Large Heel Angles Research Articles

An Improved Procedure for Strongly Coupled Prediction of Sailing Yacht Performance

Abstract In this paper, an improved procedure for strongly coupled prediction of sailing yacht performance is developed. The procedure uses 3D RANS CFD to compute the hydrodynamic forces. When coupled to a rigid body motion solver and a sail force model, along with a rudder control algorithm, this allows sailing yacht performance to be predicted within CFD software. The procedure provides improved convergence when compared to a previously published method. The grid motion scheme, partially using overset grid techniques, means that correct alignment between the free surface and the background grid is ensured even at large heel angles. The capabilities are demonstrated with performance predictions for the SYRF 14 m yacht, at one true wind speed, over a range of true wind angles, with up- and downwind sailsets. The results are compared to predictions from the ORC-VPP for a yacht with similar main particulars.

Numerical study on a heeled one-stepped boat moving forward in planing regime

Stepped planing hulls have the potential to reach high-speeds in the sea. The step on the bottom of these vessels influence the pressure distribution and thus stability of the vessel, especially in transverse plane. Understanding the behavior of these vessels in a non-zero heel condition is fundamental in the early stage design. In the current paper, numerical simulation of the viscous flow field around a heeled one-stepped planing hull is performed to evaluate influences of the asymmetric planing on the performance of the vessel. The numerical model is validated in two steps. At the first step, performance of a heeled stepless planing hull operating in calm water is simulated using the numerical model, and the computed data are compared against experimental data. At the second step, the numerical model is used to compute resisatnce anad running attitudes of a one-stepped planing hull in symmetric condition, and the obtained results are compared against experimental measurements. Numerically computed results are in good quantitative agreement with laboratory measurements, showing that the numerical model has reasonable accuracy. On the whole, it has been found that a heeled condition manages a one-stepped vessel to settle at a smaller trim angle. But, unlike stepless boats, large heel angles have less effects on the trim angle at high-speeds since significant negative pressure is caused behind the step by the air, triggering negative pitching moment. It is shown that the resistance of a one-stepped planing hull is increased up to 25% at most of speeds and heel angles when the vessel advances in asymmetric condition. Moreover, this study shows that, the heeling moment of a one-stepped vessel decreases as the speed increases. The extent of reduction in the heeling moment gets more significant by the increase in heel angle at high-speeds, and, as a result, the heeling moment required to keep the vessel at large heel angle is observed to converge to the heeling moment correspanding to small heel angles.

Analysis of Flettner Rotor ships in beam waves

Flettner Rotors are spinning cylinders which produce lift force by utilizing wind power. Today, due to global warming and necessity of reducing emissions, renewable energy resources such as wind energy become more significant than ever before. This study presents the influence of Flettner Rotors on roll motion of ships and the effects of roll motion on performance of Flettner Rotors in beam waves. The calculations are carried out by using XFlow CFD program. The sample rotor has a 4 m diameter, 27 m height and a Thom disc. The calculations are performed with 0°, 20°, 40° and 60°heel angles to obtain the heeling moment function and driving force function with respect to heeling angle. The results of roll responses of the sample ship with and without Flettner rotors are presented in time and frequency domain comparatively. The results from the calculations show that the lift power performance of Flettner Rotors decreased with increasing of heeling angle. After adding Flettner Rotor contribution on heeling moment function, it is seen that Flettner rotors do not increase the nonlinearity of roll motion. Also, it was found that at large heel angles the driving force becomes almost zero.

Mathematical modelling of operation of the tug equipped with azimuthal thrusters

Use of harbor tugs to assist vessels in berthing operations is a common practice. More often nowadays tugs are used also to provide escort service to large vessels, which cannot maneuver safely in constrained harbor waters without assistance due to their size, or to provide back-up steering ability if the vessel propulsion fails. High level of risk usually inherent to these operations stipulates the study of the tug and ship interaction dynamics. This research deals with mathematical modeling of the towing process, with the focus on azimuthal stern drive, tug maneuvering characteristics and its transverse stability. A mathematical model of an ASD tug in four degrees of freedom is presented in the paper. Following features were specifically considered: thrust degradation due to propeller interaction, transverse stability on large angles of heel, interaction via a hawser with an independently steered vessel, and the hawser tension in a dynamically unstable situation. Software developed on the basis of the proposed mathematical model allows modelling girting and capsizing of a tug. In addition, it gives an opportunity to access dangerous tug handling modes, which can be presented during tug masters training or used in the towing operations risk analysis.

준해석적 방법을 통한 파라메트릭 횡동요 해석

† This study aims the development of a semi-analytic method for the parametric roll of large containerships advancing in longitudinal waves. A 1.5 Degree-of-Freedom(DOF) model is proposed to account the change of transverse stability induced by wave elevations and vertical motions (heave and pitch). By approximating the nonlinearity of restoring moment at large heel angles, the magnitude of roll amplitude is predicted as well as susceptibility check for parametric roll occurrence. In order to increase the accuracy of the prediction, the relationship between righting arm(GZ) and metacentric height(GM) is examined in the presence of incident waves, and then a new formula is proposed. Based on the linear approximation of the mean and first harmonic component of GM, the equation of parametric roll in irregular wave excitations is introduced, and the computational results of the proposed model are validated by comparing those of weakly nonlinear simulation based on an impulse-response-function method combined with strip theory. The present semi-analytic doesn't require heavy computational effort, so that it is very efficient particularly when numerous sea conditions for the analysis of parametric roll should be considered.

Broaching prediction of a wave-piercing tumblehome vessel with twin screws and twin rudders

The new intact stability criteria which are under development at the International Maritime Organization (IMO) are required to cover a broaching phenomenon, well known as a great threat to high-speed vessels which can lead to capsizing. Some reports exist which demonstrate that their numerical models can predict a highly nonlinear phenomenon of broaching. However, additional validation studies are needed for unconventional vessels, in addition to conventional ones, to develop direct stability assessment methods for the new intact stability criteria. In this research, we selected as the subject ship a wave-piercing tumblehome vessel with twin screws and twin rudders, a design expected to be one of a new generation of high-speed monohull ships. Firstly, a series of captive model tests were conducted to measure the resistance, the manoeuvring forces, the wave-exciting forces, the heel-induced hydrodynamic forces, and the roll restoring variation for the unconventional tumblehome vessel. Secondly, the existing mathematical model which had been developed for broaching prediction of conventional vessels with a single propeller and a single rudder was extended to unconventional vessels with twin propellers and twin rudders. Finally, comparisons between numerical simulations and the existing free running model experiments were conducted. As a result, it was demonstrated that fair quantitative prediction of broaching is realised when the rudder force variation, the roll restoring variation and the heel-induced hydrodynamic force for large heel angles are taken into account.

Biomechanics and plantar ulcers in diabetes

Patients with diabetes are at risk of developing plantar ulceration. To study the frequency of abnormal biomechanics and its possible influence on the location and distribution of lesions, patients presenting with active or recently healed forefoot plantar ulcers were investigated. 28 feet were assessed. Heel bisection angles were measured with the foot in subtalar joint neutral calcaneal stance and in relaxed calcaneal stance positions. The location of the lesion was mapped. In all cases abnormal function was demonstrated, expressed as abnormal positions in neutral and/or relaxed stance, and quantified by compensatory pronation. In relaxed stance for metatarsal ulcers there was a strong correlation between heel angles and metatarsal ulcer sites. Inverted heels were associated with lateral ulcers and everted heels with medial ulcers (r=0.8696: P < 0.001). In subtalar joint neutral stance position, large heel angles (> +8.0°) were associated with lateral ulcers, and smaller angles (< +6.0°) were associated with medial ulcers (r=0.7962: P<0.001). A significant difference was found between the compensatory pronation for metatarsal lesions, and that for toe lesions ( P < 0.05). Biomechanical assessment of patients may help in the identification of those at risk of ulceration and in the fabrication of functional orthoses or pressure relieving insoles.