Course stability and yaw motion of a ship in steady wind

In order to achieve safe navigation, it is important to be able to understand and calculate the effects of an external force on the maneuvering behavior of a ship. This paper analyzes the course stability and yaw motion of a ship traveling under steady wind conditions. A course stability criterion and approximate formulae for the yaw motion in steady wind, including the aero/hydrodynamic force derivatives for the ship, are derived. To confirm the reliability of the criterion and formulae, they were used to investigate a pure car carrier in steady wind. The results of this investigation revealed that course instability appears in the head and following wind directions, mainly under the influence of aerodynamic derivatives with respect to the yaw restoring forces. However, this course instability can be reduced by applying steering control. For winds ranging from head winds to beam winds, yaw oscillation appears when the period is relatively long and the damping is small. The analytical formulae derived here can be used to gain a better understanding of ship maneuvering behavior in steady wind.     

Strip method for a laterally drifting ship in waves

Lateral drift occurs due to the effects of wind forces, wave drifting forces, or both on ships sailing in actual seas. It is important therefore to investigate the influence of lateral drift on seakeeping performance for improved ship operation. The velocity potential was expanded as an asymptotic power series in terms of the lateral speed parameter, τ, defined as ω _e V ₀/g, where ω _e is the frequency of wave encounter; V ₀ denotes the lateral velocity, which is assumed to be sufficiently small; and g is the acceleration due to gravity. By combining this technique with the strip method, two sets of motion equations of all the hydrodynamic force coefficients for ship seakeeping were derived. The first set is for ships without lateral drift and is the same as the equations in the new strip method, and the second set is for the additional motions induced by lateral drift. It was found that all ship motion modes except surge are coupled when a ship drifts laterally in waves.     

Maneuvering simulations of pusher-barge systems

Pusher-barge systems were studied in nine different configurations. Captive model tests were performed at the Hiroshima University Towing Tank and the hydrodynamic derivatives for the various configurations were obtained. At a service speed of 7 knots, pusher-barge systems with the same number of barges but arranged in a row (shorter length overall but with a larger breadth) require more power to operate than those that were arranged in a line. When the length overall increased, the tactical diameter, advance, and transfer distances also increased, mainly due to the significant increase in the moment of inertia when barges are arranged in a line, rather than in a row. All pusher-barge systems had small first and second overshoot angles. Pusher-barge systems with the same number of barges had a longer response time to the rudder angle of attack and required a longer stopping distance when arranged in a line, mainly due to the increased moment of inertia and reduced resistance when barges are arranged in this way.     

Hydrodynamic derivatives investigation of unconventionally arranged pusher-barge systems

Unconventional arrangements of pusher-barge systems were studied in this paper. Pusher-barge systems consisting of 4, 6, and 8 barges with one pusher were tested in various combinations. Captive model testing was performed on the various combinations at the Hiroshima University towing tank. Hydrodynamic derivatives of the systems were obtained from the model test data by using the least-square analysis method. For asymmetric conditions, the hydrodynamic derivatives and N′_ββ were added to the force and moment equations in order to obtain better fitting of the least-square curves. Motion equations were modified to cover the asymmetric cases of pusher-barge systems with lateral force and yaw moment due to the asymmetry arrangement. Turning simulations (with 20° sudden angle change) were carried out and a comparison of advance distance and tactical diameter made. An erratum to this article can be found at     

Validation of 6-DOF motion simulations for ship turning in regular waves

In this study, a six degrees of freedom (6-DOF) motion simulation method of a ship steering in regular waves is validated. The proposed simulation model is based on the two-time scale concept where the 6-DOF motions are expressed as the sum of the low-frequency maneuvering motions and high-frequency wave-induced motions. Turning simulations of a KCS container ship model with a rudder angle of \(\pm 35^\circ\) in calm water and regular waves are performed and the obtained results are compared with the results of a free-running model test. The model tests were conducted using a ship model of length 3.057 m in a square tank at the National Research Institute of Fisheries Engineering, Japan. The wave conditions were as follows: the wave height was 3.6 m at full-scale, ratio of wavelength to ship length was 1.0, and the ship approached in the head wave direction before it was steered. The present method can simulate both the turning motion and wave-induced motions in regular waves with practical accuracy.

     

Investigation on the rudder force of a ship in large drifting conditions with the MMG model

Journal of Marine Science and Technology - The rudder force, including interference with a ship hull in large drifting conditions, is investigated in this study by captive model tests using a ship...