Once a semi-submersible is operating at an inconvenient draft (A shallow draft with limited water column above the pontoons), the passing waves over the pontoons can not keep their linear motion and energy will be transferred to higher harmonic wave frequencies. This means the hydrodynamic response will also contain energy at these higher frequencies. Conventional linear diffraction solvers are not able to solve the combined response of the wave frequency and its higher harmonics, which then result in a non-physical solution for the wave loads, motions and internal loads. This research aims to obtain a better insight into the hydrodynamic response at the inconvenient draft and ultimately on the internal loads of the semi-submersible. In the first part of this research, model test solutions, linear potential solutions obtained with WAMIT and CFD solutions obtained with ComFLOW are compared. the comparison shows that ComFLOW is able to provide more accurate wave loads compared to the linear potential solver. The higher harmon- ics observed in the measured wave loads during model tests are correctly predicted by ComFLOW, although maximum deviations of 30% are still observed between measured and predicted wave-load amplitudes. However, ComFLOW is not able to solve the motions of a free-floating semi-submersible correctly. Due to pressure peaks in the wave-exciting forces, solving the equation of motion results in an incorrect motion response and ultimately results in incorrect internal loads. Although a significant effort was made - in close collaboration with the ComFLOW developer - to improve this functionality, the results remained unsatisfactory. Even so, in order to obtain an insight into the higher harmonic response contributions to the internal loads, a parameter study has been conducted, using tests in which the semi-submersible was held captive. This parameter study was conducted by systematically varying wave amplitudes and draft, and resulted in situational limits at which the higher harmonic response contribution becomes significant and the linear relation between the incoming wave and the hydrodynamic response is lost. This limit is shown to be dependent on the Ursell number. Furthermore, it is demonstrated that the significance of the higher harmonic response contribution increases from 10% to 40% throughout the inconvenient draft, while the most severe situations resulted in a higher harmonic response contribution of 50% of the total response amplitude. In the final part of this study, an attempt is made to couple the wave loads from ComFLOW to internal loads. A quantitative analysis is made on the effect of the higher harmonic responses of the wave loads on the internal loads. The time domain simulator aNySim is used combined with the wave- exciting forces on a captive semi-submersible calculated with ComFLOW. A multi-body analysis is used to obtain the internal loads on the aft and front part of the semi-submersible. This did not provide the correct answers because the stiffness of the spring damping between the two sections affects the higher harmonic response contribution. This method overestimated the higher harmonic response contribution. A better understanding of the joint and the joint stiffness/damping of a dual-body simulation may solve the encountered problems.

The diffuser stern is part of a design concept from Damen and the TU delft: the interceptor diffuser. This concept aims to improve the seakeeping behavior of fast ships in waves. With PIV measurements, a contribution to the development of the diffuser stern is made. The flow acceleration and detachment are studied, and a comparison with a CFD pre-study from Damen is made.

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.