Tradeoffs in ship Propulsion control: engine overloading and cavitation inception in operational conditions

Date Author Address

April 2008

A. Vrijdag, D. Stapersma and T. van Terwisga Deift University of Technology

Ship Hydromechanics Laboratory

Mekelweg 2, 26282 CD Delft

TU Deift

Deift University of Technology

Tradeoffs ¡n ship propulsion control: engine

overloading and cavitation inception in

operational conditions

by

A. Vrijdag, D. Stapersma and T. van Terwisga

Report No. 1584-P 2008

Published: Proceedings of the gth International Naval Conference and! Exhibition, Hamburg, Germany, INEC'OB,

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Rolls-Royce

' BMT Defence Services

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s e S S Y-CCH, HAMBURG, GERMANY ORGANISED BY

THE INSTITUTE OF MARINE ENGINEERING, SCIENCE AND TECHNOLOGY (IMAREST) SUPPORTED BY

THE INSTITUTION OF ENGINEERING AND TECHNOLOGY THE NAUTICAL INSTITUTE

AMERICAN SoCIETY OF NAVAL ENGINEERS

THE SOCIETY OF NAVAL ARCHITECTS & MARINE ENGINEERS

MAIN SPONSORS

Aa.

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CONFERENCE THEMES

» Concepts

» Signatures and modelling » Propulsion

» Responding to external influences » Through life support systems » System design

» Equipment design

» Ownership

» Equipment support

» Damage control ard fire fighting -,,

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.

.

.

I

I

Iñs(iI,,leof Marine Engineering, Science ¿Technolo!y

I M4iE ST

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2008

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TITE INSTITUTE OF MARINE ENGINEERING, SCIENCE AND TECHNOLOGY

80 Coleman Street, London, EC2R 5BJ

Tel: +44 (0)20 7382 2600 Fax: +44 (0)20 7382 2670

Email: events@imarest.org Web: www.imarest.org

President: Professor Yoo Sang Choo

ChiefExecutive: Keith F Read CBE

9th

International Naval Engineering Conference and Exhibition

(INEC 2008)

1 3 April 2008

Congress Center Hamburg, Germany

The Institute of Marine Engineering, Science and Technology would like to thank the following Advisory

Committee Members for their invaluable help in the organisation of this conference:

Chairman:

Capt John Newell MBE RN, Ministry of Defence, UK

Committee Members:

Dipl Ing Wolfgang Bohlayer, ThyssenKrupp Marine Systems, Germany Cdr Matt Bolton RN, Ministry of Defence, UK

Capt Mark Dannatt RN, Converteam Ltd, UK Cdr John Dannecker USN, Ministry of Defence, UK

CV Stanislas Gouriez de la Motte FN, French Ministry of Defence, France Dr Alistair Greig, University College London, UK

Mr Rob Hughes, Rolls-Royce plc, UK Mr Paul Maillardet, Ministry of Defence, UK Mr Dave Mattick OBE, Converteam Ltd, UK Mr Roy Quilliam, BMT Defence Services Ltd, UK Dr Phil Rottier, BAE SYSTEMS, UK

Lt Cdr Dr Paul Schulten RNLN, Defence Materiel Organisation, The Netherlands Mr Tim Stiven, QinetiQ, UK

Lt Cdr John Voyce RN, Ministry of Defence, UK

Published by THE INSTITUTE OF MARINE ENGiNEERING, SCIENCE AND TECHNOLOGY,

a Registered Charity, No 212992, of 80 Coleman Street, London EC2R 5BJ. ('England Reg No 1100685.)

Cöver design by Gung-Ho Design, 2 Pear Tree Street, London, EC1V 3SB

In accordance with the terms and conditions of the Copyright, Designs and Patents Act, 1988, the written consent of the publisher must be obtained before publishing more than a reasonable abstract.

Papers presented or published reflect the views of the individuals who prepared them and, unless indicated expressly in the text do not necessanly represent the views of The Institute of Manne Engineenng, Science and

Technology. Whilst every effort has been made to ensure that the information in this publication is accurate, The Institute of Marine Engineering, Science and Technology makes no representation or warranty, express or

implied, as to the accuracy, completeness or correctness of such information. The Institute of Marine

Engineering, Science and Technology accepts no responsibility whatsoever for any loss, damage or other liability arising from any use of this publication or the information which it contains.

IMarEST Conferences

Contents

¡NEC OPENING SESSION

CVF Procurement and build strategy

RAdin R Love, CVF Team Leader, Ministiy of Defence, UK

(Paper not available at the time of printing)

i

F125 - the new frigate for the German Navy

A Grudda, ThyssenKrupp Marine Systems, Germany

10

The new Dutch patrol ship

Capt MHendriks, Cdr P Knipping, Defence Materiel Organisation, The Netherlands

16

_{Existing gunboat proven design improvement New concept design}

J Tzagarakis, Hellenic Shipyards SA, Greece

Session A: CONCEPTS

26

_{The globally deployable minor warship - A conceptualisation of future solutions}

A Kimber, W Giles, BMT Defence Services Limited, UK; TDinham-Peren, BMT

Seatech, UK

38

The new Dutch JSS

- the challenges of its design and procurement

Lt Cdr Dr P J M Schulten, Cdr J W Hartman, P Everts, Deftnce Materiel Organisation,

The Netherlands

47

Submarines for the future, a new design model

WH de BruUn, B J van Oers, Delfi University of Technology, The Netherlands

63

A future RN fleet with an IFEP nuclear aircraft carrier

Dr A R Greig, S Rusling, Dr R W G Bucknall, University College London, UK

New concepts in mine warfare

FSchom, DCNS, France

(Paper not available at the time of printing)

Session B: SIGNA TURES AND MODELLING

71

Acoustic design tools - present and future integrated software

R Fischer, L Boroditsky, J Spence, Noise Control Engineering mc, USA

82

_{Tradeoffs in ship propulsion control: engine overliiäding and cavititiiijitiijÌ}

(in operational condition

4 P7däDStapeaTDelfl-Un1vergi1y

(Tvan Terwis,a, De(fl University

94

Considerations for low noise navy propellers

CJohannsen, Dr UHollenbach, HSVA, Dr C Bauer, VA TECH Escher Wyss GmbH,

Germany

Simulation of ship electric AC-networks: a mean value first principle approach

T de Lange, Delfi University of Technology, The Netherlands

(Paper not available at the time of printing)

107

On electric load characterization and categorization in ship electric installations

Prof Dr Ing 1K Hatzilau, Cdr 1K Gyparis, Hellenic Naval Academy, Ass Profi M

Prousalidis, Dr G J Tsekouras National Technical University ofAthens, Greece

123

Real time simulation of the propulsion plant dynamic behaviour of the aircraft

carrier "Cavour"

MAltosole, G Benvenuto, U Campora, MFigari, Università di Genova, Italy; Capt M

Giuliano, Capt S D'Arco, Italian Navy, Italy, V Giuffra, A Spadoni, ABB PS&S;

MRotto, S Micchetti Fincantieri, Italy

Session C: PROPULSION

131

Making Waves Faster - The MT3O enters naval service

R Tooke, R Kok, Rolls-Royce plc, UK

143

Naval gear systems and their future demands

Dr F Hoppe, RENK, Germany

Hybrid electric propulsion for Naval Auxiliaries

R Partridge, A Scott, M North, Rolls-Royce Naval, UK

(Paper not available at the time of printing)

154

Propulsion options for future frigates - Power dense solutions for medium sized

warships

R F Lamerton, N Moss, Thales Naval Ltd, UK; R E Maltby, W Ubhi, Converteam UK

Ltd, UK

164

Practical application of viscous CFD in ship design - Integration study of a

submerged waterjet

P Kaeding, C Thieme, J Ballé, ThyssenKrupp Marine Systems, Blohm and Voss GmbH,

D Jürgens, Voith Turbo Schneider Propulsion GmbH, Germany

179

Putting the power into CVF - integrating the prime movers behind the IFEP

System

Lt R Casson RN, Lt Cdr I Timbrell RN, Min istiy of Defence, UK; S Newman, C English,

Aircraft Carrier Alliance, UK

Session D: RESPONDING TO EXTERNAL INFLUENCES

189

Rise to the challenge - a newly developed CP Propeller system that fulfils the

strictest environmental requirements

L D Johansson, MSkrinning, Rolls-Royce AB, Sweden

200

UK Navy surface warships engines exhaust emissions study 1988-2006

218

Demonstrating safety by class societies: A Navy's point of view

E C A ter Bekke, Netherlands Defence Materiel Organisation/Delfi University of

Technology, The Netherlands; P Everts, Netherlands Defence Materiel Organisation,

The Netherlands

227

Maintaining Naval combatants in classification

G MAshe, Capt D H Lewis USN, American Bureau of Shipping, USA

237

Future fuels onboard UK warships

JBuckingham, BMT Defence Services Lid; Lt R Casson RN, Ministiy of Defence, UK

250

First ever classification of a Naval submarine

Dr L Grünitz, Germanischer Lloyd AG, Germany

Session E: THROUGH LIFE SUPPOR T SYSTEMS

254

Support solution design and development, in the light of the Defence Industrial

Strategy

SB Mather, TLillie, Dr P Grosse, BAE Systems UK

269

Maximising performance across a class of vessels, through optimal distribution of

system availability

N Lawson, Ministîy of Defence UK; Dr P Rottier, BAE SYSTEMS, UK

278

Enhancing value for money through model-based support solution optimisation

A Bowden, R Parker, Dr P Rottier, BAE SYSTEMS, UK

289

Improving the performance of complex support organisations - the application of

systems dynamics modelling in the UK submarine support enterprise

S Kershaw, PA Consulting Group, UK

307

Managing and supporting ship availability

G Lea, D Houghton, VT Naval Support, UK

Session F: SYSTEM DESIGN

Technology management in warship acquisition

A J Shanks, W L Mitchelmore, BMT Defence Services Ltd, UK

(Paper not available at the time of printing)

326

Facing up to failure - the defence of graceful degradation

B Salter, N Smith, Converteam Ltd, UK

341

Advanced marine power system architecture with active fault protection

J Wang, Dr P Kadanik, Dr M Sumner, Dr D WP Thomas, University of Nottingham,

UK; Lt R D Geertsma RNLN, Ministiy of Defence, UK

355

The use of a complement generation and analysis tool within ship design

H Morley, P Wotton, Quintec Associates, UK

364

Unmanned underwater vehicle (UUV) deployment and retrieval considerations

for submarines

T Hardy, G Barlow, BMTDefence Services Ltd, UK:.

Session G: L?QUIPMENT DESIGN

r

379

Simulation and performance analysis of a faulty marine diesel engine running in

realistic operating conditions

G Benvenuto, U Campora, Università di Genova, Italy

391

The influence of turbo charger matching on propulsion performance

Prof D Stapersma, Netherlands Defence Academy, The Netherlands,

Dr H T Grimmelius, Delfi University of Technology, The Netherlands

402

Energy storage systems as a mechanism for improving power quality in an IFEP

system

Dr I MElders, JD Schuddebeurs, Dr CD Booth, Dr G MBurt, ProfJR McDonald,

University of Strathclyde, UK, J McCarthy, Rolls-Royce pic, UK

413

Electrical insulation for ships

Dr R E Hebner, Dr A Ouroua, University of Texas at Austin, USA

Session H: OWNERSHIP

420

Application of re1iabilitybased inspection techniques for integrity management of

mechanical handling systems

IBottomley, DrJAustin, Frazer-Nash Consultancy Limited, UK; JBentley, IMES

Limited, UK, K Barnes, Lt Cdr S Mealing RN, Ministry of Defence, UK

430

Project managing a VICTORIA class extended docking work period (EDWP): a

snapshot naval perspective

LCdr MC Wilson, LCdr D E Hughes, CF - Canadian Forces, Canada

440

Through life support of the Collins class submarine

P Crosby, ASC PTYLTD, Australia

450

Maritime environmental challenges - impact and solutions for Naval vessels

Cdr(ret) K D Eule, Deerberg-Systems GmbH, Germany

Lessons from marine incidents for Naval Engineering

Capt MDannatt RN, P T Norton, Converteam Ltd. UK

(Paper not available at the time of printing)

Session I: EQUIPMENT SUPPORT

458

Living with the threat of microbiologically influenced corrosion in submarine sea

water systems - the Royal Navy's perspective

468

Protraction of legacy warships by judicious implementation of automation

Commodore (Dr) R K Rana, Commander S Chhabra, Indian Navy

479

The dry dock code

WBrook-Hart, G[ford, UK, R Smith, BMT Marine Projects Ltd, UK; G Skinner, Fleet

Support Marine Projects Ltd, UK

Session J: DAMAGE CONTROL AND FIRE FIGHTING

498

Recoverability in the füture damage control & fire fighting in 21st century

C S Smit, TNO, The Netherlands, Lt Cdr H Zor, Defence Materiel Organisation; The

Netherlands

508

Latest developments in damage control

MK Vierow, L-3 Communications MSUK, UK

519

Intelligent robotic local suppression system for the marine enviùonment

Dr JL D Glockling, Fire Protection Association, UK; Dr G Doherty, Rolls-Royce

Marine Electrical Systems, UK

532

Challenges to protect confined and cluttered environments like submarine

compartments with water mist

C Cueff C Le Gac and G Lucas, DCNS, France

Design methodology for increasing the survivability of warships

R A Logtmeer, K S van Bodegraven, Defence Materiel Organisation, The Netherlands

(Paper not available at the time of printing)

INEC CLOSING SESSION

542

Warship missile system integration

PNGazard, MBDA UKLtd, UK

550

The Type 45 AAW Destroyer Combat System Integration Strategy

SfLoneragan, BAE SYSTEMS, UK

The electric warship - then, now and later

CHodge, BMTDefence Services Ltd, UK; D Mattick, Converteam Ltd, UK

(Paper not available at the time of printing)

Tradeoffs in ship propulsion control:

engine overloading and cavitation

inception in operational conditions

A. Vrijdag, MSc

PhD student, Delfi University of Technology/Netherlands Defence Academy

D. Stapersma, MSc

Professor of Marine Engineering, Delfi University of Technology/ Netherlands Defence Academy

T. van Terwisga, MSc, PhD

Professor of Ship Propulsion, Delfi University of Technology! MARIN

SYNOPSIS

Off-design conditions can have a severe impact on ship propulsion system behaviour. Resistance increase for instance leads to a higher engine loading, and can also easily lead to a decrease of

cavitation inception speed with respect to calm water conditions. Wakefield variations due to ship motions, waves and manoeuvres also have effect on engine loading and on cavitation inception speed. This paper shows that one single pitch-shaft speed relation not always results in favourable propulsion

system behaviour in the great variety of ship operating conditions. It is demonstrated that with

relatively simple changes to the propulsion control system, the adverse effects of off-design

conditions can be counteracted by using a condition dependent pitch-shaft speed relation.

INTRODUCTION

During the design phaseof a ship propulsion control system, there are tradeoffs to be made between the

various goals that one pursues with the propulsion installation. These goals _{can be related to fuel} efficiency and manoeuvrability, but also to acoustic signature, thermal overloading of the (diesel) engine, or even shipboard vibration levels

A (naval) ship is required to operate in a great variety of environmental conditiOns. During_propulsion (control) system design these conditions should be taken into account to ensure that the system delivers favourable behaviour under many circumstances. These circumstances include effects given by nature

such as seastate and wind, but also dynamic conditions resulting from ship manoeuvres such as turning

circles and accelerations! decelerations.

Author's Biographies

Arthur Vrijdag graduated from the Royal Netherlands Naval College in 2004 and in the same year heobtained his masters degree in ship hydromechanics at DeIft University of Technology. He is now performing a PhD research titled 'development and implementation of an optimised ship propulsion control system' in close cooperation with the Royal Netherlands Navy, Defence Research and Development Canada, the Royal Australian Navy, Wärtsilä Propulsion Netherlands, IMTECH and MARIN.

Douwe Stapersma, after graduating in 1973 at DeIft University of Technology in the field of gas turbines, joined NEVESBU - a design bureau for naval ships - and was involved in the design and engineering of the machineiy installation of the Standard frigate. After that he co-ordinated the integration of the automatic propulsion control system for a class of export corvettes. From 1980 onward he was responsible for the design and engineering of the machineiy mstallation of the Walrus class submarmes and in particular the machinery _{automation After that he} was in charge of the design of the Moray class submarines in a joint project organisation with RLDM. Nowadays he is professor of Marine Engineering at the Netherlands Defence Academy and of Marine Diesel Engines at Delfi University of Technology.

Tom van Terwisga finished his studies in ship hydromechanics at Delfi l!Jniversity of Technology in 1985. After that he started working at the R&D department of the Maritime Research Institute Netherlands (MARIN). In 1990 he moved to the Ship Research Department and focussed on ship propulsor hydrodynamics In 1996 he finished his

PhD research on 'waterjet-hull interaction'. Currently he is working as senior researcherPropulsorsat the R&D dept. of at MARIN and is professor in ship hydromechanics at Delft University of Technology.

To demonstrate the effect of environmental conditions on the propulsion system behaviour, two

operating conditions are worked out in detail by means of a simulation model. Furthermore the benefit of making the traditionally fixed shaft speed pitch combinations dependent on the actual conditions is considered. Therefore this paper is limited to controllable pitch propeller (CPP) ships since only these ships have the possibility to achieve the same ship speed with multiple shaft speed-pitch combinations This paper focuses on the minimization of acoustic signature due to propeller cavitation combined with

the prevention of thermal overloading of the diesel engine.

TRE SIMULATION MODEL

This section describes the propulsion system simulation model that has been used in the current study. The total simulation model includes the propulsion system model:, the (simplified) currently applied

control system, and the environmental disturbance model.

The propulsion model

The propulsion simulation model as used in this paper is shown in a block diagram in Fig I. It

describes the (non-linear) dynamics of the ship propulsion plant including the engine, the propulsion control system, and the CPP. Note that the gearbox is part of the propulsion machine-block and that the hydraulic pitch actuating system is part of the pitch control system-block. The actual ship under

consideration has a similar installation for port and starboard side. Only one side is shown here.

On the right hand side of the figure, the ship translation loop is shown, based on a force balance

between propeller thrust and ship resistance. When those two forces are out of balance, a net force Will

result in an acceleration or deceleration of the ship. Integration of acceleration gives ship speed:

v3(t) =--JFdt+v0

where v is ship speed (having initial value v0 at time t = O), F is the sum of all forces working in

the longitudinal direction of the ship and m is the effective mass of ship and entrained water.

On the left hand side the shaft rotation loop is shown, dealing with the balance between propeller and engine torque. In the same way as in the translation loop, a net torque will cause an acceleration or

deceleration of the shaft. Integration of angular acceleration gives shaft speed:

n(t)=1

JMdt+n0

where n is shaft speed (having initial value n0 at time t = O), M is the sum of all torques working on the shaft and I is the effective rotational inertia of the shaft system (engine and gearbox rotating parts, shaft, propeller and entrained water).

In the middle the propeller thrust and torque are modelled. The propeller thrust Fprop and torque Q

can be computed from the open water diagram of the propeller under consideration:

v(1w)

_{k1 =f(O,J), kq =g(O,J),}

nD

Q=kqpn2D5,and Fprop k1pn2D4

where J is the advance coefficient made up from the advance speed of the water (ship speed v corrected by wake factor w) and something that is proportional to circumferential speed (propeller speed n times diameter D). The actual open water diagrams are given as thrust coefficient k, and torque coefficient kq both being a function ofpitchangle 9 and the advance coefficient. Torque and

thrust than can be determined using the definition of the two non dimensional coefficients. A more

complete description of this general propulsion block diagram is given in Stapersma'. Some particulars

of the ship under consideration are given in Table I.

Command MSh,tì M Table I Vesselparticulars Governor Propulsion machine Shaft rotation dynamics Ship translation dynamics Ship Resistance 1< Propeller Torque J e +

FIg 1 Theshippropûlsion block diagram

The control system

The real propulsion control system has many nonlinear and dynamic features such as fuelrack-limiting, transient control and overspeed/ underspeed protection. However, in static conditions the input-output relation of the propulsion control system is given by the pitch- shaft speed relation which is also called the "combinator curve". This combinator curve consists out of two lookup tables. One lookup table translates the single lever command to a shaft speed setpoint. The other iookup table translates the

single lever command to a pitch setpoint. One single command thus results in two setpoints.

An example of both iookup tables is shown in Fig 2. Three types of regimes can be distinguished: for commands between -80rpm and +67rpm the shaft speed is fixed to 80rpm, while the pitch increases (nonlinearly) from -15 to 26deg. Secondly there is a region from 67rpm to 91rpm where pitch is kept constant at 26deg and shaft speed is increased from 80rpm to 110rpm. The commands between -80rpm and -135rpm follow the same regime. Finally for commands above 91 rpm there is a regime where both pitch and rpm are changing until the maximum command is reached at 135rpm and 3ldeg pitch.

The single lever command should have the property that it allows intuitive use and it should be

monotonously increasing with ship speed In the Royal Netherlands Navy commands on the bridge are given in terms of the virtual shaft speed setpoint. This virtual setpoint is a virtual number (in rpm) that has the properties described above. For the ship under consideration the so-called virtual shaft speed

virI is given by:

0

'i

(1)

1'nom

-where 00 is the zero thrust pitch-angle and _9nom is the nominal pitch angle. n represents the actual

shaft speed and O the actual pitch. Onboard, the realised virtual rpm is constantly calculated and

presented via measurement of actual pitch and actual shaft speed via Eq 1.

84 Propeller Thrust Fprop FshjP Disturba nces Disturbances

dem _Propulsion dem Pitch Control Control System System

Type of vessel Frigate

Lpp 114m

Draught FPP/APP 4.3 m

Engines Stork - Wärtsjl5 Diesel SW280

Number ofpropellers 2

Fig2 Exampleof a combinator curve. The left figure shows theshaft speed lookup table. The right figure shows the pitch lookup table. Three different regimesare present.

The relation between _yjrj and ship speed is shown in the n - O plane in Fig 3. Note that the contours

of _vjr1 and ship speed align quite well. Also note that this figure only holds for calm water conditions. More heavy loaded conditions will shift the ship speed curves to the lower left comer. In other words: the same virtual shaft speed setting will result in lower ship speed in high resistance conditions. A well known fact is clearly illustrated by Fig 3: the same n., or the same ship speed can be reached with

many combinations of pitch and shaft speed

5

ship speed [kts]

C'virtuaI shaft speed [rpm] i example combinator curve

25

20 C.)

lo

Environmental disturbance model

85 35 -30 25 I 20 D, I

is---10 r T 1 o_o. I t5 5

---t

-i C, i I O

---I----

-I---1 20 o io command: _irt.set(rnm] -100 -50 0 50 100 command: irt.setitpmj 120 140 60 80 100 shaft[rpm]

Fig 3Contours of virtual:shaflspeed and ship speed in the n-pitch plane. Only forward speed is shown For reference the combinator curve is shown The ship speedas shown holds. for the calm water condition, 6 months out of dock.

140 130 E 120 C o 110 100 90 80

As illustrated in Fig 1, there are two main disturbances acting on the propulsion system: resistance disturbances and wakefield disturbances. The mean resistance disturbance is modelled from three

resistance components as follows:

Rincr = Rwind+R waves+Rouiing

The mean wave resistance Rwaves is determined using the wave resistance transfer function, in

combination with the spectrum undr consideration The wave resistance transfer function Rwaves shows that the wave resistance is considered proportional with squared wave amplitude . This

transfer function has been calculated offline by means of the Gerritsma-Beukelman method for

multiple values of wave frequency w, and is taken from MARIN report2. By combining these transfer functions with the wavespectrum S (w) related to a certain seastate, the predicted mean resistance

increase due to waves is obtained as follows:

Rwaves

=

2f

R_waves

(w)

S(w).dw

Mean wind resistance is calculated by application of:

R wind= _{Pair (reI})2

A C

Where Vrel is the mean relative wind speed, A is a reference area, and Cd is a wind drag-coefficient

(for headwinds in this specific case). fouling-resistance is taken as an extra increase of 13% of the calm water resistance, taken from MARIN report2. This is an estimate for the 6 months out of dock

condition, based on data supplied by the RNLN.

GOALS AND CONSTRAINTS

As shown in Fig 3 there are many combinations of shaft speed and pitch that result in the same virtual shaft speed. The choice for one of these combinations is a compromise between various goals that one pursues with the ship, limited by constraints of the ship, its installations, and its crew. Inthis paper the

goal is to decrease propeller cavitation noise in operational conditions, under the constraint that the diesel engine should not be subjected to thermal overloading. Both the goal and the most important

constraint are further described:

Goals

In the discipline of propeller hydrodynamics it is customary to present cavitation inception behaviour of a propeller in a propeller inception diagram. This is a diagram showing the dimensionless

cavitation-number a versus the thrust- or torque coefficient (k or lç) or versus the advance ratió J. a is given

by:

p0pr +pgz

n

p

where Po is the atmospheric pressure, Pv iS the vapour pressure of seawater, is the density of

seawater, g the gravitational acceleration, and z the water height above the propeller shaft. n represents

shaft speed, and D is the propeller diameter.

Inception conditions are drawn to show the locús of propeller working points where a specific type of cavitation starts. Due to their shape the inception lines are often related to as the "inception bucket". An example is shown in Fig 4. Note that a nominal operating line is also shown. Operating on the left hand of this bucket-shape will result in pressure side cavitation, while operation on the right hand side will in this case results in tip-vortex suction side cavitation. Unfortunately this bucket only holds for one single pitch angle. A change in pitch results in (a) a shift in location of the bucket with respect to KT

and (b) a different shape of the bucket and thus requires a completely new diagram.

Cavitation number Pressure side 4 t Operational curve

i

Full power Tip Vortex suction side Kl

Fig 4: Example cavitation bucket showing two inception lines

In this paper an alternative presentation of the inception diagram is presented where the variable K1 is replaced by something like an effective angle of attack of the flow encountering the blade. The effect is that a change in pitch angle will practically not result in a shift of the bucket This type of presentation is useful both during analysis of propeller design but even more in the design phase of a propulsion

controller.

The idea is as follows: cavitation takes place if the pressure somewhere around the propeller blades drops below the vapour pressure. The pressure distribution around the blades is dependent on the (local) inflow angles of the propeller sections. These inflow angles are the result of the following aspects: shaft speed, ship speed, wakefield, propeller geometry and the loading dependent induced

velocities. A simple sketch of the inflow of a specific section is shown in :Fig 5.

Fig 5: Velocity triangle of a carnberedLsection at OiR

From this figure it can be derived that the local inflow angle contains pitch:O, flow angle

_ß

and a

correction fOr the shock free entry anglea. as follows:

aeff

=G-ß-a

Where c1 is a correction factor that can be used for tuning, a1 is the shock free entry angle that is dependent on camber and on induced velocities near the leading edge.

87

Working this out gives:

a

_e

-

arctan

( '7R

I

arctanI

(

c1v_a

I a.

I

For now only the geometric (camber) part a.0 of a. is determined and incorporated in the effective inflow angle estimation process. a10 is calculated from the (pitch dependent) camber c and thickness f

and given without derivation:

By conversion of three full scale observed cavitation buckets (each observed at a different pitch), the coefficient c1 is determined such that the best overlap of the three buckets in the a versus o, plane is found. From this, since we are interested in control, a safe inner bucket is derived that is expected to

give cavitation free behaviour over a large pitch-range. This safe inner bucket is made visible

schematically in Fig 6. Operating on the left hand of this V-shape will result in pressure side cavitation, while operation on the right hand side Will result in suction side cavitation. The existing combinator curve and the RTBO-constraint (explained in the next section) are shown for reference. Contours of

virtual shaft speçd and pitch angle are also shown.

o i:

i

i'

¡I

/

'g I I n ¡

a,0

'I

/

I. Cfmax

pitch contours [deg]

Cl) virtual shaft speed contour [rpmi

combinator curve

- cavitation inception bucket

RTBO line

/ o

1/

__/ -88 II

/

th

/

ç2 4 J max -6 -2 0 2 4 6 8

eff

[deg]

Fig 6 Cavitation bucket in terms of OeS. Contours of pitch, virtual shafl speed are shown. This figure holds for the calm water condition, 6 months out.of dock.

Constraints

Examples of constraints of the propulsion system are: minimum/maximum engine speed and maximum (engine speed dependent) fuelrack setting to prevent thermal overloading of the diesel engine. In this paper the latter one is called the Reduced Time Between Overhaul (RTBO) -line. A prolonged stay on the high loaded side of the RTBO-line is undesirable from the viewpoint of maintenance and costs. Closely related to the RTBOlthe is the engine margin: this is the amount of fuelrack X (in mm) that can be added to theactûal ftielrack position (at the actual shaft speed) until the RTBO-line is crossed:

margin = XRTBO - X

This margin is important when sailing in waves since propeller torque variations require the governor to constantly adapt the fuelrack in order to keep the shaft speed constant at the requested speed. A higher seastate thus requires a higher margin to allow for fuelrack variations. An example showing the engine envelope consisting out of the RTBO-line and the maximum and minimum shaft speed is shown in Fig 7. The figure also shows the reference combinator curve that was introduced in Fig 2. The other

two combinator curves that are shown are described later. Note that the RTBO- line shown in Fig 6 is

exactly the same RTBO line as in Fig 7, but now shown in a different grid.

Other constraints may be imposed by for instance the thrust bearing or by the CPP hydraulic actuator

capabilities. These constraints are not dealt with here.

35 30 25 20 D) Q) -D 15 10 con bindo'tl%_{- %..} ccmbintoy2

--F I

"s

contour [rpm]

='cavitation bucket

-+reference combinator

-combinator 2

--combinator i

engine envelope refe re n ce cçmb Inator

---.;.--,.---89 ,ssI p...'

's-'

I' I I I

.1

£

o Operating condition I Operating condition 2

Windforce Bfi O Bfi 8

Wind direction - heathvind

Equivalent seastate 0 6

60 80 100 120 140

shaft

[rpm]

Fig 7 The engine envelope in the shaft speed-pitch plane. This figure holds for the calm water condition, 6 months'out of dock.

DESIGN EXAMPLE

The difficulty with the design of a combinator is that a specific curve may be a good compromise in one operating condition, but might not even completely fall within the system constraints (engine envelope) in another condition. A conservative approach can then easily result in a design in which the combinator curve falls just within the engine- envelope in the most demanding operating condition. Unfortunately this will most likely lead to (unnecessary) performance degradation in less demanding

operating conditions.

In this section the design-process of a combinator curve is demonstrated for the two conditions shown

in Table II: seastate O and seastate 6. It is emphasised that for the sake of the example, the two cases are

considered independently. Later on the consequences are shown if a combinator-design that is specific

for one condition is used in another condition.

Desgn for calm water

For the design in calm water the plots as shown in Fig 6 and Fig 7 are used. The design consists out of

several steps and decisions that influence each other. The main starting points are given here

The working point of the engine should lie within the operating envelope for all possible static working points.

The combinator curve should preferably lie in the middle of the cavitation bucket. Is it

acknowledged that this is not possible for the lower ship speeds where ship speed can only be

reduced by reducing the pitch. The maximum pitch is 31 deg.

Results of the decision process for the calm water condition are listed in Table III. Some choices are somewhat subjective, but it is likely that another "designer" will come up with a similar combinator curve. Especially in the lower virtual shaft speed-region there is somewhat more freedom of choice. The resulting combinator 1 is shown in Fig 7. It is clear that in calm water combinator I satisfies the goal and the constraints

Table Ill Resulting combinatôr curve for calm water. (Combinator 1)

Design for Seastate 6

The same process as for the calm water cofldition is now carried out for the "seastate 6" condition. Fig 8 and Fig 9 are the equivalents of Fig 6 and Fig 7, but now for the new operating condition. For the design process the same starting points as in the previous section apply. Results of the design process are shown Table IV. In Fig 9 it can be seen that combinator 2 satisfies the goal and the constraint in the high loaded condition.

Table IV: Resul ing combinãtor curve for seastate 6. (Combinator 2)

90

viri (command) seI 9set

Node

number remarks

135 rpm 135 rpm 31 deg 4

Only one viable option: max engine speed and

max pitch. Lies just outside the operating

envelope due to the 13% fouling resistance. 120 rpm 135 rpm 27.8 deg Bottom of the Cl-bucket. A bucket less deep_{wouldlead toa different choice.}

50rpm 56.2 rpm 27.8 deg 2 Thisis the middle of the cavitation bucket.

0rpm 56.2 rpm 1.7 deg I Zero thrust. The minimum shaft speed is fixed_{to 56.2rpm}

vjri (command) 'sef 8set

Node

number remarks

135 rpm 135 rpm 31 deg

5 Only one option.

Not achievable without

serIous overloading of the engine. 110rpm 135 rpm 26 deg 4 Bottom of the cavitation bucket. A bucket_{less deep would lead to'a different choice.}

90rpm _rpm108.5 26 deg 3 Compromise between margin and CI

30rpm 56.2 rpm 17.3 deg

2 This is to ensure that thelowest shaft speed is equal to lowest shaft speed of the other combinator curve. This is done for the

sake of comparison

C b 35 30 25 20 C) cl) -D 15 IO I j

I,

I

i/

r

pitch contours [deg]

C1ä) virtual shaft speed contour [rpm] combinator curve

cavitation inception bucket

k RTBO line

i

/

/ F

/

/ ,'l / I I

/

-.

-I o 'F ,.c::t_ - "t con bInaJori1 - -1

II

I ccmbln'toj' 2 I

\

'-.

..__I

04 -4 -2 0 2

a

_eff[deg]

Fig 8 Cavitation bucket in terms of aipha_eff. Contours of pitchandvirtual shaft speed are shown. This figure

holds for the seastate 6 condition, headwaves, 6 months out of dock.

"s»

.90

N

's ¡..!ference c.rnbinator + 60 80 100 shaft

[rpm]

FIg 9 Theengine envelope in the shaft speed-pitch plane. This figure holds for seastate 6, 6 monthsotAt of dock.. 91 -S 4 .'-- I '-4. 6 8 S..-' '4. I I 77_ I

--.

..-i

-C) nVd contour [rpm] cavitation bucket 4-- reference combinator

combinator2

--combinator 1

k engine envelope

120 --3

/

j

EVALUATION OF DESIGNED COMBINATOR CURVES IN OTHER CONDITION

The previous two combinator designs give a desirable locus of static working points in specific

conditions. In other conditions these combinator curves do not give the same desirable behaviour. To show this, combinator 1, combinator 2 and the reference combinator are all shown in condition i (calm water) in Fig 7.

This figure clearly shows that in the calm water condition, combinator i is a balanced compromise

between keeping the middle of the cavitation bucket, and keeping a reasonable distance from the

RTBO-line. The second combinator touches the pressure side of the CI bucket since it is (too) lightly loaded. Due to the high uncertainty in the CI bucket this is not a desirable situation. A very small

disturbance of the working point or of the pressure side inception line will immediately lead to

inception. The ample distance to the RTBO-line is due to the relatively high shaft speed that is used to achieve the desired virtual rpm. The same holds for the referencecombinator.

In the higher loaded condition (seastate 6) as shown in Fig 9, combinator 2 has one extra combinator "node", which was necessary to keep well clear of the RTBO-line. It can be observed that node 3 could

be positioned a little more towards the southeast in order to keep more distance from the RTBO-line in

the area around point (90 rpm, 23 deg). Combinator 1 in this case is not a viable option since a

significant part lies outside the engine envelope. To resolve this, the iso-pitch value should then be reduced from 27.75 deg to 25 deg or less. The reference combinator also isn't a good option here, since

it partly lies outside the engine envelope.

SAILING WITH CONDITION DEPENDENT SHAFT SPEED-PITCH COMBINATION The foregoing showed that it is wise to develop and use condition-dependent combinator curves. As was demonstrated, it is very well possible to develop different combinator curves beforehand for different conditions. Once these "condition dependent" combinator curves have been designed, the question remains how the propulsion control system should recognise that the ship is sailing in certain

conditions. A possible solution could be to give the operator the possibility to choose one of the combinator curves by means of an additional switch on the bridge or in the engine control room.

Another option is to let the ship system recognise the environmental conditions automatically, and then

automatically choose a suitable pitch-rpm combination that delivers the requested virtual shaft speed.

This paper is part of a PhD-study that is currently focussed on full scale implementation of a

propulsion control system onboard of one the HNLMS frigates. This control system automatically determines the desirable pitch-shaft speed combination that still delivers the desired virtual shaft speed. This means that the actual realisation of the command truly is condition dependent. This "adaptive" feature is expected to give positive system behaviour in terms of both propeller cavitation and engine

overloading prevention.

CONCLUSIONS

There are many combinations of pitch and shaft speed that deliver the same ship speed or virtual shaft speed. The freedom of choice for one of these combinations is limited by application of a combinator curve. It was shown that a defensive combinator design will lead to performance degradation in terms

of propeller cavitation in calm water. This performance degradation is unnecessary because there is still

enough engine margin to be able to operate at a higher propeller loading. On the other side, a

combinator design that is well suited for calm water conditions results in engine overloading and a

decrease in cavitation inception speed in high loaded conditions. It is shown that the reference combinator results in loss of cavitation performance in calm water conditions, while engine

overloading occurs in high loaded conditions.

It is concluded that one single combinator curve cannot give satisfactory system behaviour (in terms of engine overloading and propeller cavitation) in the great variety of conditions that (naval) ships are required to operate in. In ongoing research the possible gain in performance due to smart control of

pitch and rpm is investigated. Full scale trials are scheduled in March 2008.

REFERENCES

Stapersma, D. 'Interaction between propulsor and engine', Proceedings of the 34th WEGEMT

School, Deift, the Netherlands (2000)

MARIN report No 14449-1 -RD 'Propeller-in-service-effects'. Technical report by Maritime Research Institute Netherlands. (February 2000)

BIBLIOGRAPHY

Stapersma, D., Grimmelius, H., Schulten P., ,

'A fresh view on propulsion control'

proceedings of the 2004 INEC Conference, Amsterdam, the Netherlands (2004)

Van Terwisga, P.F., 'Hydrodynamic aspects of the application of a dynamic simulation model in frigate propulsion design and operation', Proceedings of the34th WEGEMT School, Deift,

the Netherlands (2000)

Verkuyl, J.B. et al. ,'Testing a new full scale cavitation observation system on board of Hr. Ms. Tydeman', Proceedings of the 34" WEGEMT School, Delfl, the Netherlands (2000) Van Terwisga, T. et al., 'Effect of Operational Conditions on the Cavitation Inception Speed of Naval Propellers', 25" Symp. Naval Hydrodynamics, St. John's, Canada (2004)

Morvillo, R.A., 'Application of Modern Digital Controls to Improve the Operational

Efficiency of Controllable Pitch Propellers'. SNAME transactions, Vol 104, 1996, pp.

115-136.

8 Hellström, T., 'Optimizing Control at Sea: The Experience of the Seapacer Project.'

htt ://www. cs. umu.se/thomash/re orts/sea 'acer. , d (2002)

9. Vrijdag, A., 'Cavitation Inception in Operational Conditions'. Proceedings of the 4th International Conference on Computer and IT Applications in the Maritime Industries

(Comp it 2005), Hamburg, Germany. (2005)