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Hydrofoil Configurations For Sailing Superyachts: Hydrodynamics, Stability And Performance


Abstract and Figures

Hydrofoil-assisted racing monohulls have undergone significant development phases in the past decade, yet very little scientific data has reached the public domain: an increasingly critical issue as the superyacht industry is now looking at the implementation of foils onto leisure vessels. Consequently, three contemporary configurations, namely a Dynamic Stability System, a Dali-Moustache and a Chistera have been towing tank tested to present the first complete characterisation of the hydrodynamic efficiency, quantification of the added dynamic stability and eventually the resulting impact on sailing performance. Furthermore, the considerations inherent to the design and installation of hydrofoils onto superyachts will be detailed. Building on extensive experimental work, this paper provides a comprehensive assessment of current design options with both technical and practical guidelines and recommendations to improve performance. DOI: 10.3940/rina.smy.2019.05
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Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
J-B R G Souppez, Solent University, UK.
J M M-A Dewavrin, Gurit, UK.
F Gohier, Bénéteau Group / ENSCBP University, France.
G Borba Labi, Principal Power, Portugal.
Hydrofoil-assisted racing monohulls have undergone significant development phases in the past decade, yet very little
scientific data has reached the public domain: an increasingly critical issue as the superyacht industry is now looking at
the implementation of foils onto leisure vessels. Consequently, three contemporary configurations, namely a Dynamic
Stability System, a Dali-Moustache and a Chistera have been towing tank tested to present the first complete
characterisation of the hydrodynamic efficiency, quantification of the added dynamic stability and eventually the resulting
impact on sailing performance. Furthermore, the considerations inherent to the design and installation of hydrofoils onto
superyachts will be detailed. Building on extensive experimental work, this paper provides a comprehensive assessment
of current design options with both technical and practical guidelines and recommendations to improve performance.
Form factor (-).
Planform area (m2).
 Beam overall (m).
 Beam on waterline (m).
 Mean chord (m).
Total resistance coefficient (-).
 Design waterline (m).
 Froude number (-).
Side force (N).
 Length overall (m).
 Length on waterline (m).
Induced drag (N).
Total resistance (N).
Span (m).
Temperature (°C).
Canoe body draft (m).
 Effective draft (m).
Keel draft (m).
Uncertainty (-).
Velocity (m/s)
 Wetted surface area (m²).
Sweep angle (°).
Heel angle (°).
Leeway angle (°).
Density (kg/m3).
AoA Angle of Attack.
CNC Computer Numerically Controlled.
DSS Dynamic Stability System.
DSYHS Delft Systematic Yacht Hull Series.
FP Forward Perpendicular.
IRC International Rating Certificate.
ITTC International Towing Tank Conference.
LCG Longitudinal Centre of Gravity.
NACA National Advisory Committee for
ORC Offshore Racing Congress.
VPP Velocity Prediction Program.
The implementation of hydrofoils on leisure vessel was
first featured in 1898 on powerboats, before being
employed on a sailing catamaran in 1938 under the
leadership of the National Advisory Committee for
Aeronautics. Then, circa 1954/1955, foiling monohulls
emerged, with the Baker Manufacturing Company
building various size dinghies. Eventually, the 1960s saw
their use in offshore racing. Nevertheless, despite their
historical use, the last decade sparked an unprecedented
regain of interest, with hydrofoiling yachts featured in
several forms in the most competitive and prestigious
sailing events, from the America’s Cup to the Vendée
While significant numerical and experimental work has
been conducted by the design and race teams, hardly any
technical data has been made publicly available.
Consequently, this paper aims to remedy this absence of
open source information by providing results for different
foil-assisted monohull configurations, whilst also tackling
performance prediction and design consideration for their
implementation on superyachts.
Firstly, the previous work, aims and objectives, and the
foils will be introduced, followed by a description of the
experimental setup, as well as the design and
manufacturing considerations for the model and three
hydrofoils: a Dynamic Stability System, a Dali-Moustache
and a Chistera. Then, the towing tank results will be
presented in different conditions, representative of upwind
and downwind sailing, eventually discussing the
hydrodynamic efficiency, added dynamic stability
provided, and the overall effect on the performance of the
vessel. The advantages and drawbacks of each option will
be outlined, finally concluding on practical design
considerations and recommendations.
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
For offshore racing monohulls, the literature has primarily
been focussed on the long-established use of straight
asymmetric daggerboards, as summarised by Campbell et
al. (2014). On the other hand, the design of hydrofoils for
flying dinghies, such as the International Moth class, have
been extensively investigated (Beaver & Zselczky, 2009).
Furthermore, new research emerged in the last few years,
targeted at flying catamarans and the optimisation of
flexible foils (Sacher et al., 2017) and issues associated
with ventilation (Binns et al., 2017), all heavily influenced
by the developments in the America’s Cup. The literature,
however, does not tackle foil-assisted monohulls.
The past couple of years also saw the first large scale
production of an offshore racing vessel with hydrofoils,
namely the Figaro Bénéteau 3, and more recently the first
superyacht fitted with a Dynamic Stability System (DSS),
namely the Baltic 142. Moreover, 2018 marked the
addition of foil measurements as part of the International
Rating Certificate (IRC) racing rule, reflecting
contemporary practice in racing craft design. This shows
the strong interest of yacht and superyacht designers for
foiling technology, and the necessity for published data
relative to their efficiency, stability and overall effect on
The experimental investigation into foil-assisted
monohulls aims to quantify the hydrodynamic efficiency
and ascertain the added dynamic righting moment
provided, to ultimately predict the velocity. Three main
contemporary designs will be tackled, namely the DSS,
the Dali-Moustache and the Chistera foils.
2.2 (a) Dynamic Stability System
The DSS is a retractable transverse foil deployed to
leeward, the intention being to increase the righting
moment, but also to reduce the pitching moment, allowing
a more comfortable sailing. Unlike the Chistera and Dali-
Moustache foils, the DSS only provides vertical lift due to
its solely horizontal planform.
2.2 (b) Dali-Moustache
Based on the IMOCA racing yacht design, the Dali-
Moustache is a V-shaped foiling daggerboard, intended to
improve stability, while contributing to both the side force
and vertical lift, the latter reducing the effective
displacement of the vessel. The other advantage of the foil
is the decrease in the pitch angle of the boat, improving
the longitudinal stability and sea-kindliness (i.e. damping
the pitch motion).
2.2 (c) Chistera
Finally, the Chistera foil is based on the Figaro Bénéteau
3 one-design class. In contrast with the Dali-Moustache,
the Chistera has an inward-facing V-shape, that also
provides both vertical lift and horizontal side force,
together with additional righting moment.
The tank testing of the different configurations has been
performed on a purposely designed hull (Dewavrin, 2018),
first towed bare, before the keel and bulb were added;
finally, each foil was evaluated. The main dimensions for
the 1:10 scale model, representative of a 50ft sailing yacht
then use to extrapolate the findings onto superyachts, are
presented in Table 1
Hull Particulars
Length overall - 
1.52 m
Length on waterline - 
1.43 m
Beam overall - 
0.47 m
Beam on waterline - 
0.34 m
Canoe body draft -
0.06 m
Keel draft -
0.36 m
Wetted surface area - 
0.39 m2
Keel Particulars
Span -
0.266 m
Mean chord - 
0.068 m
Planform area -
0.018 m²
Wetted surface area - 
0.037 m²
NACA 64-012
Swept back angle -
Leading edge distance aft of FP
0.636 m
Bulb Particular
Chord - 
0.270 m
Wetted surface area - 
0.023 m²
Horizontal section
NACA 65-017
Vertical section
NACA 65-012
Table 1: Tank testing model dimensions.
General modelling and scaling laws are driven by Froude's
similitude theory. Equality in Froude number between
model and full-scale will ensure that gravity forces are
correctly scaled. However, this implies that the vessel and
appendages will operate at a too small Reynolds number,
thus not replicating the full-scale laminar to turbulent
transition. As a result, transition will artificially be
triggered using sandpaper strips, in accordance with the
International Towing Tank Conference (ITTC) procedures
(ITTC, 2017).
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
The general dimensions and locations of the hydrofoils
were based on a parametric study of the existing vessels
they are featured on. The cross-sectional shape is a critical
design consideration as it directly affects the lift and drag
characteristics. For consistency, and in order to compare
the hydrodynamic results, the same section was employed
for each foil, namely the NACA 63-412. This is
commonly used for small craft, such as the International
Moth (Beaver & Zselczky, 2009) and was chosen due to
its high lift to drag ratio (Abbott & Doenhoff, 1959) and
the relative ease of manufacturing.
Table 2 presents the main dimensions for the three foils
and their leading-edge location, longitudinally aft from the
forward perpendicular (FP) and vertically upwards from
the design waterline (). Note that the spans given are
for the entire foil, not accounting for its actual immersion
at a given heel angle.
Dynamic Stability System
Span - 
0.232 m
Mean chord - 
0.070 m
Planform area - 
0.016 m²
Wetted surface area - 
0.034 m²
Leading edge distance aft of FP
0.742 m
Leading edge height above 
-0.016 m
Span - 
0.368 m
Mean chord - 
0.058 m
Planform area - 
0.021 m²
Wetted surface area - 
0.045 m²
Leading edge distance aft of FP
0.488 m
Leading edge height above 
-0.016 m
Span -
0.364 m
Mean chord -
0.056 m
Planform area -
0.020 m²
Wetted surface area - 
0.043 m²
Leading edge distance aft of FP
0.488 m
Leading edge height above 
0.142 m
Table 2: Model foil dimensions.
The positions of each foil along the hull can be visualised
in Figure 1 (a), with underwater views of the DSS, Dali-
Moustache and Chistera respectively shown in Figures 1
(b), 1 (c) and 1 (d) respectively. Note the forward position
of the Dali-Moustache: unlike the racing yachts, it is
located further forward to fit within the overall beam when
retracted, and importance consideration for leisure vessel,
further discussed in Section 7.3.
Figure 1: (a) 3D view of the appendages on the designed
model. Underwater view of (b) the Dynamic Stability
System, (c) the Dali-Moustache and (d) the Chistera foil.
The hull shape was CNC cut on a 5-axis milling machine
out of 32 kg/m3 polystyrene. The hull was hand laminated
with two layers of E-glass woven roving having a total
combined dry weight of 300 g/m2 and epoxy resin. Then,
it was sanded to a smooth finish, equivalent to that
achieved by 400 grit wet and dry sandpaper, as per the
recommended ITTC procedure (ITTC, 2017). Geometric
tolerances were well within the required allowable +/- 1
mm for the overall length, breadth and depth (ITTC,
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
The keel was constructed out of thin laser-cut plywood,
then laminated and faired. One outer layer of epoxy resin
was applied for coating.
The model keel bulb and hydrofoils were manufactured
out of ABS resin using stereolithography on a ProJet 3600
Max 3D printer. This was required to achieve the
necessary +/- 0.2 mm tolerance on such complex 3D
geometries (ITTC, 2017). Moreover, their location was
accurately ascertained to respect the permitted 0.5 mm
variation in position (ITTC, 2017). To strengthen the foils
and ensure no deformation under dynamic loading, a layer
of high modulus 200 g/m2 twill carbon fibre and epoxy
resin was applied and vacuumed consolidated at 1 atm.
Finally, all components were fitted with a 5 mm wide
sandpaper strip located to replicate the full-size flow
regime, as the model hull and foils would be operating at
a much lower Reynolds number in the towing tank.
Indeed, while the Reynolds effects on hydrofoils are not
well-understood and consequently there is no current full-
size correction for a smaller geometry being tested, the
best practice across fields of fluid dynamics is to ensure
that transition is replicated at model-scale where expected
at full-scale. The use of studs or sandpaper strips to
artificially trigger transition is, therefore, deemed suitable
(Jackson & Hawkins, 1998), and is recommended by the
ITTC (ITTC, 2017).
The locations of the rough strips were established based
on the ITTC recommended Reynolds number as a function
of the model/appendages length and Froude number
(ITTC, 2017).
The experiments were performed following the ITTC
Recommended Procedures and Guidelines for Resistance
Test (ITTC 2014), and were undertaken in the
Hydrodynamic Test Centre at Solent University. The main
characteristics of the towing tank utilized are presented in
Figure 2.
Figure 2: Towing tank characteristics (Souppez, 2018).
For the characterisation of the hydrodynamic efficiency of
each foil, the runs were performed for a defined speed, at
a constrained heel and yaw angle, with the vessel free to
heave and trim. Conversely, to quantify stability, the
model could heel freely, as later described in Section 5.
The drag, side force, heave and trim (or heel for the
stability investigation) were measured with a precision of
five decimal places, and the data sampled at 1000 Hz over
a minimum of 6 seconds, or longer at the lowest speeds
where a greater data acquisition window was available.
The installation of the model on the towing carriage and
the measurement devices are depicted in Figure 3. The
drag, side force and trim are measured by potentiometers
(P), while the heave is quantified thanks to a linear
variable displacement transducer (LVDT).
Figure 3: Model installed on the towing carriage
(Dewavrin & Souppez, 2018).
The test matrix was defined after running a standard
Velocity Prediction Program (VPP), where the
hydrodynamic model was based on the Delft Systematic
Yacht Hull Series (DSYHS) (Keuning & Katgert, 2008).
The intention was to establish a relevant set of testing
parameters representative of upwind sailing on the one
hand (low speed, high heel, high leeway), and downwind
sailing on the other (high speed, low heel, low leeway),
with also higher Froude numbers to be more in line with
the performance of racing yachts (0.35 to 0.70).
Additional tests were undertaken in the first place to
establish the form factor, 1+k, based on the Prohaska
method suggested in the ITTC procedure (ITTC, 2014).
Moreover, a preliminary study investigated the best Angle
of Attack (AoA) for each geometry. In this instance, the
AoA is defined as the angle between the chord line of the
foil at its root and the design waterline.
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
Once acquired, the model scale data was scaled up to full-
size (ITTC, 2011). However, prior to comparing the
results for each configuration, an uncertainty analysis was
performed to ensure the reliability of the data collected.
Based on the ITTC recommended procedures and
guidelines for Type A uncertainty analysis (ITTC, 2014),
the experimental precision could be quantified. The
parameters under consideration are the wetted surface area
(), speed (), water density (), total resistance ()
and associated coefficient (). The uncertainty , of each
parameter , and inherent components , is labelled .
An example of broken-down uncertainty analysis for a
resistance test undertaken at 2.25 m/s (Fn = 0.60) is shown
in Table 3.
Wetted Surface Area  (m2)
Model uncertainty - 
Displacement uncertainty - 
Wetted surface area uncertainty - 
Velocity (m/s)
Calibration uncertainty - 
Data acquisition uncertainty - 
Velocity uncertainty -
Density (kg/m3)
Temperature -
Temperature error -
Density uncertainty -
Total Resistance - (N)
Calibration uncertainty - 
Fitting uncertainty - 
Data acquisition uncertainty - 
Misalignment uncertainty - 
Resistance uncertainty -
Total Resistance Coefficient -
Resistance coefficient uncertainty -
Table 3: Example of uncertainty analysis.
Early tests were conducted to investigate the impact of the
AoA of the foils. By design, they can be given a pre-set
angle; many racing yachts are also typically able to adjust
foils by up to +/- 7°; thus, a smaller study investigating the
performance at a range of AoA was devised (Kitching,
The DSS was set at 0°, 4° and AoA, while the Dali-
Moustache and Chistera were tested with 0°, and 16°
AoA. It is important to mention that the angles defined
here are at the root of the foil, the portion that would be
controlled on the yacht. In the case of the Dali-Moustache
and Chistera, these do not reflect the actual angle adopted
by the hydrofoils, which is smaller due to the curvature
and twist. The aim is to assess the optimum AoA, to then
perform all the tests in their respective ideal condition,
thus comparing the best possible performance for each
The investigation revealed that, when using a DSS, while
a larger AoA resulted in an increase in heave and a
reduction in displacement, this came at a cost in terms of
resistance. Overall, a DSS with no AoA appeared to be the
best solution. This is consistent with the properties of the
NACA 63-412 foil that exhibits the highest lift to drag
ratio at 4° for the tested Reynolds number. Despite the foil
having no initial AoA, the vessel trim, ranging from 1° at
low speeds to 5° at higher speeds, implies the section will
naturally operate close to its most efficient AoA. It could,
however, be deemed appropriate to offer some degree of
control in order to alter the angle at low speed, and reduce
it for the higher downwind speeds, while retaining the
optimum operating angle.
For the Dali-Moustache, an increase in AoA did contribute
to an increment in heave, resulting in a lower resistance.
This was achieved for an AoA of 8° in upwind conditions
(θ=20°, λ=2°+) and 16° downwind (θ=10°, λ=0°), with
however a decrease in side force. Variations in the AoA,
therefore, alter the contribution of the lift that goes
towards the side force or heave. This is particularly
interesting as these foils are fitted on canting-keel yachts.
Upwind, the fully canted keel will provide vertical lift but
less side force; which the Dali-Moustache could easily
make up for.
Finally, the Chistera exhibited a better side force and
heave with an angle of 16°. The impact on resistance was
nevertheless negligible, thus suggesting better sailing
performance will be achieved with a higher AoA.
As a result, it can be stated that for best performance, the
DSS should be operated at the lowest AoA possible, while
the Chistera is more efficient at a higher AoA, ensuring
stall is not reached. As for the Dali-Moustache foil,
variations in AoA allow to either boost the side force and
reduce the heave, sensible for upwind, or raise the vertical
lift at the expense of the side force, a suitable option for
downwind. Consequently, the rest of the study was
conducted with the most efficient AoA for each foil
configuration and sailing condition.
The performance of appendages can be quantified by
plotting the induced drag factor, i.e. the side force squared
versus the total resistance. For the results to be
meaningful, they must be compared to the typically
required upwind side force.
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
In this instance, the ‘upwind sailing’ line corresponds to
the vessel operating in 16 knots of true wind (i.e. the upper
end of Beaufort 4, after which the boat would be expected
to reef), at a true wind angle of 35°. The results in typical
upwind sailing conditions are presented in Figure 4 (data
point at = 6° not shown for the Dali-Moustache).
Figure 4: Induced drag factor for a typical upwind
condition, Fn = 0.35 and θ = 20°; data points at = ,
, 4° and 6°.
Firstly, it is interesting to notice that the Dali-Moustache
is the only one able to provide significant side force with
no leeway. While this is no surprise for the keel alone or
DSS, it could have been expected of the Chistera to be able
to generate more side force thanks to its asymmetric
profile without any leeway. The present results, however,
demonstrate it is not the case.
Regarding the contribution of each foil to the overall side
force upwind ( = 20°, = 4°), the Chistera provides 15%
and the Dali-Moustache 45%. Those values are consistent
from Froude numbers for 0.35 to 0.50.
The best performance in terms of generating side force for
minimum drag is achieved by both the keel alone first, and
then the DSS. However, looking at the side force that
would be required to sail upwind, the keel only is superior
in that portion where the realistic operation of the vessel
would occur. Furthermore, this is assuming the keel only
contributes to the side force, thus neglecting the
asymmetry of the waterplane area, the rudder (if weather-
helm is achieved), and foil (if fitted).
Under the limitations presently considered, the
configuration without any foils appears more
hydrodynamically efficient. Nevertheless, despite creating
more resistance, the Dali-Moustache and the Chistera
would contribute to reducing the leeway angle; this could
permit the vessel to sail a shorter distance on an upwind
The hydrodynamic performance of yacht appendages is
quantified using the effective draft, , derived from the
theory of induced drag on a lifting surface,
 Effective draft (m).
Side force (N).
Density (kg/m3).
Velocity (m/s).
Induced drag (N).
It can be noted that the ratio
 is, in fact, the
reciprocal of the induced drag factor slope. The DSS
having the lowest slope, it naturally yields the highest
effective draft, as presented in Figure 5.
Figure 5: Effective draft at θ = 20°.
Those results should, however, be moderated with the
previously identified fact that, within the normal sailing
operation, the best configuration is achieved without foils.
It would, therefore, be recommended that the best design
option is assessed solely on the induced drag factor and in
relationship with the expected side force to be provided in
upwind conditions, as in this case the use of the effective
draft has been proven to be misleading.
So far, the data analysis has been focused on the total drag
and side force, critical upwind, but not accounting for the
vertical lift generated by the foils. The measured heave, in
both upwind and downwind conditions, is presented in
Figures 6 (a) and 6 (b) respectively, where 0 heave
corresponds to the static heave of the vessel.
050 100 150 200 250
No Foils
Upwind Sailing
0.35 0.40 0.45 0.50
No Foils
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
Figure 6: Heave for (a) upwind (θ=20°, =4°) and (b)
downwind condition (θ=10°, =0°).
The DSS, that primarily generates lift upwards, appears to
be the best at reducing the effective displacement of the
vessel. Moreover, due to its presence closer to the
longitudinal centre of gravity (LCG), a greater portion of
the lift contributes to reducing the displacement, although
negligible for typical cruising Froude numbers.
Conversely, the Dali-Moustache and Chistera produce a
higher trim, since they are located further forward and thus
the lift induces a higher pitch moment. On the other hand,
the Dali-Moustache, which proved to generate the most
side force (albeit with a drag penalty) did not appear to
significantly lift the vessel out of the water and was
recorded to have greater negative heave than the boat
without foils in this experiment.
The towing tank testing of the three main options for foil-
assisted monohulls has been conducted for a range of
upwind and downwind conditions. The purely
hydrodynamic analysis provided experimental evidence of
the effectiveness of hydrofoils and yielded a number of
important results.
Firstly, the induced drag factor appears a more sensible
method to assess the ideal configuration compared to the
effective draft, as the former enables to identify the typical
operating range of the yacht in terms of side force,
whereas the effective draft could suggest an erroneous
Then, to generate a given side force, the boat without foils
will create a lesser resistance than any of the three
geometries tested.
The Dali-Moustache foil is the only arrangement that
creates significant side force without leeway. This is
surprisingly not the case for the Chistera and was expected
for the DSS. Moreover, below a Froude number of 0.50,
the vertical lift is not sufficient for the displacement to be
reduced. Past that Froude number, the DSS develops the
most vertical lift (in addition to the one generated by the
vessel reaching semi-displacement mode). Moreover, at
any Froude number, the Dali-Moustache performs worse
than the configuration without foils.
Finally, when investigating the effects of an increased
AoA, the Chistera appears to respond better to a higher
angle. The DSS, however, operates best with no AoA, as
the vessel’s trim allows the section to operate very close
to its ideal lift/drag ratio. Finally, the Dali-Moustache
functions optimally at a moderate AoA upwind (8°) and a
higher AoA downwind (16°). A varying angle of
incidence can, therefore, be beneficial on a Dali-
Moustache foil to boost either the side force or the heave.
Overall, building on the experiments undertaken and
hydrodynamic data gathered, it appears that, for foil-
assisted monohulls, no resistance advantage over a design
without foils could be achieved, thus demonstrating their
inefficiency under the present test conditions and inherent
limitations, namely the pure hydrodynamic efficiency of
foil-assisted monohulls.
Nevertheless, the increasing presence of hydrofoils in
offshore racing yachts and now cruising superyachts
suggest there are indeed strong advantages. These
observations and present experimental results, therefore,
call for further work to tackle the stability and
performance aspects, and identify where the benefits of
foils truly are, so that their design can be better refined,
and the most suitable configuration selected for a vessel’s
operating profile.
The tests undertaken for the purpose of quantifying the
added dynamic stability were performed in conditions
representative of upwind (,) and
downwind (,) sailing. The slightly reduced
leeway in the upwind condition was dictated by the free-
to-heel setup that could not cope with the larger side force
generated for higher leeway angles.
These experiments featured a new aft position for the Dali-
Moustache and Chistera. This would not allow the foil to
fit within the maximum hull width as intended in the
previous experiment, but could provide greater stability
and thus be of interest for racing crafts.
0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70
Heave (mm)
No Foils
0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70
Heave (mm)
No Foils
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
It must also be emphasized that the heel angles quoted
correspond to the dynamic angle adopted by the yacht
without hydrofoils, tested at a given speed. This, therefore,
required trial and error to assess, for each Froude number,
the transverse ballast location and inherent starting static
heel angle, so that the vessel would reach the desired
dynamic angle once towed.
For this particular test campaign, the vessel was not
constrained in its heel angle. The righting moment
provided by each foil was quantified from the change in
heel angle measured. Firstly, an inclining experiment was
conducted on the model fitted onto the towing tank
carriage to establish the position of the centre of gravity.
This information was then combined with the model
geometry in a large angle stability analysis to determine
the righting moment at every heel angle. The difference
between the righting moment with and without foils,
therefore, gives the dynamic contribution to the stability
of the yacht. The results, in the form of the added
percentage of righting moment compared to the hull fitted
with a keel and bulb only, are presented in Figures 7 (a)
and 7 (b) for upwind and downwind respectively.
The Dali-Moustache foil in the aft position proved to be
the best in terms of generating righting moment at any heel
angle; it must be noted that in certain cases, the foil was
able to bring the boat back beyond the upright and into
negative heel; those results should, therefore, be
considered with care. Upwind, the performance of the
Dali-Moustache is matched by the DSS, the latter
suffering from ventilation issues due to the proximity with
the free surface at the highest Froude number, hence the
sudden decrease in righting moment. On the other hand,
the Chistera foil only provides minor improvements
downwind and reduces the dynamic stability in its forward
position upwind.
Figure 7: Added righting moment provided by the foils (a)
upwind and (b) downwind.
For both the Dali-Moustache and Chistera foils, the aft
position is far better in term of the contribution to added
dynamic stability, primarily because it is located further
away from the centerline. The aft location should,
therefore, be preferred, provided practical considerations
do not dictate a forward position, for example, so that the
retracted foil fits within the overall breadth for mooring
purposes, a vital aspect for cruising vessels.
Finally, the experiments demonstrated that a yacht or
superyachts subject to a given heeling moment onto which
a suitable foil is added will benefit from a drastic
increment in stability, with however no decrease in drag.
The results proved very consistent with the original
hydrodynamic efficiency experiment in that the lowest
resistance is always achieved without foils. Indeed,
despite the vertical heave (only significant from 
), the reduced displacement and wetted surface area are
never sufficient to overcome the added resistance and
induced drag of the foil. Notably, the configurations
providing the most righting moment, namely the Dali-
Moustache in both conditions and the DSS upwind, also
have the most drag, as shown upwind in Figure 8 (a) and
downwind in Figure 8 (b).
0.35 0.40 0.45 0.50 0.55 0.60
% Increase in RM
Dali - Fwd
Dali - Aft
Chistera - Fwd
Chistera - Aft
Negative Heel
0.35 0.40 0.45 0.50 0.55 0.60
% Increase in RM
Dali - Fwd
Dali - Aft
Chistera - Fwd
Chistera - Aft
Negative Heel
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
Figure 8: Total resistance (a) upwind and (b) downwind.
The resistance is a vital factor in the performance of a
yacht: for a given drive force, the vessel with the least
resistance will be the fastest. Furthermore, for upwind
sailing, side force is critical. Interestingly, the Dali-
Moustache generated the most side force in its aft position,
while the Chistera did so in its forward position.
Remembering that IMOCAs feature a Dali-Moustache aft
and the Figaro Bénéteau 3 has a Chistera forward, the fact
that each one is located in the position developing
maximum side force could imply this is the parameter
designers have been trying to improve for optimized
performance. For the Dali-Moustache, the aft location is
also the best position to generate stability. It is however
not the case for the Chistera foil, which could suggest its
primary objective is not to improve the stability but
provide additional side force. Ultimately, this would allow
the vessel to sail closer to the wind, and thus travel a
shorter route into the wind. This will be further analysed
in Section 6.4, taking into account the effect of the leeway
angle on upwind performance to compare the theoretical
sailing times on the water around an upwind race course.
The investigation into the ability of hydrofoils to create
dynamic stability provides an insight into the added
stability due to various configurations and positions. The
present work is consistent with the earlier findings relative
to the hydrodynamic efficiency and provides tangible
arguments regarding the influence of foils on stability.
5.4 (a) Dynamic Stability System
The DSS demonstrated a very effective contribution to the
righting moment upwind. In this particular instance, for
 and  at , i.e. a typical upwind
sailing condition at the upper end of upwind speeds, a
reduction in heel angle of 4.73° was measured. This is in
agreement with the findings of Welbourn, inventor of the
DSS: “I figured out a simple rule way back at the very
beginning of all this, and you should be looking to
optimize the foil for about 5 degrees heel equivalent of RM
at the top end of typical upwind speeds” (Welbourn,
personal communication, 14 December 2017).
5.4 (b) Dali-Moustache
The Dali-Moustache in the aft position (where it is found
on racing yachts), revealed the best ability to add stability
and reduce heel angle. While the DSS is limited by
ventilation issues due to the proximity with the free
surface, the Dali-Moustache proved to be able to bring the
vessel past upright, thus demonstrating its efficiency at
low heel angle, characteristic of downwind sailing. Since
this configuration is seen on IMOCAs, primarily
optimized for downwind, it is no surprise to see it perfectly
suited for this point of sail. Moreover, the added stability
explains the reason behind the latest generation of
IMOCAs being narrower (Beyou, 2017): with the
tremendous dynamic stability provided by the foils, the
form stability due to the width of the vessels can be
decreased, in turn resulting in a yacht with lower wetted
surface area, but also a lighter weight thanks to the
diminished size.
5.4 (c) Chistera
The Chistera exhibited a greater contribution to stability
in its aft rather than forward position, the actual amount
however being the lowest compared to other
configurations. The advantages of the Chistera in the
forward position, where it is found on the Figaro Bénéteau
3, are a lower drag and greater side force upwind. This
would, therefore, suggest its design is targeted at a faster
boat, able to sail with less leeway upwind. This would also
explain the previously not understood reason for the new
Figaro Bénéteau 3 featuring a heavier and deeper keel
despite the foils (Dewavrin, 2018). This can now be
explained as compensating with weight stability for the
minimal increase in dynamic stability. Practical
considerations also drive the forward location of this foil,
as discussed in Section 7.3.
0.35 0.40 0.45 0.50 0.55 0.60
Total Resistance (kN)
Dali - Fwd
Dali - Aft
Chistera - Fwd
Chistera - Aft
No Foils
0.35 0.40 0.45 0.50 0.55 0.60
Total Resistance (kN)
Dali - Fwd
Dali - Aft
Chistera - Fwd
Chistera - Aft
No Foils
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
5.4 (d) Findings
The DSS appeared to be most suited to upwind sailing, and
a very similar reduction in heel angle compared to the rule
of thumb developed by the DSS’ inventor has been
observed. Downwind, or at low heel angles, the proximity
to the free surface negatively affects this configuration,
with limited stability gains.
The Dali-Moustache is creating the most righting moment
in all conditions, especially in its aft position. This justifies
its presence further aft on racing yachts, as well as why the
latest generation of IMOCAs can afford to reduce the form
stability of the hull, now mostly relying on the tremendous
dynamic stability of the foils.
The Chistera foil in its forward position proved less
efficient stability-wise. This would, however, explain why
the new generation with hydrofoils features a deeper and
heavier keel. Nevertheless, with a greater side force and
lower drag upwind, it could be suggested that this is where
the benefits of this configuration reside.
With the knowledge of hydrodynamic efficiency of these
foils and the characterization of the added righting
moment, the understanding of hydrofoil-assisted
monohulls has been strongly extended. The final element
to be ascertained is the overall impact on performance.
Indeed, added stability will increase the power to carry
sail, but it has also been shown to enlarge the total
resistance. Similarly, greater side force will make for a
shorter distance upwind, but again at a cost in terms of
induced drag. Consequently, the development of a
velocity prediction program able to capture the various
behaviours of the foils will be tackled, to eventually
establish their significance to the overall sailing speeds.
While the hydrodynamic and stability influence of the
various foils have been quantified, the sailing speeds
remain to be assessed. Velocity prediction programs have
been successfully employed for the comparative
performance of racing yachts (Thomas & Souppez, 2018)
as well as cruising vessels (Guell & Souppez, 2018), but
current commercial packages do no account for the effect
of hydrofoils. Consequently, a dedicated VPP was
developed (Borba Labi, 2019).
The three degrees of freedom VPP (surge, sway, roll)
relies on hydrostatics and stability input. On the other
hand, the Offshore Racing Congress (ORC) methodology
(ORC, 2017) was adopted to quantify the sail forces, and
the DSYHS regression equations provided the
hydrodynamic hull resistance model (Keuning & Katgert,
2008). For the hydrofoils, Glauert’s biplane theory
corrected for proximity with the free surface was utilised
(Daskovsky, 2000), implementing correction coefficients
based on the towing tank results obtained previously.
Indeed, the empirical nature of the mathematical model
representing the forces generated by the foils does not
account for all the variables, hence the addition of an
efficiency factor to bring the theoretical prediction in line
with the experimental results for the various degrees of
freedom considered.
The heave has been neglected in this instance has it was
previously shown to be beneficial for typical sailing
Froude numbers (see Section 4.4), and a significant
reduction in displacement would not be expected on
superyachts, as it is on some of the small and light racing
The VPP developed, having the architecture depicted in
Figure 9, was first validated for non-foiling vessels against
commercial packages to demonstrate its validity and
suitability, before analysing the behaviour of hydrofoiling
Figure 9: Structure of the VPP (Borba Labi, 2019).
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
Due to its nature and underpinning theory, this VPP is
solely intended for foil-assisted yachts (i.e. not fully
flying), and is best utilized at an early development stage,
for the purpose of performance assessment, as illustrated
in Section 6.3, but also hydrofoil design optimisation later
highlighted in Section 6.4.
The initial assessment was conducted on the foil
geometries as tested in the towing tank. The intention
being to translate the experimental measurements into a
quantifiable performance on the water.
6.3 (a) Upwind
In upwind conditions (mainsail and jib) for a low wind
speed of 8 knots, the overall performance of each foil is
very similar, with an increasing advantage in boat speed.
The main difference, however, lies in the heel angle
adopted by the vessel (Figure 10), highlighting the added
stability of the Dali-Moustache, resulting in a lower heel
Figure 10: Upwind boat speed (a) and heel angle (b) in 8
knots true wind speed.
In low wind speeds, the small angles of heel are not yet
optimum for the DSS, which provides greater stability
further away from the free surface as the boat heels over
more in stronger wind. This is reflected in Figure 11,
where the vessel fitted with a DSS has the lowest heel
angle. In terms of performance, greater differences are
now shown, with the Chistera achieving higher velocities.
Figure 11: Upwind boat speed (a) and heel angle (b) in
16 knots true wind speed.
In those conditions, the pure boat speed is only superior to
the vessel without foils when fitted with a Chistera. The
DSS and Dali-Moustache do however contribute to a
much lower heel angle, which could be seen as more
suitable in cruising conditions for comfort.
6.3 (b) Downwind
In the downwind case (mainsail and spinnaker), the results
are less sensitive to the wind speed. Both low (Figure 12)
and high (Figure 13) wind speeds depict identical trends,
with the Chistera performing best, but at a higher heel
angle. Here again, only the Chistera proved able to surpass
the boat speed of the yacht not fitted with hydrofoils.
30 45 60 75 90 105 120
Speed (kts)
TWA (deg)
No Foils
30 45 60 75 90 105 120
Heel (Deg)
TWA (deg)
No Foils
30 45 60 75 90 105 120
Speed (kts)
TWA (deg)
No Foils
30 45 60 75 90 105 120
Heel (Deg)
TWA (deg)
No Foils
90 105 120 135 150 165 180
Speed (kts)
TWA (deg)
No Foils
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
Figure 12: Downwind boat speed (a) and heel angle (b)
in 8 knots true wind speed.
Figure 13: Downwind boat speed (a) and heel angle (b)
in 16 knots true wind speed.
Those results are however not sufficient to ascertain the
Chistera foil as the best performing one in all conditions.
On the one hand, an identical vessel has been considered,
when added sail area could, for instance, be fitted on a boat
equipped with Dali-Moustache or a DSS due to the
significant added righting moment. For a given heel angle,
those two configurations would have more power and thus
achieve better speeds. In addition, the leeway angle would
be considerably reduced if the yacht was fitted with a Dali-
Moustache or Chistera. On the other hand, the comparison
presented is for a given foil design that has not been
optimised. The original towing tank tested geometries
resulted from a parametric analysis to ensure their
representative nature, but in light of the new experimental
findings and the VPP created, their design can be refined.
Consequently, the specifications of each hydrofoil will be
altered to achieve an optimum geometry before re-
assessing the performance.
The VPP created permits to conduct a parameter study of
the hydrofoil geometries with the aim of maximising
performance. The design optimisation was targeted
around some key features, namely: the span, aspect ratio,
the angle of the foil to the hull and angle between its two
part for the Dali-Moustache and Chistera.
The results showed that very little improvement could be
made on the initial Chistera geometry that already appears
to be extremely efficient; this explains its superiority in
the previous presented section. On the contrary,
significant performance optimisation could be achieved
with both the DSS and Dali-Moustache. The gains
between the original tank tested versions and the VPP
optimised ones are presented for 16 knots of wind upwind
and 18 knots downwind in Figure 14.
Figure 14: Polar plots for the original (dashed line) and optimised (solid line) boat speed of the (a) DSS, (b) Dali-
Moustache and (c) Chistera.
90 105 120 135 150 165 180
Heel (Deg)
TWA (deg)
No Foils
90 105 120 135 150 165 180
Speed (kts)
TWA (deg)
No Foils
90 105 120 135 150 165 180
Heel (Deg)
TWA (deg)
No Foils
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
Lastly, as all VPPs, the presented one should be
considered qualitatively, allowing to compare the
performance of various boats, rather than quantitatively.
Indeed, although similar results between VPPs and sea
trials can be achieved (Souppez, 2014), there is now
evidence to suggest the force coefficients employed as
part of VPP models and originating from wind tunnel tests
could be flawed (Souppez et al., 2019)
Nevertheless, this illustrates the crucial importance of the
qualitative VPP at early design stages. Having reached an
optimal geometry for each configuration, the performance
of the three vessels could be compared again, yielding
very interesting results. Indeed, with the optimised
hydrofoil designs, virtually no differences in velocity or
heel angle were present, as illustrated for an upwind case
in Figure 15.
Figure 15: Upwind boat speed (a) and heel angle (b) in
16 knots true wind speed for optimised hydrofoils.
Therefore, it appears that provided the design of the
hydrofoil is optimised, similar velocities can be achieved,
irrelevant of the actual configuration employed on the
vessel. However, the comparison must also consider the
leeway angle, with a strong difference between the
arrangements not creating side force (no foils and DSS)
and those that do (Dali-Moustache and Chistera), the later
having a much smaller leeway angle, as quantified in
Figure 16.
Figure 16: Upwind leeway in 16 knots true wind speed for
optimised hydrofoils.
With this information, for a one nautical mile upwind
course, the Chistera would be the first at the mark,
followed by the Dali-Moustache 27.2 seconds behind, and
then the vessel without foils and the DSS, respectively
44.3 and 44.7 seconds later. This therefore provides a clear
comparison of the actual performance on the water of the
various designs.
An empirical VPP tool able to account for the effect of
hydrofoils was devised to quantify the sailing performance
and demonstrate the significant impact of their
optimisation on boat speed for foil-assisted monohulls.
One of the key findings is that, for designs that have been
ameliorated, there is no configuration superior to another
in terms of velocity, including a yacht without foils.
However, there are strong differences in terms of
performance on the water, with a clear ranking between
the different options.
It is worth noting that this study assumed that the design
of the yacht remains constant. In practice, the amount of
sail area or hull shape could be refined based on the
specificities of each hydrofoil, as tackled in Section 7.5.
Nevertheless, the choice of which option to be installed on
a yacht or superyacht is also subject to practical
Since the same performance can be attained for any of the
three hydrofoil configurations investigated, the practical
design considerations are vital factors to consider. These
primarily revolve around minimising the loss of internal
volume, ease of mooring and preventing marine growth on
the foils. These also supplement elements normally
considered as part of the development process, such as
issues associated with cavitation and ventilation, or the
structural loads, although these are currently beyond the
scope of structural design regulations (Souppez, 2018).
30 45 60 75 90 105 120
Speed (kts)
TWA (deg)
No Foils
30 45 60 75 90 105 120
Heel (Deg)
TWA (deg)
No Foils
30 45 60 75 90 105 120
Leeway (Deg)
TWA (deg)
No Foils
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
Accommodation volume is always limited and must be
maximised, even on the largest mega yachts. As a result,
the intrusiveness of the hydrofoils should be minimized.
To that effect, the DSS is the easiest to fit, as it can easily
be concealed under the floorboard, even on small crafts
(Guell & Souppez, 2018), thus having very little impact
on the interior volume. The Chistera, and to a greater
extent the Dali-Moustache, however, induce in a much
larger loss of volume.
Of course, the physical size of the yacht itself play a large
role. At present, only the DSS has been featured on
superyachts, which would appear a sensible solution to
avoid the loss of internal spaces, but also for mooring and
maintenance reasons.
An additional factor to consider in implementing
hydrofoils onto superyachts is the ability to fully retract it
within the overall breadth of the boat for mooring. Not
only is it more expensive and harder to find suitable berth
for a wider vessel, but hydrofoils are fragile, and should
be protected.
A hydrofoil such as the Dali-Moustache protrudes beyond
the overall beam of the vessel, thus requiring a larger
berthing space as well as suitable protection for the
hydrofoils. This also implies the deck edge will be further
away from the quayside, which could represent a
loading/unloading issue.
Consequently, it could be seen beneficial to prevent this
situation. In the case of the DSS, the foil retracts and can
be stored within the breath of the hull. This generally
governs its position further aft, where a greater breadth is
available. On the other hand, the Chistera, which still
practically protrudes once retracted, is located forward,
where the boat is narrower so that the outer extent of the
foil remains within the overall breadth.
Irrelevant of the size of the vessel, preventing the
hydrofoils from sticking out of the hull’s overall beam in
the harbour is to be considered by the designer. This can
also help prevent marine growth and thus minimise
maintenance by keeping the foil dry.
The performance gains obtained from the hydrofoils rely
on a high lift to drag ratio. Unfortunately, the development
of marine growth on its surface sharply hinders its
effectiveness. The hydrofoil’s surface should, therefore,
remain smooth; the easiest way being to keep the hydrofoil
dry when not in use.
On racing yachts such as the IMOCAs the foils will remain
submerged even when retracted. This issue is alleviated by
the fact that racing yachts will be regularly cleaned, and a
high level of maintenance will be available ahead of the
race start. A racing class such as the Figaro Bénéteau 3,
equipped with Chistera foils, would also be expected to
benefit from this.
For a more leisurely application of hydrofoils, which
represents most of the sailing yacht industry, a more
maintenance-free solution should be reached. Here again,
the ability of the DSS to fit within the hull shell and the
Chistera being mostly outside of the water when retracted
provide strong practical arguments for their use.
The selection and design of a given hydrofoil
configuration should consider all aspects, from the
performance to the more practical elements. With the
ability to fit within the hull’s overall breadth, be kept away
from the environment when retracted and minimal loss of
internal volumes, the DSS appears as an easy system to
install and has currently been the most widespread form of
hydrofoil on sailing yachts and superyachts.
Nevertheless, other hydrofoil configurations can lead to
alternative design philosophies. The Dali-Moustache for
instance, shown to provide the most added dynamic
righting moment, has led to new hull design for the
IMOCA class. Indeed, the latest vessels feature narrower
hulls, with less form stability, no longer required thanks to
the foils. This also diminishes the build cost and weight as
the surface area and size of the craft is effectively lowered.
Extensive experimental hydrodynamic testing has been
performed on three contemporary hydrofoils in order to
further the knowledge of hydrofoil-assisted monohulls, for
application ranging from small racing yachts to cruising
mega yachts.
Firstly, the hydrodynamic efficiency investigation
revealed that, despite their contribution to the vertical lift
and side force, none of the tested configurations could
achieve a lower drag that the hull without hydrofoils.
This prompted further work to quantify the added righting
moment provided, in a free-to-heel setup. The results
showed that, while significant dynamic righting moment
could be created, this came at a strong drag penalty.
In order to ascertain how the previous findings influence
the overall speed of the yacht, which is of paramount
importance, a dedicated velocity prediction program was
developed. This tool allowed to define the comparative
performance of the various foil types, but also to conduct
a parameter study and refine their design. Upon
optimisation of each hydrofoil, it appeared that none could
provide a greater speed than the others.
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
Furthermore, it is interesting to note that, for foil assisted-
monohulls where the foil does not a provide significant
reduction in heave (particularly on superyachts where the
lift force is very small compared to the vessel’s
displacement, and would not occur until a higher Froude
number, as demonstrated experimentally), the actual boat
speed remains virtually unchanged. There are however
some strong benefits in terms of reducing the heel angle
for comfort and leeway for performance that can be very
attractive. Overall, looking at the race time on an upwind
course, the Chistera would win, followed by the Dali-
Moustache, and eventually the DSS, the later achieving a
similar time as a yacht without foils.
Finally, the practical considerations that could influence
the selection and design of the most appropriate
arrangement have been outlined, revolving around the
ability to retain internal volume, ease of mooring and the
prevention of marine growth.
These novel findings provide new insights into the design
of hydrofoil-assisted monohulls. Future work will,
however, consider the impact on the design of the vessel
itself. Indeed, all configurations were tested on an
identical hullshape and sailplan. In practice, a narrower
hull with less drag and less mass could be designed thanks
to the added dynamic stability. Moreover, a greater sail
area could be implemented as the power to carry sail has
been increased, eventually resulting in a faster yacht.
In addition, research into the seakeeping characteristic of
the vessels should be undertaken, with a potential
reduction in motions experienced for greater comfort and
lower structural loads.
The authors would like to greatly acknowledge the support
of the following organisation and individuals:
The West Pomeranian University of Technology,
Poland, the University of Rostock, Germany and
Solent University, UK, cooperating in the framework
of the EMship+ Erasmus Mundus Joint Master’s
Degree programme, coordinated by the University of
Liège, Belgium.
The Bénéteau Group and the ENSCBP University,
France, for the funding and supporting the stability
J. Cunningham-Burley for his support during the
manufacturing of the model, C. Kitching for his
assistance with the experimental testing, and G. Kay
and H. Welbourn for their expert insight about DSS
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Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
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19. Souppez, J.-B. R. G., ‘Development and
Validation of a Computational Fluid Dynamics
Hydrodynamic Model of the Stewart 34 for
Velocity Prediction Program Applications’,
Master Thesis, The University of Auckland, 2014.
20. Souppez, J.-B. R. G., High Performance Racing
Yachts: An Experimental Comparison of the
Latest Hydrofoil Configurations’, UK Fluids
Conference, Manchester, UK, 2018.
21. Souppez, J.-B. R. G., Structural Design of High
Performance Composite Sailing Yachts under the
New BS EN ISO 12215-5’, SNAME Journal of
Sailing Technology, issue 2, pp 1-18, 2018.
22. Souppez, J.-B. R. G., Arredondo-Galeana, A. &
Viola I. M., ‘Recent Advances in Downwind Sail
Aerodynamics’, The 23rd Chesapeake Sailing
Yacht Symposium, Annapolis, Maryland, United
States, 2019.
23. Thomas, J. and Souppez, J.-B. R. G.,
Comparative Performance Prediction of
Historical Thames A Rater Class Designs’,
Historic Ships, London, UK, 2018.
Jean-Baptiste R. G. Souppez is the Senior Lecturer in
Yacht Design and Composite Engineering at Solent
University, teaching on the prestigious BEng (Hons) Yacht
and Powercraft Design, BEng (Hons) Yacht Design and
Production and MSc Superyacht Design. He contributes to
the European Master in Integrated Advanced Ship Design
(EMship+) as a Visiting Professor and Research
Supervisor, and is also the UK Principal Expert in Small
Craft Structures, in charge of representing the interests of
the British Marine Industry in the development of
international structural regulations (BS EN ISO 12215).
His research in fluid dynamics features twisted flow wind
tunnel, towing tank, wave and current flume, particle
image velocimetry, laser doppler anemometry, and full-
size instrumented testing, as well as a range of numerical
Juliette M. M.-A. Devawrin is currently a Graduate
Design Engineer at Gurit Composite Engineering, UK.
She is working on pre-studies and structural engineering
solutions for composite structures in glass fibre and
carbon fibre reinforced polymers on various projects such
as sailing and motor boats, superyachts, vehicles and
transportation, architecture and renewables. She is also
conducting an experimental research on naval composite
structures for Gurit internal development in the scheme of
her graduate programme. During her studies on the
EMship+ master in Ship Design, she conducted
experimental research in fluid dynamics at Solent
University, featuring modern hydrofoils-assisted
monohulls hydrodynamic efficiency.
Florian Gohier presently occupies the position of
apprentice engineer in composite engineering between the
ENSCBP (Bordeaux Graduate School of Chemistry,
Biology and Physics) and BJ Technology, Bénéteau
Group, France. He is responsible for the structural
calculations of hulls, hull liners, decks, deck liners and
composite components manufactured by the Bénéteau
Gaetan Borba Labi is currently a mooring engineer at
Principal Power, Portugal. His responsibilities include the
analysis and design of mooring systems for offshore
platforms used in the Eolic industry of energy generation.
His previous experience features numerical modelling of
fluid dynamic problems and development of tools for the
velocity prediction of hydrofoils-assisted monohulls.
... The heel angle is limited to [0, 26]° to keep the deck out of water, even with the heaviest sails in design space. The yaw angle is set to [0, 5]° [50]. The sail angle is set within the optimal value ±20° proposed in the literature [26]. ...
Full-text available
Wing-sailed autonomous sailing monohulls are promising platforms used in various scenarios to provide data for marine science research. These platforms need to operate long-term in changing seas; their general configurations (size matching between sail, hull, and keel) necessitate careful trade-offs to balance safety and efficiency. Since autonomous sailboats are often designed for different observation missions, scientific pay-loads and target areas, their design space is considerably large. It is also challenging to obtain prior performance estimation from historical designs. Therefore, traditional offline surrogate-based simulation-driven design frameworks suffer from a large amount of sampling required, the computational cost of which remains too expensive for such ad hoc design tasks. This paper proposes an innovative, generalised simulation-driven framework combining Bayesian optimisation and knowledge transfer. It allows for high-quality, low-cost optimisation of autonomous sailing monohulls' general configuration without initial design and prior performance estimation. The proposed optimisation framework has been used to optimise the 'Seagull' prototype within the design constraints. The optimised design exhibits significant performance improvements. At the same time, the results show that the present method is significantly superior to traditional offline methods. The authors believe that the proposed framework promises to provide the autonomous sailing community with a solution for a general design methodology.
... More recently, VPPs or performance prediction programs (PPPS) as they are more often referred to for wind-assisted ships, have been used to support the optimisation of sails [48], hulls [49] and hydrofoils [50,51], as well as maximise the performance of both wind-assisted ships [52,53] and fully wind-powered ships [54]. Similar performance optimisation strategies are employed; for instance, the established use of depowering in yachts [55] has now been applied to wind powered cargo ships [56], albeit with different constraints for the allowable heel angle, much smaller compared to yachts. ...
Conference Paper
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With the current global warming crisis and contemporary concerns for sustainability, the transport industry is developing and implementing novel solutions to reduce greenhouse gases. With close to 90% of the world's goods relying on maritime transportation, responsible for 3% of global energy-related carbon dioxide (CO2) emissions in 2019, there is a vital emphasis on reducing emissions. The latest legislation from the International Maritime Organisation has imposed even tougher sulphur oxide targets. On the other hand, emission intensity for CO2 will need to be decreased by 70% in 2050, compared to 2008 figures. While operating measures and fuel alternatives are suitable in the short-term to meet these novel regulatory constraints, as the use of fossil fuels tapers off, the long-terms solution appears to reside in wind-assisted ships. Consequently, this study aims to identify viable solutions that could reduce emissions, focussing on three prominent technologies, namely sails, rotors and kites. Furthermore, this review provides guidance on the benefits and risks associated with each technology and recommends guidelines for performance prediction and associated constraints. Ultimately, future stakes in wind-assisted propulsion are highlighted, including the need for full-scale validation, the challenge in assessing environmental and economic impact, and the structural issues associated with wind-assisted propulsion systems.
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A leading-edge vortex (LEV) can be a robust lift generation mechanism on both the wings of natural fliers and delta wings. A spinnaker-type of sail is a thin wing that promotes the formation of LEVs due to a sharp leading edge. Recent numerical simulations (Viola et al., 2014) have demonstrated that this type of sail can prevent LEV shedding and instead, keeps it trapped near the leading edge. In such cases, the LEV could enhance lift generation (Saffman and Sheffield, 1977; Huang and Chow, 1982), and so there is a need to investigate the existence of the LEV and its role for sails. To study the LEV in the context of sails, a rigid model scale spinnaker was tested in water at low Reynolds numbers and uniform flow. It was found that the flow separates at the leading edge, followed by turbulent reattachment, forming an LEV. For finite periods the LEV breaks down into weaker LEVs that are shed downstream; otherwise, the LEV remains coherent at the leading edge. On the lower half of the sail, the LEV has negligible diameter, and trailing edge separation occurs after the first quarter of the chord. To understand whether there is a benefit from having the LEV trapped near the leading edge, as opposed to being shed downstream into smaller LEVs, the local circulation was measured and its value utilised in a complex potential model. The model maps a circular arc into a rotating cylinder and assumes the Kutta condition, to provide a bound circulation value that is a function of the position and circulation of each LEV (Pitt Ford and Babinsky, 2013; Nabawy and Crowther, 2017). It is found that when the LEV is trapped near the leading edge, the LEV provides a marginally higher lift than when it breaks down and sheds. Surprisingly, with the conservative assumption of the Kutta condition, the LEV contributes between 10% to 20% to the sail’s sectional lift. In actual sailing conditions, the spinnaker experiences a twisted onset flow, that could not be replicated in the water flume, such that the angle of attack varies along the span of the sail. To explore this effect three spinnaker models were made, where the original sail was twisted from top to bottom by different angles. PIV and force measurements were compared. It was observed that a low twist sail allows the LEVs to remain close to the body of the sail, whereas a high twist sail causes them to drift away and generates counter vorticity on the surface of the sail. This viscous effect results in a marginal reduction in lift, but significant reduction of induced drag. The results presented in this PhD thesis aim to provide an improved understanding of the aerodynamics of downwind sails, where vortex flow is a dominant feature. The existence of trapped and shedding LEVs is demonstrated and an attempt is made to model LEVs through a complex potential model in order to assess their contribution to the sectional lift of the sail. Finally, the effect of twist is evaluated with regard to the aerodynamics of sails.
Conference Paper
Full-text available
Over the past two decades, the numerical and experimental progresses made in the field of downwind sail aerodynamics have contributed to a new understanding of their behaviour and improved designs. Contemporary advances include the numerical and experimental evidence of the leading-edge vortex, as well as greater correlation between model and full-scale testing. Nevertheless, much remains to be understood on the aerodynamics of downwind sails and their flow structures. In this paper, a detailed review of the different flow features of downwind sails, including the effect of separation bubbles and leading-edge vortices will be discussed. New experimental measurements of the flow field around a highly cambered thin circular arc geometry, representative of a bi-dimensional section of a spinnaker, will also be presented here for the first time. These results allow to interpret some inconsistent data from past experiments and simulations, and to provide guidance for future model testing and sail design.
Full-text available
Despite the omnipresence of hydrofoil-assisted racing monohulls and the inherent development phases to refine their designs, very little scientific data has reached the public domain. Moreover, following the trend set by racing yachts, the cruising industry is now looking at the implementation of foils onto leisure vessels, with several already built. This paper therefore presents a hydrodynamic comparison of three contemporary options, namely a Dynamic Stability System, a Dali-Moustache and a Chistera foil, that have been towing tank tested on a 1:10 scale model of a 50 ft racer-cruiser hull. The analysis presented focuses on the resistance, side force, heave and trim, as well as the induced drag factor and effective draft of each design, eventually resulting in a conclusion on the most suitable configuration for leisure craft applications, and providing experimental data relative to hydrofoils. At this stage, the interest is purely hydrodynamic, and does not yet account for the additional righting moment provided by the foils and the impact on sailing performance.
Conference Paper
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The Thames A-Rater fleet is a unique class both in appearance and in its combination of historic and modern technologies. With high aspect ratio, carbon fibre rigs fitted onto wooden hulls, many of which have survived two World Wars, the class is a demonstration of the evolution of sailing technology. In more recent decades, various attempts have been made to expand the class with new composite boats. However, due to the strict rules issued by the class association, new hulls must be exact replicas of existing A-Raters, with a 1.5 inch tolerance. Furthermore, as only one linesplan exists in the public domain, the expansion of the fleet is extremely limited. Consequently, in order to ensure the conservation of some of these historic designs, the lines of several vessels were taken off and used to create accurate linesplan and 3D models. The comparative performance of the various crafts was then assessed through a Velocity Prediction Programme, focused on the specific environmental conditions of the vessels' main operating area, eventually ascertaining the hull with the best racing potential by design.
Full-text available
The forthcoming publication of the revised BS EN ISO 12215-5 is set to transform the structural design of most small crafts for the next decade, with the implementation of significant changes to the scope and underpinning theory, such as an applicability extended up to 24 m Load Line, and the use of finite element methods as part of the compliance assessment process. This paper represents the first public release of the major changes and novelty in the standard, with a strong emphasis on high performance composite sailing yachts. The aim is to provide designers and builders with an insight into the technical background and practical applications of the new regulation for structural optimization, and how the marine industry will be impacted.
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The project consists of two stages: the design of a classic sailing yacht, including all the interior design and the development of a structure that merges wood building and carbon fiber on the more critical areas, then followed by the incorporation of a modern hydrofoil system. The aim of the hydrofoil, adapted from racing yacht technology, is to generate lift that will reduce the amount of hull in the water, make the yacht more stable and performant. The particular system to be investigated is known as a Dynamic Stability System (DSS), and will be tested on a model hull in the towing tank at Southampton Solent University to demonstrate the benefits of this new configuration. A number of hydrofoil sections will be experimented with to assess the optimum one, and eventually predict the improvements in performance of the boat. For the incorporation of the hydrofoil system, a study will be done on different wing sections and shapes inspired from the most advanced racing yachts in the world that can be added to the side of a wooden, more traditional boat. Once this is done, a model will be tested in a towing tank to look at the effects of this system and the flow interaction generated, and then, with the achieved data and calculations based on first principles, a velocity prediction program (VPP) will be developed to be able to extrapolate the information achieved to new boats that want to use this system. Based on the results achieved and the designed VPP, a conclusion will be derived to see if the DSS increases the speed of the boat by being able to add more power to the vessel or to see if all the positive effects created are neglected by the increased drag produced by the foil.
Full-text available
The prediction of the hydrodynamic behavior of sailing yachts is a key component of modern yacht design, particularly to assess the theoretical performance of a design through a velocity prediction program. The increasing computational power now offers an alternative to towing tank testing: computational fluid dynamics. The work conducted on the development and validation of a computational fluid dynamics hydrodynamic model of the Stewart 34 sailing yacht for velocity prediction program applications is presented in this report. An empirical resistance model has initially been developed based on the Delft Systematic Yacht Hull Series. The method has been described and its limitations highlighted, the main ones being the resistance prediction in semi-displacement mode and the side force prediction at high leeway angle. The method is however reliable for common designs in displacement mode and is particularly efficient since only a few design inputs are required. In a second time, the use of the Rankine source panel code FS-Flow led to a computational fluid dynamics hydrodynamic model of the Stewart 34, validated against an experimental benchmark, as done for the empirical resistance calculations. Instabilities in the panel code at high Froude numbers resulted in a loss of accuracy. Furthermore, the panel code did not prove suitable at high leeway angles. FS-Flow appears to have some limitations when handling the hydrodynamic model of sailing yachts; its intended use primarily being the comparison of ship designs. A total of three velocity prediction programs have been developed. An empirical 4 degrees of freedom one enabled to ascertain the test matrix for the Stewart 34. A second 4 degrees of freedom one was realized with WinDesign to provide a reference and comparison with the final 6 degrees of one done using FS-Equilibrium. This final velocity prediction program gathers the experimental hydrodynamic and aerodynamic models and was validated against available upwind full-scale data to ensure its reliability. A complete performance prediction for the Stewart 34 was therefore achieved based on experimental data, mostly focussing on the hydrodynamic model ascertained using both empirical and computational fluid dynamics methods, thus meeting the objectives set for this project. Finally, areas of further improvement and future work have been recommended.
Conference Paper
Full-text available
Hydrofoil sailing has been able to unlock performance characteristics previously confined to speed records, making them available to multiple racing fora. The America’s Cup is now regularly sailed at 40 knots, Moth sailing dinghies and A-Class catamarans achieve up to 30 knots on standard race courses. The systems employed to achieve these speeds have been refined to such an extent that high speeds are regularly attained. However, there are still large gaps in our understanding of the fundamental hydrodynamic phenomena to enable safe control of these machines and continued increases in performance. For example, arbitrary ventilation pathways have been noticed and yet are not fully explained. This paper provides the means to unlock the methods of quantitatively establishing a pathway for arbitrary ventilation and for measuring the flow regime complexity around such foils. These two methods have been developed over many years by the collaborators mentioned in this paper. The result is a valuable contribution to capability available to the sailing research community. An additional two methods of experimental analysis have been detailed within the paper.
Comparative hydrodynamic analysis between three hydrofoils by model testing in a towing tank following ITTC recommendation procedures (Resistance tests)
The IMOCA 60 Class has a complicated set of appendages: with canted and tilted keels, cambered dagger-boards that can be designed to be fitted to the hull in different orientations along with toed-in and twin rudders that can also be configured in different orientations. Curved dagger-boards and straight boards with positive lift inducing dihedral angles have been used in a number of recent IMOCA 60 designs and in other classes, principally multi-hulls. These were considered an option by the client for their new Open 60 design and so a research and development programme was instigated by Owen Clarke Design to compare new curved designs with conventional straight dagger-boards optimised for upwind conditions. It was felt that the modelling of the trim of the yacht was very important to the calculation and sharing of loads between all of the appendages, and so our group chose to use a combination of one third scale high speed towing tanks tests and computational fluid dynamics (CFD), rather than CFD alone to investigate the relative performance between these dagger-board types.
Results of an experimental study of the effect of surface proximity on hydrofoil lift are presented. The biplane image theory, a horseshoe vortex model and momentum theory are described in relation to the effect of surface proximity on hydrofoil lift and drag. The biplane image theory and the horseshoe vortex model are shown to predict the same effect on lift, and are seen to be in good agreement with the experimental data. The Payne momentum theory is seen to differ significantly from the measured results. The data indicate a significant reduction in lift at depths less than two chords with very little effect at greater depth.