Effects of ventilation on open water characteristics of azimuth propellers under shallow water conditions

Abstract

Inland waterway vessels can accommodate relatively large propellers due to the specially shaped sterns of these ships. Specifically, they can accommodate up to 20 percent larger diameter propellers, thereby enabling operation without ventilation. However, for an azimuth thruster with a freely positioned propeller, ventilation can occur due to the small distance between the water surface and the propeller. This can lead to dangerous operational situations, such as an inability to maneuver. Systematic investigations comprising model testing and numerical URANS simulations were performed to analyze open water characteristics of a typical azimuth thruster fitted with the newly invented shallow water duct. This so-called shallow water duct was developed to prevent air drawing of the thruster's propeller, i.e., to avoid the inception of propeller free surface ventilation inception (aeration). A typical thruster fitted with this duct was investigated at an immersion depth smaller than the minimum immersion depth recommended by the ITTC for open water tests. Compared to a thruster fitted with a conventional standard duct, the performance of the thruster fitted with this shallow water duct was improved significantly. At lower propeller advance coefficients, especially during bollard pull conditions, it generated more continuous thrust and torque with the propeller operating at the same operating point although, at higher propeller advance coefficients, this duct caused the thruster to act like a break. Especially when fitted with the shallow water duct, thrusters accommodated on inland waterway vessels represent a constructive solution to improve the maneuverability and operational safety of ships navigating in confined waters of limited depth. Fitting the shallow water duct is expected to improve the ability of the thruster to initially accelerate the ship.