Yoshiho Ikeda

A Simple Prediction Formula of Roll Damping of Conventional Cargo Ships on the Basis of Ikeda’s Method and Its Limitation

Since the roll damping of ships has significant effects of viscosity, it is difficult to calculate it theoretically. Therefore experimental results or some prediction methods are used to get the roll damping in design stage of ships. Among some prediction methods, Ikeda’s one is widely used in many ship motion computer programs. Using the method, the roll damping of various ship hulls with various bilge keels are calculated to investigate its characteristics. Ship hull forms are systematically changed by changing length, beam, draft, midship sectional coefficient and prismatic coefficient. On the basis of these predicted roll damping of various ships, a very simple prediction formula of the roll damping of ships is deduced. It is found, however, that this formula cannot be used for ships that have higher position of the center of gravity.

Porpoising oscillations of very-high-speed marine craft

Experimental and theoretical investigations on porpoising of a planing hull at high forward speed up to Froude numbers Fr = 6.0 (Fr = U √gLOA, where LOAdenotes the overall length of ship) in calm water are conducted. Captive model tests and forced motion tests are carried out to measure the hydrodynamic forces acting on the hull. The results reveal that without any exciting force, such a craft can porpoise as a self-excited oscillation originating from the existence of coupling restoring coefficients of different sign in a coupled system. The results also show that significant nonlinear effects for motion amplitudes appear in the restoring, the added mass and the damping coefficients. The measured hydrodynamic forces are compared with the results of an existing prediction method including nonlinear effects, and show a good agreement with them. Simulation of porpoising in calm water is carried out using the predicted hydrodynamic forces in a nonlinear equation of motion. The calculated results are in fairly good agreement with experimental ones.

Estimation of inertia forces on a horizontal circular cylinder in regular and irregular waves at low Keulegan-Carpenter numbers

Inertia forces acting on a horizontal circular cylinder in regular and irregular waves significantly decrease at low Keulegan-Carpenter numbers (Kc). The rapid reduction of inertia force is caused by a circulating flow around the cylinder. In order to clarify the mechanism of the occurrence of the circulation, flow visualizations around a horizontal circular cylinder in regular waves and a circular cylinder moving along a circular orbit were carried out. Simple formulae to estimate the circulation strength and the inertia force reduction in regular waves were deduced from a starting vortex model with reference to the results of the flow visualizations. The prediction method was applied to the cylinder inclined to wave crests. The estimated inertia coefficients in regular waves are in good agreement with the experimental results at Kc less than 2.5. The inertia force reduction in irregular waves was also estimated by considering the circulation effect for each half wave period which was calculated by the prediction method in regular waves. The estimated results are in good agreement with the experimental results.

Parametric Roll Resonance of a Large Passenger Ship in Dead Ship Condition in All Heading Angles

Although it has been widely believed that a ship in dead ship condition has the largest roll motion at resonance condition, the authors found that, because of parametric roll resonance, a kind of ships has significant heavy rolling in beam waves with slightly smaller period of the half value of its natural roll period if the roll damping is small. In the present paper, model experiments to measure ship motions of a large passenger ship in waves with various heading angle are carried out to confirm the region of heading angles where large parametric rolling appears. The results demonstrate that parametric rolling appears at wide region of heading angles and disappears in certain heading angles.

Experimental and Numerical Studies on Roll Motion of a Damaged Large Passenger Ship in Intermediate Stages of Flooding

Measurements of roll motion of a two-dimensional scale model of a damaged large passenger ship are carried out during flooding process. The experimental results demonstrate that large and slow roll motion sometimes appears in the intermediate stages of flooding. The appearance of the large roll motion significantly depends on location and size of damage opening. It is also confirmed that the simulated results are in fairly good agreement with experimental ones.

Experimental Investigation on Capsizing and Sinking of a Cruising Yacht in Wind

This paper describes the main finding of the investigation of the sinking accident of a 6.45 metre-long cruising yacht in September 2003 in Japan. Responding to the request from Japan’s Marine Accident Inquiry Agency, the authors executed a model experiment in beam wind for identifying the time-to-sink as well as stability calculations with and without water inside the yacht. The results indicate it could capsize when the wind velocity exceeds a threshold. Because of an opened hatch on the deck, water initially enters into the cabin. After capsizing, the water ingress process stops. However, if wind is strong enough to incline the capsized yacht, the yacht starts to return to upright condition but this transition stage provides an opportunity to further flooding. By systematically changing wind velocity in the experiment, the time to sink was recorded. In conclusion, the critical wind velocities for capsizing and sinking were estimated, and reasonably well explain the reason of this accident.

A Study on the Characteristics of Roll Damping of Multi-Hull Vessels

In this study, for a catamaran and a trimaran as multi-hull vessels, the characteristics of the roll damping are investigated experimentally. A free roll decay test and a forced roll motion test with and without forward speed are carried out. The results show that the roll damping of them is much larger than that of conventional mono10 hull vessels, and the component created by side-hull accounts for a significant rate for trimaran. Especially, at the condition without forward speed, the interference of waves created by hulls are significant, the measured roll damping values by different experiments are different on the basis of different water surface condition created by hulls. Moreover, the simplified prediction method is proposed.

A Basic Study on Wind Pressure Characteristics of a Trimaran PCC

In recent years, the marine transport of a car is activated, and the demand for a pure car carrier (PCC) has grown quickly in the world wide. However, there are lots of technical issues of the present PCC, like lack of stability, effect of strong winds on its resistance, difficulty of course keeping in rough seas, difficulty of berthing in strong winds and so on. To overcome these technical issues, one of the authors has proposed a trimaran PCC which has very limited number of transverse bulkheads in the center hull by using two side hulls as fenders. In this study, wind forces and moments acting on a scale model of the trimaran PCC are measured in the towing tank with wind fans of Osaka Prefecture University. Furthermore, in order to find the parameters affecting on wind pressure, height and width of tunnels which are between a center hull and two side hulls are changed. To analyze it qualitatively, the flow field around the body is calculated by CFD. The wind pressure acting on the body is compared with that on a mono-hull PCC. Using these experimental and theoretical results, the wind pressure characteristics of a trimaran PCC are obtained.Copyright

Experimental Study on Two Flexible Hanging-off Circular Cylinders undergoing Wake Interference

This paper presents experimental results of three different configurations of two flexible circular cylinders were towed in still water. Each cylinder has length-to-diameter ratio of 34.4 with low mass ratio of 1.24. The cylinders were submerged partially into the still water from towing carriage as hanging vertical cantilever and towed by various speeds that give Reynolds number of 1.08 times 104 up to 3.78 times 104. For side-by-side arrangement, two distinct center-to-center distances between the two cylinders were considered, while for the tandem arrangement the relative position of the cylinders is only 4D. For staggered arrangement, a distance between upstream and downstream cylinders (LUD) was fixed by 4D, and the position of the downstream cylinder in transverse direction (LT) was varied by 1D, 1.5D, 2D, and 2.5D, respectively. Measurement of the hydrodynamics forces in two, in-line (drag force) and transverse (lift force) directions are conducted simultaneously with two- component load cells. For all test cases, the characteristics of the time-dependent forces are strongly dictated by the reduced velocity (Ur) parameter. Frequency of the drag force was found approximately twice the corresponding lift force frequency at the related Ur. For all cases, the CD and CL values lie in the range of approximately 0.03 up to 0.25. The tandem case became a very special case due to an extreme drop of the CD for the downstream cylinder compared with all other cases. In addition, for staggered configuration, the wake interference only has strong influence on the forces fluctuation until the LT = 1D, especially for the downstream cylinder.