder area forward of the rudder stock, balancing the steering torque around the stock. Fully balanced designs are common on large slow-speed two-stroke vessels.

Advantages:

  • Lowest steering torque, smallest steering gear.
  • Compact.

Disadvantages:

  • Sensitive to angle of attack.
  • Risk of “over-balance” at high angles (the rudder torque can reverse, requiring active control).

Flap rudder

The flap rudder has a hinged trailing-edge flap that deflects automatically with the rudder angle, increasing the effective camber and the lift coefficient. Flap rudders provide significantly higher lift coefficient (typical 60 to 100% increase over a conventional rudder of the same area), allowing smaller rudders or higher manoeuvrability.

Flap rudders are common on:

  • Large cruise ships (manoeuvrability in port).
  • Some ferries.
  • Some specialist vessels.

The principal commercial flap rudder is the Becker FKSR (Becker Marine Systems).

Twisted leading edge rudder

The twisted leading edge rudder has a leading edge that is twisted (typically 4 to 8 degrees) to align with the rotating outflow from the propeller. The alignment reduces the angle of attack on the rudder leading edge, reducing rudder drag and reducing cavitation erosion at the rudder leading edge. The rotating outflow can also be partly converted into forward thrust by the rudder.

Twisted rudders are commonly combined with rudder bulb (a streamlined fairing on the leading edge of the rudder, immediately aft of the propeller hub) to further enhance the post-propeller energy recovery; the combined package is part of the energy-saving devices family.

Typical performance: 1 to 3% main-engine fuel saving for a twisted rudder; 2 to 5% for the combined twisted rudder + rudder bulb package.

The principal commercial twisted rudder is the Becker Schilling rudder, with several proprietary variants including the Becker Twisted Fin.

High-lift rudder (Schilling, Birdsall, Bishop)

High-lift rudders are specialised designs (typically used on tugs and harbour vessels) that achieve very high lift coefficients (up to 70 degrees of effective deflection) for extreme manoeuvrability. Examples include the Schilling Trapezoidal Rudder and the Promas integrated propulsor-rudder (Rolls-Royce / Kongsberg).

Rotor rudder (Voith-Schneider, Z-drive)

For some specialist applications, the conventional rudder is replaced by a steerable propulsor:

  • Voith-Schneider propeller (Voith Turbo): vertical-axis rotor with adjustable blades, providing 360-degree thrust direction control. Used on tugs, ferries, mine countermeasures vessels.
  • Z-drive (azimuth thruster): conventional propeller mounted on a rotating azimuth pod, providing 360-degree thrust direction. Standard for tugs, ASD vessels, dynamic positioning vessels, some cruise ships.
  • Azipod (ABB): integrated electric motor + propeller in a rotating pod beneath the hull. Standard for cruise ships, large icebreakers, some specialist vessels.

These propulsor-rudder systems eliminate the need for a separate conventional rudder.

Rudder design parameters

Rudder area

The rudder area is conventionally specified as a fraction of $L \times T$ (the longitudinal projected area of the underwater hull):

  • Bulk carriers, tankers: $A_R / (L \times T) = 1.5$ to 2.0%.
  • Container ships: $A_R / (L \times T) = 1.7$ to 2.2%.
  • Ferries: $A_R / (L \times T) = 2.0$ to 2.5%.
  • Cruise ships: $A_R / (L \times T) = 2.0$ to 3.0% (often combined with bow and stern thrusters).
  • Tugs: $A_R / (L \times T) = 3$ to 5% (or replaced by steerable propulsor).

A larger rudder provides better manoeuvrability but adds drag and weight.

Rudder profile

The rudder cross-section is typically a NACA airfoil (NACA 0015 to NACA 0021 are common), modified for marine applications. Key profile characteristics:

  • Maximum thickness: typically 18 to 20% of chord (more than aerodynamic profiles, due to structural needs and lower Reynolds number).
  • Symmetric (for conventional rudders) or slightly cambered (for twisted leading edge variants).
  • Modified leading edge: rounded to delay stall.

Rudder aspect ratio

The aspect ratio ($AR = h^2 / A_R$, where $h$ is rudder height) characterises the rudder geometry:

  • Low aspect ratio (1.0 to 1.5): typical of merchant ship rudders. Provides good lift at high angles, tolerant of unsteady flow.
  • Medium aspect ratio (1.5 to 2.5): typical of cruise ship and ferry rudders.
  • High aspect ratio (2.5 to 4): rare in marine applications; high efficiency but less stall margin.

Rudder stock and bearings

The rudder stock transmits the steering torque from the steering gear to the rudder. The stock diameter is sized for the maximum steering torque under all operating conditions, with significant safety margins per classification society rules.

The rudder is supported by:

  • Upper bearing (in the rudder trunk).
  • Carrier bearing (above the rudder, in the steering gear room).
  • Lower bearing (only for semi-balanced rudders, in the rudder horn).

Bearing materials are typically synthetic resin (Thordon, Romor) or bronze-on-bronze for older designs.

Steering gear

Electrohydraulic steering gear

The dominant steering gear architecture in modern merchant shipping is electrohydraulic:

  • Hydraulic pumps: typically two pumps (for redundancy), each driven by an electric motor. Variable displacement axial-piston pumps are standard.
  • Hydraulic accumulator: provides response speed and short-term reserve capacity.
  • Hydraulic actuators: typically rotary vane (more common on modern designs, more compact) or ram-type (older, larger).
  • Tank: hydraulic fluid reservoir with cooling coils.
  • Control valves: solenoid-operated proportional valves, controlled by the steering control system.
  • Position feedback: rudder angle sensor.

Two independent steering gear systems are standard on larger merchant vessels under SOLAS Chapter II-1 Regulation 29; either system alone must be capable of operating the rudder at maximum angle within the regulatory time limits.

Electric steering gear

Electric steering gear (typically using a high-torque electric motor with planetary gear reduction) is increasingly common on smaller vessels and on some specialist applications. It eliminates the hydraulic system, reducing maintenance and environmental risk (no oil leakage). Commercial offerings include systems from Kongsberg, Rolls-Royce (now Kongsberg), Wartsila, Mitsubishi.

Steering control system

The steering control system comprises:

  • Bridge controls: typically a steering wheel for manual steering, plus auxiliary controls (push-buttons, joystick) for harbour manoeuvring.
  • Autopilot: maintains a programmed heading or course; integrated with ECDIS, GMDSS, and the steering control system.
  • Track control: maintains a programmed track from waypoint to waypoint, compensating for current and wind drift.
  • Adaptive autopilot: tunes its parameters automatically to vessel characteristics and sea state.

Major autopilot vendors include Sperry Marine (Northrop Grumman), Raytheon Anschutz, Furuno, Tokyo Keiki, Yokogawa.

Manual steering and emergency steering

In addition to the autopilot and powered steering controls, the steering gear must support:

  • Manual steering from the bridge (steering wheel direct-controlling the steering gear).
  • Emergency steering from a steering gear room control position, in case of loss of bridge control.

The steering gear room must be permanently manned during certain operations (entering port, navigating restricted waters, hazardous weather) to allow rapid switch to emergency control if needed.

SOLAS and IMO requirements

SOLAS Chapter II-1 Regulation 29

The principal regulation is SOLAS Chapter II-1 Regulation 29, which requires:

  • Two independent steering gear systems on vessels of 70,000 GT or more (or those carrying potentially hazardous cargo).
  • Capacity to put the rudder over from 35 degrees on one side to 35 degrees on the other side at the maximum service draught and a forward speed of 22 knots (or maximum service speed if less than 22 knots) within 28 seconds.
  • Capacity of each steering gear to put the rudder over from 15 degrees on one side to 15 degrees on the other side, at the maximum service draught and one-half of the maximum service speed (or 7 knots if greater), within 60 seconds.
  • Power supply: independent power supply for each steering gear; main power supply must be from the main switchboard or from independent power source.
  • Indication of rudder angle: visible from the bridge.
  • Communication between bridge and steering gear room.
  • Emergency power supply: capable of operating one steering gear for at least 10 minutes if the main power fails.

IMO Standards for Ship Manoeuvrability (MSC.137(76))

The IMO Standards for Ship Manoeuvrability, adopted as Resolution MSC.137(76) and in force January 2004, specify quantitative manoeuvring tests:

  • Turning circle test: vessel turns 35 degrees rudder; measure advance, transfer, tactical diameter at 90 and 180 degrees of heading change. Maximum advance approximately 4.5 ship lengths; maximum tactical diameter approximately 5 ship lengths.
  • Zigzag test (10/10): vessel alternates 10 degrees of rudder; measure overshoot angle and time to reach steady state. Overshoot angle limits depend on length-to-speed ratio.
  • Zigzag test (20/20): similar test with 20 degrees of rudder.
  • Stopping test: vessel stops from full ahead service speed; measure track distance and head reach. Maximum stopping distance approximately 15 ship lengths.

Vessels must satisfy all the criteria as a condition of class certification.

Crash stop distance

The crash stop distance is the distance required to stop the vessel from full ahead by reversing the propeller. For typical merchant vessels:

  • VLCC at 14 knots: approximately 4 to 5 km stopping distance.
  • Capesize bulker at 14 knots: approximately 3 to 4 km.
  • 14,000 TEU container ship at 22 knots: approximately 3 to 4 km.
  • Cruise ship at 20 knots: approximately 1.5 to 2.5 km (smaller mass-to-thrust ratio).

The crash stop is a binding consideration in collision avoidance, particularly for confined waters and high-traffic zones.

Steering and propulsion interaction

Propeller race

The propeller race significantly enhances rudder effectiveness, particularly at low ship speeds. At zero ship speed (vessel stationary), the rudder still develops force from the propeller race; this is the basis of rudder use during port manoeuvring.

For slow-speed two-stroke propulsion, the propeller race is well-defined and consistent; the rudder is positioned to maximise interaction.

For azimuth thruster propulsion, the thruster itself provides the directional control; no separate rudder is needed.

Reverse thrust (astern operation)

When the propeller is operating astern (reversed), the rudder loses its effectiveness because the reversed flow is disorganised. Steering authority during astern operation is much reduced, requiring careful crew handling.

Propeller torque effect (transverse force)

A right-handed propeller (rotating clockwise when viewed from astern) produces a transverse force that pushes the stern to port (and bow to starboard) when going ahead, and the opposite when going astern. This creates a permanent steering bias that the autopilot or helmsman must correct.

The transverse force is most pronounced at low ship speeds, where the rudder has limited authority.

Twisted leading edge rudder for fuel saving

Energy-saving rudder concept

The twisted leading edge rudder is part of the energy-saving devices family. By aligning the leading edge with the propeller’s rotating outflow, the rudder:

  • Reduces the angle of attack at the leading edge, reducing drag.
  • Reduces cavitation erosion at the leading edge.
  • Recovers some of the rotational kinetic energy of the propeller race, converting it to forward thrust.

The combined effect is approximately 1 to 3% main-engine fuel saving, with no impact on manoeuvring performance.

Becker Schilling rudder

The Becker Schilling rudder is the dominant commercial twisted rudder, with approximately 1,500 installations by end-2024 (predominantly on larger container ships and tankers). The rudder is patented by Becker Marine Systems (Hamburg).

Combined with rudder bulb

The twisted rudder is often combined with a rudder bulb (a streamlined fairing on the rudder leading edge, immediately aft of the propeller hub). The combined package recovers more of the propeller hub vortex energy than either device alone. Combined savings are typically 2 to 5%.

The combined Becker package (twisted rudder + rudder bulb) is the Becker Twisted Fin Special. Other vendors (Hyundai, DSME / Hanwha Ocean, Wartsila, MOL Techno-Trade) offer comparable combined packages.

Notable manoeuvring incidents

Torrey Canyon (1967)

The Torrey Canyon ran aground on the Seven Stones reef off Cornwall, UK on 18 March 1967 after the steering system malfunctioned at the critical moment. Approximately 100,000 t of crude oil was released, the largest oil spill in history at that time, leading to fundamental changes in maritime safety regulation including the eventual SOLAS Chapter II-1 Regulation 29 requirements.

Amoco Cadiz (1978)

The Amoco Cadiz lost steering off the coast of Brittany, France on 16 March 1978 due to a hydraulic ram failure in the steering gear. Despite emergency response, the vessel ran aground and released approximately 220,000 t of crude oil. The incident drove subsequent SOLAS amendments requiring redundant steering systems on tankers.

Exxon Valdez (1989)

The Exxon Valdez ran aground in Prince William Sound, Alaska on 24 March 1989 after the third officer mishandled the vessel during a course alteration to avoid icebergs. The grounding released approximately 37,000 t of crude oil. While not strictly a steering system failure, the incident emphasised the importance of bridge resource management and steering control system reliability.

Costa Concordia (2012)

The Costa Concordia ran aground off the Italian island of Giglio on 13 January 2012 after the captain ordered a close-shore deviation that brought the vessel into grounding contact with a submerged rock. The vessel subsequently capsized; 32 people died. The incident emphasised the importance of human factors in steering decisions.

Ever Given (2021)

The Ever Given ran aground in the Suez Canal on 23 March 2021, blocking the canal for six days. Investigations attributed the grounding to high winds, high speed, and possibly insufficient rudder authority at low forward speed. The incident emphasised the importance of canal-specific manoeuvring constraints and rudder design margins.

Future developments

Integrated propulsor-rudder systems

The trend towards integrated propulsor-rudder systems (Promas, Mewis Twisted Fin, Wartsila EnergoFlow) is expected to continue, integrating the rudder with the propeller and the energy-saving devices into a single optimised package.

Autonomous steering integration

The development of autonomous shipping is closely connected with steering control. Modern autopilots are increasingly sophisticated, integrating with AIS and weather data, and capable of executing planned manoeuvres without human intervention. Full autonomous operation would require enhanced steering reliability and additional redundancy.

Electric steering proliferation

Electric steering gear is expected to displace electrohydraulic on smaller vessels (under approximately 20,000 GT) through the late 2020s, driven by environmental concerns about hydraulic oil leakage and lower lifecycle maintenance costs.

Energy-saving rudder packages

Combined twisted rudder + rudder bulb + propeller boss cap fin packages are expected to become standard on newbuilds from approximately 2026 to 2028, driven by EEXI and CII rating economics.

See also

Stability and naval architecture

Operational and technical efficiency

Marine fuels

Regulatory frameworks

Cargo and operations

Ship types

Calculators

References

  • IMO Resolution MSC.137(76): Standards for Ship Manoeuvrability. International Maritime Organization, 2002, in force January 2004.
  • IMO MSC/Circ.1053: Explanatory Notes to the Standards for Ship Manoeuvrability. International Maritime Organization, 2002.
  • SOLAS Chapter II-1 Regulation 29: International Convention for the Safety of Life at Sea, 1974, as amended. International Maritime Organization, 1974 with subsequent amendments.
  • IACS. UR M42: Recommendation on type approval procedure for steering gear. International Association of Classification Societies, 2024.
  • IACS. Common Structural Rules for Bulk Carriers and Oil Tankers (CSR BC and OT), Chapter 6 Rudder. International Association of Classification Societies, 2024.
  • DNV. DNV Rules for Classification of Ships, Pt 4 Ch 14 Steering. DNV, 2024 edition.
  • Lloyd’s Register. Rules and Regulations for the Classification of Ships, Part 5 Ch 18 Steering Gear. Lloyd’s Register Group, 2024 edition.
  • Brix, J. Manoeuvring Technical Manual. Seehafen Verlag, 1993.
  • Crane, C. L. and Eda, H. Controllability and Manoeuvrability. SNAME, 1989.
  • Lewis, E. V. (editor). Principles of Naval Architecture, Volume III: Motions in Waves and Controllability. SNAME, 1989.

Further reading

  • ITTC. Recommended Procedures and Guidelines: Free Running Model Tests for Manoeuvrability. International Towing Tank Conference, 2017.
  • Becker Marine Systems. Schilling Twisted Rudder Performance Brochure. Becker Marine Systems, 2023.
  • Wartsila Marine Solutions. EnergoFlow Combined Rudder System Technical Brief. Wartsila, 2022.
  • Tupper, E. C. Introduction to Naval Architecture. Butterworth-Heinemann, 5th edition, 2013.