This article presents a propeller ventilation model that is tuned on experiments performed in open water condition in a towing tank. The main purpose of performing the experiments was to obtain more data at higher advance numbers for validation purposes and to make a calculation model for thrust and torque loss due to free-surface proximity. Tests were performed at different draughts. For each draught, the propeller was tested at different propeller speeds n = 9, 12, 16, and, 16 Hz at advance number in the range from J = 0 to J = 1.0. The different advance numbers were obtained at different propeller speeds so that for the same advance number, different propeller thrust values were tested, so that the effect of propeller loading can be seen independently from the speed of advance J. The main focus of this article is to explain and validate a prediction model for thrust loss due to ventilation and out-of-water effect. 1. Introduction Ventilation is a phenomenon of air-drawing seen on structures operating below the free surface, such as hydrofoils, rudders, and propellers. Propeller ventilation is related to the propeller coming close to the free surface and "sucking" air into the propeller, or when the blades are piercing the free surface. In these cases, propeller ventilation leads to a sudden and large loss of propeller thrust and torque, which might lead to propeller racing and possibly damaging dynamic loads, as well as noise and vibration. Ventilation typically occurs when the propeller loading is high and the propeller submergence is limited, and when the relative motions at the propeller are large because of heavy seas. Propeller ventilation inception depends on different parameters, i.e., propeller loading, forward speed, and the distance from the propeller to the free surface, see e.g., Smogeli (2006); Koushan (2006a-c); Kozlowska et al. (2009); Califano (2011); Jermy and Ho (2008); Hough and Ordway (1965); and Kozlowska and Steen (2010).

This paper presents experimental and theoretical studies on the dynamic effect on the propeller loading due to ventilation by using a simulation model that generates a time domain solution for propeller forces in varying operational conditions. For ventilation modeling, the simulation model applies a formula based on the idea that the change in lift coefficient due to ventilation computes the change in the thrust coefficient. It is discussed how dynamic effects, like hysteresis effects and blade frequency dynamics, can be included in the simulation model. Simulation model validation was completed by comparison with CFD (computational fluid dynamics) calculations and model experiments. Experiments were performed for static and dynamic (heave motion) conditions in the large towing tank at the SINTEF Ocean in Trondheim and in the Marine Cybernetics Laboratories at NTNU (Norwegian University of Science and Technology). The main focus of this paper is to explain and validate the prediction model for thrust loss due to ventilation and out of water effects in static and dynamic heave conditions.

The paper presents a discussion of the ventilation inception and air drawing prediction of ships propellers, aiming to predict under what conditions ventilation will happen, and the actual physical mechanism of the ventilation.Three different types of ventilation inception mechanisms are included in our discussion: free surface vortex ventilation, ventilation by sucking down the free surface without forming a vortex as well as ventilation by propeller coming out of the water. Ventilation prediction is based on a series of model tests, where the propeller is tested in different levels of intermittent ventilation. The use of underwater video gives a visual understanding of the ventilation phenomena.Ventilation by vortex formation has analogies with other phenomena, such as the inlet vortex in pump sumps, ground vortex at the inlet of the aircraft engines and the Propeller Hull Vortex Cavitation (PHVC). The paper includes comparison between Propeller Hull Vortex Cavitation (PHVC) and Propeller Free Surface Vortex Ventilation (PFSVV) as well as comparison between PFSVV and vortex formations of aero engines during high power operation near a solid surface. Experimental data based on several different model tests shows the boundary between the vortex forming, non-vortex forming and free surface ventilation flow regimes. For comparison the following parameters, which determined the intensity of the hydrodynamic interaction between the propeller and free surface have been used: propeller load coefficient cT, tip clearance ratio c/D, propeller submergence ratio h/R, ambient velocity Vi and flow cavitation/ventilation number 𝜎 𝑐𝑎𝑣 /𝜎 𝑣𝑒𝑛𝑡 .