Predicting Thrust Loss of Ship Propellers Due to Ventilation and Out-of-Water Effect

Abstract

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).