Ship Rudder: How It Works, Its Parts and Types of Rudders

A ship’s rudder is a foil-shaped blade at the stern that steers the ship by deflecting the water flowing past it. Turning the rudder creates a sideways force that swings the stern, so the ship changes heading.

This guide is written for deck and engineering cadets and junior officers. It explains how a rudder works and why it sits behind the propeller. It also covers rudder parts and types, the maximum rudder angle and what surveyors check.

What Is the Function of a Rudder on a Ship?

The rudder’s function is to steer the ship: to turn her onto a new heading and hold her on a steady course. It does this with a sideways lift force at the stern.

• Turning: starts and controls a change of heading.
• Course keeping: small rudder movements correct yaw from wind, waves and current.
• Checking a swing: opposite rudder slows and stops the ship’s rate of turn.

The rudder sits at the stern of the ship, directly behind the propeller. It is turned by the steering gear in the steering gear room above it.

How Does a Rudder Turn a Ship?

When the rudder is put over, water flowing past it creates lift, like an aircraft wing. This force pushes the stern sideways, the ship takes up a drift angle, and the water pressure on the angled hull then turns her.

1. Starboard rudder is ordered. The rudder blade turns so its trailing edge points to starboard.
2. Water deflected by the blade creates a lift force on the rudder towards port.
3. Because the rudder is far aft, this force pushes the stern to port and the bow swings to starboard.
4. The whole ship first moves slightly to port, away from the turn, and heels briefly inwards.
5. The ship now meets the water at a drift angle, and the hull itself starts acting like a wing.
6. The hydrodynamic force on the angled hull acts forward of the centre of gravity and keeps the ship turning.

The rudder only starts the turn. Its force is small compared with the ship’s mass; it creates the drift angle, and the much larger hull force does most of the turning. Where the ship appears to rotate is explained in the article on the pivot point of a ship.

Why Does the Hull Do Most of the Turning?

At a drift angle, water strikes the hull side and creates a large sideways force. Going ahead, this force acts well forward of the centre of gravity, so it turns the ship harder than the rudder.

The rudder force acts aft of the center of gravity, the hull force forward of it. Both turn the ship the same way, which is why a ship keeps swinging and settles into a steady turning circle.

Once the ship is turning steadily, the hull force, the rudder force and the water resistance balance each other. The ship then turns at a constant rate, with a drift angle and an outward heel.

Why Is the Rudder at the Stern Behind the Propeller?

The rudder sits behind the propeller because the propeller wash multiplies the water speed over it. It sits at the stern because that is the furthest point from the pivot point, giving the longest lever for turning the ship.

• Propeller wash: rudder force rises with the square of water speed. The fast slipstream from the propeller gives a much stronger force than the ship’s speed alone.
• Steering at low speed: a short burst of engine ahead puts wash over the rudder, giving steering effect even when the ship is almost stopped.
• Lever arm: going ahead, the pivot point moves forward, so a stern rudder acts on the longest possible lever.
• Course stability: a rudder aft acts like the tail feathers of an arrow, helping the ship hold a straight course.
• Protection: aft, the rudder is sheltered by the hull and clear of anchors, ice and berthing contact at the bow.

A rudder ahead of the propeller, or at the bow, would sit in slower, disturbed water and give much less force. Bow rudders are used only on ships that often go astern, such as some double-ended ferries.

Going astern, the propeller wash is thrown forward, away from the rudder. The rudder then has little effect until the ship gathers sternway, which is why ships steer poorly astern.

What Affects the Force a Rudder Produces?

Rudder force depends mainly on rudder area, the speed of the water over the blade and the rudder angle. It grows with the square of water speed, so halving the flow cuts the force to about a quarter.

Factor	Effect on rudder force
Water speed over the rudder	Force rises with speed squared; propeller wash adds most of it at low ship speed
Rudder area	Usually about 1.5–2% of length × draft on merchant ships; more on tugs and ferries
Rudder angle	Force rises up to about 35°, then the flow separates and the rudder stalls
Immersion	A rudder not fully submerged, as in light ballast, loses force and can draw in air
Water depth	In shallow water the ship becomes sluggish to the helm and needs more rudder
Aspect ratio and section	Tall, foil-shaped rudders give more lift for the same area

Trim matters too. A ship trimmed by the head lifts the rudder and propeller, which reduces both rudder force and course stability.

What Are the Parts of a Ship’s Rudder?

A ship’s rudder consists of the blade, the rudder stock that turns it, and the bearings and pintles that carry it. The tiller and steering gear sit at the top of the stock, inside the hull.

• Rudder blade: a hollow, streamlined body of steel plating stiffened by internal webs.
• Rudder stock: the shaft that transmits the steering gear torque to the blade.
• Rudder coupling: joins the stock to the blade, usually by a cone and nut or a bolted flange.
• Rudder trunk: the tube through the stern that the stock passes up, sealed against the sea.
• Rudder carrier: the bearing in the steering gear flat that supports the weight of rudder and stock.
• Neck bearing: guides the stock where it leaves the hull.
• Pintles and bushes: hinge pins and their bearings that support the blade on a horn or sole piece.
• Rudder stops: fixed stops that limit the blade’s movement, usually just beyond 35°.
• Anodes and drain plugs: sacrificial anodes protect the steel, and drain plugs let water out of the blade in drydock.

What Are the Types of Rudder?

Rudders are classed by how much area lies forward of the stock: balanced, semi-balanced or unbalanced. They are also classed by support (spade, horn or skeg), with high-lift designs as a third group.

Balanced, Semi-Balanced and Unbalanced Rudders

Type	Area forward of stock	Main features
Balanced	Typically a quarter to a third	Low steering torque; common on modern merchant ships
Semi-balanced	Only on the lower part	Upper part hung on a horn; balance only below it
Unbalanced	None	Stock at the leading edge; needs the most torque; older ships and small craft

Balance reduces torque because the water pressure on the blade acts near the stock. The centre of pressure moves aft as rudder angle increases, so designers place the stock to keep torque low across the working range.

Too much area forward of the stock can overbalance the rudder at small angles. The blade then tends to move away from midships on its own, which is why balance stays below about a third.

Spade, Horn and Skeg Rudders

• Spade rudder: hung from the stock alone, with no lower support. Fully balanced and efficient, but the stock must be thick to take the bending load.
• Horn (Mariner) rudder: the upper part sits behind a fixed horn on the hull and turns on a pintle. Semi-balanced and strong, common on large merchant ships.
• Skeg or sole-piece rudder: supported at the bottom by a pintle on a sole piece or skeg below it. Typical of older ships and some small craft.

A deep horn reaches more than halfway down the rudder’s height and gives better support. A shallow horn covers less, leaving more of the blade balanced.

High-Lift and Special Rudders

• Flap rudder (Becker type): a hinged flap on the trailing edge, giving much more lift at the same angle.
• Schilling rudder: a fishtail-shaped blade with end plates that can work at up to about 70°.
• Twisted rudder: the leading edge is twisted to match the swirling propeller wash, reducing drag and cavitation.
• Rudder bulb: a streamlined bulb on the blade behind the propeller hub, improving propulsion efficiency.

What Is the Maximum Rudder Angle on a Ship?

The maximum rudder angle on most ships is 35° to port or starboard, called “hard over”. Beyond about 35°, a conventional rudder stalls: the flow breaks away from the blade and lift drops sharply.

SOLAS Chapter II-1, Regulation 29 bases steering gear performance on 35°. The main gear must move the rudder from 35° on one side to 30° on the other within 28 seconds.

Limit switches stop the steering gear before the rudder reaches its mechanical stops. High-lift rudders, such as flap and Schilling designs, are built to work at larger angles.

How Does a Boat Rudder Differ From a Ship’s Rudder?

A boat rudder works on the same lift principle but is smaller, usually turned by a tiller or a cable or hydraulic helm. Many powerboats have no rudder; outboard and sterndrive units steer by turning the propeller.

On a sailing yacht the rudder works together with the keel. The keel resists sideways drift, and the rudder controls heading and balances the pull of the sails.

What Do Surveyors Check on a Rudder?

Class surveyors examine the rudder at every drydocking and at the renewal survey. They measure bearing clearances and weardown, and check the blade, stock and pintles for cracks, corrosion and water inside.

• Weardown: measured at the rudder carrier with a gauge and compared with the reading at build.
• Clearances: pintle and neck bearing clearances measured against the class limits.
• Jumping clearance: the vertical gap that stops the rudder lifting and hitting the hull.
• Blade: plating checked for cracks, cavitation erosion and corrosion; drain plugs removed to check for water.
• Pressure test: carried out after repairs to confirm the blade is watertight.
• Stock and pintles: checked for cracks at the coupling and the horn, often by non-destructive testing.
• Securing: pintle nuts, coupling nuts and locking arrangements checked tight and secured.
• Anodes: renewed when wasted.

Results go into the class survey report, and repairs beyond the limits are required before the ship sails. Survey types and intervals are explained in the guide to surveys carried out on ships, and the preparation in how ships are prepared for drydocking.

Common findings are excessive bush clearance, cracks in the horn or at the pintle housing, and seawater inside the blade. Between drydockings, port state control inspectors test the steering gear and compare the rudder angle indicators with the actual rudder position.

Related Topics

• Steering a ship: helm orders and technique
• Telemotor in steering gear
• Propeller cavitation

Frequently Asked Questions