Directional Stability and Control of Ships in Waves

Abstract

The stability and oscillatory motions of ships (automatically steered and unsteered) in the horizontal plane were examined on a digital computer for the case of regular following seas. Available hydrodynamic data for Series 60 hull forms were used. Analysis of directional stability was made for the case of zero encounter frequency (i.e., the ship runs at high speeds equal to the wave celerity). The ship (which is hydrodynamically stable without automatic control in calm water) is directionally unstable in following seas except for the small region near the ascending node of the waves. Addition of automatic control can give the ship directional stability when it is located on the wave trough, but not when it is located on the wave crest. At relatively high frequency (i.e., at low speeds in following seas), the rudder and control system are almost incapable of reducing oscillatory motion. Violent rudder activity in following seas can be decreased by reducing the yaw-rate-gain control constant and by increasing the rudder-response-time constant.

An Approach for Computing Parameters for a Lagrangian Nonlinear Maneuvering and Seakeeping Model of Submerged Vessel Motion

“…With respect to the rotational component of the second term in the integral of (23), by applying ( 25)-( 27 (28) Applying ( 22), (25), and (28) to the second term in the second row of ( 7 (…”

“…However, these efforts have typically imposed strict conditions. For example, the conditions for slender-body theory have been assumed [14], [21], or that the ship's nominal motion is purely in surge and in a following sea [22], or that the ship moves only in the horizontal plane and does not deviate significantly from a straight course [23]. More recent and more advanced models allow small perturbations from a nominal speed and heading [9], [24]- [26].…”

Bioinspired morphing fins to provide optimal maneuverability, stability, and response to turbulence in rigid hull AUVs

“…Directional stability and maneuverability are desirable features of long-range underwater vehicles that are tasked to perform complex operations [1]. Directional stability ensures maintaining a straight course during transit missions, with minimal corrective control action in the presence of disturbances [2][3][4]. Rudders and fins located at the tail end of vehicles can ensure stability, but they also reduce maneuverability, which is desired for both manned and unmanned underwater vehicles in order to execute sudden course and depth changes, or perform complex tasks [5][6][7][8].…”

“…Severely reducing the directional stability of a vehicle in order to gain in maneuverability has adverse consequences [12,13]. A vehicle's ability to maintain a steady course and depth with minimum control actions is proportional to the directional stability index of the vehicle [2][3][4]. Reduced stability, especially at higher speeds, makes the vehicle more sensitive to external disturbances, requiring large and continuous active control action [14], resulting in Figure 1.…”

The Conception and Mechanism of Ship Frequency Scattering due to Maneuvering Motions in Regular Waves

“…Rydill [2] got the linear results of controllable and uncontrollable of ships in waves on the basis of Froude-Krylov force. With the linear theory, Eda [3] obtained the results of course stability by adding harmonic wave force into the linearized equations of maneuvering motion, and studied the coursekeeping stability [4] using the PD rudder action in waves. Studies on ship maneuverability in waves were based on the theory that adding high frequency wave force into low frequency equations of maneuvering motion, and it has been proved by Nonaka [5] by using two-time-scale expanding method in trend stream theory.…”