Novelties in Marine Propulsion

1. Introduction

The maritime industry is undergoing one of its most profound transitions since the shift from sail to steam, driven by increasingly stringent international regulations such as the International Convention for the Prevention of Pollution from Ships (MARPOL). Maritime transport supports over 80% of global trade and is therefore unavoidable in the current economic system [1]; at the same time, its environmental footprint is substantial and growing, with propulsion systems alone contributing up to 3% of global greenhouse gas (GHG) emissions according to recent International Maritime Organization (IMO) assessments [2]. Meeting the increasing demand for commercial shipping thus requires a fundamental redefinition of maritime transport in line with regulatory targets, including the IMO objective of halving GHG emissions by 2050 relative to 2008 levels. In this context, research has largely focused on energy supply, identifying fossil fuels—particularly heavy fuel oil—as the primary source of environmental impact. Consequently, an extensive body of literature has explored alternative energy carriers aimed at reducing GHG emissions, sometimes prioritizing decarbonization over combustion efficiency [3]. Among the most widely investigated solutions are battery-powered propulsion systems and biofuels [4,5,6], while nuclear energy is also emerging as a potential long-term option [7,8]. At the same time, the tailored design [9] and optimization [10] of propulsive units remain fundamental to fully exploit low-emission energy carriers and to further enhance system performance toward more sustainable maritime transport.

Driven by tightening regulatory constraints on maritime navigation, with the long-term objective of achieving net-zero emissions, the quest for “Novelties in Marine Propulsion” has evolved from a predominantly academic endeavor into a critical industrial necessity. To foster progress in sustainable marine propulsion, this Special Issue brings together eight pioneering studies that address these challenges through advanced Computational Fluid Dynamics (CFD), innovative design exploration, and the investigation of disruptive physical phenomena.

2. Published Articles

In the past decade, the reliance on traditional towing tank testing has been augmented—and in some design stages, replaced—by high-fidelity numerical simulations. The papers in this issue highlight the maturity of Unsteady Reynolds-Averaged Navier–Stokes (URANS) and Large Eddy Simulation (LES) techniques. As ship owners face stricter Energy Efficiency Existing Ship Index (EEXI) and Carbon Intensity Indicator (CII) regulations, the ability to predict hydrodynamic performance with sub-1% error margins (as seen in the works of Zhang et al. and Feng et al.) is feasible. These tools allow for the rapid “virtual prototyping” of complex systems, such as the ducted waterjets and multi-propeller configurations discussed herein, which would be prohibitively expensive to iterate physically.

A recurring theme in this Special Issue is the shift from optimizing individual components to modelling the integrated propulsion system. Modern vessels are no longer characterized by a simple screw propeller; they are complex assemblies of ducts, stators, and interaction zones. The work by Feng et al. on body force modeling for waterjets emphasizes that as vessels grow in complexity, interference between multiple units becomes a dominant design factor. Wang et al. provide a systematic mapping of these interactions in a two-propeller configuration, offering concrete indications for spacing and arrangement to maximize collective efficiency.

Beyond the refinement of existing technologies, this issue explores groundbreaking novelties that challenge conventional naval architecture. The exploration of the Coanda effect by Lee et al. represents a step toward circulation-control propulsion, demonstrating that bio-inspired or fluid-dynamic phenomena can yield power reductions of nearly 8%. Moreover, the work by Avanzi et al. on submerged nacelles for the Outboard Dynamic-inlet Waterjet (ODW) introduces a new aero-derivative propulsive concept with the aim of extending the operational efficiency of unducted screw propellers and surpassing the installation penalties of traditional flush-type waterjets.

Finally, the transition to green fuels necessitates a reimagining of the vessel’s internal architecture. The integration of Hydrogen Fuel Cells (PEMFCs), as analyzed by Chen et al., serves as a reminder that the propulsor of the future is only as efficient as the thermal and energy systems that support it. This comprehensive view—combining hydrodynamics, thermodynamics, and innovative physics—is the hallmark of the research presented in this Special Issue.

Summary of Published Papers

The study by Zhang et al. introduces a novel, simplified computational framework for assessing the hydrodynamic performance of ship propellers subjected to wave-induced heave and pitch motions. Addressing the critical limitation of traditional calm-water tests, the authors propose a hybrid methodology that integrates Potential Flow Theory (via Response Amplitude Operators, or RAOs) with Computational Fluid Dynamics (CFD) to model unsteady propeller forces. By prescribing heave and pitch motions derived from the potential flow method onto the propeller in a CFD environment, the research bypasses the high computational cost of full-ship–propeller coupling while maintaining engineering precision. Numerical simulations conducted on the KCS container ship and KP505 propeller reveal that vertical stern motions lead to significant performance degradation, specifically a thrust reduction and torque increase. Crucially, the results demonstrate an absolute efficiency decrease of 3 to 6 percentage points, representing an approximately 10% relative loss in propulsive performance under real sea conditions. Furthermore, the study identifies a significant nonlinear coupling between heave and pitch effects, indicating that their combined hydrodynamic impact is not merely a linear superposition of individual motions. Validated against high-fidelity CFD data, the proposed method achieves a four-fold reduction in computational time, making it a highly effective tool for systematic propulsion analysis in the early ship design phase and for the development of operational “Digital Twins” focused on energy efficiency.

The contribution by Park et al. provides a rigorous comparative analysis of numerical simulations for the open-water performance of cavitating propellers, a critical aspect in the design of high-speed General Purpose Planning Hulls (GPPHs). Utilizing two distinct computational codes—CFDShip-Iowa V5.5, an overset-based solver, and STAR-CCM+, a commercial general-purpose CFD tool—the authors evaluate the thrust and torque characteristics across a range of advance ratios (J) and cavitation numbers (σ). A pivotal finding of the study is the identification of significant performance degradation under low cavitation numbers (σ = 0.274), where averaged thrust, torque, and efficiency reductions reached approximately 63%, 50%, and 30%, respectively. The research highlights a technical discrepancy between the two solvers at high advance ratios: while CFDShip-Iowa captures complex blade-to-blade cavitation interactions and shedding, STAR-CCM+ (employing periodic boundary conditions for a single blade) tends to underpredict these violent patterns. This novelty in methodology underscores the necessity of full-propeller modeling over simplified periodic domains when simulating ultra-high-speed regimes where cavitation vapor may wrap around the entire assembly. By detailing the differences in vapor-phase iso-surfaces and pressure distributions, the work serves as a high-fidelity benchmark for naval architects aiming to refine the propulsive efficiency and noise signatures of high-speed craft. It ultimately emphasizes that for extreme operational conditions, the choice of mathematical model and boundary treatment is as decisive as the grid resolution in achieving predictive accuracy.

The study by Fraguela et al. presents a novel, data-driven framework for optimizing the operational efficiency of tugboats equipped with Voith–Schneider Propellers (VSPs). Unlike conventional screw propellers, VSP systems allow for independent control of both rotational speed (RPM) and blade pitch, providing 360-degree thrust vectoring but significantly increasing the complexity of energy management. To address the scarcity of full-scale empirical data in this field, the authors conducted extensive sea trials in the estuary of Ferrol, utilizing synchronized measurements of vessel speed, fuel consumption, and propulsion settings. The research introduces a methodology for generating interpolated fuel-consumption and efficiency maps using cubic interpolation and visual analytics, which identifies optimal operating “islands” segmented by speed ranges. A key contribution is the development of a relative efficiency indicator that allows masters to select the most energy-efficient combination of pitch and RPM for a required transit speed, bypassing the need for complex internal engine monitoring. By providing practical guidelines that could lead to significant fuel savings in port operations, this work serves as a foundational step toward the creation of Digital Twin architectures for harbor craft. It highlights that the novelty in modern propulsion research lies not only in hydrodynamic design but in the intelligent, data-led exploitation of existing high-maneuverability systems to meet the industry’s decarbonization targets.

The research presented by Feng et al. addresses the computational bottleneck of simulating submerged waterjet propulsion systems by proposing an improved body force model. While high-fidelity simulations using the sliding mesh method provide an accurate representation of rotor–stator interactions, they are computationally intensive for full-scale ship applications. To bridge this gap, the authors developed a modified body force method that replaces the physical rotor and stator geometries with virtual cylindrical disks where forces are uniformly distributed. A key progress of this study is the introduction of a velocity correction procedure: the authors used the average flow velocity at the duct inlet to refine the open-water characteristic curves of thrust coefficient versus advance ratio, ensuring that the body force inputs reflect the actual internal ducting flow rather than an idealized external velocity. Validated against Experimental Fluid Dynamics (EFD) and sliding mesh results, the improved model demonstrated remarkable precision, with thrust coefficient errors as low as −0.61%. Furthermore, the study investigated the sensitivity of the inflow velocity plane position, concluding that any plane between the duct inlet and the propeller disk can be used for correction without compromising flow-field accuracy. This simplified numerical approach offers a robust balance between accuracy and efficiency, providing naval architects with a high-speed tool for analyzing the self-propulsion performance and hull–propulsion interaction of waterjet-equipped vessels.

The study by Avanzi et al. presents a comprehensive investigation into the design principles for optimal nacelle shaping in Outboard Dynamic-inlet Waterjets (ODWs). Recognizing the critical role of the intake geometry in delivering clean flow to propulsive pumps, the authors performed a multi-point design exploration using two-phase RANS simulations. A relevant feature of this work is the simultaneous assessment of performance at both cruise and take-off conditions, the latter specifically accounting for the potential onset of internal lip cavitation. By utilizing a B-spline parameterization to manipulate the nacelle’s wall profiles, the research identifies key correlations between geometric features—such as the highlight-to-throat ratio—and propulsive metrics extracted via Thrust–Drag Bookkeeping (TDB). The findings emphasize that the Inlet Velocity Ratio (IVR) serves as a deterministic criterion for identifying operating regimes, showing how pre-diffusion or convergence in the capture streamtube directly affects the cavitation margin and internal efficiency. This multi-fidelity approach establishes ground rules for the geometric specification of submerged nacelles to ensure that next-generation propulsive pumps can operate efficiently across a wide range of vessel speeds. It bridges the gap between aero-engine research techniques and marine engineering, offering a sophisticated toolset for the optimization of outboard propulsion units.

The study by Wang et al. investigates the complex hydrodynamic interactions within a two-propeller configuration, offering design guidelines for multi-propeller propulsion systems. Utilizing an in-house viscous CFD code, the authors performed URANS simulations with a structured overset grid approach to analyze 45 different spatial arrangements of two KP505 propellers. The originality of this research lies in the systematic mapping of interference effects across a wide range of transverse and longitudinal spacings between the propellers. The results reveal that while the front propeller remains largely unaffected when spacing exceeds one diameter, the rear propeller suffers significant performance loss due to the high-velocity wake induced by the front unit. This interaction reduces its effective angle of attack, leading to substantial decreases in thrust and torque. Interestingly, the study finds that longitudinal spacing has a negligible effect on the rear propeller due to the minimal decay of wake velocity, whereas transverse spacing is the dominant factor. The authors conclude that to achieve optimal overall propulsive efficiency, surpassing that of a single propeller, the transverse spacing must be maintained at no less than one diameter. This comprehensive dataset provides a valuable benchmark for the arrangement of twin-screw vessels and the development of energy-efficient multi-propulsor architectures in modern naval architecture.

Chen et al. addressed a critical challenge in the adoption of hydrogen-powered marine vessels: the thermal–hydraulic management of Proton Exchange Membrane Fuel Cells (PEMFCs). Efficient cooling is vital for maintaining fuel cell longevity and performance, yet conventional cooling-plate designs often suffer from high pressure drops or uneven temperature distributions. The authors investigated six different cooling channel configurations (Types A to F) using COMSOL Multiphysics simulations to optimize heat dissipation. The research introduces secondary flow channels designed to enhance fluid mixing and reduce thermal gradients. Through a rigorous orthogonal experimental design, the researchers identified that channel width is the dominant factor influencing both temperature uniformity and pressure drop. The optimized Type B configuration, featuring these secondary channels, demonstrated a significant improvement in the cooling efficiency coefficient, balancing the trade-off between parasitic pumping power and heat removal capacity. By providing a detailed sensitivity analysis of channel angles and spacing, this work offers a robust engineering blueprint for the thermal design of high-power fuel cell stacks. It underscores that the transition to green propulsion requires not only new propulsors but also the optimization of the auxiliary systems that sustain zero-emission energy converters in the demanding marine environment.

The final contribution by Lee et al. introduces a transformative approach to marine propulsion by integrating the Coanda effect into propeller design to enhance self-propulsion efficiency. Using a 6.5K DWT tanker as the baseline model, the authors conducted a series of numerical simulations using URANS equations and the SST k–ω turbulence model to compare three configurations: an original propeller, a standard propeller, and a specialized Coanda propeller. In the latter, the Coanda effect was used to generate additional lift and thrust even at significantly lower rotational speeds. The study’s results are compelling, demonstrating that the Coanda-based system achieved a reduction in delivered power of approximately 7.8% compared to traditional propulsion setups. This improvement in the self-propulsion factor not only suggests a path toward more fuel-efficient vessel operations but also offers a viable alternative for noise and vibration reduction in sensitive maritime environments. By providing a rigorous numerical baseline for this technology in a ship-scale context, the authors pave the way for a new generation of high-efficiency, bio-inspired, or circulation-control propulsors that challenge the conventional limits of screw propeller theory.