Aspects of the Design Procedure for Propellers Providing Maximum Bollard Pull

SYNOPSIS The number of tugboats worldwide, their size and their power are all increasing rapidly. Their most important design and operation criterion is the available bollard pull. The propeller manufacturers are trying to increase the delivered power and the system diameter of the ducted propellers to meet all requirements. This has led to a higher power density of the ducted propeller and to a greater risk of cavitation. That is why the design process is complex and has to rely on calculations and model tests. An important aspect regarding the bollard pull of highly loaded ducted propellers is the cavitation behaviour; this is the reason that the results of model tests are increasingly necessary in the design process for tugs and their propellers. In conjunction with the model tests and the resulting thrust prognosis, the influence of Reynolds number effects has to be considered in the fullscale correction of the model thrust. This paper will present the major results of the investigations regarding cavitation of ducted CP propellers at bollard pull condition and will show the main aspects that can influence the bollard pull. Based on an example of a 220 tonne tug with ducted conventional CP propellers, it will be shown that both the shape of the aft ship and the propeller design have to be optimised in order to realise the maximum possible bollard pull.