1. CFD Modelling of Interference of
Yachts Sailing Upwind
MEngSt Project Report
Author Supervised by
Kahan Sudev Assoc.Prof.Peter.J. Richards

3. Abstract
Sailing is a sport that involves heavy sums of money which makes this sport an extremely serious
venture for all who are involved in it. Sailing, which was once considered an art has now become
a venue that invests millions of dollars in research and development which has caused this sport
to evolve into an engineering field of its own where people have discovered ways to optimize the
use of wind energy to propel at speeds up to a 100 kilometres an hour.
The researchers in this field are exploiting the every nook and corner to increase the performance
of these crafts. Studies are being carried out to escalate the efficiency of the design process,
manufacturing and operation. This project is one such research, but on a comparatively smaller
scale, on the effects of the aerodynamic interference between yachts sailing upwind.
Yachts sailing at close proximities experience a change in their performance when compared to
sailing solo or isolated conditions. This is due to the aerodynamic interference between the sails
of the yachts which causes a variation in performance significant enough to cause a team to win
or lose. The knowledge of the reason behind the occurrence of this phenomena as well as the
location of the zones of high interference will help the sailors to create a strategy based on this
information.
In this project, the CFD software Ansys CFX was used to simulate the interference between yachts
at full scale. The results from this study prove that the negative and positive interference effects
can be predicted by analysing the behaviours of the wind around a single yacht and also solidifies
the fact that the effects of interference are more due to the change of the direction of the wind that
comes on to the sails than the change in the wind speed or pressure. The results present a number
of contour plots of interference zones around a yacht, indicating the intensities of the interactions.
The data from this project would prove to be extremely valuable in the hands of a skipper or race
strategist who would have the practical knowledge and experience to identify the scenarios in
which this information would come to use and create a game plan accordingly.

5. Contents
1. Introduction ............................................................................................................................. 1
General Introduction ............................................................................................................ 1
Project Scope ....................................................................................................................... 3
2. Literature Review .................................................................................................................... 4
3. Project Outline....................................................................................................................... 10
4. CFD Modelling...................................................................................................................... 13
Geometry............................................................................................................................ 13
Meshing.............................................................................................................................. 15
Mesh Refinement Analysis................................................................................................ 17
Boundary Conditions ......................................................................................................... 18
Running the Solver and Data Processing........................................................................... 19
5. Results and Discussion ............................................................................................................. 20
Initial Single Boat Analysis or Sphere Analysis................................................................ 22
Two Boat Analysis............................................................................................................. 28
Comparing Results from Different Runs........................................................................... 32
6. Conclusion............................................................................................................................. 36

7. 1
1. Introduction
1.1.General Introduction
Sails are propelling devices that transform and utilize energy carried by the wind to move a vessel
through waters. Although various sails are used to propel vessels in different conditions and
directions, modern sails can be principally categorized as main sails, head sails, and spinnakers.
The spinnakers are used to sail downwind, i.e. in the direction of the wind and the main and head
sails are used to sail at an angle that is almost against the direction of the wind. This is known as
upwind or windward sailing. These sails act as aerofoils and generate a lift force in a direction
perpendicular to the direction of the wind and a drag force in the direction of the wind. The lift
and drag forces can be further solved as a force in the direction of the heading of the sailing vessel
termed as the drive force and a side force in the direction perpendicular to the drive force Fig (1.1).
The keels of these sailing crafts are specifically designed to generate a hydrodynamic lift and drag,
which balances the aerodynamic forces and thus propels the vessel. Apart from advantages such
as reducing fuel consumption in vessels during voyages, sailing is a source of recreation that
becomes a way of life for the people who indulge in it.
Fig(1.1)- Lift and Drag resolved to Drive and Side forces as well as the hydrodynamic forces
It is a human tendency to compete by racing anything that moves, and sailing races are events that
have been happening for more than a century. These races are primarily categorized as match races
Fig (1.2) and fleet races Fig (1.3). Match racing is events where 2 boats race each other in a course
whereas in a fleet race the competition is between a number of boats. In recent times, thousands
of millions of dollars are invested in these races for the research, development, design and
construction of these crafts as well training the sailors. America’s is one of the most famous yacht
races in the world and the first race was held in the year 1851. The Yacht Research Unit at The

8. 2
University of Auckland is an example of one such institution where intensive research that goes
into the science of sailing to improve and increase the efficiency of sails as well as the yachts.
Fig(1.2)- America’s Cup – Match Racing
Fig(1.3)- TP-52 – Fleet Racing
The Yacht Research Unit houses a 23.6m (length) by 7 m (width) twisted flow wind tunnel
(TWFT) that has a height clearance of 3.5m Fig (1.4). A maximum wind speed of 8.5 m/s can be
achieved in this experimental set up where all the wind tunnel tests on models of yachts are carried
out. The models are placed on a load balance that is connected to a Real Time Velocity Prediction
Program.
Fig(1.4)- Schematic Diagram of the Twisted Flow Wind Tunnel (TWFT) at the Yacht Research Unit

9. 3
The Real Time VPP is a program that has a set of hydrodynamic data such as underwater drag, lift
or side force produced by the keel at particular heel angles, the density of the water and more such
underwater information for a particular yacht. This program is coupled with the load balance in
the wind tunnel which possesses load gauges. The tests are run by placing the model of the yacht
on the load balance and running the wind tunnel. These load gauges determine forces acting on
the model and feed the data to the Real Time VPP which calculates the respective side forces
generated by the keel and hence deduces the appropriate heel angle. This information is fed back
into the load balance which in turn inclines the boat to the particular heel angle.
1.2.Project Scope
In a match racing or fleet racing scenario, one boat tends to have an advantage or disadvantage
over the other due to the aerodynamic interferences between the two boats. This interference
causes the decrease or increase in the driving forces and thus affects the performance of the racing
yachts to a great extent. Interference is almost unavoidable in race scenarios, especially in short
course racing, where boats come in close proximity to their competitors at some point.
Fig(1.5)- Negative effects caused due to interference can be felt up to 10 boat lengths down the wind
The aerodynamic interference is a subject of interest for skippers and race strategy technicians
who are paid large sums of money to perform in races. The fact that the worst place to be in any
race scenario would be in the aerodynamic shadow of its competitor, i.e. at a position where the
wind coming onto the sails are blocked by another boat, is obvious and is know to every sport
sailor. The reason behind this effect is widely misunderstood as it is thought that the decrease in
the drive and side force is due to the decrease in pressure.
Thus a lot of effort has gone into the understanding of this phenomena by a academics and avid
sailing enthusiasts. A number of experiments using both TWFT as well as CFD analysis have been
used at the Yacht Research Unit. The research work has led to the understanding of the behavior
of the wind around a sailing yacht and what really causes the interference effects.
The objective of this project is to understand the flow between yachts sailing upwind using the
CFD software ANSYS CFX and to understand the reasons behind the effects of the aerodynamic
interaction. The study also compares the flow around a single yacht to various two boat
interference scenarios to find out if the effects due to the interference could be predicted by
analysing the flow pattern around a single yacht.

10. 4
2. Literature Review
Every sailor knows when he or she has an advantage or disadvantage over his or her competitor
due to the effects of interference. Their knowledge is gained by nothing but experience and often
are under the impression that the change in pressures caused by the interference is the primary
reason for the reduction in the lift the sails experience. One of the earliest experimental studies on
the phenomena of interference was by Macrchaj (1964). He was one of the first to observe and
state the fact that the interference causes the change of apparent wind angles by tracing the velocity
fields around racing yachts as shown in in Fig (2.1). Gentry (1971), performed a similar study
where he analysed the velocities and angles of the flows around the sails and plotted velocity
streamlines around them to show the changes in the flow field.
Fig(2.1)- Macrchaj (1964) velocity streamlines around two yachts(left), Gentry(1971) velocity flow field around a yacht(right)
He also studied a blanketing effect caused downwind by the sails, the dead wind zone, which
causes a negative effect in the drive force for a boat sailing in its wake. This zone for yachts sailing
upwind was further studied by Jhonson (1995). He implies that there was an intense negative
interference zone downwind to the direction of the true wind. This was revised by Richards et al
(2013) where, through the wind tunnel testing, it was observed that the zone of intense negative
interference was downwind, in line with the apparent wind direction. This zone is formed due to
the decrease in wind speed. The zone of negative interference was observed to be larger than that
predicted by Jhonson(1995). It is observed that the change in wind angle is the primary cause for
both the “negative’ and ‘positive interference’ zones rather than the change in wind speed or
pressures. Richards et al(2014) broke down the effects of interference to its root causes, which are
the winds coming onto the sails or moving away from it and the wind speed. These were calculated
as the change in AWA and the change in dynamic pressures. These factors were recorded through
the cobra probe experiment, where probes were set at one third and two third of the mast heights
and moved around a yacht model in the wind tunnel. The probes recorded u,v and w velocity
components and these results were plotted as contours shown in Fig (2.2).

11. 5
Fig(2.2)- Zones created due to the interference studied by a) Gentry (1964), b) Jhonson (1995), c) Richards et al (2013)
Fig(2.3)- Richards et al(2014) Change in AWA( left) Change in Dynamic Pressure of the wind(right) at 1/3rd
of mast height

12. 6
One can observe that the zones of intense negative and positive interferences from Richards et al
(2013), Fig (2.2) resembles the negative changes in both the AWA and Dynamic Pressures on the
contours in Fig(2.3).
The most drastic effect that the interference causes is a reduction in the drive forces. A number of
tests have been carried out on this subject by various academics. The drop or raise in these forces
were studied as a ratio of the force experienced by an interfered boat to the force experienced by
a boat in an isolated condition. Little (2009) investigated the changes in the drive force of the
interfered boat, by using the Fd/Fdiso ratio. The experiments were carried out in the Twisted Flow
Wind Tunnel, by using the model on a force balance to calculate the forces. The drive forces were
calculated for the different positions. A similar study was carried out by Caponetto (1997). He
used a computational model, Vortice Lattice Method to carry out his investigations, and computed
the forces and resolved them to the force ratio.
Fig(2.4)- Caponetto’s (1997)(left) and Little’s (2009)(right) Fd/Fd iso plots.
The angles along the X axis in the graphs above are the positions of the boats. Both Caponetto and
Little used a radial grid to place the second boat around the key boat. Richards et al(2013) and
(2014) on the other hand utilized placed along a cartesian x and y axis to each other forming a
rectangular sequence, and hence are refered to as a rectangular grid system.
Fig (2.5) - Radial grid that was used in Little’s experiments

13. 7
BAÇ (2015) carried out the same investigations in the TWFT, using a VPP. The model interfered
model in this case did not have a stationary heel angle. The heeling varied depending on the force
on the sails. To set up his second model, he used a transverse grid to determine its position. The
grid is based on the idea that a start line in a race is set up at a direction perpendicular to the true
wind and hence helps simulate realistic start line and overtaking scenarios. The setup of the
transverse grid is explained in detail as it has been used to carry out the CFD investigations.
BAÇ (2015) carried out his investigations and plotted the force ratios with a free heel against the
positions and compared his data with Capenetto’s and Little’s, shown in Fig(2.6). The free heel
results do not show drastic changes in the forces like that of fixed heel. This is because the lift
forces on the sails increases as the heeling angle decreases The fixed heel force predictions tend
to exaggerate the results.
Fig (2.6) – Comparisons of the Fd/Fd iso ratios
Richards et al (2014) and Little (2009) presented their results plotting the areas around a key yacht
where a second boat would experience a certain positive or negative change in their drive forces
displayed in Fig (2.7).
Fig (2.7) – Areas of reduction in drive force - Little (left) and Richards et al (right)

14. 8
CFD is a very useful tool when it comes to modelling flows and Capennote’s Vortices Lattice
Methods is one of the first computational simulation of the interference between yachts.
Fig (2.8) – Caponetto (1997) CFD simulation of Yachts sailing up the wind.
Spenkuch et al (2011) used the lifting line method on both the Wind tunnel model and the CFD
RANS model sails for fleet racing scenarios. The results from both these tests yacht were
combined with the lifting line algorithm and was implemented within a strategy analysis tool
called Robo Race.
Fig (2.9) – Spenkuch et al (2011) CFD modelling of interference
Spenkuch also modelled the vortex development in the wake of a yacht. It displays the behaviour
of the wind downstream, but it does not predict nor analyses the effects it causes on a boat in its
wake.

15. 9
Fig (2.10) – Spenkuch et al (2011) CFD modelling of the vortex wake
Although there aren’t many modelling of interferences between yachts on CFD, there are a lot of
simulations and experimentations of yachts sailing upwind in CFD software. Gillen (2013) used
Openfoam to validate upwind sails using a structured mesh system. Hazard and Stone (2014) have
used an unstructured mesh to carry out their modelling. Both cases show pressure distribution that
is alike but has produced results to validate the use of CFD software to analyse the flow around
sails. All the above-mentioned CFD simulations have been modelled to the wind tunnel
experimental scale and the boundary conditions have been designed to suit the conditions in the
TWFT.
The above studies provide a solid platform for this project and has provided a great information
and understanding of the subject. The Ansys CFX CFD software has a great post processing
software that could be used to retrieve and manipulate various data that would be impossible to do
in an experimental set up and the visuals of the flow pattern of the fluids can be easily generated.
Thus the CFD Package CFX has been chosen to model and simulate the interaction between two
full-scale yachts sailing upwind, analyse the effects and causes and to compare the effects caused
by the interaction between the two boats with the flow behaviour around a single boat, to
comprehend the reasons for their respective behaviour.

16. 10
3. Project Outline
The CFD modelling is carried out in the CFX software on Ansys Workbench version 1.6. The
project is categorized into three sections namely
1. Initial Single Boat Analysis or the Sphere Analysis
Where the first few Initial single boat test results are analyzed to determine the mesh
settings that are used for all runs. The aim of the single boat analysis is to understand the
change in flow and pressure caused due to the aerodynamic interference of the yacht. The
reason that the initial analysis is also known as the Sphere Analysis is because in this step
the CFD results were processed to create a volumetric average of the U, V and W velocity
components for a spherical volume of diameter equal to the mast length, at numerous
locations around the single boat.
2. Single Boat Analysis at Various AWA
The single boat analysis at various AWA is carried out by changing the angle of attack of
the air flow onto the sails and computing the side and drive forces at the various angles.
The angles at which the analysis are carried out ranges from 18⁰ to 28⁰ with an increment
in angle by 1⁰. This analysis is carried out to compare the drive forces at the different AWA
to the drive forces of boats affected during interference.
3. Two Boat Analysis.
The two boat analysis are carried out with the second boat placed at various positions in a
rectangular grid system. The drive and side forces are calculated for both the interfering
boat as well as the interfered boat and these results are further processed and analyzed.
e
Fig(3.1)- Streamlines around the two boat tests

17. 11
A transverse grid is used to define the positions of the boats for the two boat analysis. This grid is
chosen over the radial and rectangular ones that were widely used for previous two boat
interference analysis because, in the situation of a real race scenario the lay line or the line of equal
advantage would be perpendicular to the direction of the true wind and the boats would maintain
a constant course direction, unless and until the skipper decides to tack or jibe. Thus, it would be
more suitable to analyse the interference experienced by a boat along its direction of heading. The
grid layout is displayed in Fig (). Each black dot in the image represents the position at which the
second boat is placed around the interfering boat for each CFD run.
Fig(3.2)- Grid Layout for the two boat Analysis
The Design Modeller is a design package on Ansys workbench which feeds the data to the Meshing
software on the workbench. The Design Modeller software uses a Cartesian coordinate system,
around which a certain geometry can be moved using the Translate command. The X, Y, and Z
coordinates have to be specified in the Translate dialogue box to move the geometry to a particular
position. This command is used to move the second boat to a particular position around the first
boat before processing the mesh around the geometries.
In this transverse grid system, the horizontal axis on the grids is perpendicular to the true wind
direction. The heading, in this project, is 42⁰ starboard to the direction of the true wind, thereby
creating an apparent wind of angle 25⁰. The arrangement of the layout is shown in detail in Fig
(3.3).
Fig(3.3)- Arrangement of the Grid System

18. 12
To incorporate this grid system into the two boat analysis carried out on Ansys, the grids were
made to intersect at a distance of 0.5 times the mast height for close proximity. The points of
intersections are the positions at which the second boat is placed. This means a certain second
boat’s position on the rectangular grid could be described as being at a distance of ‘a’ times ‘h’
longitudinally and ‘b’ times ‘h’ transversely, where ‘an’ and ‘b’ are multiples of 0.5 along the
longitudinal and transverse grids respectively and ‘h’ is the mast height. However to input this data
into the Design Modeller, the rectangular grid data has to converted into Cartesian coordinates and
are done so by using simple trigonometry as explained in Fig (3.4).
Fig(3.4)- Changing Grid Coordinates to Cartesian Coordinates
Multiplying these values by the mast length gave the X and Y distances to position the boats at the
desired positions on the transverse grid.

19. 13
4. CFD Modelling
This section describes the processes involved in setting up the geometry, mesh and physics settings
for the various scenarios of the single boat tests as well as the two boat tests.
4.1. Geometry
The CFD modelling and simulations are carried out in full scale using the boat and sail models of
an AC33. The hull and mast are modelled on the engineering designing software Solidworks and
are assembled with pre-existing sail geometries Fig (4.1). The sails were of the wind tunnel test
model size and were scaled up using the scaling factor 1:14 to create a real-time sailing yacht with
the following dimensions.
Overall Length 21 m
Overall Breadth 4.6 m
Mast Height 31.4 m
Main Sail Area 202.5 m2
Jib Sail Area 135.3 m2
Freeboard Length 1.06 m
Fig(4.1)- Principal Particulars of the AC33 geometry
Fig(4.1)- Geometry of the Yacht
The .STEP file of the geometry from Solidworks is imported to Ansys Design Modeller, where
the geometry of the boat is placed at the centre of its Cartesian coordinate system, where the length
of the boat is along the X axis, the width along Z and the height of the mast along the Y axis. The
boat was yawed around the Y axis to an angle of 25⁰ in order to model an Apparent Wind Angle
of the same and was heeled at an angle of 25⁰ for all the analysis. These heeling and apparent wind
angles were chosen in order to model close hauled sailing scenario with a wind speed of 20 knots
that is likely to cause an AC33 to heel at an angle of 25⁰. In the case of single boat analysis at
various angles, the model was rotated to the required AWA about the Y axis.

20. 14
A domain of 190.3 m length, 136.3 m breadth, and 35 m height is created to enclose the Yacht on
the Design Modeller software on Ansys’s workbench using the Enclosure command. The yacht is
placed such that it is at a distance of 3h from the upwind wall. 8h from the downwind wall and 2.5
h from the port and starboard walls of the domain, where h is the mast height. The figure below
displays the arrangement of the geometry. A box was created around the geometry of the yacht,
which is used as a control volume for the mesh around the sails, hull and mast during the meshing
stages.
Fig(4.2)- Geometry of the domain with the control volume around the yacht
In the case of two boat testing, the key boat is placed at a position on the Design Modeller, where
the X and Z coordinates are zero. The second boat is moved around the first boat and is placed at
positions specified in the transverse grid system as shown in Fig (4.4). The domain size remains
the same. The figure below shows the leeward boat placed at a - 1.5 ‘ah’, - 1.5’ bh’ from the key
boat on the transverse grid
Fig(4.3)- Geometry of the domain with the control volume around the yachts for the two boat test

21. 15
Fig(4.4)- Positions of the 2nd
boat around the key boat which is at 0,0
4.2.Meshing
The models are meshed on Ansys’s Meshing software. An unstructured mesh using a few explicit
sizing control to accurately capture the resolution of the geometry are the meshing features used
throughout the project.
The method used to mesh the domain is ‘Automatic’ and the domain is also body sized. The box
around the geometries is used as the body of influence to reduce the local mesh sizing. Fig (4.5)
shows the gradual increase in the growth rate from the concentrated area.
Fig(4.5)- A Sliced section of the domain displaying the growth rate of the mesh from the geometry
The prioritize and refine the mesh around the sails, the surfaces of the yacht’s geometry is face
sized. This controls the mesh size on the chosen faces and is the reason for the concentrated mesh
inside the body of influence in Fig (4.5) . The face size is described in detail in Fig(4.6).

22. 16
Fig(4.6)- Face Sizing the surfaces of the yacht geometry
To make the velocity gradient from no-slip wall boundary conditions, inflation layers are created.
Thus, the inflation layers are created around the sails and mast to model the boundary layer as
accurately as possible. This is done by using the Inflation command on the meshing software. The
first layer thickness inflation option is used and the first layer height that is used in 5mm. A
maximum of 12 layers are created around the yacht geometries and a growth rate of 1.1 is induced.
Fig(4.7) shows a sectional view of the sails, and mast and the dark lining due to the neatly packed
inflation layers. In Fig(4.8)(a) and (b), the image is zoomed in to display the layers on the jib,
mainsail, and mast.
Fig(4.7)- Inflation Layers around the sail geometry and mast

23. 17
(a)
(b)
Fig(4.8) The inflation layers on the jib (a) and main sail (b)
4.3.Mesh Refinement Analysis
A mesh refinement analysis is carried out to determine a certain mesh setting to carry out the
project. The forces acting in the Z and X direction on the yacht are the Lift and Drag forces and
are computed through the function calculator on CFX Post software on Ansys, which is an inbuilt
data processing software on CFX. These forces are plotted against the grid size H as shown in Fig
(4.9) and (4.10)
𝐻 = [
𝑉
𝑁
]
1/3
Where V is the volume of the domain
N is the number of elements.
Fig(4.9)- Lift vs grid size
Fig(4.10)- Drag vs grid size

24. 18
It can be observed the magnitude of lift increases and the drag decreases because of the increase
of the element size and Fig (4.9) is almost a mirror image of Fig (4.10). The lift forces are under
predicted and the drag forces are over predicted with a cores mesh. The mesh is refined to a point
where further refinement of the mesh does not bring about a significant change in the lift and drag
forces and thus the chosen mesh is made up of 520390 nodes and 1766566 elements.
Since the same number of nodes and elements cannot be used to mesh in the two boat analysis, the
mesh settings used to generate the mesh with 520390 nodes and 1766566 elements are used.
4.4.Boundary Conditions
The physics of the runs were set up on the CFX Pre module on Ansys and the same boundary
conditions are used for all runs for this project.
1. The walls upstream of the yachts are set as the inlet with a Normal velocity component of
12m/s.
2. The walls down to the aft of the yachts are set as with the opening B.C. The static
pressure and direction condition is chosen in the B.C dialogue box and the pressure
specified for the flow exiting the domain is 0 Pa.
3. The top of the domains are set as a free slip walls.
4. The walls to the port and starboard of the yacht were set to transitional periodicity,a
Periodic boundary condition to approximate an infinite condition.
5. The floor of the domain is set as no-slip walls.
6. The yacht(s) are set as no-slip walls.
7. The SST Turbulence models are used for all the runs as it behaves as a k-ω model close
to the walls and switches to behaving like a k-ɛ turbulence model at freestream .
8. The turbulence intensity was set to low – 1% to model a flow originating from a stand
still fluid.
9. The fluid used was Air at 25⁰C from the material library.
10. Heat transfer is set to isothermal.
11. The maximum number of iterations for each run is set as 10
Fig(4.11)- Boundary Conditions

25. 19
4.5.Running the Solver and Data Processing
After the stage of data input, the CFX solver is run. This iteratively solves the conservation of
mass, momentum and energy equations through each of the mesh elements throughout the domain.
The number of iterations is described on CFX Pre and all the meshing and boundary condition
information are fed into the solver which automatically calculates the Conservation equations till
the solutions converge. The convergence can be monitored on a graph in the ‘Solution’ module
that plots the convergence as the solvers run. The post processing of the results are carried out on
CFX Post, a data processing Ansys module. A number of data have been exported through this
module for this interference study and are explained in detail in the Results and Discussion section.

26. 20
5. Results and Discussion
Data are retrieved from the CFX Post module either by exporting variables such as velocity and
pressure in Spreadsheet formats or by using the function calculator to calculate the integrated or
averaged values of the selected variable. The data exported from this module are then processed
on Microsoft Excel and Tecplot.
The effect of interference between yachts are defined as the changes in the side (Fs) and drive (FT)
force acting on a boat due to the aerodynamic interference caused by another boat sailing by it.
The Fs and FT are derived by further resolving the lift and drag acting on the sails. The forces of
lift and drag are computed by using the function calculator of CFX Post.
The function, location, and the directions are specified. In the case of force, CFX integrates all the
pressures acting in the specified direction along a selected geometry and calculates the force acting
on it. Force X is along the direction of the wind and thus is ‘Drag’ and the Force Z is perpendicular
to the direction of the wind which is the ‘Lift’.
Fig(5.1)- Computing the Forces of Lift and Drag
Fig(5.1) shows the calculated force in the Z direction has as magnitude that is negative. This is
because the force is acting in a direction opposite to the Z coordinate.
The primary aim while carrying out any analysis on a computer software package is to verify and
validate the results, to make sure that the computer modelling produces reliable and realistic
results. For the first few runs of the single boat testing with an apparent wind angle of 25° the
values of the Coefficients of lift and drag were compared to the results of Jowett (2009) for the
initial verification of the tests.
To compare the results the Lift and Drag forces are brought down to their coefficients using the
equations below.

27. 21
𝐶 𝐹 =
𝐹
1
2
𝜌𝑆𝑉2
Where F is the force
S is the area of the sails
V is the velocity of the apparent wind
To further verify the computations, the forces of lift and drag on the sails at different angles of
apparent winds from the ‘Single Boat Analysis at Various AWA’ are resolved into the Side and
Drive forces. The coefficients of these forces are then compared with the results of Richards et al
(2014) as shown in Fig (5.2).
The Lifts and Drags are converted into Drive (T) and Side (S) using simple trigonometric
operations.
𝑇 = 𝐿 sin 𝛽𝐴 − 𝐷 cos 𝛽𝐴
𝑆 = 𝐷 sin 𝛽𝐴 + 𝐿 cos 𝛽𝐴
Where βA is the apparent wind angle
These forces are again converted to their coefficient forms
Fig(5.2)- Comparing the Coefficients of forces from CFD and Wind Tunnel Testing
The blue and orange lines, which are the CT Exp and CS Exp on the graph in Fig (5.2), are results
from the wind tunnel testing carried out by Richards et al (2014). The CT and CS depicted in grey
and yellow respectively, are results from the single boat CFD runs at a various angle of attacks.
The predictions of CT in both cases seem identical and although the CS values are slightly under
predicted, the results are realistic and hence are reliable.
Jowett
CFX
Results
CL 1.25 1.2736
CD 0.39 0.41096

28. 22
5.1. Initial Single Boat Analysis or Sphere Analysis
The initial single boat testing is used to gather data of the flow around a single boat, analyse and
compare these results with those of two boat testing. This testing is also referred to as the Sphere
Analysis because while processing the data in this stage, a spherical volumetric average of
velocities and pressures are calculated to analyse the behaviour of the flow a boat would experience
in the position of the respective sphere. Fig (5.3) displays the concept behind the sphere analysis.
Fig(5.3)- Sphere Analysis
After the running the solver for an isolated boat testing at 25° AWA, a three-dimensional grid is
created inside the domain on CFX Pre. The intersection of the grid are the points of interest. The
velocity variables for each of these points are exported onto a spreadsheet. The figures below
display 3D the grid layout.
Fig(5.4)- 3D Grid layout along X,Y axis

29. 23
Fig(5.5)- 3D Grid along Z,Y axis
Fig(5.6)- Sphere’s Volume covering the Sails
The sphere has a radius of 16.5 m which is approximately half the mast height. This radius was
chosen as the sphere completely envelopes the sails that are inclined at the heeling angle. Fig (5.6)
shows the sphere within the 3D grid covering the sails of the yacht. At any given location along
the Z, X axis of the domain, the sphere covers an average of 603 points of interest. The average of
the U, V and W velocity readings for the points within the sphere produces the volumetric average
of the respective readings. The volumetric average was sought to estimate the behaviour of the
wind a second yacht would be able to experience if placed at the position of the respective sphere.
This sphere is moved around the entire domain along the Z, X-plane by moving its centre every
2.5m along the X axis and Z axis creating a grid system of its own. The volumetric average of the
velocity components are calculated for every sphere’s location and is plotted as a large contour,
mapping the change in the velocities caused due to the aerodynamic interference of the sails. Fig
(5.7) (a), (b) and (c) show the volumetric averages of the U, V and W components of velocity.

30. 24
Fig(5.7)(a)- Volumetric Average of U velocity Fig(b)- Volumetric Average of V velocity
Fig(c)- Volumetric Average of W velocity
Z
X
Z
Z
X
X

31. 25
The position of the yacht is marked by the black hull shaped marking on the contour maps. The
intersections of these grids are the points at which the centre of the spheres lies. These maps show
the magnitude of change in velocities a second yacht would experience if placed anywhere within
the displayed domain.
When two boats interfere the both boats experience a change in the drive and side force. The reason
this happens is because of the wind either turning into or away from the sails and due to pressure
differences created by the wind movements. Hence to understand the interference, the velocity
components are resolved into Change in Apparent Wind Angles and change in Dynamic Pressures.
Resolving the velocities along the X and Z axis we get the change in angle of the wind, which is
measured in degrees.
𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝐴𝑊𝐴 = 𝐴𝑊𝐴° − [tan−1 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑊
𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑉
]
In this case, the AWA is 25°.
The change in dynamic pressure is the ratio of the changed dynamic pressure due to interference
to the standard dynamic pressure without the effect of interference.
𝐷𝑦𝑛𝑎𝑚𝑖𝑐 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 =
1
2
𝜌𝑉′2
Where V’ is the resultant velocity.
Change in dynamic pressure is calculated as a percentage and is
𝐹𝑟𝑎𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝐷𝑃 =
𝐷𝑃𝑖𝑛𝑡𝑒𝑟
𝐷𝑃𝑆𝑡𝑑
100
Where DPinter is the dynamic pressure at a particular point due to interference
DPSTD is the standard dynamic pressure
These values are calculated with the data available through the sphere analysis and are compared
to the results of Richards et al (2014). The experimental data that is being compared is the result
of moving a boat around the key boat and calculating the drive and side forces using the VPP. This
side and drive forces correlation with AWA was used to determine the change in the wind angle
which is plotted as the contour in Fig (5.8).
The change in the AWA from the sphere analysis is compared to the change in AWA from the two
boat analysis data. The contour on the left in Fig (5.8) displays the sphere test results and right
displays the two boat data from wind tunnel experiments. This comparison shows that a single
boat analysis of the change in AWA using the volumetrically averaged data almost accurately
predicts the change in wind angle a yacht would experience in the respective zones.

32. 26
Fig(5.8)- Comparing the change of AWA in Sphere Analysis(left) to Richards et al(2014) wind tunnel testing results (right),
Both contours display a change of 0° to 1.5° upwind of the yacht and a negative change in angle ranging from -3° to -6°
down the wind.
In Richards et al(2014) the percentage of change in dynamic pressures were recorded at one-
thirds and two-thirds of the mast heights and the results are displayed in Fig(5.9). Although the
value of the effects due to change in dynamic pressure experienced by a second yacht would be
the integrals of the change in pressure at all heights. In the case of the sphere analysis, the change
in pressures at all heights along the mast is taken into consideration. These results help in
predicting the pressure change experienced by a yacht at a certain position.
The contours display similarities where there is almost no change in pressure on the windward
side and there is a slight increase in pressure on the leeward side. Changes in pressure downwind
in the sphere analysis are not as severe as the predictions in the experimental data at the two
specific heights.
The change in wind angle causes the change in the forces generated by the sails. When the wind
heads into the sails there is an increase in the drive and side force due to the increase in AWA and
this causes a positive interference. The opposite happens when the wind moves away from the
sails which are the reason for the negative interference. Although the change in wind angle is the
major contributor in terms of the effects of interference, there is a significant contribution from the
fractional change in Dynamic Pressure as well. Only a certain percentage of the forces generated
by the sails due to the change in wind angle is experienced by a yacht that is being interfered with.
For example, if a yacht experiences an AWA of 23⁰ in a 25⁰ AWA course due to interference, the
drive force generated by this craft would approximately be 97% (depending on its position relative
to the interfering boat) the force generated by the same in an isolated condition and with an AWA
of 23⁰. Thus the fractional change in DP is expressed in percentage.

33. 27
Fig(5.9)- Change in Dynamic Pressures from Richards et al(2014) data at 1/3rd
(left) and 2/3rd
(right) mast height
Fig(5.10)- Change in Dynamic Pressures from the sphere analys

34. 28
5.2.Two Boat Analysis
Interference between two boats has been simulated, where the boats were placed at distances 0.5h,
1h, 1.5h and 2h from each other along the x’ and y’ axis on the transverse coordinate system. A
yacht at a particular position is described as (a, b), where ‘an’ is the distance along the x’ axis and
‘b’ is the distance along y’ axis. Examples are shown in Fig (5.11).
Fig(5.11)- Positioning of the second boat around the key boat( which is at (0,0))
The tests were carried out placing the boat at every half mast length through the x’ and y’
directions in the longitudinal grid system. The maximum distances at which the interfere boat
was placed were at 2 mast lengths away from the key boat along the x’ and y’ axis for leeward
upwind and downwind as well as winward upwind and downwind conditions.
The forces in the X and Z directions for the boats in these tests are calculated on CFX post, which
is the lift and drag. These forces are converted to the side and drive forces and are analysed to
determine the effects of interference. These test results are plotted for a hypothetical overtaking
scenario(Fig() for example), where the boats are placed at a certain distance from the transverse
axis (x’) of the rectangular grid and one of the boats are made to move along their respective
longitudinal axis (y’) from -2h to 2h. The change in the side force and drive force are recorded for
every position the boats are at and are plotted against the values as shown in Fig(5.12) – (5.13).
The change in the forces is defined as CT/CT iso and CS/CS iso where CT iso and CS iso are the
coefficients for the respective forces for an isolated boat condition at 25⁰ AWA.
Fig(5.12)- A real life overtaking scenario

35. 29
The orange lines in the graph represent the windward boat, which has a clear advantage when the
leeward boat is stuck in its wake. As the leeward boat advances, there is a drop in the windward’s
forces and an increase in the leeward’s. There is a point at which forces of both boats are the same.
At this point, the boats do not have any advantages over the other referred to as the crossover
position in this project. Here the drive forces and side forces are significantly lower than what
would be in an isolated scenario, The diagrams next to the graphs display the transverse distances
between the boats as well as the positions of equal advantage. Although in this position the forces
are equal, the boats have a substantial drop in the side and drive force when compared to a boat
sailing in isolated conditions.
Fig(5.13)- Fraction of the Coefficients of forces of the interfered boats by the
coefficients in isolated conditions VS distance between the yachts
when the boats are 0.5 times mast height apart
The graph above is the scenario where the boats are at a transverse distance of 0.5 times the mast
length. The horizontal axis displays the distance in the longitudinal axis on the transverse grid. In
the case of overtaking the distance between the boats close in and it can be observed that when the
leeward boat is at a distance of 2 mast height to the aft of the windward, the drive and the side
forces generated by the leeward is substantially very low whereas the windward generates a drive
force almost equal to that of a boat in isolated conditions. As the boats move in closer the forces
on the leeward increases and those generated by the windward decreases. The closer the boats are
the more the interference. The separation causes a reduced interference.
The increase in the interference at close quarters is due to the drastic change in wind angles in
these areas. The negative effects are caused by the wind moving away from the sails. The effect
of the positive interference is not as large as that which is predicted by the single boat analysis.
The images below are of scenarios where the boats are at a transverse distance of 1, 1.5 and 2 mast

36. 30
lengths apart from each other on the transverse grid system. The final couple of graphs in this
section represents the
(a)
(b)

37. 31
(c)
(d)
The Fraction of the Coefficients of forces of the interfered boats by the coefficients in isolated conditions VS distance between
the yachts at distances (a) 1h apart, (b) 1.5h apart, (c) 2h apart and (d) when the two boats are in line in direction of heading

38. 32
5.3.Comparing Results from Different Runs
The flow around the single boat was modelled to understand the reason for the effects caused by
the interference between two boats. A comparison study is carried out to realize if a two boat
analysis can be predicted by inspecting and understanding the flow around a single boat.
To carry out this study the CT/CT iso and the CS/CS iso from both the two boat tests are compared to
the results from the single boat test. To compare this the change in AWA and change in DP from
the sphere are concerned with the CT and CS from the ‘single boat test at various AWA’ results to
create a CT/CT iso and the CS/CS iso that is compared with that of the two boat testing.
An isolated yacht causes the wind to change angles and velocities. These are defined by the Change
in AWA and Dynamic Pressures which causes the interferences when yachts are sailing at a close
range. The change in AWA will cause the wind to either flow on to the sails or away form the
sails. So a yacht at a particular position with reference to another yacht will experience a certain
change in the wind angle. For example if the wind angle changes to a negative 3⁰, and moves away
from the sails of a yacht that is set to be sailing at 25⁰, it would be experiencing an AWA of 23⁰,
and hence the drive force the wind generates could be assumed as that of a yacht sailing at new
AWA of 23⁰ .
𝐴𝑊𝐴 𝑁𝐸𝑊 = 𝐴𝑊𝐴𝑖𝑛𝑖𝑡𝑖𝑎𝑙 − 𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝐴𝑊𝐴
The change in AWA readings is converted to AWANEW. Using the data from the ‘single boat test
at various AWA’ the values of CT and CS are interpolated for each of these AWANEW values.These
values are further resolved as CT’/CT iso and CS’/CS iso where CT iso and CS iso are the coefficients for
the respective forces for an isolated boat condition at 25⁰ AWA.
Fig(5.14)-CT’/CT iso (left) and CS’/CS iso (right) for the AWANEW
The above contours display the changes in drive and side forces due to the change in AWA. As
the change in Dynamic pressure plays a part in the interference, the DP/DPiso has to be taken into
account.

39. 33
Fig(5.15)-Change in Dynamic pressure from Sphere analysis
The change in dynamic pressure data from the sphere analysis is multiplied with the CT’/CT iso to
give the predicted 2 boat effects analysed using the single boat analysis. These results are then
compared with the results from the two boat testing as shown in Fig(5.16).
Fig(5.16)-C’T/CT iso from sphere analysis (left) and CT/CT iso (right) from the 2 boat testing
The areas in red are the areas of maximum positive interference and the areas marked areas marked
in dark blue are the areas of maximum negative interference. An area marked as .55 means that a
boat in that zone would generate a drive force that is equal to 55% of the drive force in isolated
conditions. The contours below are maps of the comparisons of the Side force by isolated side
force comparisons between the sphere analysis results and the two boat testing results.

40. 34
Fig(5.17)-C’S/CS iso from sphere analysis (left) and CS/CS iso (right) from the 2 boat testing
The predictions from the single boat testing are very optimistic in comparison to two boat testing.
The sphere analysis predicts a 20% increase in the drive force to the fore end of the key boat (the
boat that is displayed in the contours above) whereas, in the 2 boat testing, only an increment up
to 5% is observed at the same location. The results in the case of the side forces are similar as well
where there is a positive interference of up to 10% in the fore end of the key boat in the sphere
analysis whereas the two boat analysis predicts a difference of 2% of maximum side force.
The values of the readings from the sphere analysis where the second boat would be at close
proximities for both upwind and downwind cases are predicted well above what is experienced in
case of the two boat testing. This is because when the boats are close to each other they almost
behave as a single entity, where the interaction between the boats cannot be used to predict by
using just the change in AWA and DP. The interaction between the sails of the two boats get a bit
complicated at such close proximities and hence the readings from the sphere analysis shows an
accuracy of 91% to 94% when compared to that of the two boat analysis.
The difference in the prediction can be explained in the graph below. The x-axis on the graph the
two boat drive force by Drive force in isolated condition factor readings and the y-axis is the single
boat or sphere analysis readings of the same factor. This data for the sphere analysis was
determined for specific locations by probing the data on the software Tecplot. The locations on
which the data was determined was that same as that of the positions of the second boat around
the key boat in the two boat analysis. This was done in order to compare the data and determine
the differences in the predictions. The line going through the graph is to describe the points at
which the values of the predictions are equal. It can be observed that the sphere analysis shows
higher values of coefficients ranging from 0.4 to 0.7 are much higher than that of the two boat
analysis.

41. 35
Fig(5.18)-C’S/CS iso from sphere analysis (y-axis) vs CS/CS iso (x-axis) from the 2 boat testing

42. 36
6. Conclusion
A successful study of the aerodynamic interference has been carried out in this project through
86 CFD runs. These results have helped in the better understanding the behaviour of the wind
and the causes of the interference effects.
The data of the change in the AWA and the fractional change in Dynamic Pressure around a single
yacht can be used to predict the causes and the effects of interference between two boats. The
change in the drive force and side force experienced by a second boat due to interference is the
product forces generated due to the respective change in wind angles and the fractional change in
dynamic pressure. Data collected from the two boat analysis also predicts the change in the side
and drive forces in an overtaking scenario along with the location of the crossover point (the point
where the forces on both boats are equal).
All the data mentioned above along with the contour maps from this project would prove to be
extremely beneficial in the hands of a race technician who would be able to create his or her
tacking, jibing and course changing strategies and sequences to minimize or avoid the negative
interference. This information could also be used to put one’s competition in an intense negative
interference zone and thereby creating an advantage and thus improving the performance.
Modelling the interference on a CFD software gives the study an advantage over the experimental
methods in terms of extracting data and visualising the flow patterns. The experimental data on
this subject are collected from probes and the load balances which reduce the flexibility in
interpreting information.
The accuracy of the predictions in this project could be increased by coupling the CFD runs with
a Velocity Prediction Program to simulate a change in heel angle with reference to the change in
force generated by the sails due to interference. Also, the interaction between two boats at very
close proximity can be studied in detailed to analyse and identify the reason behind the behaviour
of the boats very close to each other and why they cannot be accurately predicted by the single
boat tests.

43. 37
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