The influence of blockage and cavitation on the hydrodynamic performance of ice class propellers in blocked flow

Ships operating in ice in Canadian waters normally use open or ducted marine screw propellers for propulsion. The operation of the vessel in an ice cover, especially during icebreaking operations, often results in the submergence of broken ice pieces at the bow, and along the length, of the vessel. As the vessel moves through the ice field, the propeller approaches the submerged piece or pieces of ice and contact or non-contact interference between the ice and the propeller occurs. -- This work examines the effects of blockage and cavitation on the hydrodynamic loads associated with non-contact propeller ice interaction. A series of experiments were done in medium sized cavitation tunnels with two 200 mm open propeller models and two 200 mm ducted propeller models. Tests were also conducted in a towing tank with one of the open propellers. The tests were conducted in uniform flow and in blocked flow using simulated ice blockages installed upstream of the propeller. Measurements were made of mean and instantaneous propeller thrust and torque, duct thrust in uniform flow, block drag load and blade pass pressure on the face of the block adjacent to the propeller. Effects of blockage, cavitation and proximity of the ice piece were examined. -- Blockage of a propeller resulted in increased mean levels of thrust and torque over uniform flow values. Furthermore the blockage increased the oscillation of the loads about their mean. Cavitation reduced the mean loads associated with blockage but further increased oscillation about the mean. The development of severe sheet and cloud cavitation posed the risk of both fatigue and erosion to the propeller.