217
1 INTRODUCTION
Marine transportation plays a crucial role in global
trade, accounting for over 80% of international
commerce due to its cost-efficiency and ability to
handle large cargo volumes. However, due to their size
and the massive volume of goods they carry, ships are
among the largest emitters of carbon dioxide (CO₂) and
other greenhouse gases (GHG) in the transportation
sector. Currently, the primary fuel used in maritime
transport is heavy fuel oil (HFO) — a low-quality diesel
fuel that poses significant environmental risks. The
combustion of HFO releases a range of harmful
pollutants into the atmosphere, including sulphur
oxides (SOx), nitrogen oxides (NOx), fine particulate
matter (PM2.5) and carbon dioxide (CO₂) – for
reference see table 1.
According to a UNCTAD report from December
2024 [1], GHG emissions from maritime transport have
increased by 20% over the past decade, driven by
sustained global economic growth, port expansions,
and intensified shipping activity. This alarming trend
underscores the urgent need for the shipping industry
to move away from an ageing, fossil fuel-dependent
fleet toward more sustainable and renewable energy
solutions [2].
Addressing the dual challenge of reducing GHG
emissions and transitioning to cleaner energy sources
has become critical [3]. As emphasised by Rebeca
Grynspan, Secretary-General of UN Trade and
Development (UNCTAD): "Building sustainable and
resilient maritime transport and future-proofing global
supply chains is not just an option – it’s a strategic
necessity."
Harnessing the Wind: The Rise of Wind-Assisted Ship
Propulsion (WASP) in the Transformation of Maritime
Transport
K. Gaul & G. Rutkowski
Gdynia Maritime University, Gdynia, Poland
ABSTRACT: The global maritime sector, which transports over 80% of international trade, is under increasing
pressure to reduce greenhouse gas (GHG) emissions, which have risen by 20% over the past decade. Among
emerging decarbonisation strategies, Wind-Assisted Ship Propulsion (WASP) stands out as a promising solution
that harnesses renewable wind energy, with the potential to reduce emissions by up to 100%. This paper explores
the technical maturity, regulatory context, and market viability of key WASP technologies, including rotor sails,
suction sails, hard wing sails, and kites. It outlines the evolving regulatory landscape, such as carbon pricing and
fuel intensity standards coming into effect from 2025, which is driving increased interest in WASP.
A comprehensive technical analysis of the three systems is provided, focusing on aerodynamic principles,
installation requirements, and operational performance. The article concludes with an assessment of market
trends, noting that as of early 2025, 52 vessels are already equipped with WASP and an additional 97 are on order.
The study affirms WASP’s strategic role in enabling near-term emissions reductions and supporting the maritime
industry's transition toward full decarbonisation.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 19
Number 1
March 2025
DOI: 10.12716/1001.19.01.25
218
In response to the environmental impact of heavy
fuel oil (HFO), the maritime industry is increasingly
turning to alternative fuels. The most considered
options include Marine Diesel Oil (MDO), Marine Gas
Oil (MGO), Liquefied Natural Gas (LNG), biofuels, and
hydrogen.
Furthermore, as illustrated in Figure 1, broad-based
decarbonisation efforts can progress across five key
areas, each offering distinct potential for GHG
reduction and presenting specific challenges to large-
scale implementation [2]. Among the wide range of
solutions — such as improvements in logistics and
digitalisation, enhancements in ship hydrodynamics
and machinery, or post-combustion carbon capture
and storage — wind-assisted ship propulsion (WASP)
stands out as a particularly promising approach.
Figure 1. Solutions for shipping decarbonisation and their
GHG reduction potentials. Source: Own research based on [2,
5, 14, 22].
In addition to its environmental benefits, wind
energy also offers tangible financial advantages for
shipowners who choose to invest in such solutions. The
potential of innovative wind propulsion is considered
substantial for green shipping, primarily due to the
abundance and availability of wind as a free energy
source [4]. This paper explores the key regulatory
drivers, technological advancements, and market
developments shaping the adoption of WASP
technologies, highlighting their pivotal role in
supporting the maritime industry's decarbonisation
objectives.
2 METHODOLOGY
The content of this article is based on statistical data
collected from shipowners, as well as from reputable
sources such as the International Maritime
Organization (IMO), the European Commission (EC),
and classification societies including DNV, PRS, LR,
ABS, RINA, among others. It also draws upon the
insights of environmental protection experts and the
authors’ personal experience and observations
accumulated over more than twenty years in the
transport industry.
3 DRIVERS FOR WASP ADOPTION BY
REGULATORY FRAMEWORKS
Decarbonisation throughout the 2020s and beyond is
expected to be driven by three fundamental pillars:
regulatory frameworks and policies, access to investors
and capital, and the evolving expectations of cargo
owners and consumers [2]. These drivers, supported
by structured frameworks and standards that define
sustainability assessment criteria, GHG emission
calculation methodologies, and reporting
requirements, are creating favourable conditions for
the continued development and implementation of
various WASP technologies.
The implementation of the GHG Strategy has
become one of the primaries focuses of the
International Maritime Organization (IMO), which
now works to ensure the industry's alignment with key
decarbonisation milestones. These include a reduction
in total GHG emissions by 20%—with an ambition of
reaching 30% by 2030; a 70% reduction striving for 80%
by 2040 (all relative to 2008 levels); and ultimately
achieving net-zero GHG emissions by or around 2050
[5]. At the 81st session of the Marine Environment
Protection Committee (MEPC) held in March 2024 [6],
an agreement was reached on how the proposed “IMO
Net-Zero Framework” could be formally incorporated
as amendments to MARPOL Annex VI. This
framework is structurally similar to the previously
introduced Carbon Intensity Indicator (CII) and
Energy Efficiency Existing Ship Index (EEXI).
According to Lloyd’s Register (LR), this evolving
framework is expected to serve as the foundational
structure for future regulations, once consensus is
reached on the implementation mechanisms and
selected policy measures.
While it remains, uncertain which specific measures
will ultimately be adopted, it is already clear that both
a fuel standard and economic incentives will be
integral components of the forthcoming regulatory
framework [2,6]. The development of these regulations
is ongoing at the IMO, and according to the agreed
timeline, they are expected to be adopted in 2025 and
enter into force around mid-2027.
Two additional regulatory processes are currently
progressing independently [7]. The first concerns the
Carbon Intensity Indicator (CII), which establishes the
annual reduction factor required to ensure continuous
improvement in a ship's operational carbon intensity
within a defined rating level. The second is the Energy
Efficiency Existing Ship Index (EEXI), which measures
a ship’s energy efficiency against a designated baseline.
Both regulations are scheduled for review by the
end of 2025, with proposed amendments to their
provisions and associated guidelines. Expected
updates include revised CII reduction targets for the
2026–2030 period, the introduction of new or modified
correction factors, and potentially the inclusion of
additional performance metrics [8]. The review may
also propose a strengthened enforcement mechanism
and the broader integration of life-cycle assessment
(LCA) methodologies to capture the full emissions
profile of non-fossil fuels.
The infographic (see Figure 2) illustrates the
evolving regulatory framework within the maritime
industry, focusing on decarbonisation measures
expected between 2025 and 2027. It begins with
“Forthcoming Regulations”, which will likely include
fuel standards and economic incentives. These
regulations are scheduled for implementation between
2025 and 2027. From this central framework, three
regulatory components are highlighted:
1. Carbon Intensity Indicator (CII): Scheduled for
review by the end of 2025, CII determines annual
reduction targets for ships' carbon intensity based
on operational data.
219
2. Energy Efficiency Existing Ship Index (EEXI): Also
set for review by the end of 2025, EEXI assesses the
technical energy efficiency of existing ships
compared to a reference line.
3. Life-Cycle Assessment (LCA): Positioned as a
complementary measure, LCA may be proposed for
broader application. It would evaluate emissions
across the full lifecycle of fuels and technologies,
including alternative and non-fossil fuels.
The diagram effectively conveys how these
initiatives are interrelated and contribute to the
broader maritime decarbonisation strategy.
Figure 2. The infographic illustrates the evolving regulatory
framework within the maritime industry, focusing on
decarbonisation measures expected between 2025 and 2027.
Source: Authors own researches.
Of particular relevance to vessels operating within
EU and European Economic Area (EEA) ports is the EU
Emissions Trading System (EU ETS), which requires
emitters to pay for their greenhouse gas (GHG)
emissions. The system applies to all voyages to and
from ports within the EEA—including Iceland,
Norway, and the outermost regions under the
jurisdiction of EU Member States. For voyages
involving ports outside the EEA, ship operators will be
required to surrender allowances covering 50% of the
emissions generated during the journey. In contrast,
voyages between EEA ports, as well as emissions
produced while the vessel is at berth, will necessitate
surrendering allowances for 100% of the associated
emissions [9].
The inclusion of the maritime sector in the EU
Emissions Trading System (EU ETS) will be
implemented gradually. Beginning in 2025, general
cargo vessels between 400 and 5,000 gross tonnage
(GT) and offshore vessels from 400 GT will be required
to report their greenhouse gas (GHG) emissions;
however, they will not yet be subject to the ETS.
Offshore vessels exceeding 5,000 GT will fall under the
EU ETS starting in 2027, while the inclusion of general
cargo and offshore vessels between 400 and 5,000 GT
will be reconsidered following a review in 2026.
Additionally, the reporting scope is expanding to cover
more GHGs. Methane (CH₄) and nitrous oxide (N₂O)
emissions must be reported from 2024, and from 2026,
these gases will also be subject to allowance surrender
under the EU ETS [10].
Another key European Union regulation aimed at
reducing greenhouse gas (GHG) emissions in the
maritime sector is FuelEU Maritime, which promotes
the adoption of renewable and low-carbon fuels and
energy sources. Introduced as part of the European
Commission’s Fit for 55 legislative packages, the
regulation came into full effect on 1 January 2025. It
applies to ships above 5,000 gross tonnage (GT) that
transport cargo or passengers for commercial
purposes. FuelEU Maritime sets maximum limits on
the annual GHG intensity of the energy used onboard,
requiring a 2% reduction starting in 2025, progressing
incrementally to an 80% reduction by 2050 [11]. Ship
operators can achieve compliance through several
options: by using fossil LNG or LPG, qualified low-
GHG-intensity fuels (with an incentive factor for
Renewable Fuels of Non-Biological Origin), connecting
to shore power while at berth, or utilising wind-
assisted propulsion systems.
As part of the broader effort to reduce air pollution
in ports, passenger and container ships will be required
to use on-shore power supply (OPS) or other
alternative zero-emission technologies while at berth
or moored alongside. This mandate will take effect on
1 January 2030 in ports covered under Article 9 of the
Alternative Fuels Infrastructure Regulation (AFIR).
The requirement will be further extended to all EU
ports equipped with OPS infrastructure starting from 1
January 2035. Additionally, EU Member States may
choose to enforce this obligation earlier—from 2030
onward—in ports not explicitly covered by Article 9
[12].
Complementing the EU ETS, the FuelEU Maritime
Regulation adopts a goal-based and technology-
neutral framework, aimed at fostering innovation and
supporting the development of sustainable fuels and
energy conversion technologies. This flexible approach
allows ship operators to select fuels and solutions best
suited to their vessels' operational profiles, without
mandating a specific technological pathway. The
regulation also introduces several flexibility
mechanisms to facilitate compliance across the existing
fleet, while offering incentives for early adopters and
investors engaged in the maritime energy transition
[11,12]. Together, the GHG pricing structure under the
EU ETS and the performance-based standards of
FuelEU Maritime create a new landscape of regulatory
challenges. These include increased demands in
administration, reporting, third-party verification,
contractual structuring, and cost forecasting (see
Figure 3). However, as with any evolving regulatory
framework, these challenges are accompanied by
strategic opportunities. Stakeholders who effectively
streamline and optimise their compliance strategies
may gain a competitive edge in an increasingly
decarbonised maritime sector.
Figure 3. EU ETS and FuelEU Maritime cost comparison.
Source: [13]. Note: EUA and Fuel EU penalty costs for vessel
emitting 9,725 tonnes of CO2 equivalent (CO2 eq.) on
voyages to and from the EU, and 1,399 CO2 eq. tonnes on
intra-EU voyages or at berth in EU ports; excludes EUA price
changes, potential impact of FuelEU penalties and penalty
multipliers port calls without using onshore power supply or
failing to meet 2% RFNBO usage.
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Table 1. Comparison of Gas Emissions from 1 kg of Marine Fuel Combusted by Ship vs. Wind-Assisted Ship Propulsion
(WASP)
Type of
Emission
MDO
MGO
LNG
Biofuel
(e.g. FAME)
Hydrogen
(H₂, via fuel cell)
WASP
CO₂
3.20 kg
3.15 kg
2.75 kg
0–3.1 kg (biogenic)
0 kg (if green
hydrogen)
0 kg
SOₓ
1–3 g (low sulphur
content)
0.5–1.5 g (very
low)
< 0.001 g
< 0.01 g (negligible)
0 g
0 g
NOₓ
5–10 g
4–9 g
1–3 g
2–6 g
< 1 g (in fuel cells)
0 g
CO
0.3–1.5 g
0.3–1.5 g
0.1–1 g
0.1–0.5 g
0 g
0 g
PM2.5
0.3–1 g
0.1–0.5 g
< 0.01 g
0.05–0.5 g
0 g
0 g
CH₄
almost none
almost none
0–5 g (fugitive)
negligible
negligible
0 g
Note: Emissions values for HFO are approximate and depend on engine type, fuel quality, and operating conditions. Wind-assisted systems
use mechanical energy generated by wind and do not emit pollutants directly. In hybrid systems (wind + engine), emissions are reduced
proportionally depending on wind contribution. Source: Authors own research based on: [2,4,6,8,14,16,23,30].
All of this underscores the urgent need for decisive
action. The maritime sector must embrace major
operational transformations, invest in innovation, and
commit to the modernisation of fleets through more
efficient, environmentally responsible technologies.
Central to this transformation is a transition to cleaner
energy sources and alternative fuels. While the
financial cost of decarbonisation will undoubtedly be
significant, retreating from the sector’s climate and
sustainability objectives is not a viable option [13]. In
this context, supporting the adoption of wind-assisted
propulsion - a technology capable of reducing both
emissions and operational costs - emerges as a
strategically sound and forward-thinking decision.
4 MARKET AND INVESTMENT DRIVERS
Beyond regulatory compliance, market forces and
investment dynamics are playing an increasingly
pivotal role in accelerating the adoption of Wind-
Assisted Ship Propulsion (WASP) technologies [2]. As
the shipping industry faces rising fuel costs, increased
investor scrutiny, and shifting customer expectations,
WASP is emerging as a commercially viable and
future-proof solution.
The potential savings achievable through the
implementation of WASP technology are difficult to
quantify due to the multitude of factors influencing
system efficiency. Different levels of fuel savings can
be expected depending on the type of WASP installed.
It should also be noted that all WASP technologies
require at least a small amount of energy to operate —
for example, to adjust orientation or activate tilting
mechanisms — while dynamic systems such as rotor
sails or suction wings demand greater energy input to
generate thrust. Increasing the number and size of
WASP units generally leads to greater thrust and,
consequently, higher potential savings. However,
interaction effects between multiple units can reduce
overall efficiency, necessitating careful consideration
of their arrangement. When WASPs are installed on
newbuilds, greater opportunities exist for design
optimisation, which can further enhance system
performance [14].
Given the complexity and the numerous factors
influencing WASP performance, it is more appropriate
to determine the average annual fuel savings of each
system on a case-by-case basis rather than by vessel
segment. Such assessments should consider the
vessel’s specific operational profile, intended routes,
and the distribution of weather conditions along those
routes.
As wind propulsion technologies generate thrust
from wind energy, greater fuel savings could
potentially be achieved if routes are optimised based
on prevailing weather conditions. Therefore, more
significant savings can be expected when voyage
(route) optimisation is implemented. Available data on
the fuel-saving potential of wing sails and suction sails
remain limited. Reported savings vary widely, from
approximately 2% to 50%, depending on the vessel
type, number of sails, and operational conditions.
Table 2. Review of fuel-saving performance of rotor sails on different type of ships.
Ship’s name
Performance parameters
Ship type
Routes
An average emission
reduction and fuel saving
target
M/V Annika Braren
one 18x3 m rotor sail
general cargo ship
Not given
2-4.5%
M/V Estraden
two 18x3 m rotor sails
Ro-Ro vessel
Rotterdam and Teesport
(UK)
6.1%
M/V Copenhagen
one 30x5m rotor sail
hybrid ferry
Rostock (DE) and
Gedser (DK)
4%
M/V Viking Grace
one 24x4 m rotor sail
Ro-Pax vessel
Turku (FI) to Stockholm
(SE)
reduced power consumption
of 207 kW to 282 kW / 231 to
315 tonnes of fuel per year
M/T Maersk Pelican
two 30x5m rotor sails
tanker
mainly trading between
Middle and Far East
8.2%
M/V SC Connector
two 35x5 m rotor sails
Ro-Ro ship
Not given
20-25%
M/V Afros
four 16x2 m rotor sails
bulk carrier
Nantong (CN) to
Vancouver (CA)
12.5%
M/T Sea Zhoushan
five 24x4 m rotor sails
VLOC
Not given
8%
Source: Own research based on [22, 25].
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For instance, the Wind Challenger project aims to
achieve a 50% reduction in fuel consumption by
equipping a capsize bulk carrier with nine telescopic
sails, while route-specific simulations (Yokohama–
Seattle) suggest savings in the range of 20–30%. Single-
sail installations, such as on the Shofu Maru, have
demonstrated fuel reductions of 5% and 8% on the
Japan–Australia and Japan–North America routes,
respectively [15]. Tests conducted on Ro-Pax vessels,
such as Ciudad de Mahón, indicated potential fuel
savings ranging from 7% to 22%, depending on the
operational scenario. These findings stem from a case
study based on simulations involving two 35×12 m²
wing sails and the assumption that the vessel operates
in the Mediterranean Sea at a design speed of 21 knots
[16].
A manufacturer of suction wings reports that their
systems could reduce fuel consumption by up to 20%
and 40%, respectively, depending on the number and
size of wings installed, as well as the vessel type [17].
However, only limited performance data is currently
available for suction wings. Speed trials were
conducted on the multi-purpose dry cargo vessel M/V
Frisian Sea, which was fitted with two suction wings,
each measuring 10 × 3 metres [18]. During testing, the
vessel maintained a constant speed of 10 knots, with
wind energy partially replacing engine output. Data
from these trials was used to estimate power savings
across a range of wind conditions, with calculated
savings on specific routes ranging from 0.7% to 4%, and
an average of 2.2%.
During 2022–2023, several speed trials were
conducted with the general cargo vessel M/V Ankie,
which was equipped with two suction wings, each
measuring 13 × 2.1 metres, to verify their power-saving
potential. During testing, the true wind speed was 10
m/s and the vessel maintained a speed of 9.5 knots.
Power savings reached up to 15% at the most
favourable wind angle, while the estimated average
power reduction on typical routes was approximately
3.5%, corresponding to a saving of around 40 kW [19].
One of the strongest commercial incentives for
adopting WASP technologies is their potential to
deliver immediate and measurable fuel cost
reductions. However, the lack of robust empirical data
confirming the predicted savings remains a barrier to
the wider adoption of WASP systems. Further research
on these technologies is therefore of significant
importance. Nevertheless, all savings achieved to date
translate directly into lower operational expenditures,
thereby enhancing the competitiveness of WASP-
equipped vessels.
5 OVERVIEW OF WIND PROPULSION
TECHNOLOGIES
Wind-Assisted Ship Propulsion systems, which are
recognised as a promising solution to reduce, and in
some cases replace, conventional fuel use in shipping,
convert wind energy into propulsion power, allowing
ships to partially, or depending on the design,
significantly substitute main engine output with wind
power. When properly applied and appropriately
designed to suit a vessel’s operational profile, WASP
can reduce greenhouse gas (GHG) emissions, air
pollution, fuel consumption, and underwater noise
[20]. While all Wind Propulsion Systems operate based
on the same fundamental physical principles, the
specific technologies differ in terms of their
mechanisms and performance characteristics. Selecting
the most suitable WASP for a particular vessel depends
on multiple factors, including average sailing speed,
operational routes, prevailing weather conditions, and
practical considerations such as available deck space
and compatibility with cargo operations [21].
Optimising ship design and navigation strategies to
align with the dynamics of wind propulsion is essential
for maximising the effectiveness of WASP.
Furthermore, ship structure and weather routing play
a critical role in the overall system performance.
Up to January 2025, 52 vessels registered in DNV
[2,22] have been equipped with modern wind-assisted
propulsion systems. While this represents only a small
fraction of the global fleet, adoption is expected to
increase significantly. The recent acceleration in uptake
is clearly evident, with 44 of these installations
occurring on ships built or retrofitted after 2020. Larger
vessels dominate this trend, accounting for a combined
total of 3.4 million deadweight tons (DWT) equipped
with WASP. As illustrated in Figure 1, the adoption of
WASP in the global fleet is currently concentrated
around four main technologies: rotor sails, suction
sails, wing sails and kites [22]. Therefore, this section of
the article will focus on and describe only these
technologies.
Flettner rotors, also known as rotor sails, were first
applied on a merchant vessel in 1924. These are
rotating cylindrical sails that harness wind energy to
assist with ship propulsion. Their operation is based on
the Magnus effect, a phenomenon in which a spinning
cylinder generates lift perpendicular to the airflow.
While the initial trial was technically successful, it
lacked economic viability, and as a result, the concept
remained largely dormant for nearly a century.
However, in recent years, renewed interest has
emerged, driven by the push for more sustainable
shipping solutions [23]. A major advantage of this
technology is that the sails can be installed on
newbuilds or retrofitted to existing ships, provided
there is sufficient deck space and unobstructed airflow,
even if the vessel was not originally designed to
accommodate sails. However, newbuilds offer better
optimisation by integrating WASP from the start. Rotor
sails are particularly suited to vessel types such as
tankers, LNG carriers, RoRos, RoPaxes, general cargo
ships, bulk carriers, as well as cruise ships and ferries
[24]. Currently, bulk carriers and tankers dominate the
use of rotor sails, accounting for 54% of all WASP
technologies installed on vessels in operation.
Rotors offer the advantage of being easily adjusted
to the wind direction by varying their rotational speed,
enabling effective wind utilization on both legs of a
voyage—something that is not always achievable with
other wind-assisted propulsion technologies.
However, a key drawback is the additional drag they
generate when not in operation, particularly when
sailing close to the wind. This added resistance can
increase engine power demand and fuel consumption.
In response, recent innovations, such as folding rotors,
have been developed to reduce this drag penalty when
the system is inactive.
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Another consideration is that rotor sails require a
continuous supply of electrical power to maintain the
rotational speed necessary for optimal aerodynamic
performance. However, the power demand is
relatively low compared to the thrust generated. The
aerodynamic efficiency of a rotor sail is primarily
influenced by the ratio between wind speed and the
surface speed of the rotating cylinder, with rotational
speed constrained by practical and operational
limitations. Interrupting the power supply halts the
rotor’s rotation, thereby ceasing lift generation [22].
Suction sails, also known as suction wings, are
vertical, aerofoil-shaped sails affixed to the vessel's
main deck. Unlike Flettner rotors, the outer surfaces of
suction wings remain stationary. However, these sails
are capable of automatically adjusting to align with
prevailing wind directions. The wing sails are
equipped with integrated fans and vents that utilize
boundary layer suction to augment the aerodynamic
force produced, in addition to the conventional thrust
generated by the sail's shape.
Similar to rotor sails, suction wings achieve
maximum efficiency when exposed to crosswinds. In
contrast, they generate negligible or zero thrust when
subjected to headwinds or tailwinds [21]. To optimize
aerodynamic efficiency, wind suction quantities and
pressures must be adjusted for different wind
conditions. Interrupting the electrical power supply
halts the operation and lift generation. Suction wing
sails can reach heights of up to 36 meters. Typically,
vessels are fitted with two or four units, although
configurations with a single suction wing are also in
operation. Smaller units, with heights under 10 meters,
are available as containerized systems, allowing for
easy transfer between vessels. When foldability is
required, some manufacturers offer a tilting system
that moves the suction sail from a vertical to a
horizontal position. This feature can help lower the air
draft of the vessel in specific situations, such as when
sailing under bridges or during cargo loading and
unloading operations [22].
Wing sails, also known as hard sails, operate on
similar aerodynamic principles as conventional soft
sails, utilizing wind interaction to generate both drag
and lift forces. However, they differ significantly in
construction materials and structural design. Hard
sails are typically made from lightweight, high-
strength materials such as carbon fibre and have a
rigid, fixed geometry. These sails can be rotated to
align with the wind direction, optimizing propulsion
efficiency. This adjustment process is usually fully
automated. The aerodynamic design of hard sails is
based on aviation principles, with shapes modelled
after aircraft wings to maximize performance. As a
result, they achieve a higher lift-to-drag ratio and
generate greater lift compared to traditional soft sails
[22]. Wing sails come in various sizes, with the largest
reaching heights of up to 50 meters and surface areas
of up to 1,000 square meters. To ensure maximum
aerodynamic efficiency, they must be aligned with the
incoming wind direction at an optimal angle of attack.
In configurations comprising multiple elements, the
wing sail can be cambered to further enhance
aerodynamic force generation. For operational
flexibility, wing sails are often designed to be tiltable.
This feature facilitates port operations, reduces air
draft when necessary, and provides protection during
periods of high wind conditions [21, 27]. One challenge
associated with wing sail systems—particularly when
more than two units are installed on board, concerns
compliance with IMO visibility and safe navigation
regulations. This issue is currently under review and is
being assessed on a case-by-case basis.
Kite Sails are tethered sails constructed from
lightweight and high strength fabric materials,
controlled using ropes, and flown at significant
altitudes to utilise stronger winds. They can use wind
to propel them forward, reducing the need for engine
power and they can also generate electricity, which can
be used to power the ship or stored for later use.
During operation, the kite moves in a dynamic figure
of 8 motion, increasing its apparent wind speed to over
ten times that of the vessel. By rapidly flying into the
oncoming wind, the kite significantly increases traction
force, effectively towing the ship. Despite having a
considerably smaller surface area than other on-deck
wind propulsion systems, the kite generates
substantially greater thrust due to its dynamic
manoeuvring. Additionally, kites operate at altitudes
where wind speeds are approximately double those
experienced at sea level. Unlike the previously
described systems, a kite achieves optimal
performance primarily when positioned in the
downwind area.
Kite sails offer fuel savings from 5% to 20%,
depending on weather conditions and route. Their
advantages are minimal structural interference with
the vessel and therefore they have relatively little
impact on heel and yaw angles on the ship and have a
positive contribution to the course keeping of the ship
and it requires much less corrections on the rudder and
much less rudder action than deck mounted systems.
Kite sails can be retrofitted on existing ships. As for
disadvantages kite sails require advanced automation
and control systems. They have limited effectiveness in
downwind conditions and ports. Kites require
designated space for deployment and storage. Another
down factor is that the surface material and the towing
rope need regular replacement, counted in few
thousands of operating hours.
6 WASP LIMITATIONS AND CHALLENGES
Most Wind-Assisted Ship Propulsion systems require
significant deck space for installation, which presents a
greater challenge for retrofitting than for newbuilds.
The availability of deck space varies depending on ship
type and size for instance, container and passenger
vessels typically offer less space compared to bulk
carriers or tankers. Interference with cargo handling
operations and onshore infrastructure is another
important consideration. This issue is often mitigated
by the use of foldable or tiltable systems, which are
now common and help reduce safety risks during high
wind conditions. Additional placement requirements
must also be considered to ensure the safe and practical
use of WASP, such as avoiding the creation of blind
spots or installing units too close to passenger
accommodation areas [20].
223
Figure 4. WASP installations on different types of ships.
Source: Own research based on Source: Based on [24, 25, 26,
28].
The weight of WASP devices varies across
technologies, but the impact on cargo capacity is
generally considered minimal. From a design
perspective, the ship’s structure must be capable of
safely transferring the forces generated by the WASP,
which may require local reinforcement, though this is
not regarded as a significant technical barrier. Wind
availability significantly affects the efficiency of WASP,
depending on the vessel’s route, direction, seasonal
weather patterns, and proximity to land [28]. To
maximise efficiency, route optimisation is essential,
balancing favourable wind conditions with overall
route length. Proper integration of the WASP into the
ship's design and operations further enhances
performance.
Since 2021, the adoption of Wind-Assisted Ship
Propulsion systems within the global fleet has
accelerated significantly. As of January 2025, a total of
52 seagoing vessels equipped with WASP are in
operation, with an additional 97 newbuilds featuring
WASP currently on order. These systems have been
implemented across a broad spectrum of vessel types,
with bulk carriers, tankers, and general cargo ships
representing the primary categories. Recent industry
developments have demonstrated that the installation
of Wind-Assisted Ship Propulsion Systems is not
limited to specific ship types. Retrofitting WASP is
technically feasible on nearly any vessel with adequate
deck space and unobstructed airflow, regardless of
whether it was originally designed to accommodate
sail systems. Currently, approximately 75% of the
WASP-equipped fleet consists of retrofitted vessels.
This adaptability enables the deployment of WASP
across a wide range of existing ships and operational
profiles [22]. To date, only two significant studies have
attempted to forecast the future market potential of
Wind-Assisted Ship Propulsion Systems (WASP). The
first was commissioned by the UK government as part
of the Clean Maritime Plan (July 2019). This study
assessed the global annual market for wind propulsion
systems in the context of broader alternative
propulsion technologies and fuels. The market for
wind technologies, which included both WASP and
vessels utilizing primary wind propulsion, was
projected to grow from a conservative estimate of £300
million per year in the 2020s to approximately £2
billion annually by the 2050s [29]. In this analysis, wind
propulsion technologies were ranked as the second
most significant category of maritime propulsion
innovation, following alternative fuels (estimated at
£8–11 billion per year by the 2050s), and were expected
to represent around 15% of the total propulsion
systems market.
The second key study, conducted by CE Delft for
the European Commission in 2017, preceded the surge
in WASP adoption post-2018 and the commercial
maturation of several technologies, such as suction
sails. The report projected that, if wind propulsion
technologies reached market viability by 2020, the
potential market for installations across bulk carriers,
tankers, and container vessels could range from 3,700
to 10,700 systems by 2030, including both retrofits and
newbuilds. These figures were based on variables such
as bunker fuel prices, vessel speeds, and applied
discount rates [30]. While some WASP technologies
did achieve maturity before 2020, and regulatory
clarity has improved since 2017, the initial uptake fell
significantly short of expectations. By the end of 2023,
only 29 WASP installations had been completed, far
below the several hundred forecasted in the CE Delft
model.
However, the underlying assumptions of the CE
Delft analysis remain relevant. The study posited that,
once the number of installations surpassed 100, the
industry would experience sufficient learning effects to
significantly reduce costs, making WASP a financially
viable option for all suitable newbuilds and retrofits
under the modelled economic conditions. With 101
planned installations now on record, the industry may
be approaching the inflection point of the model’s
projected S-curve: a phase of accelerated adoption
followed by a plateau, as WASP become a standard
feature on appropriate newbuild vessels.
7 CONCLUSION
Comparison of four main WASP technologies
including kite sails, wing sails, rotor sails (Flettner
rotors), and suction sails (Ventifoils) has been
presented in Table 3 considering fuel savings, technical
and operational complexity, aero efficiency,
installation and operational cost, spatial and structural
constraints, integration with existing ship propulsion
system.
Table 3. Comparison of Wind-Assisted Ship Propulsion
(WASP) systems considering fuel savings, technical and
operational complexity, aero efficiency, installation and
operational cost and retrofit friendly.
Techno
logy
Market share by
WASP Type [%]
Fuel Savings
[%]
Installation Ease
[Factor 1 to 5]
Aero Efficiency
[Factor 1
-5]
Cost &
Complexity
[Factor 1
-5]
Retrofit Friendly
[Factor 1
-5]
Kite Sails
2%
5–20%
4
2
3
5
Wing
Sails
19%
10–
30%
2
5
2
2
Rotor
Sails
48%
5–25%
3
4
3
4
Suction
Sails
31%
10–
20%
4
4
3
4
Note: Rating factor on a scale of 1 to 5, where 1 indicates the
lowest rating, least efficiency, 5 the highest rating, most
efficiency. Source: Own researches based on [2,22,30].
Based on our researches it must be noted, that wing
sails offer the highest aerodynamic potential, but
require significant space and are more difficult to
224
retrofit. Kite sails and suction sails are the easiest to
implement, especially on existing vessels. Rotor sails
are the most proven commercial solution, with a
realistic return on investment within a few years. The
best choice depends on ship type, route, vessel design,
and shipowner priorities (e.g., ROI, sustainability,
operational costs).
Kite sails offer fuel savings from 5% to 20%,
depending on weather conditions and route.
Advantages: Minimal structural interference with the
vessel. Operates at high altitudes (stronger winds). Can
be retrofitted on existing ships. Disadvantages:
Requires advanced automation and control systems.
Limited effectiveness in downwind conditions and
ports. Needs space for deployment and storage.
Wing Sails offer fuel savings: 10–30%, depending on
size and alignment. Advantages: High aerodynamic
efficiency. Can be automated (angle of attack
adjustment). Effective on beam and broad reach
courses. Disadvantages: Large space and height
requirements. May interfere with cargo operations.
Retrofitting is more challenging (deck reinforcement
needed).
Rotor Sails (Flettner Rotors) offer fuel savings: 5–
20%, sometimes more (>25% in favourable conditions).
Advantages: Effective even in side wind conditions.
Fully automated operation. Successfully implemented
in commercial shipping (e.g., Norsepower).
Disadvantages: Requires electrical power for rotation.
Tall structures may impact stability and visibility.
Requires robust deck structure.
Suction Sails (Ventifoils) offer fuel savings: 10–20%
(according to manufacturer Econowind). Advantages:
High efficiency in a compact design. Easy installation
(containerized or foldable units). Fully automated.
Disadvantages: Requires power for air compressors.
Lower thrust compared to larger wings or rotors.
ACKNOWLEDGEMENTS
This research was supported by the statutory activities of
Gdynia Maritime University (grant number
WN/2025/PZ/07), which actively support activities aimed at
reducing energy (fuel) consumption and reducing toxic
greenhouse gas (GHG) emissions. This paper explores the
technical maturity, regulatory context, and market viability
of key WASP technologies, including rotor sails, suction sails,
and hard wing sails.
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