The problem of not driven propellers in Multi - screw ships propulsion arrangements

Power requirement for ship's movement depends on their size (water displacement) and sailing speed. The problem is especially important for fast ships, because the speed of movement is generating significant high power requirement, while limited displacement, diminishing a drag quantity from one hand, is a reason of limited draft and consequently limited capacity of a hull from the other. Above facts have strong influence at a number, type, power and size of main engines and propellers. Fast ships are designed for reaching maximum speed, but majority of exploitation time, are cruising at lower speed, called partial speed. That exploitation profile requires stopping of some engines or braking of propellers. Not driven propellers are dragged behind the hull, and work at a turbine mode, giving a torque at a propulsion shaft and creates a negative thrust what means the additional drag. The additional drag of not driven propellers must be calculated during the propulsion design stage, and analysis of speed value during different variants of the ship's movement . Torque values and thrusts can be evaluated basing on the universal hydrodynamic characteristics of propellers. the method and example of calculation of the drag created by free-rotating and stopped propellers of the exemplary propulsion set are presented in the paper.