When propellers are operating near the free surface,  phenomenon called ventilation might occur. Due to insufficient immersion and high thrust loading, the propeller draws air, resulting in a reduced thrust. Reduced thrust may have consequences such as loss of propulsive power, control and steerability, and may therefore be leading to safety issues. Long time exposure to ventilation’s unsteady torque loading can also lead to propulsive unit malfunctioning. As propeller diameters tend to grow bigger, free surface clearance decreases and room is left for air to be drawn. To increase understanding of the phenomenon, current experimental and numerical research was executed.  The used propeller was a Wageningen C4.55-propeller, an in design condition lightly-loaded propeller with low blade area, fitting to the trend of increasing diameters. The research was bound by perfect conditions to capture ventilation in the purest form; no influences of wake, waves and ship motion were taken into account. Experimental research showed that free surface ventilation appeared to be the most stable and predictable ventilation regime. Inception through free surface breaking mainly depends on the pressure gradient between the propeller tip and free surface, the tip immersion rate and the ability to draw the free surface. Increased ventilation thrust breakdown showed to be influenced by the local velocity on the blade, which mainly depends on the propeller rotation rate. Vortex ventilation was the most unstable regime in the experiments. Inception seems to be independent of the propeller loading, but influenced by local flow phenomena in the area above the propeller and propeller characteristics . It is believed that vortex inception, shape and wash-out resembles the appearance of the cavitating propeller-hull vortex. Vortex ventilation showed a bistability effect. Experimental results were obtained using a statistical research planning/Design of Experiments, such that polynomial models could be constructed. The model fitted the data well, demonstrated by the fitting coefficient r2 exceeding 0.9. Structural shortcomings were found in capturing the highly unsteady vortex ventilation, variations in mixed ventilation and increased thrust breakdown in free surface ventilating. Numerically, ventilation was simulated using the incompressible VoF-solver ReFRESCO. Vortex ventilation inception was not found, even when a scale resolving simulation was conducted. This is ascribed to insufficient application of the SRS-model in the near blade area, due to insufficient convergence of the omega-equation. Also application of Boussinesqs assumption in the k-equation might be stringent. Free surface ventilation inception was accurately found, both in simulations with a for ventilation adapted actuator disk model and with the propeller. Thrust breakdown was underestimated by CFD. Only breakdown due to surface piercing was found. Underestimation is ascribed to the absence of air entrainment. Application of TNT-EARSM-model (which is not using Boussinesqs assumption) and application of free-slip boundary conditions did not improve the shortcoming. As in literature, other free surface discretization schemes showed the same lack of air engtrainment, the origin might be in the VoF-assumption, being the increased interpolated density used in the momentum equation which prevents air to be convected.    

An accurate prediction of propeller hull interaction is an important step in the design of a new vessel. The prediction of full-scale flow phenomena, which eliminates scale effects, is becoming available due to increasing computational power. However, the complexity of full-scale CFD calculations combined
with a lack of validation data results in unknown uncertainties. This study contributes to the uncertainty estimation for full-scale calculations by answering the question With what uncertainty can we currently numerically predict resistance and propeller power on full-scale Reynolds numbers?.
The resistance and propeller flow predictions are done for the general cargo vessel MV-regal for three cases; a double body, a free surface resistance simulation and an open water propeller simulation. The simulations are performed for the design speed of 14 knots, resulting in a full-scale Reynolds
number of 푅푒 = 1.12 ⋅ 10ዃ. The discretization error is determined by the grid refinement study as presented by Eça and Hoekstra for the propulsion parameters; resistance coefficients and the wake factor. For each case the flow field is analysed, an uncertainty assessment is made and the results are compared to a group of numerical results for the same simulation performed by 20 participants of a case study organised by Lloyd’s register on the same vessel as is considered in this thesis.
Modelling the boundary layer of a flat plate on model and full-scale Reynolds numbers encourages the use of unstructured grids for full-scale Reynolds numbers. The uncertainties as predicted by the grid refinement study, vary between 0.6 and 24.5 percent for the friction coefficient. For the 푅푒 = 10^7 the values are compared to a structured grid study, which had a better trend over the grid refinement series. Comparison to theoretical friction coefficient calculations confirmed the absolute friction result.
The double body simulation, performed on the full-scale number 푅푒 = 1.12 ⋅ 10^9, demonstrated the use of the unstructured grids on the full-scale Reynolds numbers. The iterative error had to be closely monitored in order to get a stable solution. While the iterative errors had the same order of magnitude, the uncertainties as predicted by the grid refinement study for the propulsion parameters varied between 1.5 and 140 percent. This is an unacceptable large scatter in uncertainty which calls for another method to determine the uncertainty.
The free surface simulation added complexity, by modelling the free surface resistance of the vessel. The order of convergence is lower for the free surface simulation, which creates a higher iterative error in the uncertainty assessment. The discretization uncertainty prediction varies between 2.7 and 23.5 percent.
The open water simulation, which is based on a hybrid grid of structured and unstructured grids, showed for the monitored parameters a sufficiently low iterative error and a low discretization uncertainty (between 0.1 and 3.4 percent) for the thrust and torque coefficients. Although all different cases showed mixed results for the discretization error, the absolute values are well within the range of results from the test group of 20 participants. This is a good starting point of the repeatability of the flow parameters. It is noted that the current uncertainty estimation is larger than the difference between two grids.