THE STABILITY OF A FREELY FLOATING SHIP

The paper presents the problem of calculating the righting arms (GZ-curve) for a freely floating ship, longitudinally bal- anced at each heel angle. In such cases the GZ-curve is ambiguous, as it depends on the way the ship is balanced. Three cases are discussed: when the ship is balanced by rotating her around the trace of water in the midships, around a normal to the ship plane of symmetry (PS), and around a normal to the initial waterplane, fixed to the ship, identical with the curve of minimum stability. In all these cases the direction of the righting moment in space and area under the GZ-curves, which is the lowest possible, are preserved. Angular displacements (heel and trim) are the Euler's angles related to the relevant reference axis. The most important features of the GZ-curve with free trim are provided. Exemplary calculations illustrate how the way of balancing affects the GZ-curves.

Split-Flaps – a Way to Improve the Heel Stability of T-Foil Supported Craft

Horizontal T-foils allow for maximum lift generation within a given span. However, the lift force on a T-foil acts on the symmetry plane of the hull, thereby producing no righting moment. It results in a lack of transverse stability during foil-borne sailing. In this paper, we propose a system, where the height-regulating flap on the trailing edge of the foil is split into a port and a starboard part, whose deflection angles are adjusted to shift the centre of effort of the lift force. Similar to the ailerons which help in steering aircraft, the split-flaps produce an additional righting moment for stabilizing the boat. The improved stability comes, however, at a cost of additional induced resistance. To investigate the performance of the split-flap system a new Dynamic Velocity Prediction Program (DVPP) is developed. Since it is very important for the performance evaluation of the proposed system it is described in some detail in the paper. A complicated effect to model in the DVPP is the flow in the slot between the two flaps and the induced resistance due to the generated vorticity. Therefore, a detailed CFD investigation is carried out to validate a model for the resistance due to the slot effect. Two applications of the split-flap system: an Automated Heel Stability System (AHSS) and a manual offset system for performance increase are studied using a DVPP for a custom-made double-handed skiff. It is shown that the AHSS system can assist the sailors while stabilizing the boat during unsteady wind conditions. The manual offset enables the sailors to adjust the difference between the deflection angles of the two flaps while sailing, thus creating a righting moment whenever required. Such a system would be an advantage whilst sailing with a windward heel. Due to the additional righting moment from the manual offset system, the sails could be less depowered by the sailors resulting in a faster boat despite the additional induced resistance. It is shown in the paper that the control systems for the ride height and the heel stability need to be decoupled. The paper ends with a description of a mechanical system that satisfies this requirement.

The Stability of a Freely Floating Ship

The paper presents the problem of calculating the righting arms (GZ-curve) for a freely floating ship, longitudinally bal-anced at each heel angle. In such cases the GZ-curve is ambiguous, as it depends on the way the ship is balanced. Three cases are discussed: when the ship is balanced by rotating her around the trace of water in the midships, around a normal to the ship plane of symmetry (PS), and around a normal to the initial waterplane, fixed to the ship, identical with the curve of minimum stability. In all these cases the direction of the righting moment in space and area under the GZ-curves, which is the lowest possible, are preserved. Angular displacements (heel and trim) are the Euler's angles related to the relevant reference axis. The most important features of the GZ-curve with free trim are provided. Exemplary calculations illustrate how the way of balancing affects the GZ-curves.

Experimental Observation on a Controllable Underwater Towed Vehicle With Vertical Airfoil Main Body

An alternative type of controllable underwater towed vehicle is proposed. The vehicle is composed of horizontal fixed main wing, adjustable wing flap, and a vertical airfoil main body above which two torpedo-shaped buoyant cylinders are symmetrically fixed. The adjustable wing flap serves as an actuator for deflection of the horizontal fixed main wing to produce enough downward lifting force to the towed vehicle running at a required water depth. Principal function of the torpedo-shaped buoyant cylinders is to create a righting moment or a favorite roll damping for attitude stability of the vehicle during towing operation. Meanwhile two ducted propellers are installed at the sterns of the buoyant cylinders to provide an induced turning moment for the vertical airfoil main body, which in turn produce a driving force for the vehicle in lateral motion. Results of our laboratory experiments indicate that flexible attitude and trajectory manipulations to the vehicle in multiple degrees of freedom can be achieved with the structural style and control mechanisms of the proposed vehicle. The manipulations to the vehicle with the proposed control mechanisms are accomplished by a joint operation of controlling the rotational speeds and directions of the ducted propellers and/or adjusting the deflection of the wing flap. By means of the proposed structural style and control manner, difficulty in designing control system of the vehicle can be reduced greatly, and stronger self stability of the vehicle during its survey towing can be guaranteed.

System Identification of Unmanned Planning Boat Rolling Motion Mode

In this paper, the author took an unmanned planning boat as the object of study and carried out a series of rolling decay ship model test by changing the draft. The author established nine kinds of mathematical model of rolling decay motion model system identification by using different damping and righting moment and established the optimization calculation of the objective function based on the principle of system identification. Then the author adapted the genetic algorithm of system identification program which is based on the Visual Basic 6.0 and got 15 kinds of identification programs. By doing research on the first three cycles of the series of rolling angular velocity curve and identifying respectively the resulting 15 kinds of identification programs, the author confirmed the feasibility of the adapted program. Comparing different drafts and the initial roll angle identification results, the author found a reasonable hydrostatic roll motion equation of the unmanned planning boat in the case of different drafts and the initial roll angle, and made a preliminary analysis.

Stability Requirements of Multi-Purpose Ships for Heavy Lifts

New minimum intact stability criteria are presented to ensure safety against capsizing invoked by sudden loss of crane load during heavy lifts at sea, followed by typical sample stability assessments for a lifting operation on four multipurpose ships. For added stability, two of these ships had a pontoon attached at their sides opposite the lift. Two numerical time-domain methods assessed the transient dynamic heel after a sudden loss of crane load. With the ship at equilibrium, both analyses started by releasing the crane load, simulating a sudden failure of the lifting gear. The first method solved the roll motion equation as a one-degree-of-freedom system; the second method used a Reynolds-averaged Navier-Stokes equations solver. The first method relied on appropriately chosen linearized roll damping coefficients, and the nonlinearity of the righting moment function had to be accounted for. The second method required creating extensive numerical grids to idealize the ship’s hull, including the counter balancing stability pontoon, rudder and bilge keels, as well as all parts of the ship’s superstructure that effect the righting moment at large heeling angles.

Comparisons Between Prediction and Experiment for Lift Force and Heel Moment for a Planing Hull

Even in calm water, high-speed vessels can display unstable behaviors such chine walking, sudden large heel, and porpoising. Large heel results from the loss of transverse stability at high forward speed. When a planing craft begins to plane, the hydrodynamic lift forces raise the hull out of the water. The available righting moment resulting from the hydrostatic buoyancy is, therefore, reduced. As the righting moment resulting from hydrostatic buoyancy is reduced, the righting moment resulting from dynamic effects becomes important. These hydrodynamic righting effects are related to the hydrodynamic lift. This article explores the relationship between the hydrostatic lift and righting moment, the hydrodynamic lift and righting moment, and the total lift and heel-restoring moment of a planing craft operating at planing speeds. A series of tow tests using a prismatic hull with a constant deadrise of 20 measured the lift force and righting moment at various angles of heel and at various model velocities. The model was completely constrained in surge, sway, heave, roll, pitch, and yaw. The underwater volume is determined from the known hull configuration and the underwater photography of the keel and chine wetted lengths. The results presented include the total lift and righting moment with the hydrostatic and hydrodynamic contributions for various model speeds at two model displacements.

STABILITAS STATIS KAPAL IKAN TIPE LAMBUT TERSANJUNG YANG BERPANGKALAN DI PELABUHAN PERIKANAN SAMUDERA AERTEMBAGA KOTA BITUNG PROPINSI SULAWESI UTARA

Ship is water vehicle with any shape and any purpose, mechanically powered or by windvor tug and categorized as a multipurpose platform. It could be operated underwater or in the surface of water and could function merely as a floating static structure. The natural condition is the fish catching area are unpredictable as the wave an current of the water could disturbing the maneuverability and the stabilty of the ship. For that purpose the fish catching ship should have and meet the criterion of robust construction and strong structure to deal with the extreme condition in the sea and also should have a good maneuverability and enough power to make it move steadily. The total lenght of the sample ship Tersanjung are 15.85 m, the width are 2.90 m and inside the ship measured as 1.32 m. The average comparison among the 3 parameters are 5.50 for L/B, 13.05 for L/D and 2.37 for B/D. The coefficient of the Tersanjung ship are Cb=0.58, Cp=0.96, C =0.60 and Cw=0.87. “Tersanjung” ship has reverse velocity 3.7 second (1 perod). Maximum rolling reaching at 120 with righting moment of 0.21 m.