Propellers are often the sole form of ship propulsion, making their design uniquely important in the vessel's operation. Despite many advantages, they suffer from issues like cavitation, vibrations and underwater radiated noise. Flexible composite materials have been proposed as a way to address these issues and increase the overall efficiency envelope, achieved by a passive pitch distribution reduction in the vessel's wake. Due to an increase in variables, optimization is increasingly dependent on the accuracy and validity of numeric simulations.

The objective of this report is to study experimental deformations of flexible propellersin terms of their underlying parametric properties. These are compared to simulations using the unsteady Reynolds averaged Navier-Stokes equations for fluid calculations, coupled with a finite element model for the structural calculations.

A custom preprocessor was developed and used to reconstruct the full parametric blade geometry from digital image correlation measurements. The first 10 mode shapes of the undeformed mesh were found using modal decomposition, which was fed as input to a flexible point cloud registration. The participation factors for each mode shape were optimized to minimize an error function, the symmetric chamfer distance. The deformed mesh was processed with a software library, PropArt, to recover the underlying geometry parameters.

It was found that the hydrodynamic pitch β was the best predictor of rake, skew, and camber deformations, with increased inertial forces increasing the magnitude of the deformation. Pitch deformations are largely predicted by the blades' inertial loading, and is mostly independent of β. The blade thickness and chord length deformations were negligible. Simulations tended to over predict the deformations of the propeller, especially at low β, being exacerbated by increased inertial loading. At high β, the deformations were small and well predicted.

The relation between pitch deformation and inertial loading suggests an optimal material stiffness per operating condition exists, to be tuned for an appropriate amount of pitch reduction. The lack of correlation to β also poses a challenge in the design as the wake peak is where the change in pitch is desired. The increase in camber also leads to an increase in thrust at low β, which is contrary to the desired wake peak thrust reduction. These relations should be investigated further, for a broader range of material stiffnesses to see if this holds. Finally, the difference between experimental results and simulations suggests a systematic error due to the error scaling with the inertial forces, which should be further investigated.
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Composite materials can be used to fabricate flexible marine propellers, which can improve efficiency and reduce underwater radiated noise. Since the hydrodynamic performance of flexible propellers is determined by their deformation under fluid loading, similarity laws for flexible propeller scaling should account for the deformation in model experiments. This study introduces a non-dimensional parameter to characterise the deformation of flexible propellers and evaluates it through time-domain fluid–structure interaction simulations. A coupled solver combines unsteady Reynolds-Averaged Navier–Stokes equations with a finite-element structural solver. The study focuses on the Wageningen C4-40 propeller geometry under uniform inflow and is limited to isotropic materials.

A set of non-dimensional relations is derived through dimensional analysis, with a form of the Cauchy number expressing deformation amplitude. Validation using a Reynolds–Cauchy similarity approach on two geometrically similar propellers of different diameters confirms consistent deformation and less than 5\% difference in thrust and torque coefficients between model and full-scale propeller. The disparities in performance results are attributed to numerical artefacts in the fluid solver, as the results indicate that the $k-\omega$ SST turbulence model is sensitive to near-wall resolution.

Achieving full-scale Reynolds numbers in propeller test facilities is not feasible, yet simulations demonstrate that flexible propellers are sensitive to viscous forces. The study observes disparities in deformation extent across Reynolds numbers. The deformation of flexible propellers improves flow attachment over the blades. The overall Reynolds-number trends remain similar to the rigid results: thrust coefficients increase and torque coefficients decrease as Reynolds numbers increase. The open-water efficiency depends on both coefficients, and larger Reynolds numbers result in higher efficiencies.

The Froude–Cauchy scaling approach proves suitable for model experiments; however, material availability limits practical implementation. This study indicates that the steady-state deformation is primarily governed by stiffness, with negligible impact from the structural-to-fluid density ratio. In contrast, in unsteady conditions, the structural-to-fluid density ratio affects the modal frequencies, which describe the dynamic behaviour of propeller blades. Particularly, propellers with high skew, rake, or with anisotropic material properties are affected by structural-to-fluid density. Furthermore, this analysis observes that the first blade mode of a zero-skew angle propeller is pure bending, and a 30\% variation in structural density does not alter the natural frequency of this blade. However, coupled bend-twist modes are sensitive to the structural density, which therefore affects blade deformation in unsteady conditions.

In conclusion, the extent of propeller deformation can be controlled by a non-dimensional parameter expressing the ratio of elastic to hydrodynamic forces. For steady open-water conditions, deformation is additionally a function of Reynolds number. In unsteady conditions, the structural-to-fluid density ratio becomes relevant, as it alters coupled bend-twist modal frequencies. Thus, the deformation extent in this regime is, in addition to the Reynolds number, a function of fluid damping and the ratio of natural frequency to revolution rate. This study offers a basis for accurate scaling of flexible propellers in both experimental and computational studies. ... Read more

This study aims to characterize the aerodynamics of rotor sails with the focus on the effects of end plate configuration. This is achieved through an experimental campaign on the wake development behind a scaled rotor sail (1:100) with varying end plate sizes in the SLT wind tunnel at TU Delft. Measurements were conducted for Re=15000, spin ratio up to 3, using a rotor of aspect ratio AR=5.2 and end plate diameters of De/D=1.0, 1.6 and 3.0. To omit heavy vibrations, a modal analysis was performed to determine the eigen frequency of the model.

Wake measurements were obtained using both a pressure rake and a 7-hole probe, making this the first experimental study to report results on all three velocity components in the wake of a scaled rotor sail. Three main wake characteristics were analyzed: 1) flow direction of the wake, 2) vortical topology of the wake flow and 3) shedding of the flow.

Results demonstrate that the end plate configuration has a large influence on the development and structure of tip vortices near the rotor’s free ends. A key limitation of this research was the increase in wake blockage for higher spin ratios affecting the pressure measurements. These findings provide new insights into the role of end plate geometry in rotor sail performance and wake dynamics. ... Read more

The objective of this thesis is to explore the possibilities of utilising machine learning to predict the axial velocities in a nominal wake field of a single-propeller vessel, given a database with only basic parameters describing hull geometries, such as length, beam, and block coefficient. The nominal wake field is of primary importance for the propeller design process, as it represents the influence of the hull on the flow field that a propeller encounters. Furthermore, propeller-hull interactions can be derived from it.

Five different machine learning models have been developed, trained and evaluated. Visual inspection of predictions made by all five models revealed that none of the models has successfully captured the underlying physical phenomena that drive the wake field: all predictions show highly generalised wake fields. This finding was supported by a feature
importance study, which showed that the features that supposedly contribute the most to understanding the hydrodynamic phenomena that drive the wake field do not have the highest relative importance.

The limited performance of the developed models is primarily attributed to two factors: the limited dataset size and a lack of feature informativeness, meaning that the used features contain insufficient information about the problem to let a model effectively capture the underlying physical phenomena. To a lesser extent, computational limitations also play a role. ... Read more

Cavitation is one of the main reasons for various undesirable effects in marine propulsion applications, such as reduction in efficiency, increased underwater radiated noise, and in extreme cases, blade erosion. These phenomena have traditionally been studied in controlled test environments at Model Scale. However, Computational Fluid Dynamics (CFD) is now increasingly utilized to visualize and estimate the physical behavior of fluid flows, particularly in the marine industry to analyze flow around propellers and predict their performance characteristics.
Despite advances in CFD for propeller performance prediction, cavitation behavior prediction still mainly relies on experimental methods conducted in model scale, which are then scaled up to full scale using similarity approaches. Even if conducted through CFD, these simulations require relatively complex settings involving multiple physics models and significantly higher mesh density. While standard mesh settings and computational models suffice for open water simulations, cavitation inception prediction demands more sophisticated configurations.
Physically, cavitation occurs when local fluid pressure drops below the vapor pressure at the operating temperature. In absolute pressure terms, this value is always positive. However, when using Incompressible RANS models, in Wetted Flow conditions for both model and full-scale propeller simulations, numerical uncertainties can produce physically impossible negative absolute pressure cells. These anomalies significantly compromise the accuracy of cavitation inception predictions when compared to model test results.
The issue is particularly problematic because cavitation inception criteria in CFD are typically triggered when pressure drops below vapor pressure. Even a single cell with an erroneously low-pressure value can prematurely indicate cavitation onset, leading to inaccurate predictions. This creates a critical need to identify and filter out these numerical anomalies.
This thesis focuses on identifying the causes of these negative pressure cells, developing an alternative approach to handle these anomalies, and implementing effective filtering methods for contaminated cells. The research proposes unsupervised machine learning models in a hybrid setup that combines statistical techniques with clustering algorithms to process simulation results in behind-hull condition. The obtained improvements aim to enhance the accuracy of cavitation inception prediction in full-scale propeller simulations.
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The shipping industry, responsible for over 80% of global trade, contributes significantly to climate change. Ocean-going ships emit over 3% of global carbon dioxide emissions. Air lubrication techniques offer a promising solution to lower a ship’s frictional drag, fuel consumption, and ecological footprint. However, the field lacks a complete understanding of the relevant two-phase flow physics involved with air lubrication techniques, hampering the application of these techniques in the shipping industry. The next step towards applying these techniques could be a numerical model designed to model the flow around air-lubricated ships. It would enable ship designers and operators to accurately predict the total propulsive power requirements and operability profile in the early design stage of these ships.
However, despite the many computational fluid dynamics (CFD) models available for predicting the multiphase flows around ships and, e.g. cavitating propellers, there currently is a need for an available model that can accurately predict the flow around an air-lubricated ship. This thesis first identifies air cavity techniques as most promising for application in the shipping industry. It then analyses air cavity flows through a literature review, dedicated experiments and multiphase Computational Fluid Dynamics (CFD) simulations. It explores different available numerical models and aims to give directions for developing a novel model providing insight into these flows.
The air loss from the air cavities determines the techniques’ success in total propulsive energy gains. This thesis identifies four air loss mechanisms: re-entrant jet, wave pinch-off, ligament stretching and turbulent separation-zone shedding. Then, it is shown that the current industry-standard Reynolds-Averaged Navier-Stokes (RANS) solvers are not yet capable of modelling the air loss mechanisms and, thus, incapable of modelling air cavity flows around ships.
RANS simulations over-predict the eddy-viscosity around liquid-gas interfaces and hinder a physically accurate simulation of the ligament stretching and turbulent separation-zone shedding mechanisms, which are shear-induced. The RANS simulations also do not show the re-entrant jet shedding behaviour, which is pressure-driven. For the modelling of the re-entrant jet, the additional eddy viscosity impedes the formation of the re-entrant jet due to the stiffened interface.
We also know in advance that RANS solvers are insufficient for simulating the wave pinch-off mechanism. Wave pinch-off is induced by the time-dependent velocity fluctuations of the turbulent flow in the boundary layer upstream of the air cavity and RANS solves these only in an ensemble-averaged manner… ... Read more

The ongoing pursuit of efficiency in the shipping industry is driven by both economic and environmental objectives. While historically focused on reducing fuel consumption to enhance market competitiveness, recent regulations such as the International Maritime Organisation’s Energy Efficiency Design Index (EEDI) and Energy Efficiency Existing Index (EEXI) have emphasized the need to minimize harmful emissions. To achieve these goals, Simulation-Based Design (SBD) and Computational Fluid Dynamics (CFD) have become essential tools for optimizing ship design. This research aims to develop and implement an optimization strategy to minimize the propeller power required for a vessel to maintain a constant speed by refining the aftbody shape while accounting for propeller-hull interaction effects.

The study addresses a critical gap in current methodologies, where many existing optimization strategies neglect propeller effects and studies including propeller effects use too computationally intensive methods to be used in an optimisation strategy. The research explores strategies to reduce computational load, ensuring a balance between accuracy and efficiency. Key questions include determining the optimal geometrical parameters for aftbody design, refining CFD procedures to be computationally light yet accurate, incorporating propeller effects efficiently, and identifying the most effective optimization algorithm.

The research findings reveal that by varying specific geometrical parameters, such as the aft arc angle and transom angle, an approximately 10\% reduction in required propeller power can be achieved. The study also evaluates different strategies for reducing computational load, including a grid refinement study and the exclusion of free surface effects, while noting the trade-offs in accuracy. The use of the virtual disk method to model propeller effects is identified as the most practical approach given computational constraints. Among the optimization algorithms tested, the NSGA-III algorithm is found to be the most effective, offering significant improvements with fewer computational resources compared to alternatives.

Overall, the research demonstrates that the proposed optimization setup can lead to a significant reduction in propeller power, contributing to the development of more efficient ship designs. However, further research is needed to refine the exclusion and estimation of free surface effects to ensure broader applicability across different vessel designs. ... Read more

Shipping is one of the most cost-effective and environmentally sustainable modes of transportation. Given that approximately 60% of a typical ship’s propulsive power is used to overcome frictional drag, implementing practices to reduce this resistance stands to yield substantial economic and environmental benefits, (Larsson & Raven, 2010). A promising drag reduction technique for a ship is air lubrication. Damen Shipyards Group is currently making this technology commercially available as the Damen Air Cavity System (DACS). The system reduces the frictional resistance of a ship by creating stable air cavities on the bottom hull of a ship. The air cavities cause a reduction in friction drag by decreasing the wetted area of a ship’s bottom. During the development of the DACS system, it was observed that air cavities also change the inflow into the propeller. Both the changed inflow and the frictional drag reduction affect the propulsive efficiency and required propulsive power of the vessel. This thesis aims to provide a better understanding of how the propulsive performance of a ship is affected by air cavities. Additionally, a key application for air lubrication systems is on inland waterway vessels, which frequently operate in shallow waters. However, the impact of shallow water conditions on the performance of the air cavity system is currently unknown. The research goals of this thesis are investigated with the help of computational fluid dynamics (CFD). A literature review identified the most feasible method to model a ship with air cavities i.e. representing the cavities as surfaces with a slip boundary condition. The influence of the air cavities on propulsive performance is investigated by studying the change in nominal wake field, thrust deduction, and propeller efficiency. Three ships were investigated: a cargo ship, a cruise ship, and an inland ship. The CFD results of the cargo ship were compared to sea trial data. Model test data was available for comparison of the cruise ship results. First, a grid convergence study and a sensitivity analysis were performed to investigate the accuracy of the numerical simulations and the power predictions. The biggest source of numerical uncertainty arose from the pressure drag. From the sensitivity analysis, it was found that the uncertainty of the power prediction is mainly affected by the uncertainty of the thrust prediction, followed by the uncertain propeller geometry for the cargo ship. For the cargo ship, it was found that the air cavities cause a significant frictional drag reduction. It was also found that the air cavities caused a decrease in pressure drag because they decreased flow separation at the stern. Furthermore, a strong decrease of the nominal wake fraction was observed for this ship, because the air cavities change the boundary layer on the bottom of the ship. A change in propeller efficiency was also observed because the propeller working point changes. The magnitude of the change was larger than for the other ships because this ship has a controllable pitch propeller running at a fixed rpm. When comparing sea trial data to the CFD results, it was found that CFD underpredicts the power, especially at higher speeds. Next to this, CFD predicted a larger reduction in power than measured during the trials. The prediction of the cavity length was identified as the most likely cause for the difference. A good comparison between model tests and CFD results was found for the cruise ship. It was found that the drag reduction and change in propulsive performance could be predicted reasonably accurately by modeling the air cavities as surfaces with a slip boundary condition. Furthermore, it was observed that the air cavities caused little change in propulsive efficiency on this ship. This is because the propellers of the cruise ship are located further away from the boundary layer of the ship and the wake field is therefore only slightly affected by the air cavities. On the inland ship, it was observed that the pressure drag and flow separation at the stern were influenced due to the air cavities. However, no comprehensive conclusions could be made due to scatter in the data. This is most likely due to the uncertainty present when modeling flow separation. Furthermore also for this ship, a decrease in propulsive efficiency was found because the air cavities decreased the wake fraction of the ship. Additionally, CFD simulations were conducted for the inland ship at varying water depths to assess how shallow water conditions impact the performance of the air cavity system. Since the ship’s frictional drag increased in shallow water, the total drag reduction from the air cavities also increased. Next to this, a change between pressure and flow separation when comparing air on and air off was observed. Also here, no strong conclusion could be made due to scatter in the data. Lastly, it was found that the change in wake field caused by the air cavities is larger in shallow water than in deep water. ii Based on the results of the three ships studied it can be concluded that an air cavity system affects the propulsive performance of a ship. It was found that an air cavity system reduces the wake fraction of a ship because it limits the growth of the boundary layer on the bottom of a ship. The reduction increases for ships with a high block coefficient and a large air-covered area. For a twin screw ship, the change of the wake field is less significant. The change in propeller efficiency depends on the resistance reduction, possible change of the wake field, and the original working point of the propeller. Furthermore, it was found that the thrust deduction effect is not affected by the air cavity system. It can also be concluded from the results of the cruise ship that the flow around a ship with air cavities can modeled reasonably accurately provided that the shape of the air layer under the ship is known. More research is recommended on how air cavities change the flow separation and pressure drag of a ship. ... Read more

Drag reduction in ships is a key approach to decreasing their energy consumption. Besides saving fuel costs, it also decreases greenhouse gas emissions. One of the most promising techniques for drag reduction in ships is air lubrication, which reduces drag due to friction. Despite its great potential, application of air lubrication in the maritime industry is limited, owing to a lack of understanding of its underlying physical phenomena and stability issues.

The present work aims to experimentally study drag reduction on a flat plate resulting from air lubrication at a representative free-stream velocity of 2.5 m/s over a set of increasing air injection rates covering all the three air layer regimes (BDR, TALDR, ALDR) and assess its dependence on air morphology in terms of the plate’s non-wetted area. The experiments were conducted at the Multi-Phase Flow Tunnel located at the Ship Hydromechanics group of TU Delft.

To achieve this, first, a custom force balance comprising a spring system was designed to facilitate the measurement of the total drag acting on the plate by means of a load cell. Drag measurements in single-phase flow were conducted on a conventional flat plate to obtain a reference dataset and validate the drag measurement system. An uncertainty analysis was performed to quantify the measurement accuracy. Then, single-phase and dual-phase experiments were conducted on another plate of similar dimensions fitted with additional parts necessary for the generation of air lubrication. For the quantification of the plate’s non-wetted area in dual phase flow, these experiments were accompanied by image capture with cameras positioned vertically below the plate. The recorded images were processed using a binary approach to distinguish regions under the plate covered with water and air. Finally, the non-wetted area ratio of the plate was computed based on these processed images. Uncertainty in both drag reduction and the non-wetted area ratio was also determined before
analysing the results.

Results from the present work indicate a positive linear correlation between drag reduction and the associated non-wetted area ratio achieved in BDR and ALDR. However, this correlation varies in slope per air layer regime, possibly due to the physical phenomena governing the regime. Thus, further investigation into the underlying physical phenomena governing each regime could shed light on the reason(s) for different slopes in the
correlation. To improve the current work, estimation of the non-wetted area ratio in the TALDR regime may be carried out to increase the resolution of the correlation found. Moreover, the non-wetted area resulting from the thickness of the air layer may also be studied to define the non-wetted area ratio of the plate more accurately. ... Read more

In the face of escalating global emissions and rising awareness of the dangers of climate change, there is a need for sustainability initiatives across all industry sectors. Within the shipping industry, this is leading to a resurgence of interest in wind-assisted propulsion systems. This research investigates the hydrodynamic design of a wind-assisted cruise ship, aiming to redefine the image of the historically very polluting cruise industry by designing a vessel aimed at achieving 50% propulsive fuel savings through wind-assisted propulsion. The literature review provides the broad theoretical and computational background necessary for the design evaluation. The description of the historical evolution of wind propulsion, as well as state-of-the- art wind-assisted vessels and emerging concepts, provides the inspiration for the design investigation. A detailed hydrodynamic analysis was conducted, focusing on variations in hull design (B/T ratio, dead- rise angle) and appendage design (skegs, passive and active anti-drift fins). The efficacy of the mod- ifications was evaluated on an operational level using power prediction programs, allowing for perfor- mance assessments in varying conditions. A semi-empirical approach for the prediction of hydrodynamic forces and moments under drift using the maneuvering tool SURSIM is hypothesized as a computationally light alternative to numerical methods. However, SURSIM was found not to have the required fidelity to accurately predict the lift coefficient of the hull at small drift angles. Therefore, a computational fluid dynamics (CFD) approach was adopted for the determination of the hydrodynamic loads. The optimal unappended cruise ship design resulted in fuel savings of 34.5%. Optimal hydrodynamic efficiency favors a hull optimized for minimal resistance, and a retractable, active angle-of-attack fin for improved side force and yaw moment balance. The introduction of the fin led to an increase in fuel savings to 37.3%. Notable improvements in ship maneuverability can also be expected based on significantly reduced rudder actuation in sailing conditions. Operational variations have a large impact on fuel savings. Using the wind statistics of the N. Atlantic Holland America Line increased fuel savings to 48%, compared to the MEPC.1/Circ.896 standard. Con- siderable further gains can be achieved through the efficient handling of surplus wind power. Allowing a variable operational speed between 10.6kn and 20kn instead of a fixed speed of 12kn increased the fuel savings on the N. Atlantic route to 56.7%. Wind-assisted propulsion can be a viable strategy for significant fuel savings beyond 50%, and efficient appendage design can meaningfully improve the ship’s performance. Accurate prediction of fuel sav- ings requires clear knowledge of the operational conditions and control strategies. Further investigation of the detailed appendage design, and comparison of variable speed operation to the employment of a regenerative propeller mode is recommended. This thesis contributes to the evolving discourse on sustainable maritime transport, specifically within the cruise ship sector, by providing a comprehensive analysis of wind-assisted propulsion’s potential benefits. ... Read more

The increased focus on the reduction of emissions in the maritime sector has been the driving force behind many technological advancements related to marine engineering. Among those is the invention of the gate rudder system, which, while still at an early stage of development, promises improvements in propulsion efficiency and ship manoeuvrability. This thesis investigates the concept from the perspective of time domain simulation in order to pave the way towards identification of the working principle of the gate rudder power savings.

First, an extensive literature review is presented. A number of subjects related to the project was studied, starting from the principles of ship manoeuvrability, which comprises of ship hull hydrodynamics, hydrodynamics of control surfaces and regulations on manoeuvrability of ships. Afterwards, the current state of research into the gate rudder concept is investigated. Then, techniques of modelling of ship motion are laid out, with a focus on mathematical modelling of ship motion and rudders. Machine learning is identified as a potentially useful tool in the research, therefore a dedicated section consists of a brief introduction of the concept, some example regression algorithms and practical techniques utilized in conjunction with their use. Finally, the concept of time domain simulation in the context of maritime engineering is investigated and the options of simulator software available for the project are reviewed.

Initial decisions made for the execution phase of the project are described, including the choice of Manwav as the preferred time domain simulation software and S175 container ship as the benchmark vessel to carry out model tests on. An effort to tune the ship's model in Manwav to better match its performance found in literature is also described at the end of the chapter.

A physics-based gate rudder model is adapted into Manwav from a scientific paper. It is based on a mathematical model of a rudder derived from the MMG methodology, and the goal of its implementation was to pave the way in terms of code structure in Manwav and to serve as a reference point to the focus of the project, which is the data-driven model.

The data-driven gate rudder model uses a machine learning regression algorithm to build a set of rudder force coefficient predictors using a CFD dataset containing the values of force coefficients within a range of rudder and drift angles of a vessel equipped with a gate rudder system. A well-behaving model is successfully built using support vector regression and deployed in Manwav.

Both gate rudder models are put to test in the case study, where three ships with different rudder models are seen performing a set of benchmark manoeuvres, comprising of turning circles and zig-zag manoeuvres at different vessel speeds and rudder deflection settings. While the physics based model was found to be generating abnormal vessel behaviour, the data-driven model performed as expected, providing some insight into a potential impact a gate rudder system might have on a real ship's manoeuvrability and power consumption.

The thesis is concluded with a discussion on the data driven model's applications, along with limitations that prevent it from reaching its true potential. What follows is an extensive list of recommendations that go into detail on how the project could be improved in the future. The overall conclusion from the project is that a relatively limited amount of CFD data was successfully used to produce a rudder model that provides simulation possibilities otherwise beyond the reach of high fidelity methods due to scenario complexity and computational cost associated with them. With not a lot of additional effort, the model will make it possible to test whether or not gate rudder power savings reported in literature come from sailing in realistic weather conditions. ... Read more

The development of biofouling on ship hulls is known to be deeply damaging for the overall performance of the vessel. The real effects of soft fouling, especially due to biofilms and diatoms, are however not yet understood and precisely defined. The current work numerically assesses the influence of an increase in surface roughness due to soft biofouling over the hydrodynamic performance of a containership. The simulations are carried out through MARIN’s in-house software ReFRESCO. Unsteady free-surface computations are used to evaluate both the resulting frictional drag, and the wave profile modification for different input surface roughness values. The results show that frictional resistance at 14 knots is increased of up to 45% for a limit equivalent sand-grain roughness height ks = 300 μm, correspondent to biofilms. Residuary resistance at the same speed suffers decreases of maximum 2.5% in the same range. These consist in hull pressure variations in the aft and transom areas of the vessel, and in small height differences in the wave pattern. A dataset obtained from monitoring of the containership for five years is used to study the added frictional resistance evolution due to roughness. Holtrop & Mennen
procedure integrated with environmental variables show a maximum increase of ΔCF = 1.4 · 10−3. The methodology proves useful to identify potential drydocking periods, and to provide an overview
of the added resistance trend. It should however be implemented with Machine Learning models to improve accuracy, in case the precise influence needs to be assessed. A novel comprehensive soft
fouling growth model is created to assess the surface roughness ks and ΔCF time evolution. This constitutes one of the few attempts in the literature to create a model that simultaneously considers the
effect of multiple environmental and operational variables based on the GPS position. Light intensity, sea surface temperature, draft and concentration of nutrients. Increases of up to 100 kN in resistance
and of ΔCF = 0.001 are observed if only diatoms are considered are integrated. Temperature and nutrients result to be more relevant for growth, in comparison to light intensity and draft. A sensitivity
study is also carried out to prove the small dependence on the choice of the analytical approach. A comparison of the growth model + CFD results with data processing show that the analytical procedure
provides a good indication on the growth trend, while added resistance intensity is underestimated in the order of ΔCF = 4.65 · 10−4. ... Read more

Engineering has become more and more optimisation and simulation based. This causes an increase in the use of optimisation methods for complex problems. This thesis focuses on the implementation of a surrogate-assisted optimisation method for propeller optimisation, to design a better propeller in a shorter timespan. Therewith reducing both: computational expenses, and fuel consumption when a propeller has been built. This thesis tries to reach two objectives: The first goal is to implement a surrogate-assisted method for propeller optimisation. The other objective is to design a propeller for both propulsive as well as for regenerative operations. To succeed, this thesis has been limited to the testing and coupling of SAMO-COBRA (A Fast Surrogate Assisted Constrained Multi-objective Optimization Algorithm) with PropArt. The testing has been based on benchmark tests where SAMO-COBRA was tested against CMOPSO and NSGA-II (the two algorithms that MARIN currently uses for propeller optimisation). Here, SAMO-COBRA outperformed NSGA-II and CMOPSO on over half of the test cases and is therefore considered for the rest of the research.

The thesis evaluates propellers using a Boundary Element Momentum theory, which is a mathematical method that is the basis of MARINs in-house propeller tool PROCAL. To be able to model propellers that operate at high J values correctly certain improvements regarding the wake expansion and alignment have been made. For the wake expansion three methods are proposed. After comparing the open water diagram of a F4-63-0.6 propeller, that is evaluated by PROCAL, and the one that was made after physical testing. It can be concluded that the disk theory is the most versatile implementation for propellers that operate at a range of advance ratio. Results show that from the two proposed methods, the method where the wake pitch is prescribed by the advance ratio results in the best wake alignment.

The test case optimises 3 configurations, two separate designs, one for propulsion and one for regeneration. A normal CPP that is loaded under a negative angle of attack for regeneration and a propeller that can be fully reversed for regenerative operations (the propeller is loaded from the trailing edge in regeneration). The propeller is optimised to reach a maximum regenerative power whilst minimising the propulsive power. The optimum propeller has to satisfy multiple constraints based on cavitational-, geometrical- and separation of flow constraints. The optimisation using SAMO-COBRA did not yield feasible results for any of the three cases. After removing the cavitational constraints a new analysis of the results has been done, it is found that a propeller that is rotated 180 degrees can provide the highest regenerative power. This thesis proposes multiple hypotheses that cause the lack of feasible results of the test case. The proposed causes are: errors in the PropArt model, problems in the compiled PropArt version or a too low convergence level of COBYLA.
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In this thesis, the influence of a combined drift angle, heeling angle and forward speed on the stability of a tug is researched. This is done to gain insight in the stability of tugs during escorting, where they operate at high drift angles, heeling angles and forward speed. The Damen RSD 2513 tug was simulated in steady CFD simulation using the software FINE/Marine. The simulations were performed for 0, 15, 30 and 45 degrees drift angle, 0 to 10 degrees heeling angle for every 2 degrees of heel, and at 8 knots forward speed. Curves of the righting moment and heeling moment were created with the results of these simulations. These curves were analysed to see the effect of the drift angle, heeling angle and forward speed. Furthermore, a comparison was made between the restoring moment of the RSD 2513 calculated by SARC PIAS.

The following conclusions were drawn from the righting moment:
1. Without a drift angle, forward speed doesn’t influence the righting moment significantly. This results a minimal difference between the restoring and the righting moment of the tug.
2. Introducing a drift angle will result in a decrease of the righting moment.
3. Even with a drift angle, increasing the heeling angle will result in an increasing righting moment. In fact, the heeling angle and drift angle can be assumed to have independent effects on the righting moment.

Furthermore, two conclusions were drawn from the heeling moment:
1. The heeling moment increases when a drift angle is introduced.
2. Increasing the heeling angle will result in a slight increase in the heeling moment.

There are however concerns about the result’s numerical uncertainties and convergence. Firstly, the error estimation in the verification exercise is unreliable and therefore has a large safety factor leading to high uncertainties. Secondly, issues with convergence were experienced for higher drift angles. This was caused by large separation areas with high turbulent kinetic energy. Because of these two things, the results have limited reliability and the conclusions should be viewed with scepticism. ... Read more

Tip vortex cavitation around the marine propeller is the primary source of the underwater noises emitted by ships. Investigating the tip vortex inception is important to understand the aforementioned side effect. The water quality, highly related to the bubbles population, surrounding the tip vortex is crucial to the tip vortex cavitation inception. This research explores how bubble moves around the tip vortex and investigates its background mechanisms. A one-way coupled Lagrangian bubble tracking model is applied to the Lamb-Oseen vortex and the hydrofoil tip vortex flow field, where the tip vortex is simulated with an open-usage CFD software, ReFRESCO. A series of capture time models have been developed by exploiting the bubble force (acceleration) balance in the Lamb-Oseen vortex to identify the crucial parameters. The other aspect of the acceleration analysis reflects the role of the lift force as the system's stabilizer, which has long been ignored. In the CFD tip vortex flow simulation, the bubble population evolution is investigated from upstream to downstream. Several kinds of bubble behaviors around the tip vortex flow have been identified via the numerical simulation, and simplified models based on the bubble force analysis have been proposed in order to explain them. Mainly due to the stagnation bubble events, clustering of large bubbles has been discovered a short distance downstream of the foil tip, implying that this may be a possible hotspot for tip vortex cavitation inception. The stagnation bubble phenomenon still needs to be further investigated, particularly in terms of quantifying its dependence on various flow parameters. The results also need to be rigorously validated which has not been possible as of yet due to insufficient measurement data. Still, this research manages to explore the bubble motion around the tip vortex and their fundamental mechanism. ... Read more

Propellers are used to propel the vast majority of ships. They are most commonly made out of Nickel-Aluminum-Bronze alloys. This is due to their superior characteristics over other metals in terms of strength and corrosion resistance. Metal propellers are assumed to be rigid and undeformable. Current research opts to construct propellers out of more flexible materials such as composites. The potential of this material lies in the theory that this results in propellers which could unload themselves in the wake peak, which should lead to significant improvements in the cavitation- and noise behaviour of the propeller.
Since the material is much less stiff, the deformation can no longer be neglected and must be computed simultaneously with the hydrodynamic solution. This is done in a branch of fluid dynamics known as Fluid-Structure Interaction (FSI). Maritime Research Institute Netherlands (MARIN) is involved in the development of a program known as the ComPropApp.

The first goal of this thesis is to validate the unsteady FSI module of the ComPropApp which uses a coupled BEM-FEM code to calculate the hydrodynamic and structural performance of marine propellers in non-uniform inflow conditions. The numerical simulations are validated by comparison with conducted experiments at MARIN. It shows that the average deformation of the experiment and simulation are in good agreement but the ComPropApp overestimates the wake peak deformation. This is most likely the effect of divergence issues which do not allow for sufficiently small step sizes. This step size also does not allow for analysis of vibrations happening within a revolution as much more points should be considered. The most important recommendation, therefore, is to improve the numerical stability of the application.

The second goal is the exploration of the design space of flexible propellers. By altering propeller geometry parameters it is investigated which parameters have the potential to design adequate flexible propellers in the future. To quickly assess the cavitation risk of the propellers, the amount of negative pressure coefficients on the blade is used to quantify the cavitation risk in the initial propeller design stage. Through this method, it is shown that suction side cavitation risk is decreased easily by altering pitch, skew, chord length and camber. Pressure side cavitation risk is however much harder to relieve, mostly because of the deformation of flexible propellers. Yet, cavitation risk can be decreased with a combination of skew and camber. It is also shown that unfavourable tip pressures (thus risk of tip vortices) can be further decreased by applying a positive tip rake.
All these propellers were simulated in open water (because of the above-mentioned divergence issues in the unsteady FSI module). Thus it is no conclusion that these propellers also perform adequately in non-uniform inflow conditions. It is shown with a demonstration that two of the three most potential propellers of the open water study do indeed unload themselves and maintain better cavitation behaviour at the velocity in the wake peak compared to a metal reference propeller.



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While the awareness of global warming rises, transport over sea remains a large contributor to green house gas emissions. New regulations imposed by the International Maritime Organization (IMO) aim to reduce the emission of green house gasses. Consequently, ship owners turn to technological solutions in order to achieve this. The Ventifoil from Econowind is a promising device in the field of Wind Assisted Ship Propulsion (WASP). By applying boundary layer suction, this airfoil shaped profile can generate large aerodynamic forces, which are
used as additional propulsion to reduce fuel consumption. Often multiple devices are installed on the deck of a vessel.

Literature shows that the aerodynamic interaction between multiple airfoils can significantly influence the local flow conditions in which each individual device operates. Nonetheless, literature agrees that it is possible to mitigate detrimental effects by adapting to the local conditions. The operational guideline of the Ventifoils are based on the far-field wind conditions. Consequently, aerodynamic interaction between Ventifoils is not considered. This leads to the main research question: "To what extend does aerodynamic interaction between two Ventifoils mutually affect aerodynamic performance?". This thesis aims to analyse aerodynamic interaction using a numerical Lifting Line Model (LLM). This method has the potential of evaluating a broad range of operational and environmental conditions in limited computational time.

Aerodynamic interaction between two Ventifoils will be evaluated over a range of apparent wind angles between 0◦ and 180◦, for two different absolute distances. The results fromthe LLM will be validated by means of three dimensional, steady Reynolds Averaged Navier Stokes (RANS) simulations. Five different interaction components will be analysed, being; changes in flow angle, changes in flow velocity, viscous interaction, pressure field interaction,
and boundary layer suction interaction.

The results reveal that aerodynamic interaction can reduce the lift and drag coefficients by multiple tens of percentages. It is found that the reduction ratio of the thrust force coefficient, CX, varies between −16% and −1% relative to a single isolated Ventifoil. The magnitude is mainly depending on the relative position of the devices. It is concluded that the interaction is most significant if the devices are positioned closely near each other and parallel to the flow
direction. Both methods show good qualitative agreement. It is concluded that discrepancies in the magnitude of reduction ratios can be attributed to modelling differences in terms of viscosity, pressure fields and boundary layer suction. Better quantitative agreement is found for larger absolute distances. The LLM is furthermore used to maximize CX by optimizing the angle of attack of each Ventifoil independently. The resulting increase in CX showed to be between +5% and +11% added to the non-optimized reduction ratio. ... Read more

Over 90% of international trade is carried out over seas. Shipping is currently the cheapest mode of transoceanic transport. The traffic density of shipping lanes on seas, oceans, and also rivers is likely to increase. Consequently, the GHG, NOx, SOx and noise emissions from shipping will rise making it more difficult to meet stricter emission regulations which the IMO aims to achieve. One opportunity to reduce emissions is by designing more efficient and quieter propellers.

To design quieter and more efficient propellers an optimal blade loading solution is required. For a rigid propeller, the blade loading distribution is optimized by modifying the geometry. The propeller geometry must be modified to achieve optimal loading that maximizes efficiency and minimizes acoustic emissions. In addition to efficiency and noise considerations, propeller optimization must consider thrust, ship speed, fairing constraints as well as unsteady wake of the vessel.... ... Read more

During berthing operations of ships, the horizontal thrusters cause a hydraulic load on the quay wall and bed. This could eventually lead to scour holes in the bed, which may affect the structural integrity of the quay wall. This thesis is about a numerical simulation of a bowthruster jet which is deflected by a quay wall and the focus of this study will be on the resulting bottom velocities. In 2019, field measurements were executed with an inland vessel and the measured bottom velocities were low compared to the design guidelines. The goal of this study is to simulate the field measurements in a numerical model to gather more knowledge about the flow field induced by bowthrusters next to quay walls. The obtained numerical results are compared to the field measurement and to the design guidelines of PIANC. As it turns out, the velocity sensors probably did not measure the maximum bottom velocity during the field measurements, since they are located slightly above the highest velocities at the bottom, as predicted by the numerical simulations. The maximum bottom velocities from the numerical simulation are similar to the expected near-bed bottom velocities by PIANC. However, there is room for improvement in these design guidelines, e.g., adjusting the efflux velocity for tunnel thrusters to account for the variation in cross-sectional area between propeller and tunnel exit. ... Read more

Throughout the last century, high energy-density fossil fuels have been the main energy source for merchant shipping. As the world tries to minimize the effects of global warming, shipowners are forced to significantly reduce their $CO_2$ emissions. Because alternative fuels are expensive, the interest in wind energy, which has been the single energy source for centuries, is renewed. There is a desire for passive and inexpensive wind-assisted ship propulsion (WASP) systems. The two-element wingsail is a promising option. This combination of a wing with a slotted flap can generate more than one and a half times more lift compared to a one-element alternative. It is used in the aviation industry as a high-lift device and has been applied on board yachts in the America's Cup and other races. Used as WASP system places new requirements as limited deck space and the ability to tack. This report describes the effects of the geometry on the performance of the wingsail and advises an ideal configuration that maximizes the overall thrust contribution. Both fixed design parameters such as the profiles and chord lengths and adjustable settings as the angle of attack and flap angle are examined. This is achieved by doing systematical geometry variations at a Reynolds number of 1.86 $\cdot 10^6$ using 2D CFD simulations. The design goal was to maximize the lift coefficient. The variations show that the lift coefficient is very sensitive to the flap angle. Increasing this angle increases the lift, but requires a well-positioned slot to ensure attachment of the flow along the suction side of the flap. The influence of the profile thickness is small. However, a minimum flap thickness was required to avoid flow separation at low angles of attack. Stall was always observed around the leading edge of the flap. The maximum lift coefficient of 2.73 at an angle of attack of 10 degrees was found for a configuration with a NACA0024 wing and NACA0012 flap profile, equal chord lengths, and a flap angle of 25 degrees. The corresponding slot had an offset of 0.5 and a side placement of 2.5 percent of the total chord length. Using the same method, the effects of adding a steering flap downstream of the wingsail were investigated. This third flap should keep the wingsail stable at a specific angle of attack. This ability is interesting for WASP purposes because it eliminates the need for fast-responding software-based control systems. The simulations show that the steering flap, which had a fixed chord length of 30 percent of the wingsail, can stabilize the system. Compared to the standard configuration the decrease of the lift was approximately 10 percent. The steering flap operates in the wake of the wingsail, which causes a decrease of drag of approximately 5 percent. This is an interesting feature because it reduces the negative effects of the steering flap. Depending on their positions, the interaction between all three elements can further enhance the performance of the whole system. ... Read more