The Specialist Committee on Azimuth Ing Podded Propulsion

International Towing Tank Conference, 4-10 September 2005, Edinburgh

THE SPECIALIST COMMITTEE ONAZIMUTHING PODDED

PROPULSION

Report and Recommendations to the 24th ITTC


1. MEMBERS & MEETINGS• Ir. J. H. Allema, Maritime Research Institute, The Netherlands (3)• Prof. M. Atlar, (Chairman), University of Newcastle, U.K. (4)• Mr. S. Ishikawa, Mitsubishi Heavy Industries Ltd., Japan (3) • Dr. P. Liu (Secretary), National Research Council Canada (4)• Dr. S-E Kim, Samsung Heavy Industries Co. Ltd., Korea (3)• Dr. A. V. Poustoshniy, Krylov Shipbuilding Research Institute,

Russian Federation (4)• Dr. A. Sanchez-Caja, VTT Industrial Systems, Finland (3)• Dr. N. Sasaki, Sumitomo Heavy Industries ltd., Japan (3)• Dr. A. Traverso, Centro Per Gli Studi Di Tecnica Navale, Italy (1.5)


1. MEMBERS & MEETINGS

• Newcastle-upon-Tyne, UK, 18-19 Nov 2002, Host: University of Newcastle

• Genova, Italy, 2-3 Oct 2003, Host: CETENA (in conjunction with 6th Numerical Towing Tank Symp, Rome and 7th Intl. Conf. on Fast Sea Transportation, Ischia)

• St John’s, Canada, 13-14 Aug 2004, Host: NRC-IOT (In conjunction with 25th Symp. on Naval Hydrodynamics, St. John’s)

• Wageningen, the Netherlands, 31 Jan ~ 1 Feb 2005, Host: MARIN.


2. RECOMMENDATIONS OF THE 23RD ITTC (THE COMMITTEE TASKS)

1. Review and make improvements to the procedures 7.5-02-03-01.3 for podded propulsor tests and extrapolation.

2. Recommend procedures for carrying out poddedpropulsor cavitation experiments

3. Establish guidelines for extrapolation to full-scale4. Review impact on off-design conditions to loads and

stability5. Review impact on IMO manoeuvring criteria


3. INTRODUCTION (General remarks)

• Considerable increase in number, power range,application speed and types of pod drives;

• Landmark applications

D.A.T QM2

Hybrid CRP-Pod driven Ropax



• Increased number of high profile R&D programmes in EU, Canada, JAPAN, USA(e.g. OPTIPOD, PODS-in-Service, FASTPOD, ENVIROPAX, Systematic Investigation of A.P.P.Perf., Super-Ecoship, HTS motors, Rim-Drive pod)

TDP

• 1st Intl Conference on technological advances in PoddedPropulsion (T-POD) was held in April 2004 in Newcastle upon Tyne, UK



• In 2004 Committee received a letter from a prominent pod manufacturer who raised concerns on different scaling methods of pod housing drag & unit open water performance estimations

J

η ο

Model scaleScaled 2 - Eta oScaled 1 - Eta oDesign point

Fig. 1: The comparison of full-scale pod unit efficiencies from two different basins based on the same model scale value, (Matilla et al, 2004)


3. INTRODUCTION (Report layout)

• Section 5: Podded propulsor tests and extrapolations (Task 1)• Section 6: Guidelines on extrapolation to full-scale (Task 3)• Section 7: Procedures for model-scale cavitation experiments (Task 2)• Section 8: Impact on off-design conditions to loads and stability (Task 4)• Section 9: Impact on IMO Manoeuvring Criteria (Task 5)• Section 10: Special applications for podded propulsion• Section 11: Technical conclusions

• Appendix A: Improved interim procedures (7.5-02-03-01.3) for “PoddedPropulsor Tests and Extrapolation”

-----------------------• ITTC Recommended Procedures (7.5-02-03-0.3.5) for “Podded

Propulsor Model-Scale Cavitation Test”


5. TESTING AND EXTRAPOLATION (TASK 1)• Propeller Open Water Test (P.O.T)

- Strongly tapered hub issues are missing- Hub should correspond to full-scale hub- Hub cap for puller-type pods should correspond to full-scale cap- Aft fairing is very important (due to separation concerns)

Aft fairing:>Rotates with propeller (gap effect is contained in the pod o/wcharacteristics, preferred method)>Does not rotate with propeller (gap effect is contained in the blade o/w characteristics)

7.5o

Aft fairing Forward cap

Hub Fig. 2: Hub geometry for an open water test with a puller-type propeller


Podded Propulsor Open Water Test• Complete pod unit [propeller + lower part of pod + upper part of pod

(strut) + fin] is tested • A special test set-up is required.

(see e.g. Mac Neil et al (2004) for a recent special pod model device)

Propeller boat

Motor Balance forunit thrust

Shaft

Shafthousing

Dynamometer for propellertorque and thrust

End plate

Wedge

Podhousing

Streamlinedbody

Strut gap

Propellergap

Fig.3 - Podded propulsorin open water test set-up

•Points of concerns:•Air leakage along ver. shaft

•Laminar flow / separation

•Low turbulence levels in pusher type pods


Podded Propulsor Open Water Test

Problematic issues: Propeller-pod housing gap effect

• First reported by (Mewis 2001) and only influences propeller thrust• Limited other studies reported in supporting (e.g. Holtrop & Rijsbergen

2004) and in conflicting nature (e.g. Ukon et al 2003)

0.10.20.30.40.50.60.70.8

0 0.5 1Advance Coefficient, J

ηo -

Prop

elle

r

1.2 mm gap2.2 mm gap3.2 mm gap4.2 mm gap

Fig 4: Open water characteristics of a puller-type pod based on thrust for different propeller gap widths

(Holtrop & Rijsbergen 2004)


Problematic issues: Propeller gap effect

• Gap effect may not be an immediate obstacle for power prediction, But:• Propulsion factors (particularly, wake fraction) obtained from a

propeller thrust identity can be important for propeller designThus:• Further investigations are required to determine how reliable

propeller thrust can be measured

0.0 0.1 0.20.00

0.01

0.02

KT

ΔJ/J

MewisUkon et al.

Right-Rot. direction

44Blade number

0.2800.276Boss ratio

0.550.58BAR (expanded)

0.8001.104Pitch ratio

200.00215.15Diameter (mm)

Ukon et al.MewisParticulars


Problematic issues: Strut gap effect• Gap size required at model-scale between strut top and the lower

end-plate is currently unknown• This effect on the pod performance is considered to be small, see

(Mewis, 2001)• Gap size should be kept as small as possible• Wedge may be required at the top to set the propeller shaft in

horizontal position.

0.0 0.4 0.8 1.2

0.2

0.4

0.6

0.8

J

KTP ,

10K

Q, η

1 mm gap3 mm gap5 mm gap

Fig 6: Effect of strut gap on pod open water characteristics, based on measured thrust of the unit for different strut gap widths (Mewis, 2001)


Problematic issues: Streamlined body effect• Streamlined body over the strut should be made similar to the strut

but mirrored to create a double body flow. This is not time and cost economical. Alternatively:

• A thin metal end plate fitted horizontally – not too long to affect the propeller

Propeller boat

Motor Balance forunit thrust

Shaft

Shafthousing

Dynamometer for propellertorque and thrust

End plate

Wedge

Podhousing

Streamlinedbody

Strut gap

Propellergap


Propulsion test

• Required mainly to predict the ships calm water performance in full-scale

• There are 2 approaches to predict the performance using propulsion tests:

1. Propeller propulsor ; pod housing appendage(Resistance tests with pod housing + Open water propeller tests + Propulsion tests)

2. Whole pod unit regarded propulsor(Resistance tests without pod housing + Pod unit open water test + Propulsion tests)

Method 1 does not take into account strong propeller-pod interaction in correct way.

Method 2 is strongly recommended because it keeps the pod unit with all its internal interaction resulting in more realistic propulsive coefficients and thus better full-scale performance prediction

(See e.g. Nakatake et al (2004) for comparison of two methods)


Propulsion test• Tests should be conducted with both the ship speed and

the propulsor load varied independently• Propeller thrust and torque are to be measured close to

the propeller• Committee recommends that unit thrust should be

measured by means of at least 2-component balance• Air leakage and Reynolds scale effects can be special

concern particularly for pusher units• Applying towing force requires decision to be made for

the model friction correction as well as the pod housing drag correction


Extrapolation Procedure• Propeller open water test results are treated in the same

way as for conventional propellers – For pulling propellers dummy hub run is required to determine the blade performance

• Open water test on a pod unit is also required dummy hub run to determine the blade performance

• Unit thrust (Tunit): KTunit ; Propeller blade thrust (T): KT

0 and Torque (Q unit): KQ

unit-0 are used to create the well-known open water table and diagram as function of advance coeff, J, where:

• KTunit-0 has to be corrected for “pod housing drag correction”

(FDhousing) :

ΔKThousing to obtain KT

unit-0 S(J) curve for the full-scale

• (FDhousing) will be discussed in the next section


Concluding remarks (TASK 1)

• Podded propulsor model tests have now been carried out for about two decades

• There are still problems to be solved -Two important ones: (1) Scaling of pod housing drag(2) Propeller gap effect Further investigations are required in these areas

• Full-scale pod performance measurements are strongly required for better understanding of scale effects


6. GUIDELINES ON EXTRAPOLATION TO FULL-SCALE (TASK 3)

• Recommended “reference (speed-power) extrapolation procedure” (included in Appendix-A) is based on a method similar to the ITTC-78 procedure where:

• Pod-unit replaces the single propeller

• Resistance, open water and propulsion tests are the basis for the extrapolation

• In its present state the procedure is applicable to vessels with single and twin podded propulsors


Speed-Power Extrapolation Procedure

• Resistance tests are conducted without the pod unit(s)

• Open water tests are required for the pod unit and are optional for single propeller

• Propulsion tests are performed by applying a suitable towing force

• Scaling of pod unit open water tests requires special care for correction to pod-housing drag which is not easy to determine as discussed in the following:


Correction to pod-housing drag

Several methods have been proposed, which are used at leading testing institutes, and reportedby:

a. (Mewis & Preafke, 2003) at HSVA b. (Holtrop, 2001) at MARIN c. (Sasaki et al., 2004) at SSPA d. (Sasaki et al., 2004) at Sumitomo e. (E.g. Chicherin et al., 2004) at KSRI


Pod housing drag scaling• a. (HSVA): Frictional drag is approximated as function of Rn;

Effect of working propeller is included as function of CT; Overlooks the propeller rotational effects; Ignores scaling of pressure drag

• b. (MARIN): Form factor based approach – too simplistic; Overlooks rotation effects; Neglects the effect of strut on the pod drag

• c. (SSPA): Semi-empirical drag formulae for torpedo shape bodies are used; Neither the effect of working propeller nor that of the strut is included; Torpedo shapes may not be representative

• d. (Sumitomo): Semi-empirical formulae are used for pod components including interference; Effect of working propeller is included; Physics of the formulae and interference effects are not clear; Also over looks rotational effects

• e. (KSRI): Measured model pod housing drag is multiplied by a factor, which is the drag ratio of the pod housing in full-scale to the model scale based on CFD (RANS) based prediction.


Correction to pod-housing drag

0.000

0.002

0.004

0.006

0.008

0.010

0.8 1 1.2 1.4

Ship speed (m/s)

ΔKTho

usin

g

Sumitomo

MARIN

SSPA

HSVA

KSRI

Pod-housing drag scaling correction using different methods

42

sinsin

DnFK

s

ghouDghou

T ρ=Δ



• Lack of full-scale data and supporting model tests make the committee’s task inconclusive

• Committee makes a call for collaborative work

• In the absence of such investigation the Committee proposes the reference method (similar to ITTC-78 method presented in Appendix –A) with special attention to pod housing-drag correction

• For the pod-housing drag correction the committee recommends RANS based prediction methods


7. PROCEDURES FOR MODEL-SCALE CAVITATION EXPERIMENTS

(TASK 2)General issues

• Main objectives: A common base for cavitation tests to give consistent, reliable and comparable results

• Approach: To adopt similar format to that used for the conventional propellers but make emphasis wherever necessary for poddedpropulsor

• Tests should be conducted with strictly scaled, complete pod unitwith or without a hull model at the highest possible Rn numberwith an acceptable level of test-section-blockage

• Basic calibrations of equipment have to be performed and some knowledge of water quality should be provided as recommended.


Propulsion Unit Operating Conditions

• Should be mutually established

• Detailed test parameters: >Cavitation number, σ,>Advance coefficient, JA,>Full-scale propeller thrust coefficient, KT

BUT due to current problem associated with the gap effect

• The Committee recommends to test:>At a “torque identity” condition, satisfying a target full-scale torque coefficient, KQ, value of

propulsor


Wake simulation

• Should be mutually agreed and documented with wake survey procedures

• For “Pusher Pods” strong velocity deficit at the top sector, stronger than conventional single screws; large scale effects are expected. It is recommended to tests at the highest Rn and use means to stimulate turbulence

• “Puller Pods” display more or less uniform flow; presence of the model pod housing is sufficient for a good simulation of the blockage of the full-scale propeller; Scale effects are expected to be smaller compared to the pusher types due to accelerated flow

• For propeller outside the hull boundary layer compliance with the properly scaled propulsor and proper flow alignment is sufficient

• If part of the propeller operates in hull boundary layerconventional procedures for hull wake simulation are used


Marking, observations & reporting• Propeller (SS/PS side) and Pod-housing (P/SB side)

are suitably marked

90o

PS 0

.25

PFP

PS 0

.50

PS 0

.75

PA

PPS

1.1

25

PS -0

.125

DSL

SL -0.25

SL -0.375

SL +0.25

SL +0.5

SL +0.627

PS 1

.25

DSL = Datum Shaft LineSL = Shaft LinePFP = Pod Fwd PerpendicularPAP = Pod Aft PerpendicularPS = Pod Station

• Reporting should be made by paying attention to suction/pressure sides as well as flow asymmetry due to helm/flap angle

Recommended Pod-body grid definition


Description of Cavitation Appearances• Should contain info’ on: cavitation type; location; size; structure;

dynamics and refs’ to prevailing flow dynamics

FIN

PROPELLER

Pod bodystreak cavitation

(suction side)

Strut sheet cavitation(suction side)

Blade leading edgeattached tip vortexFin tip

vortex

FLAP

PODBODY

STRUT

Detachedsheet cavitation

Pod bodyattached

tail vortex

Strut streak cavitation

(suction side)

Fig. 9 Some cavitation type definitions for a puller type podded propulsor


Concluding remarks (TASK 2)• Committee presents a new set of procedures (ITTC,

2005) based upon the procedures for conventional propellers but making appropriate amendments

• Committee recommends to use the full-scale torque coefficient (through Torque ID) to run tests

• There is tendency to perform tests at varying static and dynamically controlled helm angles which are useful but require care.

• Committee particularly recognizes the importance of cavitation tests at dynamically controlled helm angles but this requires further investigation for appropriate procedures and interpretation of test results


8. IMPACT ON OFF-DESIGN CONDITIONS TO LOADS AND STABILITY (TASK 4)

• Off-Design Conditions occur in steering and manoeuvring operations resulting in structural and stability concerns.

• Classification of Off-Design conditions(Pustoshniy & Kaprantsev, 2001):

AccelerationNormal/Extreme SteeringCrash stopCrabbing and dynamic positioning


Acceleration• Large hydrodynamic loads on the blades and high risk of tip

vortex cavitation – Consequences are comparable to those with bollard-pull ahead scenario which can be assumed the most dangerous situation for blade strength

Normal Steering• Helm angles within ±7~10o around the straight-ahead condition:

They pose no danger for the blade strength or cavitation, either for the propeller or strut. If only a lower fin or flap is present, cavitation risk due to “the effect of tip vortex on a short foil” may develop

Extreme Steering• Helm angles exceeding ±7~10o, in practical terms 15~30o:

High probability of cavitation due to lower J and high α; Danger of acceleration dependent Spike (side) Loads; and hence danger of a large initial rolling angle


Extreme Steering – Sample consequences

0

50

100

150

200

0 10 20

time, t (s)

Con

trol f

orce

, Yp

(N) .

0

20

40

60

80

Hel

m a

ngle

, δ (d

eg)

δ

Yp

-4-3-2-101234567

0 25 50

time, t (s)

Rol

l ang

le,

θ (d

eg) .

-40

-35

-30

-25

-20

-15

-10

-5

0

5

Hel

m a

ngle

, δ

(deg

) .θ

δ

Side force measured on podded modelin turning circle (Woodward et al., 2005b)

Turn initiated roll recorded on a pod driven model (Woodward et al., 2005b )

Spike loads = f (acceleration, dynamic course stability); Strut has the most important contribution


Crash Stop

• Main concerns are forces (in steady/unsteady nature) on both propeller and pod housing as well as the ship behaviour

• Podded propulsors offer more options for crash stop:


Crash Stop• Conventional way: Propeller strength issue is even more vital than

ahead/astern bollard-pull condition; Unsteady character of the involved forces; Cavitation and flow separations (vibration & noise) are other main concerns

• Turning the propulsor around: Not applicable for single pod applications; Can be more effective than the conventional method; Key problem is the blade loading (due to combined effect of helmangle & propeller operation opposite to ship speed)

• Indirect Manoeuvre: Not applicable for single pod applications; Pods can be turned to 60o in opposite direction while reversing thrust; Induced lift on the struts is used as effective braking force; While it has more sustained braking force it can provide shortest stopping time and distance and lower peak loads


Crash Stop - Comparisons

05

1015202530

0 50 100 150 200 250 300 350

time (s)

Forw

ard

Spee

d (k

nots)

CSMSSM1SSM2ISM

-1-0.5

00.5

11.5

0 50 100 150 200 250 300 350

time (s)

Indu

ced

Sway

For

ce (M

N)

CSMSSM1SSM2ISM

Comparison of ship speed (top) and side loads (bottom) for different crash stop modes (Woodward et al 2005a)


Review of literature

• Experimental investigations(Szantyr, 2001a; Szantyr, 2001b);(Grygorowicz & Szantyr, 2004); (Woodward et al., 2004); (Heinke, 2004); (Stettler et al., 2004);(Moukhina and Yakovlev, 2001)

– (Heinke, 2004) and (Stettler et al, 2004) are noteworthy since: Dynamic tests with rotating pods; Unique PIV images, res.

• CFD based investigations(Junglewitz et al., 2004a); (Junglewitz et al., 2004b);

(Cheng & Stern, 1998); (Jessup et al., 2004)- (Junglewitz et al., 2004b) and (Jessup et al., 2004) are noteworthy since they present comparisons with model test data for pods andconventional propeller (crash back), respectively.


Concluding remarks (TASK 4)• Main challenges: To find steady and unsteady loads

on the propeller and other components of the pod in manoeuvring and crash stop modes

• For the steady loads: CFD is recommended• For the unsteady loads: Pseudo-steady and dynamic

tests supported by flow observations are required• Conduct of dynamic tests require special procedures

to be set• Podded propulsors experience significant “spike

loads” in off-design conditions that are in origin related to dynamic manoeuvring

• Spike loads may have significant implications for the structural design as well as impact on the roll stability


9. IMPACT ON THE IMO MANOEUVRING CRITERIA (TASK5)

Reasons for the impact:

• Unlike a conventional rudder, the propeller race (of a pulling pod) stays parallel to the pod when slewed

• In this condition, propeller receives asymmetric inflow producing forces different than in the straight-ahead condition

• “Prammed" aft end to accommodate azimuthing pods further affects the dynamic behaviour of ship through:> Change in LCF, LCB and hence LCG> Removal of central skeg / deadwood and tendency to be less course stable


Critical Review of IMO Manoeuvring Criteria regarding pod-driven ships

Turning circle criterion

No question of the applicability of “Advance” and “Tactical diameter” requirements

Current literature indicates that these parameters are easily obtainable with podded ships

However what is less clear is the application of helm angle for pods (35o therefore has little meaning when you have 360o to choose from)

For the pilot of a pod-driven ship it is not immediately clear what helm angle would produce either the most efficient manoeuvre or the fastest response in an emergency


Critical Review

Initial turning criterion

• A measure of the transient response to a specific helm angle

• A certain level of directional course stability is necessary for the “safe and operational” ship as opposed to a “super stable” ship

• Test is significantly influenced by the time-domain response of the steering gear (note that Pod mass >> rudder mass) – i.e. slewing acceleration far more influential

• Also definition of helm angle is less clear(a 10o rudder angle amounts to 25~30% of the total available control force but it is not clear if a 10o applied helm angle amounts to a greater or lesser proportion of the available control force afforded by a podded propulsor)


Critical ReviewYaw checking criterion (Zig-zag manoeuvre)

• To enable testing within the confines of a test tank and gives some measure of transient response of the ship

• (Nomoto et al., 1957) show how the equations of motion can be re-arranged in a Time and Gain constant format and to allow useful experiment when measuring only the yaw rate

• Using this (Clarke, 1992) demonstrates how the response of the ship is described by the Phase and Gain of the closed loop system. And later on (Clarke & Yap, 2001), using criteria maps, demonstrated that standard zig-zag manoeuvres provides a good approximation for the close loop system.

• Though the over shoot criteria is a good approximation of the closed loop phase margin for conventionally propelled ship, no such validation exists for the podded vessels

• Again, it is not entirely clear how appropriate the 10o or 20o applied helm angle requirement is for a podded propulsor


Critical Review

Stopping criterion

• It is perfectly satisfactory to apply conventional way of stopping BUT

• There are other options that may be more effective or less demanding on the propeller


Literature review

Full-scale, experimental and numerical investigations by

(Kurimo, 1998); (Lepeix, 2001); (Hamalainen & van Heerd, 2001); (Toxopeus and Loeff, 2002); (Woodward et al., 2002a,b and 2003); (Kurimo and Bystom, 2003); (Pustoshny and Kaprantsev, 2001); (Boushkovsky et al., 2003b); (Woodward et al., 2005a)

indicate that :No example were found where pod-driven ships have failed to meet the “Turning Circle” and “Initial Turning” criterionSome pod-driven ships fail to meet the “yaw checking” criteriondue to “hull-pramming” related tendency for less course stabilityConcerns raised over manoeuvring induced heel angles and cavitationAlternative crash stopping methods (e.g. turning pod around, indirect manoeuvre) can be more effective than conventional one


Applicability of the Criteria ‘Resolution MSC.137(76)’, (IMO, 2002)

• Turning ability criterion

3

4

5

6

7

8

0 20 40 60

Applied helm angle, δ p (deg)

A

dvan

ce, X

90/L

(Shi

p le

ngth

s)

Ship AShip BShip C

Assessment of advance criterion , Woodward (2005)

Systematic manoeuvre simulations indicate that:

• 35o applied helm angle is entirely appropriatefor testing turning ability of pod-driven ships



• Initial turning ability criterion

1

2

3

4

0 5 10 15 20

Applied helm angle, δ p (deg)

In

itial

turn

ing,

X 1

0/L(S

hip

leng

ths) 1 (deg/s)

3 (deg/s)5 (deg/s)

Pod slew rate

Assessment of initial turning criterion, Woodward (2005)

•10 o applied helm angle is entirely appropriate for testing the initial turning ability of pod-driven ships



• Yaw-checking criterion

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5δ

δ

YY′′Δ

Phas

e M

argi

n

1 st10/10

1 st20/20

2 nd10/10

15o

10o

5o

0o

-5o

-10o

-15o

Initia

l Turn Advance

Tactical Diameter

{ }SternYY vv ′′Δ

Assessment of yaw-checking criterion, Woodward (2005)

•10/10 and 20/20 test criteria are entirely appropriate for testing yaw-checking ability of pod driven ships as they well approximate the -5o

phase margin



• Stopping criterion

• The IMO stopping ability criterion is perfectly valid for pod-driven ships

• Other operations exist to stop a pod-driven ship more efficiently, perhaps less load on the propeller



• The IMO manoeuvring criteria ‘Resolution MSC. 137(76)’, provide equivalent information about the manoeuvring response of pod-driven ships as for conventionally propelled ships; and can thus be applied directly. However, the application of specific helm angle is less clear

• Some pod-driven ships cannot meet the yaw-checking criteria. However this is a design issue and the existing criteria provide an adequate benchmark to impede the development of poor designs


DISCUSSIONS, Q & A


10. SPECIAL APPLICATIONS FOR PODDED PROPULSION

• A brief survey on the following applications / propulsorswithin the framework of the Committee’s tasks

• Double Acting Tanker (DAT)

• CRP-Podded Propulsion Systems:

Hybrid CRP- Podded Propulsor

Pure CRP- Podded Propulsor

• Rim-Driven Podded Propulsor

• New pods for smaller power range


Concluding remarks (Special applications for pods)

• Various different features of these applicationsrequire rather sophisticated experimental facilities, techniques and more support from CFD procedures and full-scale data.

• While the ITTC is currently concentrating on the conventional applications, the review of developments for these special applications should not be overlooked (Particularly D.A.T and Hybrid CRP-Pod)


11. TECHNICAL CONCLUSIONS1. Last decade has witnessed rapid developments in concept and

real design applications of pods accompanied by vast research programmes

2. Committee’s efforts on “Procedures” for:Podded propulsor tests (TASK1),Full-scale propulsion prediction (TASK 3)Model-scale cavitation tests (TASK2)

Indicate that:

2a) A podded propulsor is considered as a ”unit” and requires special testing device for open water tests

2b) The unit O/W test, hull resistance test (without pods) and propulsion test provide a basis for the speed-power prediction in full-scale

2c) In full-scale power prediction, there is still uncertainty associated with the prediction of “pod-housing drag” in full-scale and this requires further investigation. The committee therefore is unable to formulate a single guideline


TECHNICAL CONCLUSIONS2d) The Committee recommends further collaborative work in the

area of extrapolation. This work should investigate different approaches, including RANS based modelling of the housing drag, and the use of consistent model test data supported by reliable full-scale measurements

2e) The measurement of the propeller thrust at the pod unit provides invaluable information for the pod designer. However the uncertainty associated with “gap-effect” requires further investigation.

2f) The Committee present procedure for model-scale cavitation tests and appearances adapted from that for the conventional propellers with necessary updates

2g) Cavitation procedure is valid for conditions at straight aheadand slight off-course at steady angles. Conditions at large helm-angles and in dynamically controlled mode can be of great interest but require further investigations


TECHNICAL CONCLUSIONS3. Off-design conditions (TASK 4) are critical stages in the

design and operations of podded propulsion systems since the loads may reach maximum values at these conditions

3a) Unsteady nature of propeller operation will induce large blade forces as well as “spike” loads on the pod housing, due to dynamic manoeuvring behaviour, that will have significant implications for the structural design and may also impact onthe vessel roll stability

3b) The nature of off-design loads are complex requiring further investigations both numerical and experimental:

CFD has rather limited success needs further developmentsThere is a need for procedures to simulate the off-design conditions as realistic as possible in testing facilities


TECHNICAL CONCLUSIONS4. The performance limits given by the IMO Manoeuvring Criteria

provide an adequate benchmark to compare all ships regardless of propulsion type. However, the application of specific helm angle is less well defined

4a) Dedicated simulation studies and limited amount manoeuvring tests with pods in full-scale suggest that the IMO manoeuvring criteria “Resolution MSC 137(76)” provide equivalent information as for conventionally propelled ships and thus be applied directly

4b) Similar investigations indicate that hull-forms suited for pod drives can have poor course-stability characteristics resulting in a ship that cannot meet the yaw checking criteria. However this is primarily a design issue and the existing IMO criteria provide adequate benchmark to impede the development of poor designs.


RECOMMENDATIONS TO THE CONFERENCE

1. Further development of the procedure “Propulsion, Performance-Podded Propulsor Tests and Extrapolation (7.5-02-03-01.3)” with specific emphasis on the procedure of scaling and validation of full scale propulsion prediction

2. Review and development of CFD investigations to aid podded propulsion prediction in full-scale and to understand unsteady flow behaviour in off-design conditions


RECOMMENDATIONS TO THE CONFERENCE

3. Review new trends in experimental techniques for investigation of podded propulsor operations in design and off-design conditions. Determination on the needs of development of the propulsion and cavitation test procedures for special modes of operations

4. Review and development of investigations on the manoeuvring induced dynamic loading for the safe and reliable design and operation of pod driven ships.

5. Review developments in special applications of poddedpropulsors, particularly for ice class ships and high speed CRP applications, with specific emphasis on their propulsion and manoeuvring characteristics, and associated experimental and numerical prediction techniques.


STEP 1: Consider complete pod unit to be the ship propulsor

STEP 2: Perform (appended) hull “resistance test” without pod unit(s)

STEP 3: Perform “open water test” with the pod unit to measure the unit open Performance data, including the propeller thrust, if possible

STEP 4: Obtain open water performance data of the propeller model

STEP 5: Assume that the unit thrust is the difference of the propeller thrust and housing drag; and the unit torque is equal to the propeller torqueattached to the pod

STEP 6: If thrust of the propeller cannot be measured in ( 3 ): estimate torque identity wake fraction corrected to full scale; apply it to determine actual operation point

Reference extrapolation procedure


STEP 7: Apply suitable correction method to take into account the effects of the propeller blade drag in full-scale

STEP 8: Calculate model pod unit housing drag based on (3), (5)& (6 if necessary)

STEP 9: Select suitable method to determine the model pod unit housing dragcorrection to full-scale and calculate housing drag correction coefficient

STEP 10: Combine the pod unit performance data obtained in (3) with the scale effect corrections in (7) & (9) to obtain pod unit open water performance data in full-scale

STEP11: Perform “pod propulsion test” by applying suitable towing force including the scale effect of pod housing drag obtained in (9)


STEP 12: Evaluate pod unit-hull interaction coefficients

STEP 13: Evaluate shaft speed and delivered power using the full-scale advance ratio and the torque coefficient to be read-off from the full scale pod unit performance data and apply trial corrections

END


Propulsion Unit Operating Conditions

• Some testing organisations opt for full-scale KTunit (based on thrust

identity) rather than propeller KT. This is not recommended by the Committe since it results in incorrect propeller loading

Cavitation patterns on suction side of propeller blade at KT = 0.122 (Using propeller thrust identity)

Cavitation patterns on suction side of propeller blade at KT unit = 0.127 (Using unit thrust identity)

The Specialist Committee on Azimuth Ing Podded Propulsion

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