Dieudonné’s direct spiral maneuver identifies the directional stability characteristics of a ship. The numerical measures, obtained from this spiral maneuver, are the steady yawing rates as a function of rudder angle. A plot of these values is indicative of the stability characteristics of a ship. For example, if the plot is a single line going from starboard rudder to port rudder and back again - as shown for Ship A in figure 4.20 - the ship
possesses controls-fixed straight-line stability. If, however, the plot consists of two branches joined together to form a ”hysteresis” loop - as shown for Ships B and C in figure 4.20 the ship is unstable. The solid line curves show the steady turning rates. Arrows along the curves show the sequence of results of the spiral tests. Dotted lines indicate the jump in steady-turning rate during spiral tests of the dynamically unstable Ships B and C.