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Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
HYDROFOIL CONFIGURATIONS FOR SAILING SUPERYACHTS: HYDRODYNAMICS, STABILITY AND
PERFORMANCE
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.
SUMMARY
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.
NOMENCLATURE
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
Aeronautics.
ORC Offshore Racing Congress.
VPP Velocity Prediction Program.
1. INTRODUCTION
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
Globe.
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
2. BACKGROUND
2.1 PREVIOUS WORK
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
performance.
2.2 AIMS AND OBJECTIVES
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.
3. EXPERIMENTAL TESTING
3.1 MODEL
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
Section
NACA 64-012
Swept back angle -
3°
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
3.2 HYDROFOILS DESIGN AND LOCATION
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
Dali-Moustache
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
Chistera
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.
3.3 MANUFACTURING
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,
2017).
a
b
c
d
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).
3.4 EXPERIMENTAL SETUP
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).
3.5 TEST MATRIX
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.
3.6 UNCERTAINTY ANALYSIS
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)
0.453
Model uncertainty -
0.781%
Displacement uncertainty -
0.025%
Wetted surface area uncertainty -
0.782%
Velocity – (m/s)
2.322m/s
Calibration uncertainty -
0.002%
Data acquisition uncertainty -
0.002%
Velocity uncertainty -
0.003%
Density – (kg/m3)
998.403
Temperature -
19°C
Temperature error -
1.316%
Density uncertainty -
0.010%
Total Resistance - (N)
11.049
Calibration uncertainty -
0.002%
Fitting uncertainty -
1.288%
Data acquisition uncertainty -
4.937%
Misalignment uncertainty -
0.934%
Resistance uncertainty -
5.186%
Total Resistance Coefficient -
0.024
Resistance coefficient uncertainty -
6.245%
Table 3: Example of uncertainty analysis.
4. HYDRODYNAMICS
4.1 ANGLE OF ATTACK INVESTIGATION
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,
2018).
The DSS was set at 0°, 4° and 8° AoA, while the Dali-
Moustache and Chistera were tested with 0°, 8° 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
configuration.
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.
4.2 INDUCED DRAG FACTOR
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 = 0°,
2°, 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
course.
4.3 EFFECTIVE DRAFT
The hydrodynamic performance of yacht appendages is
quantified using the effective draft, , derived from the
theory of induced drag on a lifting surface,
mathematically:
Where:
• 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.
4.4 HEAVE
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.
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
050 100 150 200 250
RT(kN)
Fh2(kN2)
No Foils
DSS
Dali
Chistera
Upwind Sailing
30%
35%
40%
45%
50%
55%
0.35 0.40 0.45 0.50
TEFF
Fn
No Foils
DSS
Dali
Chistera
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.
4.5 DISCUSSION
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
interpretation.
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.
5. STABILITY
5.1 INTRODUCTION
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.
-200
-150
-100
-50
0
50
100
0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70
Heave (mm)
Fn
No Foils
DSS
Dali
Chistera
a)
-200
-150
-100
-50
0
50
100
0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70
Heave (mm)
Fn
No Foils
DSS
Dali
Chistera
b)
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.
5.2 RIGHTING MOMENT
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.
5.3 RESISTANCE
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).
-25%
0%
25%
50%
75%
100%
125%
150%
175%
0.35 0.40 0.45 0.50 0.55 0.60
% Increase in RM
Fn
DSS
Dali - Fwd
Dali - Aft
Chistera - Fwd
Chistera - Aft
Ventilation
Negative Heel
a)
-25%
0%
25%
50%
75%
100%
125%
150%
175%
200%
225%
0.35 0.40 0.45 0.50 0.55 0.60
% Increase in RM
Fn
DSS
Dali - Fwd
Dali - Aft
Chistera - Fwd
Chistera - Aft
Ventilation
Negative Heel
b)
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.
5.4 DISCUSSION
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
2
4
6
8
10
12
14
16
18
0.35 0.40 0.45 0.50 0.55 0.60
Total Resistance (kN)
Fn
DSS
Dali - Fwd
Dali - Aft
Chistera - Fwd
Chistera - Aft
No Foils
a)
0
2
4
6
8
10
12
14
16
18
0.35 0.40 0.45 0.50 0.55 0.60
Total Resistance (kN)
Fn
DSS
Dali - Fwd
Dali - Aft
Chistera - Fwd
Chistera - Aft
No Foils
b)
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.
6. PERFORMANCE
6.1 INTRODUCTION
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).
6.2 METHODOLOGY
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
crafts.
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
yachts.
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.
6.3 RESULTS PRE-OPTIMISATION
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
angle.
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.
2
3
4
5
6
7
8
9
10
30 45 60 75 90 105 120
Speed (kts)
TWA (deg)
No Foils
DSS
Dali
Chistera
a)
0
5
10
15
20
30 45 60 75 90 105 120
Heel (Deg)
TWA (deg)
No Foils
DSS
Dali
Chistera
b)
4
6
8
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12
14
30 45 60 75 90 105 120
Speed (kts)
TWA (deg)
No Foils
DSS
Dali
Chistera
a)
0
5
10
15
20
25
30
30 45 60 75 90 105 120
Heel (Deg)
TWA (deg)
No Foils
DSS
Dali
Chistera
b)
3
4
5
6
7
8
9
10
90 105 120 135 150 165 180
Speed (kts)
TWA (deg)
No Foils
DSS
Dali
Chistera
a)
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.
6.4 RESULTS POST-OPTIMISATION
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.
0
5
10
15
20
25
90 105 120 135 150 165 180
Heel (Deg)
TWA (deg)
No Foils
DSS
Dali
Chistera
b)
4
6
8
10
12
14
90 105 120 135 150 165 180
Speed (kts)
TWA (deg)
No Foils
DSS
Dali
Chistera
a)
0
5
10
15
20
25
30
90 105 120 135 150 165 180
Heel (Deg)
TWA (deg)
No Foils
DSS
Dali
Chistera
b)
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.
6.5 DISCUSSION
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
considerations.
7. PRACTICAL DESIGN CONSIDERATIONS
7.1 INTRODUCTION
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).
4
6
8
10
12
14
30 45 60 75 90 105 120
Speed (kts)
TWA (deg)
No Foils
DSS
Dali
Chistera
a)
5
10
15
20
25
30
30 45 60 75 90 105 120
Heel (Deg)
TWA (deg)
No Foils
DSS
Dali
Chistera
b)
0
1
2
3
4
5
30 45 60 75 90 105 120
Leeway (Deg)
TWA (deg)
No Foils
DSS
Dali
Chistera
Design & Construction of Super & Mega Yachts, 14h -15th May 2019, Genoa, Italy
© 2019: The Royal Institution of Naval Architects
7.2 INTERNAL VOLUME
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.
7.3 MOORING
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.
7.4 PREVENTING MARINE GROWTH
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.
7.5 DISCUSSION
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.
8. CONCLUSIONS
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.
9. ACKNOWLEDGEMENTS
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
investigation.
• 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
hydrofoils.
10. REFERENCES
1. Abbott, I. H. & Doenhoff, A. E., ‘Theory of wing
sections, Including a Summary of Airfoil Data’,
New York: Dover Publications Inc, 1959.
2. Beaver, B. & Szeleczky, J., ‘Full Scale
Measurements on a Hydrofoil International
Moth’, The 19th Chesapeake Sailing Yacht
Symposium, Annapolis, USA, 2009.
3. Beyou, J., ‘On Peut Envoyer Fort Dans La
Grosse Brise!’, Voiles et Voiliers, 2017.
4. Binns, J. R., Ashworth-Briggs, A., Fleming, A.,
Duffy, J., Haase, M., & Kermarec, M.,
‘Unlocking Hydrofoils Hydrodynamics with
Experimental Results’, The 4th International
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© 2019: The Royal Institution of Naval Architects
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11. AUTHORS BIOGRAPHY
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
methods.
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
Group.
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.
