Propulsor configuration for increased efficiency

2

Content

Chapter 1:Introduction………………………………………………………1

Chapter 2:Devices before the propeller……………………………………..8

The wake equalizing duct…………………………………………………...8

Grothues spoilers…………………………………………………………..14

Chapter 3:Devices at the propeller………………………………………...16

Ducted propellers…………………………………………………...……..16

Application………………………………………………………………...25

Grim vane wheels………………………………………………………….32

Chapter 4: Devices after the propeller…………………….………………38

Contra-rotating propellers…………………………………………………38

Chapter 5: Conclusion…………………………………………………….45

Bibliography………………………………………………………………46

3

Chapter 1

Introduction

Great increases in the cost of fuel and the advent of very large tankers

and bulk carriers have focused the attention, during the last decades, on

means to enhance the efficiency of ship propulsion. An obvious way of

obtaining an efficiency increase is to use propellers of large diameter driven

by engines at low revolutions. Such a solution is, however, in many cases

not practically possible. This has then given impulse to the study and

adoption of unconventional propulsion arrangements, consisting, in general,

of static or moving surfaces in the vicinity of propellers.

A distinct indication of the serious and extensive activity in the

development and use of unconventional propulsors may be seen in the report

of the Propulsor Committee of the 19th ITTC (1990b).

Emphasis is given in the following to a variational procedure which,

in a unified fashion enables nearly optimum design of several of these

configurations.

In addressing the problem of energy efficiency it is useful to

understand just where the energy is expended in a fishing vessel and what

aspects of this can be influenced by the operator, boatbuilder or mechanic.

In a small slow-speed vessel, the approximate distribution of energy created

from the burning of fuel is shown in figure below.

Fig 1.1 Energy losses in a fishing vessel

Only about one-third of the energy generated by the engine reaches the

propeller.

4

• 35 percent is used to turn the propeller;

• 27 percent to overcome wave resistance;

• 18 percent to overcome skin friction;

• 17 percent to overcome resistance from the wake;

• 3 percent to overcome air resistance.

So, where can gains be made, or at least losses minimized?

1.Engine:

Most of the energy generated by the fuel burnt in the engine is lost as heat

via the exhaust and cooling system.

2.Propeller:

The energy lost in turning the propeller is controlled by two principle factors

- the design of the propeller and its condition.

3.Mode of operation:

The effect of wave resistance, although determined principally by the

dimensions and form of the vessel, increases dramatically with speed.

Significant fuel savings can be made by maintaining a reasonable speed

for the hull, irrespective of vessel type.

4.Hull maintenance:

The significance of skin friction is controlled principally by the quality of

the hull's finish - hull roughness as well as the amount of weed and marine

growth that is allowed to accumulate on the hull. Both of these factors are

under the direct influence of the operator's maintenance programme but,

depending on the type of vessel, a significant expenditure on hull finish is

not always worthwhile.

Propulsion Efficiency:

Efficiency is conventionally thought of as the product of the open-

water efficiency of the propulsor, the hull efficiency and a factor termed the

relative-rotative efficiency.

The relative-rotative efficiency is considered as a correction to the

propulsor (propeller) efficiency arising from the alteration in the radial

distribution of loading when operating in the hull viscous and potential

wake. The hull efficiency is given in terms of the wake fraction and the

thrust-deduction fraction as (1 - t)/(l - w). Although it is necessary to

consider the total propulsive efficiency the propeller designer has mainly

influence on the propulsor efficiency to which we limit our considerations

here.

The losses for an average propeller can be traced to 3 physical phenomena:

5

Axial losses:

A propeller generates thrust, due to the acceleration of the incoming water.

Behind the vessel, the outcoming flow mixes with the environmental flow.

Due to turbulence, energy will be lost.

Frictional losses:

Water in contact with the propeller blade surface causes friction, and thus

losses. The total blade surface, speed of rotation and surface roughness are

the dominating factors concerning frictional losses.

Rotational loses:

Rotation of the blade causes a rotation in the wake too; consequently this

energy is lost to generate a thrust in axial direction.

In addition a very significant loss is caused by the viscous drag of the blades.

Fig.1.2 Example of type of losses general ship propeller.

An indication of the relative importance of these three losses in

efficiency has been provided by Glover (1987) for representative vessels

with conventional propellers over a wide range of disc loadings as displayed

in Table 1.1.

6

CTh

Losses

[%] total efficiency

[%] axial rotational blade drag

0,56 15,5 6,7 16,4 61,4

1,43 22,7 5,6 13,9 57,8

3,44 32,1 4,8 14,3 48,8

5,98 40,6 6,9 10,7 41,8

Table 1.1.Values of propeller efficiency and it’s components for a range of thrust loading

values

It is clear that the addition of auxiliary surfaces which can reduce the

propeller loading by producing thrust and by reducing the rotational losses,

although these are relatively small, provide the possibility of higher

efficiency than the propeller alone with several provisos. These are:

The thrust loading of the optimum propeller is sufficiently high.

The added viscous and induced drag of the added surfaces is

sufficiently less than their production of thrust.

The cost of the installation can be recovered within (generally) five

years.

There are no or little "side effects" requiring costly maintenance or

failure of reliable operation such as cavitation or vibration.

The 1980’s saw a proliferation of hydrodynamically based energy-saving

devices enter the marine market.

For discussion purposes these energy-saving devices can be considered as

operating in three basic zones of the hull. Some are located before the

propeller, some at the propeller station and some after the propeller. Figure

1.3 defines, these three stages as Zones I, II and III respectively for devices

which act before, at or astern of the propeller location. Clearly some devices

transcend these boundaries but these three zones are useful to broadly group

the various devices.

7

In Zone I the thrust augmentation device is reacting with the final stages of

the growth of the boundary layer over the stern of the ship in order to gain

some direct benefit or present the propeller with a more advantageous flow

regime - in some cases perhaps both.

In Zones II and III they are working within both the hull wake field and the

slipstream of the propeller and attempting to recover energy which would

otherwise be lost. Table 1.2 identifies some of the principal thrust

augmentation devices and attempts to categorize them into their /ones of

operation.

Energy-saving device Zones of operation

Wake equalizing duct

Grothues spoilers

I

I

Ducted propellers

Grim vane wheels

II

II

Contra-rotative propeller III

Table 1.2. Zones of operation of energy-saving devices

Fig.1.3 Zones of classification of energy-saving devices

8

Chapter 2:Devices before the propeller

The wake equalizing duct

The wake equalizing duct (WED) is a ring-shaped flow vane with

foil-type cross-section fitted to the hull in front of the upper propeller area

(Fig.2.1).

The WED is by far the most frequently installed propulsion improving

device (Table 2.1).In contrast to the Kort nozzle, which shrouds the

propeller, these ducts are less than half as big in diameter and section length

and are arranged in the wake.

They are fitted to the hull in the form of two half-ring ducts in front of

the propeller. Their upper ends may be integrated to the hull ahead of the

stern frame or they may extend into the stern aperture, in which case the gap

at the trailing edge aft of the stern frame is given a horizontal filling.

WEDs consist usually of two centro-symmetric halves which are

connected by straight foiltype parts to the hull. For an asymmetric stern

fitting a half-ring duct on only one side can be more beneficial than the

double-sided arrangement.

Fig 2.1.Wake equalizing duct

9

Table 2.1.Installations of propulsion improving devices up to 1991

The duct is most effective on the side with larger curvature of the

waterlines. The basic principle underlying the application of this device is

that the flow creates a circulation around the foil section of half-ring ducts

which accelerates the flow in the area enclosed by them and retards it in

their outer environment.

Fig.2.2 a Schematic diagram of flows:

Flow along a waterline at a height about ¾ propeller diameter.In stern region

separation occurs

Fig.2.2 b Schematic diagram of flows:

Flow with duct, no separation

Type number

Wake equalizing duct >500

Asymmetric aftbody 75

Vane wheel 60

Grothues spoilers 35

10

Thus, such a nozzle channels the flow in the upper quadrants where it

matters most. The inward-directed circulation guides the water into the duct,

and ahead of it presses the flow on to the hull. The flow is then better

attached to the hull and separation prior to the duct is reduced (Fig.2.2).

A normal longitudinal section across the duct explains the circulation

effect relating to the speed distribution in the upper and lower halves of the

propeller.The inflow of the propeller is accelerated in the upper region

where it is slow, corresponding to the fuller form of the ship, and in the

lower region, where the speed of inflow is normally higher, it will be

retarded.

In practice the average and effective wake will hardly be changed

(Fig.2.3). In accelerating nozzles and ducts the open cross-section at the

trailing edge is usually smaller than that at the leading edge. This often may

not be so in WEDs.

The flow in the WED region has divergent flowlines due to the ship hull

form. The WED decreases this divergence by locally accelerating the flow in

this region.

Fig.2.3 Circulation in vertical direction

11

Advantages:

The main advantage lies in power savings resulting from various effects:

Improved propeller efficiency:

Results from more axial flow and more uniform velocity distribution over

the disc area.The asymmetrical arrangement of half ducts gives a rotational

direction to the water entering the propeller, which is opposite to that which

the propeller will impart. Thus the loss from rotation energy in the propeller

wake is less.

Reduction of flow separation at the aftbody:

This effect is strong and reduces resistance and the thrust deduction

fraction.

Pre-rotation:

The nozzle axes are oriented such that the propeller inflow is given a slight

pre-rotation which counteracts the propeller rotation.

Improved steering qualities:

Results from more straightened flow to the rudder. In spade rudders the

longer upper sections become more effective because of the higher flow

velocity.

Improved course-keeping ability.

No constructional changes and no modifications in propeller design

are involved when the duct is fitted to an existing ship.

Possibility to integrate devices for ice protection to propeller:

Even without special ice protection, ducts protect propellers. Up to 1997,

almost 900 ducts had been installed, many in ships on ice-infested routes.

No damage to ducts has been reported and ice-damage to propellers has been

reduced.

Reduction of propeller-excited vibrations:

Caused by decreased propeller tip loading in upper quadrants to less than

half the amplitudes. This allows reduction of propeller clearances in new

designs. Reduced vibrations have in practice also decreased malfunctions of

electronic equipment. The reduction in vibration amplitudes by the WED is

easily explained by the velocity distribution. Larger inflow velocity means

smaller angle of attack – between profile zero lift position and inflow

direction .The hydrodynamic forces and thus pressure impulses are roughly

proportional to the angle of attack for small angles of attack. The WED also

smooths the torque and thus reduces the tendency for torsional vibrations.

12

Higher speeds:

The power savings can be used to obtain higher speed. For a given speed,

the power savings are converted to a lower rate of rotation.Model tests have

determined a maximum energy saving of 14% at the same speed in several

cases.

Fig.2.4 Speed versus engine power

Reduced cavitation:

Unlike ducted propellers, which are hardly used in ocean vessels due, in

part,to problems of tip clearances and cavitation, the WED does not pose

such problems. The more uniform flow into the propeller reduces the

dangers of propeller cavitation. The duct itself is less exposed to this

problem than the rudders because of the considerably lower flow velocity in

the wake at its location, which is often less than half the ship speed. Another

advantage here is that normally the WED is considered for moderate speed

vessels with block coefficient above 0.6; fast ships, which tend to have

cavitation problems, are less suited to its use.

Cost aspects:

In newbuilds the costs of the duct can be lower than those costs saved by

choice of a smaller engine, made possible by the power savings.

13

The investment for fabrication and fitting is invariably recovered in 6–20

months, depending on ship form and fuel price.

Simpler design:

- The ship hull can be kept simpler. The stern bulb can be built

less pronounced and the counter can be placed lower. Concave waterlines

at the height of the WED are not necessary, thus the hull is cheaper to

produce and the resistance lower.

- Simpler propellers with fewer blades and less skew. The

propellers can be more highly loaded at the tips. Thus the propellers are

cheaper, yet more efficient.

Fig 2.5. Axial velocity distributions at the propeller plane, without and with the duct

Disadvantages:

Above certain speeds, the drag created by the ducts is greater than the

advantages.

14

Grothues spoilers

This device, named after its inventor, Grotheus-Spork (1988), consists

of a set of fins attached to the hull ahead of the propeller. They consist of a

set of triangular fins attached to the hull just ahead of the propeller, as show

in Fig.2.6.The purpose is to straighten the boundary layer flow right before

the propeller, thus creating direct thrust and improving the propeller

efficiency. These devices are also referred to as Grothues Fins.

Fig.2.6 Grothues spoilers

The idea is to straighten the flow before it enters the propeller plane.

The fins provide thrust and the propeller inflow is more axially directed. In

principle such fins may be beneficial: wake velocities in transverse direction

create a non-uniform propeller loading and is lost energy. They may even

avoid separation. The problem is control of the flow around these fins. When

they are not in the proper position their effect is negative, and even if they

are, the own viscous drag of he fins may annihilate any gains.

The fins are designed to improve the flow into the propeller by

converting the vertical component of the flow due to bilge vortices into a

horizontal flow, and thus recover energy. The fins produce a small forward

thrust, increase the mass flow through the propeller disc and reduce angular

velocity variations in the flow. Grothues spoilers thereby reduce resistance,

improve propeller efficiency and reduce vibration.

The shape, position and number of fins must be determined by model

tests. Power savings ranging from about 3 percent for fine vessels with low

breadth-draught ratios to about 9 percent for full tankers are possible using

Grothues spoilers, greater improvements being achieved in the ballast

condition.

15

Fig.2.7 The water flow around grothues spoilers

During the drive for energy saving devices in the seventies these fins

have been applied incidentally in combination with the Schneekluth duct.

The advantages of these fins are minor, if any. The own drag of the fins is

often larger than the gain in propeller efficiency. The main advantage is the

suppression of separation, which sometimes occurs involuntarily. In that

case such devices may be useful as retrofits to correct problems.

Advantages:

*Power savings measured in model tests were:

Tankers and bulkers, fully loaded up to 6%

Tankers and bulkers, in ballast up to 9%

Ships of medium fullness with B=T < 2:8 up to 6%

Fine vessels with small B=T up to 3%

*The expense of manufacturing and fitting spoilers is generally low.

Disadvantages:

*If not placed in proper position, they can increase the drag of the ship.

16

Chapter 3:Devices at the propeller

Ducted propellers

To obtain the most thrust, a propeller must move as much water as

possible in a given time. A nozzle will assist the propeller in doing this,

especially when a high thrust is needed at a low ship speed. As we already

know, as the propeller blades rotate in the water, they generate high-pressure

areas behind the each blade and low pressure areas in front, and it is this

pressure differential that provides the force to drive the vessel. However,

losses occur at the tip of each blade as water escapes from the high pressure

side of the blade to the low pressure side, resulting in little benefit in terms

of pushing the vessel forward. The presence of a close fitting duct around

the propeller reduces these loses by restricting water flow to the propeller

tips.

A ducted propeller consists of a screw propeller surrounded by a non-

rotating duct (shroud or nozzle) usually in the form of an axisymmetric

(annular) aerofoil with a very small gap between the propeller blade tips and

the internal surface of the duct, Fig. 1. The duct cross-section is normally

circular. However, in ships with very blunt waterlines aft, the forward part of

the duct may be widened laterally, giving the forward part of the duct an

elliptical cross-section.

The concept of ducted propellers is due to Stipe (1931) and to Kort

(1934).

fig.3.1 Ducted propeller

17

a b Fig.3.2 Circulation around accelerating (a) and decelerating nozzle (b)

dT=thrust; dL=lift; pu=negative pressure; p0=positive pressure

There are two types of ducts:

1.Accelerating ducts:

An accelerating duct produces positive thrust and increases the

efficiency of heavily-loaded props (a heavily-loaded prop is one where the

power being put through it is close to the maximum absorbable for that prop

design).

With accelerating ducts, the inflow velocity and efficiency of the

propeller is increased. This is the type that is used on heavily loaded

propellers or propellers with limited diameter. As Ludwig Kort performed

extensive research on this type, it is often called Kort nozzle.

The Kort nozzle is a shrouded, ducted propeller assembly for marine

propulsion. The hydrodynamic design of the shroud, which is shaped like a

foil, offers advantages for certain conditions over bare propellers.

Kort nozzles or ducted propellers can be significantly more efficient than

unducted propellers at low speeds, producing greater thrust in a smaller

18

package. Tugboats are the most common application for Kort nozzles as

highly loaded propellers on slow moving vessels benefit the most.

The additional shrouding adds drag, however, and Kort nozzles lose

their advantage over propellers at about 10 knots (18.5 km/h).

Kort nozzles may be fixed, with directional control coming from a rudder set

in the water flow, or pivoting, where their flow controls the vessel's steering.

Shrouding of this type is also beneficial to navigation in ice fields since it

protects the propeller tips to some extent.

In a Kort nozzle, the inflow velocity is increased, reducing pressure.

This lowers thrust and torque of the propeller. At the same time, a

circulation occurs, resulting in an inward aimed force, that has a forward

component. The duct therefore has a positive thrust. This is normally larger

than the thrust reduction of the propeller. The small clearance between the

propeller and duct reduces tip vortex, increasing efficiency.

As drag increases with increasing speed, eventually this will become

larger then the added thrust. Vessels that normally operate above this speed

are therefore normally not fitted with ducts. When towing, tugboats sail with

low speed and heavily loaded propellers, and are often fitted with ducts.

Bollard pull can increase up to 30% with ducts.

Accelerating ducts, Fig. 3.2 (a), are used in heavily loaded propellers.

The small clearance between the propeller blade tips and the duct suppresses

the trailing free vortices shed by the blades, the bound vortices on the blades

joining the bound vortex ring on the duct. The shape of the accelerating duct

at the forward end (leading edge) increases the mass flow to the propeller.

At the after end, the duct is so shaped that the cross-section increases going

aft and the normal slipstream contraction is suppressed. The increased

inflow velocity causes a decrease in the thrust and torque of the propeller.

At the same time, a circulation develops around the duct section

resulting in an inward directed force which has a forward component, the

duct thrust. The duct also has a drag directed aft, and this should be

substantially less than the duct thrust. The total thrust of the propeller and

duct taken together is then usually greater than that of an equivalent open

propeller (i.e. one without a duct) whereas the torque is smaller. The

efficiency of the ducted propeller is therefore greater than that of the open

propeller. The improved efficiency of the ducted propeller may also be

explained by the reduced kinetic energy losses in the slipstream due to the

suppression of the trailing vortices and the reduction of the slipstream

contraction.

19

2.Decelerating ducts:

With the second type,Fig.3.2 (b), the inflow velocity into the propeller

is reduced, whereby pressure at the propeller location is increased, and

delays the cavitation.

The duct has a circulation around it which produces an outward

directed lift with a component directed aft, i.e. the duct thrust is negative.

The efficiency of a ducted propeller with a decelerating duct is lower than

that of an equivalent open propeller, but its cavitation properties are su-

perior. Decelerating ducts are therefore used for high speed hydrodynarnic

bodies in which it is necessary to minimise cavitation and underwater noise.

A decelerating duct produces a negative thrust and is valuable for

reducing cavitation. A decelerating duct is a major contributor to noise

reduction which is why it is getting more popular with modern warship

designs. The most used decelerating ducts systems the ones are called pump-

jets, used in high-speed or shallow draft operating ships.

Theoretical comparison between accelerating and decelerating ducts:

An insight into the performance of ducted propellers is provided by

applying the axial momentum theory. Consider the fluid column flowing

through a ducted propeller of cross-sectional area A0, Fig. 3.3.

The pressures far ahead, just ahead, just behind and far behind the

propeller are '

110 ,, ppp and 2p respectively, the propeller being regarded as

an actuator disc causing'an abrupt increase in pressure from 1p to '

1p .The

velocities of the fluid with respect to the propeller far ahead, at the propeller

and far behind are 1, vVV AA and 2vVA . The mass of fluid flowing through

the propeller per-unit time is:

1vVm AO (1)

The total thrust of the propeller is equal to the rate of change of momentum

of the fluid in the slipstream, and is given by:

AA VvVmT 2 = 21 vvVAO (2)

20

Fig.3.3 Flow through a Ducted Propeller

Applying the Bernoulli theorem to the sections far ahead and just ahead of

the propeller, one obtains:

2

11

2

02

1

2

1vVpVp AA (3)

Similarly, for the sections far behind and just behind the propeller:

21

'

1

2

222

1

2

1vvpvVp AA (4)

Far astern of the propeller the flow is parallel to the axis and hence there is

no radial pressure gradient, so that 02 pp and:

22

22

21

'

12

1

2

1

2

1vvVVvVpp AAA

(5)

The thrust for the propeller alone is then:

221

'

12

1vvVAAppT AOOP

(6)

21

A thrust ratio is now defined as the ratio of the propeller thrust pT to the

total thrust Dp TTT , where DT is the thrust of the duct:

T

T

TT

T P

DP

P

A

A

V

v

V

v

1

2

1

2

1

1

(7)

The thrust loading coefficient is given by:

AAAO

A

AO

TLV

v

V

v

VA

vvVA

VA

TC 2

2

2

21

2

2

1

12

2

1

2

1

(8)

This gives the following results:

5.02

112

12

1

1 TL

A

CV

v

5.01 112

11 TL

A

CV

v

(9)

5.02

1 1112

1

TL

AAA

D CV

v

V

v

V

v

where Dv is the induced velocity due to the duct.

The delivered power of the propeller is equal to the increase in the kinetic

energy of the fluid per unit time:

22

22

1

2

1AAD mVvVmP 221

2

1

2

1vvVvVA AAO

(10)

The ideal efficiency of the ducted propeller is then:

221

21

2

1

2

1vvVvVA

VvvVA

P

TV

AAO

AAO

D

Ai

AV

v22

1

1

1

5.011

2

TLC (11)

22

Putting '112

1ppp , the pressure coefficient at the propeller is given

by:

2

0

2

1A

p

V

ppC

2

5.0

2

11

21

TLTL CC (12)

The effect of the drag of the duct on the efficiency of the ducted propeller

can be taken into account approximately as fallows:

DAD CDlVD 2

2

1

TLAO

DAD

D

CVA

CDlV

T

DTk

2

2

2

12

1

1

TL

D

C

C

D

l41

noting that 4

2

0

DA

. Then:

5.011

241

TLTL

DiD

CC

C

D

lk

(13)

where:

DD – duct drag

D – propeller diameter

l – duct length

CD – duct drag coefficient

KD – drag correction factor

- ducted propeller efficiency including duct drag.

Conclusions:

Equations (11), (12) and (13) yield some interesting conclusions:

The ideal efficiency of the ducted propeller increases as r decreases.

In practice, reducing below a certain limit would cause the flow

past the inner surface of the duct to break down due to boundary layer

separation, resulting in a sharp drop in the efficiency of the ducted

propeller.

23

The static pressure at the propeller increases as increases, thereby

delaying cavitation. By definition, if is greater than 1 the duct has a

negative thrust.

The effect of duct drag reduces as the thrust loading of the ducted

propeller increases. For values of TLC exceeding about 1,5 the drag of

the duct is insignificant compared to the kinetic energy losses.

The effect of duct drag on efficiency can be reduced by minimising

the length of the duct. In practice, however, there is a lower limit to

the duct length for a given duct loading below which the flow through

the duct breaks down resulting in a severe degradation of

performance.

Real-life test:

To demonstrate the difference in design efficiencies, tests were

conducted using (2) 21,9456 meters trawlers of 375 Hp @ 1800rpm with 6:1

reduction gear each performing the same work. One was fitted with an open

propeller, and the second with a Kort nozzle and a Kaplan propeller.

Test results :

Open Prop 66" x 44":

– Bollard Pull: 4708 kg

– Max Speed: 9.3 knots

– Fuel: 1512 l/day

Kort Nozzle 61" x 56":

– Bollard Pull: 6646 kg

– Max Speed: 9.0 knots

– Fuel: 1264.5 l/day

24

Advantages:

At high thrust-loading coefficients, better efficiency is obtainable. For

tugs and pusher boats, efficiency improvements of around 20% are

frequently achievable. Bollard pull can be raised by more than 30%.

The reduction of propeller efficiency in a seaway is lower for nozzle

propellers than for non-ducted propellers.

Course stability is substantially improved by the nozzle.

In ‘steerable nozzle’ versions, the nozzle replaces the rudder. The hull

waterlines at nozzle height can be run further aft and thus the waterline

endings can be made finer and ship resistance reduced. The steerable

nozzle, however, has a somewhat lower efficiency than the fixed nozzle,

since the gap between propeller blade tips and nozzle internal wall must

be kept slightly larger. There is also less space for the propeller diameter,

since the steerable nozzle, unlike conventional fixed nozzles, cannot fit

into the stern counter.

Decelerating ducts minimise cavitation and underwater noise ( useful

for military ships)

Disadvantages:

Course-changing ability during astern operation is somewhat

impaired.

Owing to circulation in shallow water, the nozzle propeller tends to

draw into itself shingle and stones. Also possible is damage due to

operation in ice. This explains the infrequent application on seagoing

ships.

Due to the pressure drop in the nozzle, cavitation occurs earlier.

25

Application:

A tug has a propeller of 3,1 m diameter and 0,8 pitch ratio, the open

water characteristics of which are as given in table 1.The tug has a wake

fraction of 0,180 in the free running condition and 0,050 in the bollard pull

condition, a relative rotative efficiency of 1,000 and a shafting efficiency of

0,950.The effective power of the tug is as follows:

Speed, knots 10,0 11,0 12,0 13,0

Effective power, kW 137,7 192,2 260,7 344,9

If the propeller is run at 150 rpm in both the free running condition and the

bollard pull condition, determine the free running speed and the bollard pull,

and the corresponding brake powers.

The propeller is replaced by a ducted propeller of the same diameter and

0,912 pitch ratio, whose open water characteristics are as follows:

Determine the free running speed and the bollard pull and the corresponding

brake powers.

D = 3,1 m w = 0,200 t0 = 0,050 t1 = 0,180

000,1R 950,0S n = 150 rpm = 2,5 s-1

J 0 0,100 0,200 0,300 0,400 0,500 0,600 0,700

KT 0,4427 0,3957 0,3493 0,3019 0,2518 0,1976 0,1376 0,0702

10KQ 0,3497 0,3467 0,3382 0,3237 0,3026 0,2739 0,2365 0,1887

26

Table 1.Open water characteristics of a methodical series.

27

28

Free running condition:

V

PR E

T t

RT T

1

V

V

Dn

VwJ

6875,9

1

1,35,2

200,011

06,6061,35,205,1 4242

TT

Dn

TKT

V k 10 11 12 13

m s-1

5,1440 5,6584 6,1728 6,6872

PE [kW] 137,7 192,2 260,7 344,9

RT [kN] 26,769 33,967 42,233 51,576

T [kN] 32,645 41,423 51,504 62,897

J 0,5487 0,5840 0,6371 0,6902

KT 0,0538 0,0683 0,0849 0,1037

Open propeller

The KT – J curves from the above table and from table 1 for P/D = 0,800

intersect at:

J = 0,6584

KT = 0,0849

10KQ = 0,1525

29

Free running speed:

V = 9,6875 J = 9,6875 x 0,6875 = 6,1719 m s-1

= 11,99723 knots

Delivered power:

R

Q

D

KDnP

532

314,439000,1

01577,05,2025,12 3 DP [kW]

Brake power:

435,462950,0

314,439

S

DB

PP

[kW]

Bollard pull condition:

From table 1 for P/D = 0,800:

KT0 = 0,3415

KQ0 = 0,04021 at J = 0

Bollard pull:

42

00000 11 DnKtTtBP T

939,1911,35,2025,13415,0050,01 42 BP [kW]

Delivered power:

R

Q

D

KDnP

053

02

42,1158000,1

04021,01,35,2025,12 53 DP [kW]

Brake power:

39,1219950,0

42,1158

S

DB

PP

[kW]

30

Ducted propeller:

Free running condition:

The KT – J curve for the ducted propeller intersects the KT – J curve from

the foregoing at the same values of J and KT as the open propeller; i.e. at the

free running speed :

J = 0,6371

KT = 0,0849

10KQ = 0,2022

Delivered power:

R

Q

D

KDnP

053

02

523,582000,1

02022,01,35,2025,12 53 DP [kW]

Brake power:

182,613950,0

523,582

S

DB

PP

[kW]

Bollard pull condition:

Bollard pull :

42

00000 11 DnKtTtBP T

819,2481,35,2025,14427,0050,01 43 BP [kN]

Delivered power

R

Q

D

KDnP

053

02

46,1007000,1

03497,01,35,2025,12 53 DP [kW]

31

Brake power:

484,1060950,0

46,1007

S

DB

PP

[kW]

Conclusion:

Both the open propeller and the ducted thus give the same running speed

in this example, but the ducted propeller provides a higher bollard pull at

a lower power.

32

Grim vane wheels

The vane wheel is a second propeller downstream of the main

propeller, which runs freely without torque on the shaft.

A vane wheel is composed of a set of equally-spaced narrow blades

mounted on a hub which freely rotates around the propeller shaft which is

extended abaft the propeller. This device, invented by Professor Dr.Otto

Grimm, is characterized by a diameter of some 25 per cent greater than that

of the propeller with pitch distributions such that the region in the propeller

race performs as a turbine and in the portion outside the race as a propeller.

Fig.3.4 Queen Elizabeth grim wheels

A vane wheel basically increases the propeller diameter. Since the

vane wheel reduces the axial and tangential velocity in the wake of the main

propeller, the optimum pitch of the propeller may be increased and the

rotation rate decreased, similar to a contra-rotating propeller. That increases

efficiency. It means that the main propeller has to be designed in

combination with the vane wheel is not a retrofit, which can be added to any

propeller.

33

The inner part of the vane wheel, the impeller part or turbine part, has

a pitch such that the vane wheel is driven, by the wake of the main propeller.

The outer part of the blades of the vane wheel, the propeller part, has a

different pitch, which causes the vane wheel to generate thrust of the main

propeller.

Vane wheels are subjected to strong fluctuations in loading. Problems

with the strength of the blades have been encountered frequently and this has

led to its limited application.

Advantages:

Substantial recovery of rotational energy

Less resistance from rudder behind the vane wheel,that is better

relative efficiency

Greater possible jet cross-section of vane wheel, since the low rpm

rate and large number of blades enable smaller vertical clearances to

be accepted.

Less resistance from rudder behind the vane wheel. This is reflected in

the relative rotative efficiency.

Better stopping ability

Reduces cavity

Moreover, the higher rpm rate associated with the smaller diameter of

the engine-driven propeller improves the weight and cost of the

propulsion unit.

Disadvantages:

Cost of the vane

If not designed carefully, the resulting drag can create more resistance

than the energy retrieved.

Strong currents can destroy the blades of the vane wheel

It is clear that the vane wheel which rotates in the same direction as the

propeller at an angular velocity in the range of 35 to 50 per cent of that of

the propeller makes use of both the axial and the tangential momentum

losses of the propeller.

The propeller and vane wheel has a larger diameter than the propeller

alone, entraining a greater volume of water and hence an increment in

efficiency as dictated by momentum considerations. A similar increment

can, of course, be secured by using a single propeller of the same, larger

34

diameter, but this would require lower shaft speed and in addition,increased

vibratory excitation attending reduced tip clearance. In contrast, the lightly

loaded vanes can extend much nearer to the hull without producing

significant vibratory forces.

The principal parameter indicative of the gain in propulsor efficiency is

the thrust-loading coefficient ThC ,the gain rising with increasing ThC . The

minimum ThC for a zero gain in efficiency is 0.50. In practice the minimum

ThC must be greater than 1.0 to obtain a reasonably short pay-back period of

the installation costs.

It is interesting to note that the first model tests carried out with the vane-

wheel design displayed much lower performance than predicted by Grim's

calculations. This was later understood to be caused by the low Reynolds

numbers at which open-water and particularly self-propulsion tests are

normally conducted. Later experiments by Blaurock & Jacob (1987) using

large models were conducted in the cavitation tunnel at Hamburgische

Schiffbau-Versuchsanstalt (HSVA) over a range of Reynolds numbers.

The results displayed in Figure 3.5 show the dominating effect of

Reynolds number on the gain in efficiency over a wide range in thrust-

loading coefficients.

Fig.3.5 Gain in efficiency by means of a vane wheel

predicted by model tests and achieved in full scale

35

Model tests at HSVA have since then been made without the vane

wheel. The full-scale prediction of the effect of the wheel is then made by

use of Grim's theory which has been verified by many sea trials.

Fig.3.6 Optimum circulation distribution for propeller

and vane-wheel combination

These measurements have shown gains in propulsive efficiency in the

range of 7 to 10 per cent depending upon CTh as summarized in Figure 3.5.

Blaurock relates that measurements of fluctuating pressures on sister hulls in

way of the propeller without and with vane wheel showed amplitudes with

the wheel to be halved at blade frequency and some 30 per cent less at twice

blade frequency. This is ascribed to reduced propeller cavitation attending

the reduction in propeller loading.

In application of the unified variational procedure by Kerwin et al.

(1986), to determine optimum propeller and vane-wheel loading

distributions this propulsor is analogous to a pair of tandem propellers of

unequal diameter with the aft unit rotating at a rate of revolutions at a

fraction of that of the forward element and as the torque on the vane wheel is

zero, then Q2 =0, giving the constraint parameter q = 0.

The optimum radial distributions of circulations for a propeller and

vane wheel are given in Figure 3.6. The operation conditions are the same

for the single and contrarotating propeller. It is seen that the propeller

component is more lightly loaded, near the tip, relative to the single

propeller.

36

First ship to use Grim Wheels:

The first passenger ship which uses Grim Wheels was Queen

Elizabeth 2. During retrofit, the fixed-pitch propellers were replaced with

variable-pitch types.

.

The pitch of the new variable-pitch blades, however, could simply be

reversed, causing a reversal of propeller thrust while maintaining the same

direction of propeller rotation, allowing the ship better stopping times and

improved handling characteristics.

The new propellers originally were fitted with Grim Wheels, which

were free-spinning propeller blades that were fitted behind the main

propellers with long vanes protruding from the centre hub.

Fig.3.7 Propeller side view plan Sketch of the propeller

and Grim Wheel arrangement on the QE2

37

Fig.3.8 Graph showing power absorption by the propellers at constant rev/min and

corresponding ship speeds

These were designed to recover lost propeller thrust and reduce fuel

consumption by 2.5 to 3%.

However, after the trial of these wheels, when the ship was

drydocked, the majority of the vanes on each wheel were discovered to have

broken off, and so the wheels were removed and the project abandoned.

38

Chapter 4: Devices after the propeller

Contra-rotating propellers

Contra-rotating propellers (CRP) have been known since 1836. They

are commonly used in torpedoes and are often met in fast motorboats.

Although their good hydrodynamic properties were well recognized they did

not gain much popularity as ship propulsors because of the mechanical

complexity associated with long shafts, their bearings and sealing.

Contra-rotating propulsion systems have the hydro-dynamic

advantage of recovering part of the slip-stream rotational energy which

would otherwise be lost to a conventional single screw system.

When a propeller accelerates water into itself from ahead and expels it

astern, it generates thrust. Unfortunately, a sizeable percentage of the power

delivered to the propeller also goes into twisting the water around, creating

the helically shaped propeller wake. This rotational energy is doing nothing

to drive the boat—it is just waste.

Fig. 4.1.Contra-rotating propeller

Contra-rotating propellers eliminate this waste. In this type of

installation, two propellers are positioned one immediately ahead of the

other on the same shaft line, but rotating in opposite directions. The

39

rotational energy imparted to the water by the forward propeller is cancelled

out by the opposite rotation of the aft propeller.

Furthermore, because of the two propeller configuration,contra-

rotating propellers possess a capability for balancing the torque reaction

from the propulsor which is an important matter for torpedo and other

similar propulsion problems.

In marine applications of contra-rotating propulsion is normal for the

aftermost propeller to have a smaller diameter than the forward propeller,

and, in this way, accommodate the slipstream contraction effects.

Similarly, the blade numbers of the forward and aft propellers are usually

different.

This is because the astern propeller is working in a faster water flow

than the ahead propeller, and as a result, it must have a smaller diameter and

steeper pitch. At the same time, both propellers should be absorbing the

same horsepower—hence the additional blade on the after propeller.

Because of the large number of blades—usually seven or nine, in

total—a contra-rotating propeller system has more blade area and thus lower

blade loading than a comparable single propeller at the same horsepower.

This reduces cavitation problems, and the large number of blades greatly

diminishes vibration.

A new concept of CRP propulsion is the hybrid CRP-POD propulsion

system, shown to have the best hydrodynamic as well as total propulsion

efficiency when CRP, twin-screw twin-skeg and single-screw systems were

compared in model tests.

Fig 4.2. CRP POD propulsion system

40

CRP has been developed by combining an electric pod propulsion unit

and conventional diesel propulsion system. With this system, energy is

saved by more than 13% as compared with the conventional twin shafts and

propellers system, contributing to a reduction of operation costs and of CO2

emission.

The CRP Azipod propulsion system consists of two contra-rotating

propellers.Located on the same axis, but without physical connection, the

pulling propeller of the Azipod unit will contra-rotate in relation to the shaft-

driven main propeller. There is no need for a conventional rudder because

the pulling Azipod unit acts as one.

The pod propulsion unit is an azimuth type one driving a propeller

directly coupled with a motor by incorporating the motor in a pod unit. This

propulsion unit is at the stern of the hull, and also the unit has a function of

rudder due to rotation. Thanks to its 360-degree free rotation, excellent

steering performance is realized in harbor and pier operations, together with

powerful propulsion.This compact system incorporating a propulsion motor

in the pod was jointly developed by European electric manufacturer and

shipyard in the early 1980s for ice breakers.

The pod propulsion unit, a revolutionary system in those days, has

been employed in more than 70 vessels, but since it is an expensive system,

it has been used mainly in cruise passenger ships so as to make

the best of its features, including excellent steering performance, vibration

and noise suppressing effect,and high flexibility of inboard layout.

As shown in Fig. 4.2, in this ship, the pod propulsion unit is located

immediately behind the coaxial line of the main propeller of one shaft, and

two propellers are arranged like one set of contra-rotating propellers. The

main propeller is a controllable pitch propeller, and is driven directly by two

sets of medium speed diesel main engines by way of reduction gears with

clutch and intermediate shaft. The pod propeller positioned behind is an

electric propulsion unit driven by an electric motor in the pod,using electric

power from the power generation plant.

As a result, two sets of propellers can be installed without any

appendages such as shaft bracket, and the resistance performance is

significantly improved as compared with the conventional twin shafts ship.

In addition,rotating the adjacent propellers in opposite directions could

realize high propulsion efficiency by reducing tangential water flow.

41

A comparison between classical CRP system and the CRP-POD can

be made if we are comparing the results of the mean axial velocity at one

diameter distance from the rear propeller plane it is clear how in the contra-

rotating propeller pod drive model the axial momentum reaches higher

values, while presenting a more uniform distribution. Near the axis, the

mean axial velocity shows a strongly marked depression, which is present in

both the CRP and the CRP-POD drive models. While for the first the

velocity drops to almost half of the propeller advance speed, VA for the last

model the velocity never falls below 1A

A

V

U,the low values of axial

momentum near the propeller axis evidence the velocity defect due to the

influence of the propeller hub, which is not producing thrust.

Fig 4.3.Distribution mean axial velocity

42

In this region there is nearly no dependence of the phase, that is.

The velocity here is independent of the propeller position. At a radial

position of about 80% of the propeller radius a strong dependence of the

phase is evidenced by the changing profiles. In both cases the tip vortex is

made visible by a solid body rotation pattern in line = 80° at position

r/Ri≈ 0.8 (Figs. 4.3(a) and 4.3(b)).

In the far wake the angle dependency of the flow is much attenuated,

which is recognized by the overlapping of the six curves in Figs. 4.3(c) and

4.3(d). This results in a weak correlation between the rear propeller angular

position and the flow field in the far wake. In both models the effect of

distance in the axial momentum profile is to make it more uniform and to

flatten the depression in the axis. Yet, the mean axial velocity distribution is

much different in both cases.

The CRP model shows a distribution which is characterized by the

hub velocity defect at inner radii and an area of momentum increase in the

interval 0.2 < r/R1 < 1.0, which coincides with the area of thrust production.

The distribution of mean axial velocity in the case of the CRP-POD model is

much more homogeneous at this axial position and the hub velocity defect is

imperceptible.

Fig. 4.4 Plant comparison

Fig. 4.4 compares the conventional mechanical drive twin-shafts system and

the hybrid system, and shows that the energy saving effect is as high as 13%.

43

Advantages

*The propeller-induced heeling moment is compensated (negligible for

larger ships).

*The slipstream from contra-rotating propellers is nearly smooth and

straight, with little twist. Such propellers are, in theory, between 5 and 20

percent more efficient than standard single propellers, but 8 to 10 percent is

a realistic practical range.

*More power can be transmitted for a given propeller radius.

*The propeller efficiency is usually increased.

* Less emissions, environmentally friendly

Particularly, for the CRP-POD hybrid system ,the advantages are:

*The energy gain is even higher, because the resistance of the appendages

(propeller shafts, brackets, skegs, rudders etc.) is absent. This significantly

decreases the ship’s resistance since the propulsor is located in the

decelerated flow region.

* Two independent propulsion systems give high redundancy

*No need for stern thrusters

*Flexible general arrangement

*Less pressure pulses on vessel hull

*Easily adjustable propulsion machinery to match required propulsion

power.

* The hybrid concept is especially attractive when the main propulsion is

very heavily loaded due to the large ship speed or power requirements of the

vessel. Adding a pod to the main propulsion will raise the

overall speed and power.

Disadvantages

* The main drawback to contra-rotating propellers is their tremendous

complexity. Not only docs this increase the cost of contra-rotating propellers

to far above that of comparable standard single propellers, but it also

increases the level of maintenance required. Further, some of the gain in

efficiency from thrust is lost to additional frictional resistance in the many

extra gears and bearings.

*The hydrodynamic gains are partially reduced by mechanical losses in

shafting.

44

* The sealing of the shafting against the ingress of water from outside is also

a major problem.

*The resistance of the pod parts (the motor housing and the strut).

Apart from their long-standing use in torpedoes, contra-rotating

propellers have been tried out in submarines, and more recently in merchant

ships.

The Grim wheel, is related to the contra-rotating propeller, but

the ‘aft’ propeller is not driven by a shaft. Unlike a contra-rotating propeller,

the Grim wheel turns in the same direction as the propeller.

45

Chapter 5: Conclusion

Real life tests revealed the following results for the energy-saving

solutions shown before. Some of the solutions are useful, regarding the

present day high prices for fuel, but the power gain of others proved to be

less than expected, and they don’t deserve the costs involving retrofitting.

46

Bibliography

[1]- Marine Propellers and propulsion ,John Carlton

[2]- Hydrodynamics of Ship Propellers, John P. Breslin, Poul Andersen

[3]- Ship Design for Efficiency and Economy, Herbert Schneekluth, Volker

Bertram

[4]- Navgathi Marine Design & Constructions Pvt. Ltd.

[5]- Det Norske Veritas AS

Figures:

1.1- http://www.propellerpages.com, FAO - Food and Agriculture

Organization of the United Nations; FAO FISHERIES TECHNICAL

PAPER 383

1.2- Retrofit propulsion improvement, Wartsila

1.3- Marine Propellers and propulsion ,John Carlton

2.1- Schneekluth

2.2,2.3- Hydrodynamics of Ship Propellers, John P. Breslin, Poul Andersen

2.4- Wartsila

2.5- Year Book 2007 : Progress of Marine Engineering Technology in the

year 2006

2.6,2.7- Schneekluth, Ship Design for Efficiency and Economy, Butterworth

& Co Ltd.

3.1,3.3,- Hydrodynamics of Ship Propellers, John P. Breslin, Poul Andersen

3.2- Wikimedia Commons

3.4,3.5,3.6,3.7,3.8- http://www.roblightbody.com

4.1- Marine Propellers and propulsion ,John Carlton

4.2,4.3- New Results in Numerical and Experimental Fluid Mechanics VI

4.4,4.5- Varldssjofartens_Dag_2007_presentationer

5.1- SSPA Sweden AB