Power Loss Related to Automatic Course-keeping

Abstract

The design trend of large industrial carriers brought out a new type of ship hull : high block coefficient and small length-beam ratio. Such hull proportions are liable to make a ship unstable on course. Since then minimum necessary course-stability has been of common interest.There are two criteria for the minimum necessary course-stability. The first relates to safe navigation normally under manual control. The second point is the propulsive power loss in a seaway under automatic coursekeeping ; the less stable is a ship, the more power loss from this reason assuming the same performance of the autopilot.The power loss is composed of three parts : elongation of ship path caused by course fluctuation, inertial drag caused by yawing and the drag of activated rudder. We'made theoretical analysis on them, taking account of the on-off (non-linear) performance of the “power unit”, a hydraulic servomechanism to control the steering gear. Also we paid attention to some non-linear elements normally called “weather adjust”, that is to avoid too frequent rudder activity in a seaway.Next we made numerical calculation of the power loss of a series of assumed ships whose course-stability vary systematically. Realistic sea and wind spectra are employed in this analysis.The conclusions are :(1) provided that the rudder gain and the yawrate gain of an auto-pilot is properly adjusted, even a very unstable ship can navigate with very small power loss, unless any low-frequency external disturbance exists ;(2) but this optimum rudder gain is so small that any frequency disturbance, very probably induced by wind gust, will cause a severe course deviation and then a considerable path elongation ;(3) from this reason, any spiral loop width (a measure of instability) greater than 15 degrees should not be permitted for fuel economy under automatic course-keeping in a seaway.