Ship Rudder

10/3/2019 1

Rudder

Rudders are a means of directional control of ship

Rudders are fitted at aft end because…

a. derive benefit of increased water velocity induced by the propeller – esp. at low speed

b. to create the maximum distance from the ship’s neutral point where lateral force does not cause ship to deviate from a constant direction…’neutral point’ is forward of midship

10/3/2019 2

Rudder

Neutral point is the point in the length of the ship at which an applied force (lateral) DOES NOT cause the ship to deviate from a constant direction

Generally, neutral point is forward of midship. Hence rudder is most effective when located aft, and it gives maximum turning effect.

10/3/2019 3

Rudder

Size and shape of rudder is governed by:

a. shape and type of stern

b. area of rudder deemed necessary

c. capacity of steering gear

d. service condition of ship

Three types of rudder

a. balanced

b. unbalanced

c. semi-balanced

10/3/2019 4

Types of rudder

The main purpose of balancing rudder is to achieve the reduction in the torque required of the steering gear.

There are 3 types of rudders:

Balanced: a portion of the blade area is disposed symmetricallythrough the rudder height and fwd of stock

Unbalanced: blade is entirely aft of stock

Semi-balanced: area fwd of stock does not extend to the fullheight of the blade aft of the stock – upper portion may beconsidered unbalanced and the lower portion, balanced

10/3/2019 5

10/3/2019 6

TYPES OF

RUDDER

Balanced Rudder

Axle fitted at its turning axis with large area forward ofthe axis (25% to 30 %).

Upper and lower bearings are fitted in the rudder.

The bearing consists of a stainless steel bush in therudder and a stainless steel liner on the axle.

The stainless steel bush is spirally grooved to permitlubrication.

Other materials are in use, such as gun metal for linerand lignum vitae or tufnol for the bush.

10/3/2019 7

BOLTED PALM

UPPER

BEARING

HORIZONTAL WEB

LOWER BEARING

DRAIN PLUG

RUDDER STOCK

LIFTING TUBE

VERTICAL WEBRUDDER AXLE

BALANCEDRUDDER

10/3/2019 8

Unbalanced Rudder

Full area aft of the axis.

Fitted with upper, mid and lower arms rested on

gudgeons.

At the bearing pintle fitted with hard steel disc and

locking nut.

The ordinary pintle arranged with bush and locking

nut and the locking pintle at the top part of the

rudder secured by locking nut.

Turn on pintles and have a small portion of their

lateral area forward of the turning axis (less than

20%).

Commonly fitted on twin screw vessels where most

of it hinged on a body post by pintles and gudgeons.

10/3/2019 9

Unbalanced Rudder

10/3/2019 10

HORIZONTAL

WEB

LIFTING TUBE

VERTICAL

WEB

RUDDER POST

BEARING PINTLE

DRAIN PLUG

LOCKING PINTLE

GUDGEON

UNBALANCED RUDDER

Rudder Stock

10/3/2019 11

Semi-Balanced Rudder

Which are very common

today. Have less than 20%

of the lateral area forward

of the axis.

Modern ship’s use this

design of rudder.

10/3/2019 12

Semi-balanced Rudder

10/3/2019 13

Rudder Pintles

Pintles on which the rudder turns in the gudgeons have a taper

on the radius, and a bearing length which exceeds the diameter.

Older ships may have a brass or bronze liner shrunk on the

pintles which turn in lignum vitae (hardwood) bearings fitted in the

gudgeons.

Modern practice is to use synthetic materials like ‘Tufnol’ for the

bearings, and in some cases stainless steels for the liners. In

either case lubrication of the bearing is provided by the water in

which it is immersed.

10/3/2019 14

Locking pintles are designed to prevent the

rudder lifting.

Bearing pintles take part of the rudder’s

weight

Function of the Rudder Pintles

10/3/2019 15

10/3/2019 16

Bearing Pintle

10/3/2019 17

Locking Pintle

10/3/2019 18

Ordinary Pintle

10/3/2019 19

RUDDER BEARING:

The weight of the rudder may be carried partly by

the lower pintle and partly by a rudder bearer within

the hull.

In some rudder types, for example, the spade type

which is only supported within the hull, the full weight

is borne by the bearer.

A rudder bearer may incorporate the watertight gland

fitted at the upper end of the rudder trunk.

Most of the rudder’s weight may come onto the

bearer if excessive wear down of the lower pintle

occurs, and the bearers illustrated have cast iron

cones which limit their wear down.

10/3/2019 20

Rudder angle normally does not exceed 35 degrees. Why?

avoid stalling of rudder

avoid flow separation on low pressure side

avoid reduction of effectiveness of control surface

avoid sudden discontinuity of lift force on downstream surface of rudder OR increase in drag force

10/3/2019 21

Lift to Drag ratio of Rudder

10/3/2019 22

• Effect on rudder stock of different rudder configuration

Balanced rudder….centre of pressure on turning axis at about 15 degrees, no torque on rudder stock

Unbalance rudder….centre of pressure is furthest from turning axis, high torque on rudder stock

10/3/2019 23

Rudder Pintles and Carriers

A head is fitted to the upper pintle to prevent undue

vertical movement of the rudder – locking pintle

The bottom pintle is known as a bearing pintle since it

rests on a hardened steel pad

Bearing pintles are only required to support the weight

of the rudder in the event of the rudder carrier failing

It is essential that the centreline of stock and pintles are

in the same line, otherwise the rudder will not turn

Appreciable wear of bearing generally is due to the

misalignment of the rudder stock

The major part of the rudder’s weight is carried by the

rudder carrier

10/3/2019 24

10/3/2019 25

Watertight Gland for Rudder Stock

A separate watertight gland is often fitted where the stock enters the rudder trunk.

A small opening with watertight cover may be provided in one side of the trunk, which allows access to a greater length of the rudder stock

It also removes the need for a watertight construction of the carrier bearing and reduces the unsupported length of the stock

10/3/2019 26

10/3/2019 27

10/3/2019 28

10/3/2019 29

Construction of Rudder

Modern rudder is of double plate construction and the steel plate sides are welded to an internal webbed framework.

Rudder framework can be of cast steel or fabricated of web plates welded together

Integral with the internal framework may be heavy forging which form the gudgeons or bearing housing for the rudder

The upper face of the rudder is formed into a usually horizontally/vertically flat palm of coupling for the stock

A lifting hole is provided for lifting when fitting/removal

Closing plate slot-welded to flat bars, which are welded to edge of internal frames.

10/3/2019 30

Construction of RudderInside of the rudder is coated with bitumastic preservative or filled

with inert foam

Vent plugs are provided for venting and draining

The rudder stock is connected to the rudder by vertical or horizontal coupling with fitted bolts and are locked by pins

10/3/2019 31

10/3/2019 32

Advantages of a Double Plate Rudder

Lighter , stronger

Reduce appendages resistance because of its streamlined , smooth surface/shape

Greater lift force produce due to aerofoil cross-section

Buoyancy reduced carrier bearing and coupling load

Perforation of one side due to corrosion/damage does not reduce its effective area.

10/3/2019 33

Special Rudders

In addition to the conventional types of rudder, there are some special types of rudder in use depending on the service condition of the ships.

Spade Rudders

Also known as ‘skeg rudder’, it is a free hanging fully balanced rudder in the shape of a spade.

It has no external supports in the form of pintles and is entirely supported by its stock…purpose of this design?

The stock is 30% heavier; shape tends to shift the C.P to turning axis and reduce B.M on stock; no pintles benefits

10/3/2019 34

10/3/2019 35

Flettner/Becker Rudders

Flap or flaps of small area are hinged to the trailing edge of the

rudder

These flaps are independently controlled and can improve

steering capability when activated without increasing its drag

force at large angle

The flap having a high aspect ratio gives a high lift to drag ratio

which improves steering capability.

10/3/2019 36

Borg Rudders

It is a rotary unit that provide maximum manoeuvrability

while operating at low speeds abd while carrying heavy

load

It uses the magnus effect to steer the ship..a cylindrical

body, rotating in a fluid develops a high lifting force at

right angles to the flow on the side of the rotor turning in

accord with the fluid flow.

Lift force increases rapidly as the surface speed of the

cylinder increases

10/3/2019 37

10/3/2019 38

Pleuger Active Rudders

It is an active rudder that does not depend on the speed of the ship to be effective

It has a streamlined body mounted on it that houses an electric motor coupled to a ducted propeller

This arrangement can actually turn the ship around its own length even when she is at rest

10/3/2019 39

10/3/2019 40