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An Experimental Investigation of Asymmetric Spinnaker Aerodynamics Using Pressure and Sail Shape Measurements

Conference Paper

An Experimental Investigation of Asymmetric Spinnaker Aerodynamics Using Pressure and Sail Shape Measurements

Abstract and Figures

A method for determining the aerodynamic forces and moments produced by sails at full-scale is investigated in this work. It combines simultaneous on-water pressure and sail shape measurements. The system has been given the acronym FEPV (Force Evaluation via Pressures and VSPARS). The experimental pressure and sail shape data were obtained from on-water tests conducted on a Stewart 34 Class yacht equipped with an asymmetric spinnaker. Data were recorded for a range of apparent wind angles in light winds, in order to check the reliability, accuracy and repeatability of the system. The flow around the sails is studied qualitatively by analysing the pressure distributions and sail shape. It was found that the results showed similar trends to the published literature, in spite of the low wind speeds during the tests. The accuracy of the system was investigated by wind tunnel tests, with particular reference to the determination of the entire sail shape from the stripe images and the VSPARS outputs, and was found to be relatively good, even for the foot shape which is outside the camera viewing region.
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Experimental investigation of asymmetric spinnaker aerodynamics
using pressure and sail shape measurements
D. Motta
a
, R.G.J. Flay
a,
n
, P.J. Richards
a
, D.J. Le Pelley
a
, J. Deparday
b
, P. Bot
b
a
Yacht Research Unit, University of Auckland, Auckland, New Zealand
b
Naval Academy Research Institute, Brest, France
article info
Article history:
Received 19 January 2014
Accepted 30 July 2014
Available online 28 August 2014
Keywords:
Asymmetric spinnaker
Sail shape
Pressure distribution
Yacht
Sail force
Gennaker
abstract
An innovative method combining simultaneous on-water pressure and sail shape measurements for
determining aerodynamic forces produced by sails is described and used on Stewart 34 and J80 Class
yachts ying asymmetric spinnakers. Data were recorded in light and medium winds in order to check
the reliability, accuracy and repeatability of the system. Results showed similar trends to the published
literature. The accuracy of the system was investigated by wind tunnel tests, with determination of the
entire sail shape from the stripe images recorded by the camera-based (VSPARS) system, and was found
to be relatively good. Generally the pressure distributions show a leading edge suction peak, occurring at
5 to 10% of the chord length, followed by a pressure recovery and then a suction increase due to the sail
curvature, with nally a reduction in suction near the trailing edge. The drive force coefcient measured
on the Stewart 34 is lower than for the J80 because of a non-optimal sail shape due to light winds. On a
reaching course, the standard deviation of the pressure signals was largest near the luff, reducing in the
stream-wise direction, while it was high on the entire sail section when sailing on a running course.
&2014 Elsevier Ltd. All rights reserved.
1. Introduction
Sail aerodynamics is commonly investigated by using wind
tunnel testing (Le Pelley and Richards, 2011; Viola and Flay, 2009)
and numerical methods (Richards and Lasher, 2008; Viola, 2009;
Lasher and Sonnenmeier, 2008). However, both methods have
various drawbacks (Wright et al., 2010). Full-scale testing is usually
required to validate results from these methods. Moreover, full-
scale testing allows the investigation of yacht performance in real
sailing conditions, quantication of the actual forces at work (Le
Pelley et al., 2012; Lozej et al., 2012; Augier et al., 2012) and, for
example, studies of the effects of the rigging on yacht performance
(Augier et al., 2012; Bergsma et al., 2012a, 2012b). Several full-
scale sail pressure measurements have been carried out in recent
years (Lozej et al., 2012; Viola and Flay, 2010b, 2011; Graves et al.,
2008; Puddu et al., 2006). Difculties in carrying out pressure
measurements include the interference of the taps on the sails, the
effects of long tubing to connect the taps to the transducers, the
recording of an undisturbed static reference pressure, and zeroing
of the pressure transducers (Puddu et al., 2006; Flay and Millar,
2006), but these difculties are being overcome with the devel-
oping experience of active research groups.
Capturing sail shape at full scale is now commonplace on many
racing yachts. Many investigators have developed their own sys-
tems for determining sail shape (Lozej et al., 2012, Augier
et al., 2012; Le Pelley and Modral, 2008). Various full-scale techni-
ques for the assessment of aerodynamic loads have been developed
to date for sailing applications. The use of sail boat dynamometers
(Herman, 1988; Masuyama and Fukasawa, 2009; Hochkirch, 2000)
has been signicant in improving performance prediction. Strain
gauging the rigging and sails (Augier et al., 2012) has provided
useful information on wind/rig/sail interaction. However, the deter-
mination of aerodynamic forces by combining pressure and sail
shape measurements at full-scale enables useful insights into steady
and unsteady sail aerodynamics to be obtained (Le Pelley et al.,
2012; Lozej et al., 2012; Bergsma et al., 2012a, 2012b) by providing
considerable detail on how and where the forces are developed.
This paper reports on research on sail aerodynamics which
is a continuation of previous work at the University of Auckland
and the Naval Academy Research Institute, France, aimed at
developing reliable and accurate methods for carrying out full-
scale experiments on sailing yachts (Le Pelley et al., 2012; Bergsma
et al., 2012a, 2012b). The system developed has been named
FEPV (Force Evaluation via Pressures and VSPARS, where VSPARS
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/oceaneng
Ocean Engineering
http://dx.doi.org/10.1016/j.oceaneng.2014.07.023
0029-8018/&2014 Elsevier Ltd. All rights reserved.
n
Corresponding author.
E-mail addresses: dmot267@aucklanduni.ac.nz (D. Motta),
r.ay@auckland.ac.nz (R.G.J. Flay), pj.richards@auckland.ac.nz (P.J. Richards),
d.lepelley@auckland.ac.nz (D.J.L. Pelley),
julien.deparday@ecole-navale.fr (J. Deparday), patrick.bot@ecole-navale.fr (P. Bot).
Ocean Engineering 90 (2014) 104118
stands for Visual Sail Position and Rig Shape). The recording
method combines pressure and sail shape measurements to
obtain the aerodynamic forces and moments produced by sails
at full scale. Le Pelley et al. (2012) presented the results of the
rst full-scale test carried out using the FEPV system and
a validation of the full system through wind tunnel testing
for upwind sailing. Bergsma et al. (2012a, 2012b) describe an
application of the FEPV system to upwind sailing, where the
effects of shroud tension on upwind sailing performance were
investigated.
The present study extends the previous research from upwind
to downwind sailing and presents the rst published results from
simultaneous pressure and shape measurements of downwind
sails recorded on the water. The results from pressure and sail
shape acquisitions from full scale testing on a Stewart 34 Class
yacht in very light winds in New Zealand, and on a J80 Class yacht
in stronger winds in France are presented. On the day of the
scheduled testing in NZ the wind strength was lower than ideal,
but testing could not be changed to another day due to the
considerable setup and people commitments. An assessment of
the accuracy of the sail shape interpolation procedure was
determined by comparing sail shape predictions from VSPARS
data with physical measurements in the wind tunnel.
In the present study only the steady approach was used, i.e.
time averaged pressures and shapes were investigated. Future
work will be dedicated to the dynamic regime.
2. Components of FEPV system
2.1. VSPARS and sail shape measurement
TheVSPARSsystemwasdevelopedintheYachtResearchUnit
(YRU)attheUniversityofAucklandbyLe Pelley and Modral (2008).It
is designed to capture sail shape both in the wind tunnel and whilst
sailing. It uses cameras mounted at deck level looking upwards at the
sails and rig. The system determines the global locations in Cartesian
coordinates of specic targets on the sails and rig. For the rig, these
targets comprise coloured dots which are placed at different heights
on the mast, typically under the spreaders or at diagonal crosses. On
the sails, coloured horizontal stripes are applied to the mainsail, jib
and downwind sails. The system is able to dynamically track the
Nomenclature
Atotal sail area (m
2
)
A
main
main sail area (m
2
)
A
spi
spinnaker or gennaker area (m
2
)
AWA apparent wind angle (1)
AWS apparent wind speed (m/s)
CFx total driving force coefcient (dimensionless)
CFx
main
driving force coefcient for the main sail only
(dimensionless)
CFx
spi
driving force coefcient for the spinnaker only
(dimensionless)
CMh total heeling moment coefcient (dimensionless)
CMh
main
heeling moment coefcient for the main sail only
(dimensionless)
CMh
spi
heeling moment coefcient for the spinnaker only
(dimensionless)
Cp pressure coefcient (dimensionless)
FEPV force evaluation via pressures and VSPARS
Fx total driving force (N)
Mh total heeling moment (N m)
VSPARS visual sail position and rig shape
TWS true wind speed (m/s)
Vs boat speed (m/s)
Fig. 1. VSPARS system: screenshot of VSPARS operating in the wind tunnel and system ow chart. (For interpretation of the references to color in this gure legend, the
reader is referred to the web version of this article.)
D. Motta et al. / Ocean Engineering 90 (2014) 104118 105
stripes, calculate the stripe coordinates in 3D space and link the stripe
positions to the rig deection.
The main advantage of VSPARS over other systems is that it is
able to deal with large perspective effects. Even systems that look
up or down at the stripes from the centre of the chord can still
have signicant perspective effects at the luff and leech ends of the
stripes. By accounting for these effects, it is possible to place a
camera in the optimum position to see as much of the sail as
possible whilst still producing an accurate sail shape, as is done in
the VSPARS system. This also enables the system to cope with
large changes in sheeting angle. This has been shown to work well
even for the highly curved stripes in off-wind sails. An example
application of the VSPARS system in the wind tunnel is shown in
Fig. 1. The location of the camera on the model, at the end of the
bowsprit in this case, is indicated by the blue arrow.
The main steps of the software can be seen in Fig. 1. The
program essentially takes images using the required camera(s),
automatically nds the sail stripes and rig targets, and then
combines the results of all the data to give the global x,yand z
coordinates of the sail stripes and rig relative to the boat origin of
the coordinate system.
Further details on the VSPARS performance can be found in Le
Pelley and Modral (2008).
2.2. Pressure measurement system
The pressure measurement system was custom-built at the Yacht
Research Unit at the University of Auckland. The generic layout of
the system, as applied to each sail, is shown in Fig. 2a. Ultra-low
range differential pressure sensors (Honeywell XSCL04DC) are the
core of the system. The sensorsresolution and range t the criteria
for sailing applications. The pressure sensors are mounted in custom
plastic housings, approximately 40 mm in diameter and 10 mm
thick. On one side, they are stuck on the sail with a small hole
melted through the sail to a pressure port on the bottom surface
of the housing. On the other side, a light sail cloth patch,
approximately 150 mm 150 mm in area, is applied with another
hole through to the opposite pressure port, as shown in Fig. 2b.
Using this setup, transducers are placed directly at the measuring
locations, thus avoiding the issues associated with the use of long
tubing and the recording of a reliable static reference pressure (Flay
and Millar, 2006).
An operation amplier (op-amp) is connected directly onto
each transducer which amplies its analogue output (in mV) to a
signal in the 72.5 V range. Using ribbon cable and IDC connectors
the transducers are connected to a ribbon cable running along the
chord of the sail. Each chord-wise cable terminates on an
analogue-to-digital (ADC) converter chip which converts the
analogue voltage signal into a 12-bit digital signal. A maximum
of 8 taps can be connected to each ADC. For upwind sails this
seems to be a sufcient number of taps to catch an accurate chord-
wise pressure distribution. For downwind sails it is necessary to
increase the number of taps per row because of their more
highly varying pressure distributions, and thus two separate
systems have been mounted in parallelon the sail in the present
measurements.
One system can handle 10 of these chips, and therefore 10 sets
of 8 transducers. The ADC chips are connected to a continuous
ribbon cable along the luff of the sail which terminates at a USB-
driven microcontroller box placed at the tack. The microcontroller
combines the data from all of the taps on the sail and sends them
in a single sentence back to the data acquisition PC. Sampling of a
single tap can occur at a frequency up to about 3000 Hz, therefore
the system can run over 150 sensors at a sampling rate of 20 Hz,
which is higher than required for sailing applications (a frequency
of 45 Hz would be enough). If necessary, in order to reduce the
effect of any high frequency noise coming from the power supply,
the signal can be averaged over a number of readings from each
transducer, resulting in a lower effective frequency.
Although the use of a very large number of pressure sensors
can lead to a highly accurate interpolated pressure distribution,
the FEPV system is intended to be a cost- and time-effective
Fig. 2. (a) Generic layout of the YRU pressure system as applied to a sail; (b) example of application of pressure sensors on the sail showing the pockets containing
the sensors.
D. Motta et al. / Ocean Engineering 90 (2014) 104118106
system that could be used by yacht racing syndicates to improve
their knowledge of sail design. Therefore a self-imposed limit of 24
sensors for the mainsail and 44 for the gennaker has been used so
far, although based on present experience further experiments
may be done with a higher chord-wise resolution of pressure taps
in order to get an improved description of the pressure distribu-
tions, and to reduce the inuence of any malfunctioning pressure
sensors. Further details on the pressure system can be found in
Morris (2011).
2.3. FEPV data analysis
The FEPV analysis was coded in Matlab, and uses the output les
from VSPARS and the pressure system to obtain the aerodynamic
forces and moments. The whole sail surface is created from the
recorded stripe shapes and the known tack and head positions from
physical measurements a priori. The head is assumed to be at
(with no camber) and to have a small nite length. A spline curve,
joining the leech points of the recorded stripes, is extrapolated
upwards to the known head height position and also downwards
using the known leech length of the sail, to give the head and foot
twists respectively, together with the rst estimate of the clew
position. Unfortunately the foot shape cannot be captured by the
camera as it is out of the viewing area with the present VSPARS
setup. Therefore an initial foot shape is estimated by tting a spline
curve through the known tack and clew positions together with a
3rd point given by an estimated foot depth and draft position,
obtained by extrapolating the depth and draft position of the
known stripes. This foot shape is then scaled in both the long-
itudinal and transverse directions to match the known foot length.
Starting from the low resolutionsail shape dened by the VSPARS
stripes and the foot and head positions, a ne quadrilateral mesh is
then interpolated over the sail surface.
The sail pressure distributions are obtained from the discrete
pressure values recorded by the pressure system which are inter-
polated using a radial basis interpolation of order 1 (linear). This
interpolation scheme is based on the Radial Basis Function, which is
a real-valued function whose value depends only on the distance
from the reference points, called centres (the pressure taps in this
application). Pressure tap positions are dened intrinsically to the
sail shape in terms of chord-wise and span-wise percentages.
Moreover the use of this interpolation scheme allows a scattered
set of pressure measurements to be extrapolated over the sail. The
pressures are interpolated to the centre of each geometrical cell in
order to obtain a pressure map distribution over the entire surface
of both sails, as shown in Fig. 3. The VSPARS stripes and pressure tap
locations are also shown in the gure. Forces in specied directions
are computed by integrating the known pressures acting over the
cell areas taking into account their surface normal directions.
Moment contributions from each cell are calculated about the
specied yacht moment reference centre. In the present case the
moment reference centre was xed at an assumed buoyancy centre.
3. FEPV system validation
In an earlier study (Le Pelley et al., 2012) the FEPV system was
validated for upwind sailing through wind tunnel testing. Results
from the FEPV system were compared in terms of forces and
moments to measurements from the wind tunnel force balance,
and good agreement was found. The tests for the upwind valida-
tion were conducted at an apparent wind angle (AWA) of 251and a
heel angle of 201. Three types of trim change were investigated.
First, the main was swept through 8 trim settings from hard
sheeted to fully eased using a combination of both mainsheet and
traveller, whilst the jib was left in a standard trim position. Second,
the jib was swept from hard sheeted to fully eased using the jib
sheet, whilst the main remained at a standard trim. Finally, both
sails were eased together over 8 settings. The trends shown by the
FEPV calculations compared well with the force balance results.
The driving force and rolling moment from the sails predicted by
the FEPV method were 10% more and 5% less respectively than
measured by the force balance. These differences are thought to be
due to the additional windage from the mast, rigging etc., which is
not measured by the sail pressure integrations, which would cause
the force balance to give a lower driving force and a higher heeling
moment for the upwind AWA of 251which was used for the
validation tests. A more detailed description of the validation
results can be found in (Le Pelley et al., 2012).
A complete validation of the FEPV system in terms of aero-
dynamic forces for downwind sailing needs much more effort and
has not been carried out to date, but is planned in future work. As
arst step, the determination of downwind sail shape has been
validated in the wind tunnel as described below. Pressure mea-
surements on downwind sails have been carried out both in the
wind tunnel and at full scale (e.g. Viola and Flay, 2009; Le Pelley
et al., 2012; Bot et al., 2013) and the measurement system used in
this work is judged to be reliable. The VSPARS system itself had
been previously validated both for upwind and downwind sails
(Le Pelley and Modral, 2008). Indeed, particular attention was
needed to assess the accuracy of the foot shape and the determi-
nation of the clew position, as these positions are obtained from
extrapolations rather than from direct VSPARS measurements.
Wind tunnel tests were carried out on a model scale VO70 yacht,
shown in Fig. 4, to obtain data for this assessment. Two different
gennakers were tested at AWAs varying from 601to 1201in order
to cover the full range of AWAs of interest at full scale.
The clew, foot depth, and draft positions were measured physi-
cally during each test, and the sail stripe positions were also recorded
by VSPARS, and used by the FEPV software to determine the sail
shape. The results of this comparison are shown in Table 1.Forthese
shape validation measurements and computations, the moment
reference centre is located at the base of the mast, with xpositive
forward, ypositive towards port and zpositive upwards. Differences
Fig. 3. Pressure map distribution over the entire surfaces of the two sails.
D. Motta et al. / Ocean Engineering 90 (2014) 104118 107
in clew positions in the x,yand zdirections are given in Table 1.Foot
depth and draft position are expressed as a percentage of the chord
length. Average chord lengths of 1400 mm and 1100 mm for sails
1 and 2, respectively, can be used for reference.
The results show that the FEPV system can predict the clew
position with an accuracy of up to 770 mm (but usually much
less). Fairly good agreement in foot shape is obtained as well, with
errors within 5% of the chord length. As a general pattern, the
present FEPV analysis software overestimates the foot depth and
underestimates the draft position. It was observed during these
FEPV validation tests that the foot of the sail was constantly
moving, probably due to shedding of the foot vortex, which is a
common characteristic of downwind sailing. Therefore the physi-
cal location of the sail could not be determined to better than a
few tens of mm (3050 mm) during the tests, and so a distance of
about 40 mm is representative of the validation accuracy.
4. Downwind full-scale testing
4.1. Stewart 34 and J80 characteristics
As mentioned in Section 1, in this work two different tests are
presented in order to show the feasibility of the FEPV system as a
measuring technique at full scale. Table 2 lists the main features of
the downwind sails used on the Stewart 34 and J80 yachts respec-
tively, together with a summary of the test conditions for the tests.
Fig. 5a and b show images of the S34 gennaker and J80
spinnaker respectively, as recorded by the VSPARS cameras when
sailing at an AWA of about 901.
4.2. Stewart 34 test setup
A Stewart 34 class yacht was used for the full-scale testing in
NZ. It was decided to equip the yacht with an available gennaker,
which was a 1/3rd-scale AC33 gennaker sized to t a smaller boat,
namely an International Platu25, which is about 7.5 m long. The
gennaker was hoisted from a pole held against the forestay.
Although this setup was not ideal, the gennaker ew in a reason-
able manner, as can be seen in Fig. 5a.
Both the mainsail and gennaker were equipped with VSPARS
stripes and differential pressure transducers. A GPS unit, sampling
at a rate of 2.5 Hz, was used to record the speed over ground and
boat location, while the boat instruments logged boat speed
through the water at 1 Hz, and wind speed and direction at
0.2 Hz. An Inertial Measurement Unit (IMU) was placed in the
yacht cabin and logged the boat motion at 10 Hz. The VSPARS
stripe recording system uses a sampling frequency of about 0.3 Hz
which enabled several images to be averaged to obtain the shapes
of the stripes for the FEPV calculations. The sampling frequency for
the pressures was 60 Hz, but these were averaged over 30
measurements to lter out higher frequency uctuations and
resulted in an effective sampling rate of 2 Hz. A custom-made
data acquisition unit recorded all these data, each one at its own
sampling rate, and so the data were all time stamped to enable
subsequent synchronous processing of the data streams.
For the Stewart 34 tests, the mainsail was equipped with three
rows of 8 sensors placed at 1/4, 1/2 and 3/4 sail heights, while the
gennaker was equipped with three stripes of 12 sensors at 1/4, 1/2
and 3/4 sail heights plus a 4th stripe of 8 sensors placed at 7/8 of
the height. The additional stripe at 7/8 height was used because
previous studies in the wind tunnel have shown that the chord-
wise pressure distribution on a gennaker can change dramatically
between 3/4 and 7/8 heights. Therefore it was felt that a simple
interpolation up to the head using the 3/4 stripe data would not be
sufciently accurate. The pressure sensors on the mainsail were
covered with sail-cloth patches, while the sensors on the gennaker
were placed into pockets created by the overlap of adjacent sail
panels.
Table 1
Comparison of clew coordinates and foot shape between FEPV and physical
measurements.
Sail 1 Sail 2
60AWA 80AWA 100AWA 120AWA
Coordinate Difference [mm] between FEPV and Exp measure
x_Clew 27 41 10 3
y_Clew 4 63 27 34
z_Clew 50 16 36 46
Sail 1
[%Chord] 60 AWA 80 AWA
Experimental FEPV Experimental FEPV
Foot depth 16.7 18.1 25 23
Foot draft 34.1 39.4 39 41.2
Sail 2
[%Chord] 100 AWA 120 AWA
Experimental FEPV Experimental FEPV
Foot depth 30.9 27.8 36.6 30
Foot draft 44.9 50.5 44.7 50.7
Fig. 4. VO70 model scale yacht used for the FEPV wind tunnel validation where the
VSPARS calculated sail positions were compared with physical measurements.
D. Motta et al. / Ocean Engineering 90 (2014) 104118108
Table 2
Stewart 34 and J80 downwind sails characteristics.
Stewart 34 J80
Spinnaker area 32 63 m
2
Luff length 8550 12000 mm
Leech length 7960 9550 mm
Mast height 12600 11450 mm
Sail features Flat luff Rounded luff
Flatter spinnaker Fuller spinnaker
Spinnaker size small compared to yacht size
VSPARS stripe 3/4 length 3453 5860 mm
VSPARS stripe 1/2 length 5208 7720 mm
VSPARS stripe 1/4 length 5327 7700 mm
Average TWS 3.6 6.7 m/s
Average boat speed 1.3 3.6 m/s
Max boat speed 1.9 m/s at 4.7 m/s at
721AWA, 4.4 m/s AWS 911AWA, 7.3 m/s AWS
Fig. 5. Images from VSPARS camera at about 901AWA (a) S34 gennaker, and (b) J80 spinnaker.
Fig. 6. S34 gennaker pressure distributions for AWAs of 721,891,1051and 1131.
D. Motta et al. / Ocean Engineering 90 (2014) 104118 109
The measurements were performed in the Hauraki Gulf, Auck-
land, NZ, in a fairly constant but very light breeze of between 6 and
8 knots, in an area with insignicant tidal ow with almost at
water. In this light breeze the sails were just able to y. Such low
wind speeds, which varied from 0 to 30 Pa for the gennaker and
from 0 to 15 Pa for the mainsail, made it difcult to accurately
measure the pressures across the sails due to the sensitivity of the
pressure transducers. More wind would have been preferred, but
the tests were planned for a certain day and could not be
rescheduled, and the wind was light on the day. Nevertheless,
the FEPV system proved to be effective and provided repeatable
results, as discussed in Section 4.3
The aim of the tests was to check the reliability and accuracy of
the FEPV system, the repeatability of the test results, and to
qualitatively study the ow around the sails by analysing the
pressure distributions and the sail shape. The yacht was sailed at
its optimum trim on starboard tack for AWAs varying from 651to
115 1. A total of 24 runs were carried out, each about 60 s long. Sail
trim (optimal sail trim with gennaker on the verge of lufng) was
kept constant for each run and the boat heading was kept as
straight as possible to enable the results to be averaged over the
run time (4560 s). Measurements from the instruments on board
(including the pressures and sail shapes) were averaged over the
run-time, and the FEPV code used the average values for the
computations. The VSPARS software allows an average sail shape
to be obtained from a given set of images by simply averaging the
positions of each recorded point on the sail stripes.
In this study only the steady approach is considered, as
explained above. Future work will be dedicated to the investiga-
tion of the dynamics involved in sailing (oscillations, waves
propagating on the sail, correlation between time-dependent
shape and pressures, etc.)
4.3. Stewart 34 results
Viola and Flay (2009) carried out wind tunnel tests on asym-
metric spinnakers. Their results show that on the leeward side of
the spinnaker the pressure, in separated ow, has a negative peak at
the leading edge, followed by a slow pressure recovery up to the
trailing edge. In attached ow the suction peak at the leading edge
is followed by a quick pressure recovery at around 10% of the curve
length followed by a second suction peak due to the section
curvature. Downstream of the second suction peak, which occurs
between 10% and 40% of the curve length, the pressure becomes
less negative, and then constant due to the trailing edge separation.
Figs. 6 and 7 show typical full-scale pressure coefcient
distributions for the gennaker and mainsail respectively at differ-
ent AWAs plotted against the sail curve length percentage. In all
the gures showing pressure and force coefcients, the dynamic
pressure was calculated from the apparent wind speed (AWS), and
the pressure differences are leeward minus windward, thus giving
negative values. The pressure coefcient plots have the negative
direction upwards, as is common in presenting pressure
Fig. 7. S34 mainsail pressure distributions for AWAs of 721,891,1051and 1131.
Fig. 8. Stewart 34 gennaker standard deviation pressure coefcients for a run
performed at AWA¼891.
Fig. 9. VSPARS recorded sail shape for the S34 gennaker stripe at 1/2 height for a
range of AWAs.
D. Motta et al. / Ocean Engineering 90 (2014) 104118110
distributions on wings. The suctions are generally higher over the
entire surface for lower AWAs. This trend is conrmed in terms of
driving force determined by integration, which is higher for the
lower AWAs. The ow around the top stripe of the gennaker is
stalled for all AWAs, as can be seen from the lack of a pressure
recovery after the leading edge peak, which occurs at around 5% of
the curve length. The rows at 3/4 and 1/2 of the height show
similar behaviour; the leading edge suction peak, occurring at 5 to
10% of the chord length is followed by a pressure recovery
(perhaps due to an intermittent leading edge separation-bubble
reattachment), a suction increase due to the sail curvature, and
then a reduction in suction as the trailing edge is approached.
However the sail is not able to generate much suction, probably
due to the very light winds, and therefore the suction due to
curvature is very small. This can be conrmed by the small values
Fig. 11. Total (gennaker plus mainsail) drive force and heeling moment coefcients
vs. AWA for the S34.
Fig. 12. Total (gennaker plus mainsail) Stewart 34 drive force coefcients vs. true
wind speed.
Fig. 13. Drive force coefcients and heeling moment coefcients for S34 gennaker
and mainsail separately vs. AWA.
Fig. 14. Heel angle vs. heeling moment for the S34.
Fig. 10. Cp at row 1/2 of sail height for several runs performed at similar AWAs for
the S34.
D. Motta et al. / Ocean Engineering 90 (2014) 104118 111
of the pressure differences, which range between 10 and 30 Pa.
The bottom row at 1/4 height has similar chord-wise distributions,
with even smaller suctions generated by the sail curvature, and
only for the lowest AWAs. There is something unexplained
happening at 25% of the chord for AWA¼891, where the suctions
are lowest, perhaps due to a problem with the pressure tap, or a
crease in the sail. Increased AWAs over 1001drastically atten the
pressure distributions in the proximity of the leading edge.
It is worth commenting on the consistency of the pressure
distributions obtained in such light airs. When testing at full-scale,
zeroing of the pressure sensors is not an easy task because the
wind cannot be turned off and because of the sensitivity of the
transducers to their orientation. In practice, zeroing was carried
out with the gennaker inside a bag to obtain a uniform pressure
and was repeated after turning the bag over so that it was upside
down, and thus all transducers were rotated through 1801about a
horizontal axis. This was done because the sensorszeroes are
sensitive to their orientation.
The pressure differences on the mainsail shown in Fig. 7 are
even lower than on the gennaker, having maximum values of only
15 Pa. For the distributions at an AWA of 721an error bar has been
added which shows plus or minus one standard deviation of the
pressure coefcient for the runs analysed. The standard deviation
is usually higher for the pressure taps in close proximity of the
leading edge, and lower when approaching the trailing edge.
Moreover, higher values of the standard deviation are achieved
in the higher pressure rows, particularly on the 7/8 stripe, for
which the ow is stalled, as shown in Fig. 8 for an AWA of 891. The
standard deviation is a useful tool to help understand if the ow is
attached or not. A high standard deviation is indicative of
separated ow because of its very unsteady behaviour as eddies
form, grow and then are shed from the sail causing large changes
in local surface pressures.
The ow on mainsails is affected by the presence of the mast
(Viola and Flay, 2010a) which usually produces a separation
bubble behind it with a low recirculation ow velocity and a low
pressure core on the leeward side of the front part of the mainsail.
This helps to explain the suction peak at 7 to 15% of the chord
exhibited in Fig. 7, followed by pressure recovery where the ow
reattaches. Fig. 7 shows two further suction peaks at all heights
and for all AWAs. The reasons for these are not clear, but might be
due to the sail curvature not being very fair due to the lack of
pressure difference across it, thus resulting in a wavy sail surface.
Another atypical behaviour is the presence of positive values of
differential pressures before and after the leading edge suction
peak. Again, this might be due to some reverse ow in the
separated area. This behaviour is not likely to be caused by
incorrect zeroing of the pressure transducers, as they were zeroed
several times on shore (before and after the tests) and at sea
during the measurements. Taking into account the drift of the
sensors with time and temperature, their sensitivity to their
orientation, and the noise during the measurements, the esti-
mated accuracy of the pressure measurements for the Stewart 34
tests is about 72.5 Pa, and thus 70.3 in terms of pressure
coefcients for the actual wind conditions. Hence this atypical
behaviour may be the result of experimental error.
It is worth noting that the pressure coefcient distributions
show very similar shapes for a wide range of AWAs, in spite of the
large values of the standard deviation especially in the close
proximity of the leading edge. This can be explained by analysing
the shape of the sail sections as outputted by VSPARS and shown
in Fig. 9. Note that the bow of the yacht is orientated in the þx
direction. Only the section at 1/2 of the height is presented as
indicative. As the AWA is increased, the luff moves more to
windward, towards and across the centreline of the boat and the
leach moves aft and outboard, thus opening the sail up. However,
the section shape does not change signicantly in spite of the large
401change in AWA shown. This might be due to the light winds
encountered during the tests, and therefore the small loads acting
on the gennaker. This behaviour might also be due to the
sub-optimal hoisting method used for the gennaker with the tack
attached to a pole on the forestay, but this could not be established
with certainty.
The tests showed very good repeatability in terms of pressure,
as discussed herewith, and in terms of forces, as discussed later in
this section. Fig. 10 shows pressure coefcients for several runs
performed at similar AWAs at about 851. Only the pressures at row
1/2 are shown, but similar behaviour was found at all heights.
Fig. 15. (a) Total drive force vs. AWA, (b) boat speed vs. AWA for the S34.
Fig. 16. Stewart 34 test: extract of a typical signal time series.
D. Motta et al. / Ocean Engineering 90 (2014) 104118112
To analyse aerodynamic forces, total and single sail drive force
coefcients are computed as follows.
CFx
main
¼Fx
main
=ð0:5ρA
main
AWS
2
Þ
CFx
spi
¼Fx
spi
=ð0:5ρA
spi
AWS
2
Þ
CFx ¼ðCFx
spi
A
spi
þCFx
main
A
main
Þ=ðA
main
þA
spi
Þð1Þ
Heeling moment coefcients are dened similarly, except that
the sail area is at the power 3/2 in the normalisation.
The variations of the total drive force (CFx) and heeling
moment coefcients (CMh) with AWA are shown in Fig. 11. The
values obtained are quite small for CFx and high for CMh, in the
authorsexperience, for the whole range of AWAs investigated.
This is thought to be due to the light winds experienced during
this test, as supported by Fig. 12 where CFx is shown to increase
with increasing T WS in the investigated range (3 to 5 m/s), for
similar apparent wind angles (see also Section 4.6). Hence it
appears that the sails become more efcient as the TWS increases.
For all AWAs the mainsail contributes only a very small amount
to the overall drive force compared to the gennaker. Indeed, CFx
spi
varies between 0.45 and 0.85 for the gennaker and CFx
main
is only
up to 0.11 for the mainsail, as shown in Fig. 13. This is as-expected,
but note that the presence of the mainsail increases the loading on
the gennaker due to the upwash it generates upwind of the sail.
Similarly, the gennaker contributes mostly to the heeling moment,
as shown in Fig. 13. CFx
spi,main
for each sail is normalised by each
single sail area (gennaker or mainsail respectively), while CMh
spi,main
is normalised by the area of each single sail to the power 3/2.
The heeling moment coefcients from the more highly loaded
gennaker generally decrease with increase in the AWA, as shown
in Fig. 13. The scatter in the mainsail and gennaker results might
be due to the different behaviour of the boat at lower and higher
wind speeds. The values of heel angle are generally low (Fig. 14)
and increase in an approximately linear manner with increase in
the heeling moment (and thus decrease with increase in AWA).
Fig. 15a and b show the overall drive force (Fx) and boat speed
(Vs) plotted against the AWA. In this case a clear trend of
increasing Fx for decreasing AWA can be identied, as well as
the expected increase in Fx for the runs performed in slightly
stronger winds (square symbols in Fig. 15). The boat speed is
generally higher for low AWAs (giving a higher AWS), and this is
associated with a small increment in heel angle. This is as-
expected since the lower AWAs give the higher thrust.
In this section discussing the results from the full-scale mea-
surements on the Stewart 34 class yacht, the measurements were
averaged over each run, under the assumption that the conditions
and yacht/sail responses were steady. To clarify this assumption,
an extract from the raw measurements is shown in Fig. 16. Time
histories of AWA, boat speed, heel angle and pitch angle are
shown. Each sample is shown at its own sampling rate. For the
present tests, boat speed and AWA were logged with the boat
instruments, and therefore they were measured at a very low
sampling rate compared to the other measurements. Regarding
the steadiness, the authors were particularly focused on the AWAs.
As a rule of thumb, a variation not greater than 561in the AWA
was considered to be small enough to assume a steady run.
4.4. J80 test setup
As stated above, an Ecole Navale J80 yacht was used to under-
take pressure measurements on sails in stronger winds, in the
Brest Gulf, France. This yacht ies an asymmetric spinnaker
(Fig. 5b) of area 63 m
2
. Both the mainsail and spinnaker were
Fig. 17. Photographs of the J80 mainsail (left) and asymmetric spinnaker (right).
D. Motta et al. / Ocean Engineering 90 (2014) 104118 113
equipped with VSPARS, although only the spinnaker was instru-
mented with differential pressure transducers, as shown in Fig. 17.
In a similar manner to what was done for the Stewart 34, the
J80 spinnaker was equipped with four rows of pressure taps at
approximately 1/4, 1/2, 3/4 and 7/8 of the sail height. Sensors were
placed on the sail along lines that joined the 1/4, 1/2, 3/4 and 7/8
height positions on the luff and leach, as the luff was much longer
than the leech on the J80 spinnaker. Hence the lines of the
pressure taps were not parallel to each other. Sensors were placed
on the port side of the spinnaker and covered with sail-cloth
patches to reduce their interference with the ow. Unfortunately
the analogue to digital converter connected to rst 8 taps of the 1/4
height row unplugged during the tests because of the excessive
stretching of the spinnaker cloth due to the sail motions. Therefore,
pressure data for this row are only available for those few runs
performed before this failure occurred.
An Inertial Measurement Unit (Xsens MTi-G) was placed at the
rotation centre of the hull (for small angles of heel) and was used
to record boat motions. An ultrasonic 3D anemometer was xed to
the mast top and a boat speed indicator was installed onto the J80
hull. A uxgate compass and a GPS were deployed inside the boat.
All sensors on board were linked to an inboard computer.
Acquisition was controlled by RTmaps, a dedicated piece of soft-
ware for synchronisation and date stamping developed by Intem-
pora. RTmaps is well suited for real-time data acquisition, as
sensors were free to communicate with the computer at their
own frequency and each sample was stored in the buffer at its own
sampling rate. Re-sampling was applied before off-line analysis to
obtain synchronous data.
The measurements were performed in the Brest Gulf, France,
with a sea breeze varying between 10 and 15 knots and almost at
water. The yacht was sailed at its optimum trim (it was a dynamic
trim, trying to keep the spinnaker always on the verge of lufng,
and boat heading was kept as straight as possible in order to keep a
constant AWA) on a wide range of AWAs varying from 581to 1431,
both on port and starboard tacks. Starting with the time series, all
measurements were averaged over chosen run-times characterised
by relatively small changes in AWA. This was intended to give
representative mean values of pressures for particular apparent
wind speed and direction characteristics.
4.5. J80 test results
In this section pressure coefcients are plotted against the sail
curve percentage at different AWAs. The pressure coefcients are
calculated as average pressure differences (leeward minus wind-
ward) divided by the dynamic pressure formed from the average
apparent wind speed (AWS) over the run in a similar manner as
used for the Stewart 34 analysis. Fig. 18 is an extract of the results
showing the pressure coefcient distributions for a wide range of
AWAs. It can be seen that the highest suctions are achieved at
AWAs of about 801901, particularly for the higher stripes.
A few interesting observations are evident by comparing
pressure coefcient distributions for runs with similar AWAs, as
in Fig. 19 which shows Cps for runs with the AWA close to 851on
starboard tack. The Cp distributions are very similar among the
three different runs, but two different situations can occur in the
proximity of the leading edge, either a high Cp or a low Cp (visible
at 1/2 and 1/4 of the sail height). It is believed that this difference
in pressure distributions is due to different trims of the spinnaker
during the runs, and suggests the idea that pressures close to the
leading edge are very sensitive to small changes in trim and to
small variations in instantaneous AWA. This is in accordance with
the way that sailors y a spinnaker at its optimum point of sailing,
i.e. on the verge of lufng. This difference in pressure distributions
is usually observed at 1/4 and 1/2 of sail height, and may
sometimes be observed on the higher level stripes. Note also the
error bars corresponding to 71 standard deviations in Fig. 1 for
only one of the runs for clarity. It is evident that the standard
deviation is usually much higher in the close proximity of the
leading edge (up to 20% of the curve) because of the sail being on
the verge of lufng, and sometimes apping, while it is quite small
towards the trailing edge.
Fig. 20 shows the standard deviation of the Cps for several runs
from 781to 1381. While the observation mentioned above still
stands for the tightest AWAs (801901) with a standard deviation
of about 0.20.3 on most of the sail section, but reaching 0.8 near
the luff, the behaviour is different for deep AWAs (11511401). In
the latter case, the standard deviations are high (1.5 to 2.5) on the
whole sail section, corresponding to a variation amplitude of the
same order of magnitude, if not larger, than the Cp mean value.
This is thought to be related to a more unsteady ow because of
massive separation over the high camber high incidence sections.
Moreover, the sail shape is expected to be less stiff and more
unstable for deep AWAs as it is less stretched and more curved.
This would result in higher lateral displacements and then higher
pressure oscillations over the entire sail, not only around the luff.
Fig. 18. Pressure coefcient distributions for a range of AWAs along stripes at 1/2,
3/4 and 7/8 of the sail height. Error bars represent 71 standard deviation.
D. Motta et al. / Ocean Engineering 90 (2014) 104118114
To support the results shown in Fig. 20, samples of several
pressure time histories are shown in Fig. 21, namely time histories
of 100 s duration from the second tap on the 1/2 height stripe at
10% of the curve length for two ranges of AWA: running and
reaching. It can be seen that pressure variations are much larger
for the deep running AWAs, while the pressures vary signicantly
less when the AWAs are less than 1001for the reaching runs. Note
that pressure data presented in Fig. 21 are not raw data, but are
smoothed and re-sampled at 100 Hz to facilitate the plotting.
The raw data exhibited the same features, so the oversampling
did not affect the conclusions that can be drawn from the time
histories.
The comparison between the pressure distributions at similar
AWAs but for runs sailed on different tacks in Fig. 22 gives an idea
of the relatively good repeatability of the tests and of the relatively
small inuence of the pressure sensors on the ow. The pressure
sensors were taped onto one side of the sail and covered with
sail-cloth patches, as discussed in Section 2.2. It was expected that
these patches would have some effect on the ow over the sail and
therefore the resulting pressures, especially when they are on the
leeward side of the sail. Fig. 22 shows the Cp distributions for two
runs sailed at an AWA of about 771on starboard and port tacks
respectively. Note that the pressure sensors are placed on the port
Fig. 19. Pressure coefcients for three runs at AWAs close to 851.
Fig. 20. Standard deviation of pressure coefcients at several different AWAs.
Fig. 21. Pressure time series from second pressure sensor from luff at row 1/2 at
10% of the curve.
D. Motta et al. / Ocean Engineering 90 (2014) 104118 115
side of the spinnaker, and therefore they are on the windward side
of the sail when sailing on port tack, while they are on the leeward
side when sailing on starboard tack. The distributions are very
similar in shape and the observed differences are well lower than
the standard deviations. It is reassuring that these results conrm
that the presence of the pressure sensors does not signicantly
affect the ow over the sail, as also observed by Herman (1988).
The variation of the drive force (CFx
spi
) and heeling moment
coefcients (CMh
spi
) with AWA as obtained through FEPV are
shown in Fig. 23 for both the S34 and J80. As mentioned above,
one of the ADCs unplugged during the tests, so pressures at 1/4 of
the height are available only for a limited number of runs. There-
fore, the results presented herewith refer only to those runs in
which all the pressures were measured. Only pressures on the
Fig. 22. Pressure coefcient comparison between port and starboard tacks at
different heights for an AWA close to 771.
Fig. 23. Drive force and heeling moment coefcients from J80 and S34 tests.
Fig. 24. Comparison between S34 and J80 pressure coefcients from tests at about
851AWA.
D. Motta et al. / Ocean Engineering 90 (2014) 104118116
spinnaker were recorded during the test, thus the forces and
moments are those produced by the spinnaker alone. The values of
CFx
spi
are of the order of 0.81 for AWAs between 801and 1001,
with the greatest CFx
spi
achieved at an AWA of about 901. The
CMh
spi
decreases with AWA as expected, and reaches its maximum
value of about 1.63 at an AWA of 671.
4.6. Comparison between J80 and S34 test results
The aim of this work is to demonstrate the feasibility of on-
water force measurements by measuring pressures and sail shapes
(the FEPV method) rather than to give a deep insight into sail
aerodynamics. However, it is possible to compare the results
obtained on the two downwind tests performed with the J80
and S34 yachts and make some comments.
Fig. 24 shows a comparison of pressure coefcients obtained
from the S34 and J80 yachts. The Cp trends are discussed
separately in Sections 4.3 and 4.5. In this section the results from
an apparent wind angle of 851are compared. Although there are
some differences due to the different sail shape/size and different
wind/sea conditions, the Cp distributions have similar features and
the same order of magnitude. At the higher levels (3/4 and 7/8 of
the sail height) the Cps for the J80 are usually higher. Several Cp
distributions (at similar AWAs) are plotted from the S34 tests to
give an idea of the variability of the measured pressures.
Fig. 23 shows a comparison of drive force and heeling moment
coefcients from both tests. The forces produced by the spinnaker-
only are shown in this comparison (the J80 mainsail was not
equipped with pressure sensors). There is a noticeable difference
in the value of the coefcients. Indeed, values of CFx
spi
are
signicantly higher on the J80 compared to the S34, while CMh
spi
produced by the S34 gennaker are higher compared to the J80,
especially at low AWAs. Overall, the aerodynamic force coefcients
(force magnitude normalised by the dynamic pressure and sail
area) are the same order of magnitude, and the Cp distributions
are similar on both tests. Hence, the source of this difference in
CFx
spi
and CMh
spi
is expected to come from the shape and
orientation of the sails. Fig. 25 shows the shape of the sails at
sections 1/2 as determined from VSPARS for the J80 and S34, at
AWAs of 911and 891, respectively. The sections are plotted in the
horizontal plane, in real coordinates from the mast base with þx
in the direction of the bow. Fig. 25 shows that the J80 shape
opens upmore (leech more outboard) compared to the S34
shape, which is instead much more closed, i.e. with a lower
angle between the section chord and yacht centreline. Similar
behaviour is found at all heights. This may be due to the light
winds in the S34 tests, and is very likely to be the source of the
higher CMh/CFx ratio for the S34, since the pressure difference
across the sail will result in a larger force in the þy(heeling)
direction, than for the J80.
5. Conclusions
An innovative method, which combines simultaneous on-water
pressure and sail shape measurements, for determining the aero-
dynamic forces and moments produced by highly curved down-
wind yacht sails at full scale is investigated in this research. The
results show that the method works well. The sail shape measure-
ment component of the system has been investigated for highly
curved asymmetric spinnakers through wind tunnel testing, and
has been shown to predict accurate sail shapes. The system has
been used for downwind sailing at full scale in low and moderate
wind conditions and with different sails. It was found that more
reliable results were obtained in higher wind speeds because the
sensors were able to record higher pressure differences, and the
sails developed fairer shapes. However, it worked well and
provided repeatable pressure distributions even in the lower wind
speeds used for the Stewart 34 testing. The comparison of runs
performed at similar AWAs on opposite tacks showed that the
presence of the pressure sensors did not signicantly affect the
ow over the sails, as the pressure distributions were very similar
on opposite tacks.
In the present study only the steady approach was considered,
i.e. measurements from the instruments on board (including the
pressures and sail shapes) were averaged over chosen run-times
characterised by relatively small changes in AWA. This was
intended to give representative mean values of pressures for
particular apparent wind speed and direction characteristics.
However, the observed uctuations and the high values of stan-
dard deviation measured for the pressures (especially near the
luff) suggest that an unsteady approach is necessary for a better
understanding of downwind sails, and further investigation of the
dynamic behaviour of the sail shape, pressures and forces is
underway. Nevertheless, the method is shown to be relevant for
evaluating time-averaged forces and some important conclusions
can be drawn.
It was shown that drive force coefcients determined using the
FEPV system give much higher values for the gennaker than for
the mainsail in full-scale, thus conrming similar results measured
in the wind tunnel. In the Stewart 34 tests the drive force
coefcients were shown to increase with TWS over the range of
true wind speeds from 3 to 5 m/s, suggesting higher efciency of
the sails in stronger winds. The pressure distributions showed
similar characteristics to other published results obtained from
wind-tunnel experiments and numerical computations. Except
when the ow over a sail section is clearly stalled, generally the
pressure distributions showed a leading edge suction peak fol-
lowed by a pressure recovery and then a suction increase due to
the sail curvature, with nally a reduction in suction as the trailing
edge is approached.
In the sets of measurements discussed in the paper, the ow
around the 7/8 stripe of the spinnaker is stalled for all AWAs, both
in light and moderate winds. This stalled ow behaviour is
conrmed by the standard deviations of the pressure signals
which were higher for the 7/8 stripe than for attached ows at
lower heights.
Fig. 25. Sail shape at section 1/2 as outputted from VSPARS.
D. Motta et al. / Ocean Engineering 90 (2014) 104118 117
Pressures close to the leading edge were shown to be very
sensitive to small changes in trim and to small variations in
instantaneous AWA, with high suctions when it was near to
lufng, and this observation agrees with the way that sailors trim
a spinnaker to have it on the verge of lufng for optimum
performance. For the tightest AWAs on a reaching course, when
the ow is mostly attached over the sail section, the standard
deviation of the pressure signals is largest near the luff, reducing
in the stream-wise direction. For the deepest AWAs on a running
course, the standard deviation of the pressures is high over the
whole sail section, probably due to a more detached ow and an
unstable sail shape. In a dynamic investigation, it would be
interesting to analyse the pressures and shape oscillations and
their phase to look for possible pumping or propagating pressure
waves in the sail structure.
The next step in this project is to use the FEPV system to
investigate unsteady sail aerodynamics at full scale for both upwind
and downwind sailing.
Acknowledgements
This project has received funding from the European Unions
Seventh Programme for research, technological development and
demonstration under grant agreement no PIRSES-GA-2012-
318924, and from the Royal Society of New Zealand for the UK-
France-NZ collaboration project SAILING FLUIDS.
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D. Motta et al. / Ocean Engineering 90 (2014) 104118118
... On-water experiments can focus on different aspects of sail aerodynamics and yacht design in general. Determination of the pressure distributions on the sails789, measuring loads on the rigging [10], investigation the influence of rigging on yacht performance [11], determining the total aerodynamic forces and loads by means of sailing dynamometers1213, are all possible applications of full-scale testing. ...
... Bergsma et al. [11] describe an application of the FEPV system to upwind sailing, where the effects of shroud tension on upwind sailing performance were investigated. Motta et al. [8] extended the application of FEPV from upwind to downwind sailing, presenting the results from downwind full-scale tests on a Stewart 34 Class yacht in very light winds in New Zealand, and on a J80 Class yacht in stronger winds in France. In all the mentioned works, sail aerodynamics were studied by analysing time-averaged values of pressures, sail shape and forces. ...
... Tap locations as a percentage of the curve length from the luff are shown inTable 1. Based on previous experience [8], the authors decided to use the maximum number of pressure taps available, namely 72. Some taps were placed as close as possible to the sail foot and close to the sail head, since these have never been measured at full-scale to best of the authors' knowledge. ...
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This paper presents new results obtained from analysing on-the-water pressure, shape, force, speed and direction data that were obtained from experiments in the Hauraki Gulf in Auckland, New Zealand in April 2014. A fully instrumented Stewart 34 Class yacht sailing downwind was used for the tests. Details of the analysis of the results from simultaneous time-resolved measurements of pressure, sail shape and loads are presented. The dynamic behaviour of the fluid-structure system made up of a light sail cloth and highly curved flow is investigated. Aerodynamic forces on the asymmetric spinnaker are determined from the combination of point pressure measurements across the sail with simultaneous shape measurements. Simultaneous time histories show a strong correlation between the variations of pressure distributions, flapping sail shape and the forces at the corners. Periodic curling and filling of the spinnaker luff influences suctions, in particular at the leading edge, and forces, which can dynamically change on the order of 40- 50%. The results are similar to, and extend, those that were presented by the authors at the 2013 Innov’sail Conference in Lorient, France. It is expected that the results from this work will give reliable benchmark data which may be used to validate unsteady fluidstructure interaction numerical simulations of downwind sails.
... Le measured the forces and the directions on the three corners of spinnakers. Viola and Flay (2012), Le Pelley et al. (2012), Lozej et al. (2012), and Motta et al. (2014) measured pressures on sails for upwind and downwind sails. ...
... The pressure sensors have a sampling frequency of approximatively 10 Hz with a maximum range of ±1 kPa and a resolution of 0.5 Pa. The pressure acquisition system is further described in Motta et al. 2014. ...
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While sailing offwind, the trimmer typically adjusts the downwind sail “on the verge of luff- ing”, occasionally letting the luff of the sail flap. Due to the unsteadiness of the spinnaker itself, main- taining the luff on the verge of luffing requires continual adjustments. The propulsive force generated by the offwind sail depends on this trimming and is highly fluctuating. During a flapping sequence, the aerodynamic load can fluctuate by 50% of the average load. On a J/80 class yacht, we simultane- ously measured time-resolved pressures on the spinnaker, aerodynamic loads, boat data and wind data. Significant spatio-temporal patterns were detected in the pressure distribution. In this paper we present averages and main fluctuations of pressure distributions and of load coefficients for dif- ferent apparent wind angles as well as a refined analysis of pressure fluctuations, using the Proper Orthogonal Decomposition (POD) method. POD shows that pressure fluctuations due to luffing of the spinnaker can be well represented by only one proper mode related to a unique spatial pressure pattern and a dynamic behavior evolving with the Apparent Wind Angles. The time evolution of this proper mode is highly correlated with load fluctuations. Moreover, POD can be employed to filter the measured pressures more efficiently than basic filters. The reconstruction using the first few modes makes it possible to restrict the flapping analysis to the most energetic part of the signal and remove insignificant variations and noises. This might be helpful for comparison with other measurements and numerical simulations.
... Additional sensors provided data about wind angles and speed at various height from sea level, heel and pitch angles, and boat speed. A more recent work was presented by (Motta et al., 2013), where aerodynamic forces and sail shape measurements using High Resolution Cameras (HRC) were simultaneously collected on a mainsail and a spinnaker concurrently used. It is also worth mentioning a new 10 m dynamometer-boat recently built by the Politecnico di Milano, endowed of sensors on mast, rigging and sails, and of cameras for sail shapes measurements. ...
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... In the last few years there has been a revival of pressure measurements on yacht sails and recently several contributions can be found in literature aiming to assess sail pressure distribution detection ( [2][3][4][5][6][7][8][9]). ...
... Several previous research programs have developed instrumented yachts to obtain full-scale experimental data via sailing dynamometers like Fujin (Masuyama, 2014), MIT Sailing Dynamometer (Milgram et al., 1993), DYNA (Hochkirch and Brandt, 1999), and LECCO (Fossati et al., 2015a). Other full scale specific instrumented yachts have been developed to measure simultaneously the loads in all the tension points of the rig, the flying shape, the wind data and attitude of the boat (Augier, 2012) or the pressure on sails (Viola and Flay, 2010;Motta et al., 2014). Wind tunnel studies (Flay, 1996a,b;Lasher et al., 2005) have proven to a be a great tool to study the aero-elastic problem of sails in wind. ...
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... In the last few years there has been a revival of pressure measurements on yacht sails and recently several contributions can be found in literature aiming to assess sail pressure distribution detection ( [2][3][4][5][6][7][8][9]). ...
Conference Paper
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The paper presents an overview of a joint project developed among Politecnico di Milano, CSEM and North Sails, aiming at developing a new sail pressure measurement system based on MEMS sensors (an excellent compromise between size, performance, costs and operational conditions) and pressure strips and pads technology. These devices were designed and produced to give differential measurement between the leeward and windward side of the sails. The project has been developed within the Lecco Innovation Hub Sailing Yacht Lab, a 10 m length sailing dynamometer which intend to be the reference contemporary full scale measurement device in the sailing yacht engineering research field, to enhance the insight of sail steady and unsteady aerodynamics [1]. The pressure system is described in details as well as the data acquisition process and system metrological validation is provided; furthermore, some results obtained during a wind tunnel campaign carried out at Politecnico di Milano Wind Tunnel, as a benchmark of the whole measuring system for future full scale application, are reported and discussed in details. Moreover, the system configuration for full scale testing, which is still under development, is also described.
... The pressure sensors have a sampling frequency of approximatively 10 Hz with a maximum range of ±1 kPa and a resolution of 0.5 Pa. The pressure acquisition system is more described in Motta et al. (2014). ...
Conference Paper
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While sailing offwind, the trimmer typically adjusts the downwind sail "on the verge of luffing", letting occasionally the luff of the sail flapping. Due to the unsteadiness of the spinnaker itself, maintaining the luff on the verge of luffing needs continual adjustments. The propulsive force generated by the offwind sail depends on this trimming and is highly fluctuating. During a flapping sequence, the aerodynamic load can fluctuate by 50% of the average load. On a J/80 class yacht, we simultaneously measured time-resolved pressures on the spinnaker, aerodynamic loads, boat and wind data. Significant spatio-temporal patterns are detected in the pressure distribution. In this paper we present averages and main fluctuations of pressure distributions and of load coefficients for different apparent wind angles as well as a refined analysis of pressure fluctuations, using the Proper Orthogonal Decomposition (POD) method. POD shows that pressure fluctuations due to luffing of the spinnaker can be well represented by only one proper mode related to a unique spatial pressure pattern and a dynamic behavior evolving with the Apparent Wind Angles. The time evolution of this proper mode is highly correlated with load fluctuations. Moreover, POD can be employed to filter the measured pressures more efficiently than basic filters. The reconstruction using the first few modes allows to restrict to the most energetic part of the signal and remove insignificant variations and noises. This might be helpful for comparison with other measurements and numerical simulations.
... Two methods to measure sail forces have been developed by the Yacht Research Unit at the University of Auckland. The first, known as Force Evaluation via Pressures and VSPARS (FEPV) [12, 13] uses a combination of the shape of the sail measured by the VSPARS camera system [14] and differential pressure data measured on the sail surface using an array of pressure transducers. The pressures are interpolated and extrapolated over the sail surface, and the shape is used to find the surface normal direction so that the orthogonal force components at each discretised cell on the sail surface can be calculated. ...
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