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Ship Hull Appendages: A Case Study

Conference Paper · December 2012

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SHIP HULL APPENDAGES: A CASE STUDY

Shiju John, Md. Kareem Khan, P C Praveen, Manu Korulla and P K Panigrahi
Naval Science & Technological Laboratory, India

ABSTRACT

Hydrodynamic model studies were carried out at Naval Science & Technological Laboratory (NSTL), Vishakhapatnam,
India to estimate the resistance contribution of appendages. This paper discusses categorization of ship hull appendages
from a hydrodynamic viewpoint. An attempt is made to qualitatively assess the resistance contribution of each of these
categories of appendages to the total ship resistance. A relative appraisal of the resistance contribution of appendages on
a naval surface combatant, a submarine, a tanker, a container, a dredger and a single screw ship is made to emphasize the
large appendage drag on naval ships. Results obtained from hydrodynamic model experiments conducted at High Speed
Towing Tank (HSTT) facility at NSTL on a fast displacement hull form used for naval applications (FDHFNA) are
discussed at length as a case study. A criterion for rating appendages is proposed.

1. INTRODUCTION limited due to the vibration associated with the flow

separation around the fins. At the same time, fixed fins at
The ship’s hull is a streamlined body designed to create the stern have long been used to improve the flow and
favourable pressure gradients so that it encounters reduce the propeller vibration. Thus, proper knowledge
minimum resistance to forward motion. However, we of the effect of a particular appendage at a particular
have to impose on the hull surface certain add-ons so as location on the hull is a vital input for the designer.
to improve the controllability and maneuverability, sea-
keeping, strength and structural aspects or to fulfil Thus, the net effect of appendages is thought to bring
operational requirements. These attachments which alter about the following changes.
the flow around the hull are called ship hull appendages.
Appendages can also be designed to improve resistance i. Increase in frictional resistance
and propulsion characteristics. [1] ii. Alteration of flow around the hull
iii. Alteration of ship motions
Needless to say, the inclusion of appendages will alter iv. Induced vibration/noise/cavitation
the resistance of the ship. The wetted surface area of the
appendages causes an increase in the total frictional drag. It is highly recommended that some standards be adopted
If the appendages have abrupt curvatures, flow may for ensuring the quality of appendage design from all
separate and cause separation drag. Also, as the aforementioned perspectives. However, it is difficult to
appendage modifies the flow around the hull it may generalise this process as appendages are of different
affect propulsion performance. An appendage such as a types depending on their requirement and working
wake adaptive fin may improve the wake characteristics principle. Thus it would make the picture lucid if we
of the inflow coming into the propeller, thereby could categorise the appendages based on their
improving the propulsion performance. At the same time, functionality. The authors have worked in this direction
an A-bracket supporting the propeller shaft reduces the in section 2.
flow between the strut arms causing hydrodynamic
problems such as wake peaks. Positioning of the In the present paper, section 3 highlights the importance
appendages has to be optimized for a favourable of appendage design in the case of a warship.
influence on the flow. Hydrodynamic model tests on the appended hull model
of a fast displacement hull form used for naval
Appendages may also affect the trim, heave and roll applications (FDHFNA) are carried out at HSTT. The
motions. Stabilizer fins are appendages used to control results are discussed in a case study in section 4. The use
roll and pitch motion of ships. These are more often of flow visualization and wake survey techniques in
active stabilizers and assist in ride control. Bilge keels appendage evaluation is illustrated. The resistance
which are passive stabilizers also improve the roll contribution of appendage categories is interestingly
stability of ships. Stern wedges/flaps and interceptors are brought out in this study. A brief discussion is made on
used to control the trim and heave of high speed vessels. the use of energy saving devices for reducing resistance.
In doing so, they primarily reduce the ship resistance and
improve overall propulsive efficiency. [2] In section 5, the authors propose a rating method for
appendages which will enable to assess the
Vibration or noise caused by the faulty orientation of an hydrodynamic quality of the appendage coupled with its
appendage is a matter of concern in ship design. For functionality. Overall, this paper attempts to emphasize
instance, a pair of fixed fins near the bow is used in some the need to standardise hydrodynamic design for ship
ships to reduce the pitching motion. However their use is hull appendages.
 23
3

21 1. Rudders
2. Lifting foils
3. Skegs
21. Bulbous Bow 4. Thrusters
22. Pre swirl stator
23. Stern wedge /flap /tab
tab
/interceptor 6
24. Rudder bulb fin
25. Ducts for thrust gain Manoeuvring
Aids

5. Fin stabilizers
Energy 6. Bilge keel
Motion 7. Stern wedge/flap
Saving Inhibitors
Appendages 8. Interceptors
9. Ventilated foils

SHIP HULL 7
19 APPENDAGES

19. Sonar Dome

20. Underwater Operational Propulsion
surveillance/ Aids Aids
weapon units

20
Structural 10. Propeller shaft
Members 11. Propeller hub
12. Wake adaptive fins
13. Propeller pod/cowl
15. Shaft brackets
16. Shaft bossings 14. Vortex generators
17. Hydrofoil
struts
18. Rudder posts 10, 11
12

17

15
13

Figure 1: Classification of ship hull appendages

2. CLASSIFICATION OF APPENDAGES 3. APPENDAGES ON NAVAL SHIPS

Ship hull appendages can be classified into the following Generally, naval platforms have more appendages as
categories from a hydrodynamic viewpoint: compared to a conventional merchant ship. Appendages
such as sonar domes, dynamic positioning systems, ‘A’
i. Maneuvering Aids and ‘P’ brackets, twin propeller shafts, twin rudders and
ii. Motion Inhibitors other operational aids seen on a warship call for serious
iii. Propulsion Aids design considerations. Location, alignment and fairing of
these appendages are essential in avoiding cavitation,
iv. Structural Members
decreased propulsive efficiency, vibration and noise apart
v. Operational Aids from incremental resistance.
vi. Energy Saving Appendages
25
Maneuvering Aids are control surfaces which contribute

% Increase in Resistance by Appendages

to the controllability of the ship. Examples are rudders,
lift generating foils and skegs. Bow thrusters can also be 20
treated as appendages in this category.

Motion Inhibitors are control surfaces such as fin 15

stabilizers and bilge keels. Interceptors, stern
wedges/flaps, ventilated foils are also ride control
devices and fall into this category. 10

Propulsion Aids include those appendages which are a

part of the propulsion unit of the ship. These include 5
propeller shaft, propeller hub, wake adaptive fins, vortex
generators, pod of a podded propeller and the cowl in the
case of a pump-jet propulsor. The propeller as a whole is 0
not considered as an appendage because it is the dynamic
0.0 0.2 0.4 0.6
effect of the propeller blades that generate the thrust to
Froude Number (Fn)
propel the ship ahead. Conveniently, the drag of the
propeller blades is accommodated in the propulsion Naval Surface Combatant
calculations (as CD in open water calculations). Since the Submarine
propeller hub contributes consistently to the drag it is Oil Tanker
treated as an appendage. Container
Dredger
Structural Members such as shaft bossing, shaft
brackets/struts, rudder posts and struts of foils among Single Screw Ships
others are also appendages. A high percentage of the
structural members are for supporting propulsion aids. In Figure 2: Comparison of appendage resistance for
many cases, it may be difficult to isolate these two various ship types
classes. However, for the purpose of providing a clear
distinction they are categorized separately. A comparison of the appendage resistance of various
types of ships is given in Figure 2. Extrapolated full scale
Operational Aids are those appendages which are fitted data from model resistance tests conducted at HSTT is
to serve the operational requirement of the ship. used for this comparison. It may be seen that the
Examples are sonar domes, underwater cameras and percentage of appendage drag, which is primarily due to
underwater weapon carriers among others. frictional resistance and the hull-appendage interaction,
is more at lower Froude numbers. The amount of wave
Energy Saving Appendages are those which are used for making drag also increases with ship speed. This is the
reducing the powering requirement of the ship. Bulbous reason why the appendage resistance contribution drops
bow is a classic example in this category. Motion down at higher speeds. Model resistance tests are based
inhibitors such as interceptors, stern wedges and flaps on the assumption that appendage contribution to wave
also help in reducing the ship powering and may be resistance is nil.
accounted in this category as well. Vanes fitted ahead of
the propeller to reduce fuel consumption are another The high percentage of appendage drag on a naval ship is
example. Propeller ducts for thrust gain, pre-swirl stators demonstrated in the above comparison. Appendage drag
and rudder bulb fins (Kawasaki)/thrust fins (Hyundai) on naval ships generally varies from 20% to 10% across
which improve propulsive efficiency are other examples. the speed ranges. Case in point hydrodynamic evaluation
Figure 1 elucidates the above classification of of the appendages on a similar hull form is presented in
appendages. Section 4.
4. CASE STUDY

A fast displacement hull form used for naval applications

(FDHFNA) is chosen for case study. [3]

4.1 HULL PARTICULARS

The non-dimensionalised particulars of the hull form are

as follows:
L/∆1/3 7.49
L/B 8.57
B/T 3.13
CB 0.55
FnMAX 0.42
FnOPERATION 0.25

The list of appendages on this hull form is as follows:

i. Bow mounted sonar dome

ii. Stabilizer fins (2 pairs -P&S)
Figure 3: Streamlines and Contours of Static Pressure
iii. Bilge keels (2 pairs - P&S)
Coefficient for sonar dome located at bow,
iv. Rudders (P&S) quarter ship and midship using SHIPFLOW
v. Shafts (P&S) (top); Comparison of Total Resistance
vi. Cylindrical bossings (P&S) Coefficient (CT) at Fn = 0.25, 0.30 and 0.35
vii. 'A' Brackets (P&S)
viii. 'P' Brackets (P&S)

A scaled down model of the ship with the above

appendages was made for hydrodynamic tests at HSTT.

4.2 FLOW VISUALISATION

Bare hull resistance tests were conducted initially.

Positioning the sonar dome and its effect on ship
resistance was studied using CFD suite SHIPFLOW. A
comparison of the pressure profile and total resistance
coefficients for different sonar dome locations is given in
Figure 3. The bow mounted sonar dome configuration
was selected for the present hull form. [4][5]

Flow visualization test was carried out on the bare hull

model with bow mounted sonar dome at a model speed
corresponding to the maximum speed to visualize the
flow over sonar dome and to arrive at the hydrodynamic
position of bilge keels & stabilizer fins. The results of the
test were examined for specific areas of hull at sonar
dome, bilge keel & stabilizer fins location. It was
observed that streamlines developed without separation
over the bow mounted sonar dome. Over the bilge keel
and stabilizer fin locations, the streamlines were running
parallel indicating “neutral flow orientation” of these
appendages. The streamlines at the “A” bracket strut
location were parallel but at “P” bracket the stream lines
Figure 4: Paint flow results over P bracket indicating 10
were inclined to the strut by 10 degrees which may result
deg angle to streamlines (top); Original P
in considerable flow separation and vibrations. Hence the
bracket design (middle); P bracket realigned
“P” bracket strut was reoriented to align along the flow
by 10 deg after paint flow(bottom)
as shown in Figure 4. [3]
4.3 WAKE SURVEY 25

Propeller inflow was examined by carrying out 3D

% Increase in Resistance
20 D
nominal wake survey in the propeller plane at Fn = 0.25
and Fn = 0.42. Results suggested that the inflow to the 15
propeller was distributed properly with uniformly
C
varying velocity ratios, thus ruling out undesired 10
fluctuations in propeller loading. Cutting an arc of about B
30° to either side of the top dead centre, a region of 5
A
wake peak can be seen. This clearly shows the shadow
effect of “A” bracket in front of the propeller. The axial 0
iso-wake plot at Fn = 0.42 is reproduced in Figure 5. [6] 0.10 0.20 0.30 0.40 0.50
-5
Froude Number (Fn)
A: Bare hull + sonar dome
B: A+ stabilizer fins
C: B + bilge keel
D: C + shafts, bossing, brackets, rudders

Figure 6: % Increase in resistance by different

appendages

4.5 USE OF ENERGY SAVING APPENDAGES

To further reduce the powering requirement, energy

saving devices (ESDs) were considered. Stern wedge,
flap, wedge-flap combination, interceptor and an
innovative wedge-interceptor combination were
attempted through a systematic test approach. The best
Figure 5: Axial iso wake plot in Port propeller plane at result in each series is plotted in Figure 7. The y-axis
Fn = 0.42 represents the ratio of ship resistance with ESDs to that
without ESDs. Similar studies in reference [7] suggest
It was observed that the transverse velocity components that the higher ratios at lower Froude numbers may be
in the propeller plane are outward dominated. The due to Reynolds scale effects. On an average, the
tangential velocity components were negative in the Delivered Power is brought down by 5% to 10% during
region of higher wake. To have lowest loading on the self-propulsion tests between Froude number ranges of
propeller while entering this axial wake peak zone, the 0.30 to 0.40. [8]
propeller should have the same direction of the tangential
velocity. Thus, for the twin propellers outward rotation 1.08
was selected. [6] 1.06
Resistance ratios

4.4 APPENDAGE RESISTANCE 1.04

1.02
Appended hull resistance tests were done in different
stages to gauge the resistance contribution of each of 1.00
these appendages. The percentage increase in resistance 0.98
created by the appendages is plotted in Figure 6. It may
be noted that the bow mounted sonar dome cancels out 0.96
wave making drag between Froude number ranges of 0.94
0.20 to 0.30, thereby reducing total resistance. The 0.10 0.20 0.30 0.40 0.50
marginal hump in the resistance curve which was Froude Number (Fn)
observed for bare hull resistance tests corresponding to a Wedge 10 deg, 1% chord
Froude number of 0.30 was reduced by the bow mounted Flap 10 deg, 1% chord
sonar dome. Overall, it increases resistance by about 2%. Wedge-Flap 10 deg, 1% chord
The stabilizer fins and the bilge keel increases the Interceptor 1mm
resistance at all speed ranges by about 2% and 3% Wedge-Interceptor 10 deg, 1% chord, 1mm
respectively. The propulsive aids and structural members
increase resistance by a bigger margin of about 9%.
Condition D represents the fully appended hull. Figure 7: Resistance ratio with Energy Saving Devices
4.6 RESISTANCE CONTRIBUTION OF ship without this appendage.
APPENDAGE CLASSES
For example, let us consider a rudder. Functional
From this study, the resistance contributions of the Effectiveness for a rudder will be a function of the
appendage classes were identified. Four distinct speed control forces generated/degree of turn, turn rate, roll
ranges are identified for this: slow speeds (Fn = 0.10 to stabilizing effect, etc. Inhibitive Capacity for the rudder
0.20), operational speeds (Fn = 0.20 to 0.30), medium will encompass the additional drag, cavitation, induced
range speeds (Fn = 0.30 to 0.40) and sprint speeds (Fn = vibrations and any adverse impact on propulsive
0.40 to 0.45). efficiency among others. Each of these characteristics
should be rated on a scale that varies from 0.0 to 1.0.
10 Such a scale is to be arrived at after considering all
available data on the performance of such an appendage.
% Increase in Resistance

8
The minimum point on the scale should be for the worst
6 performance and the maximum for the best known
4 performance at present. This offers flexibility in allowing
for future improvements in appendage design. This
2 means that an appendage that is rated as ‘good’ today
0 may not be good enough tomorrow when the benchmarks
-2 have improved.
0.10 to 0.20 0.20 to 0.30 0.30 to 0.40 0.40 to 0.45
-4 Let us consider the example of a stern wedge fitted to a
Froude Number (Fn)
Operational Aids planing craft which is used as a survey boat. [9]
Maneuvring Aids
Motion Inhibitors Purpose of fitment : Reduction of trim and heave for
Propulsive & Structural Aids proper immersion of sensors
housed on hull bottom.
Energy Saving Appendages
Speed : Maximum Fn = 1.94
Figure 8: % Increase in resistance by appendage classes Operational Fn = 0.65
The mean percentage variation in resistance in these
speed zones were calculated for these appendage To meet the above requirement, hydrodynamic model
categories. The resistance contributions are summarised tests were planned on a scaled down model. Stern
in Figure 8. The propulsive and structural appendages wedges were designed and manufactured at model scale.
contribute maximum to the overall appendage drag. Model tests were conducted with the stern wedge fitted
ESDs are found to reduce resistance at higher speeds. to the model and the resistance, heave and trim were
measured. The performance of the optimised wedge
5. APPENDAGE EFFICIENCY INDEX (wedge angle = 4 deg, chord length = 2 % of LBP) is
presented in Table 1.
While designing appendages for a ship’s hull form, the
designer would like to rate his design of appendages. The PE_wedge/ H_wedge/ T_wedge/
Fn
quality of an appendage should be benchmarked by PE_bare hull H_bare hull T_bare hull
weighing its operational effectiveness against the
0.12 1.19 0.25 0.50
negative impact it has on the bare hull hydrodynamics of
the ship. A universal standard for reviewing the 0.24 1.11 0.98 0.50
effectiveness of an appendage will serve as a handy tool 0.36 1.12 0.91 0.47
for the designer. In an attempt to compare and improve 0.49 1.02 1.04 0.72
the performance of an appendage, the authors would like
to propose such a system in this section as a starting 0.65 1.01 0.26 0.80
point. To begin with, an Appendage Effectiveness Index 0.79 0.97 1.74 0.72
(AEI) is put forward which represents the sum of 0.96 0.95 1.17 0.70
Functional Effectiveness and Inhibitive Capacity of the
appendage. 1.18 0.96 1.13 0.72
1.34 0.94 1.13 0.72
Appendage Effectiveness Index 1.53 0.94 1.14 0.72
=Functional Effectiveness + Inhibitive Capacity 1.70 0.93 1.13 0.70
1.79 0.93 1.11 0.68
Functional Effectiveness will reflect the capability of the
appendage to perform its intended function. Inhibitive 1.94 0.94 1.16 0.66
Capacity will cover all the negative/positive impacts that
the appendage will have on the hydrodynamics of the Table 1: Performance of stern wedge on planing boat
 It may be seen that the wedge effectively reduces the An Appendage Effectiveness Index is proposed for rating
running trim of the model. The upwards heave motion is and reviewing the quality of any appendage. At this
also substantially reduced around the operational speed stage, due to lack of a global database it is not possible to
(Fn = 0.65). To compliment, the powering performance estimate AEIs for all appendages. However, an example
of the craft is also greatly reduced at Froude numbers is provided just to give a feel of the AEI estimation
greater than 0.65. [9] which the authors have in mind. Such a simple and
flexible system would allow for comparing any
Functional Effectiveness (+1.0): appendage and thus improving its performance
1. Reduces trim and heave as required around 10 periodically.
knots. (+1.0)
2. The increase in heave beyond 10 knots is 7. ACKNOWLEDGEMENTS
irrelevant from operational perspective. (NA)
We would like to thank Shri VBS Ayyangar, Scientist,
NSTL and the entire testing team of HSTT, NSTL for
Inhibitive Capacity (-0.2):
their help in pursuing this work.
1. The powering requirement around 10 knots is
increased by 19% to 1% from 2 to 10 knots. 8. REFERENCES
This powering increment is comparable to data
from available literature. Nevertheless, there are 1. SHIJU, J. et al, ‘Hydrodynamic performance
few optimised cases where the powering is enhancement using stern wedges, stern flaps and
increased by only 2% to 1%. Hence a rating of interceptors’, RINA International Conference on Ships
(-0.5) is given for this performance. and Offshore Technology – India, 2011.
2. The powering is improved above 10 knots by 3
2. KARAFIATH, G. et al, ‘Stern Wedges and Stern
% to 7%. (+ 0.3) Flaps for Improved Powering – US Navy Experience’,
SNAME Transactions, Vol. 107, 1999.
Appendage Effectiveness Index = (+1.0) + (-0.2) = 0.8
3. NSTL Hydrodynamic Model Test Report No.
On a scale of 0.0 to 1.0, an AEI rating of 0.8 seem to NSTL/HR/HSTT/221/1, June 2010.
reflect very good design and performance of the
appendage. The ratings used in this example are just for 4. NSTL CFD Report No. NSTL/HR/CFD/2012/007,
demonstration and can be standardised only with a September 2012.
performance database of similar devices.
5. PRADEEP, J.S. B. Et al, ‘Comparative study for a
6. CONCLUSIONS ship hull with sonar dome at different positions’, RINA
International Conference on Ships and Offshore
The criticality of appendages in powering performance of Technology – India, 2011.
ships was emphasized in this paper. Ship hull appendages
were hydrodynamically classified into six categories. The 6. NSTL Hydrodynamic Model Test Report No.
resistance contributions of each of these categories of NSTL/HR/HSTT/221/2, November 2010.
appendages were assessed through examples. Key points
in design and evaluation of appendages were touched 7. CUSANELLI, D.S., ‘Scaling Effects on Stern flap
upon. The huge appendage drag on a warship was Powering Progress Report’, NSWC Carderock Division,
highlighted through a comparison with other ships. Sonar Technical Report, September 2009
dome positioning was discussed for minimum added
resistance. 8. NSTL Hydrodynamic Model Test Report No.
NSTL/HR/HSTT/221/3, August 2011.
The process of evaluating and redesigning appendages on
a FDHFNA was demonstrated through a case study. 9. NSTL Hydrodynamic Model Test Report No.
Through the case study, the minimum appendages NSTL/HR/HSTT/230, December 2011.
present on a warship, CFD studies for initial
configurations, their evaluation and realignment through 9. AUTHORS BIOGRAPHY
flow visualization studies and overall effect on ship
resistance are brought out. The resistance contribution of The authors are scientists at the Hydrodynamic Research
each appendage was extracted. The application of Energy Wing of the Naval Science & Technological Laboratory,
Saving Devices for reducing the powering requirement of India. Their areas of work include improving
the ship was also illustrated in this study. Overall, the hydrodynamic performance of marine vehicles through
resistance contributions of the different categories of computational fluid dynamics, ship model experiments
appendages for the warship in picture were outlined. and full-scale sea trials.

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