Effects of Propellers and External Forces

Although a propeller is meant to produce a longitudinal force to propel the ship, a transverse force and a turning moment are also generated by propeller operation. How this transverse force and turning moment arise in a single screw ship is explained. In a twin screw ship, the transverse forces and turning moments of the two propellers turning in opposite directions cancel each other. However, twin propellers also affect manoeuvring since during a turn the flow to the two propellers is not identical. This chapter also considers the effect of external forces on the manoeuvring of a ship and describes the important case of a ship being towed.