Fast Ferries Research Articles

Analysis and Prediction of Slamming-Induced Loads of a High-Speed Monohull in Regular Waves

In recent years, an important trend in the shipbuilding industry has been the increase in the length and speed of high-speed crafts, thus demanding lighter structures. High-speed vehicles with their increased flexibility are more likely to be excited by impulsive loads, such as slamming, which has been extensively studied and discussed by the scientific community. Nevertheless, ship design still demands plain and reliable procedures (numerical and/or experimental) to evaluate the time-dependent global loads in structural dynamics. In this paper, the aim is to explore the possibility of combining the conservation of fluid momentum with the two-dimensional numerical estimation of the effective wetted length in order to improve the prediction of the impact loads without losing the simplicity and efficiency of analytical methods. In order to evaluate the prediction capability of the proposed formulation, the numerical computation of the slamming force is based on processing the model test data relative to the incoming waves and rigid-body motion, and the attained results are compared with the hydrodynamic force experimentally identified. The presented analysis is applied to the slamming tests in regular head waves of a segmented model, supported by an elastically scaled beam, of a fast ferry. By using the slamming load obtained with the theoretical model, the elastic response in terms of bending moments is computed and compared with that provided by direct measurement with strain gauges. Finally, the uncertainty analysis relative to both numerical and experimental results is performed.

Far-field vessel wakes in Tallinn Bay

The properties of wave fields induced by high-speed ferries and recently introduced conventional ferries with increased cruise speeds are analysed for a site in Tallinn Bay, the Gulf of Finland, the Baltic Sea, located about 3 km from the sailing line and up to 8 km from the wave production area. The analysis is based on high-resolution profiling of the water surface for about 650 wakes from fast ferries, measured during 4 weeks in June–July 2008. The new large conventional ferries with cruise speeds of 25–30 knots (~ 45–55 km/h) sail at near-critical speeds along extensive sections of eastern Tallinn Bay, and excite wakes equivalent to those of high-speed ferries. The peak periods of these wakes are between 10 and 13 s. The typical daily highest ship wave is approximately 1.2 m, measured prior to wake breaking. The largest recorded ship wave in calm conditions had a height of 1.5 m and in the presence of some wind wave background 1.7 m. The cumulative impact of ship wakes results in a gradual increase in the suspended matter concentration in near-bottom water over the course of a day. The largest and longest ship waves produce considerable wave runup at the coast and prevent several coastal sections from achieving an equilibrium state. The largest ship waves have an asymmetric shape both in terms of the water surface elevation above and below the mean level and in terms of the shape of the wave front and back. The overall intensity of anthropogenic waves has remained at the same level as it was in the year 2002, although the ships that produced the highest waves in the past are no longer in service.

Identification of longitudinal and transversal dynamics of a fast ferry

An analysis of the system identification methods have been carried out and a new alternative approach is proposed in order to estimate models for heave, pitch and roll dynamics of a high speed craft. As starting point, a first approach resolves the identification subject as an optimization problem to fit the best model, and uses genetic algorithms and nonlinear least squares with constraints methods applied in the frequency domain. The second and definitive one suggests a new parameterization which facilitates obtaining high quality starting values and avoids non-quadratic functions in the cost function. At last it is shown an example in which the two approximations are applied and compared.

Frequency Domain Study of Longitudinal Motion Attenuation of a Fast Ferry Using a T-Foil

Longitudinal, heave and pitch, motion of ships in response to encountered waves can be smoothed using moving submerged wings, like transom flaps or a T-foil under the bow. Recently a 3 DOF detailed model of the surge, pitch and heave motions of a fast ferry has been developed, in terms of a structure of twelve transfer functions. On the basis of this model the most effective control for motion attenuation using a T-foil has been determined in the frequency domain, both for unlimited or saturated action. The results have been obtained by point to point exploration and depict amplitude and phase profiles of the controller. This result is useful to orientate linear control design. A first preliminary linear controller is presented.

Simulation of patterns of wakes from high-speed ferries in Tallinn Bay

We analyse spatial patterns and far field properties of the long-wave part of wakes of fast ferries in Tallinn Bay with the use of the B oussinesq-type shallow-water model COULWAVE, forced by realistic ship motions. The calculated he ights of ship waves exhibit substantial spatial variability. The largest waves were created when the ship, sailing to Talli nn, entered into super- critical regime when moving over the coastal slope. The maximum wave height eventually reached 3 m, whereas along other sections of the track the wave height was about 1 m. The highest waves hit the area of Pirita Beach that apparently has much larger ship wave loads than the adjacent sections of the Viimsi Peninsula.

Airfoil Assisted Catamarans: The price of lift is drag

Scarcity of fossil fuels and a worldwide development of metropolises in coastal areas and estuaries lead to an increasing demand for fast ferries with economical fuel consumption. The Airfoil Assisted Catamaran (AAC) should fulfil these requirements. An AAC is a catamaran assisted by wings above the water surface; it combines the beneficial properties of both the catamaran (manoeuvrability, large deck area) and the wing in ground vehicle (good lift-to-drag ratio). To undertake research after the AAC concept, a design tool has been developed in which hydrodynamics, aerodynamics, a weight calculation, strength calculation and cost estimation are integrated. Finally an AAC has been designed as an alternative for an existing catamaran.

Inter-Island modal competition: the case of Gran Canaria-Tenerife

In this paper, we use discrete choice modelling to analyse inter-modal competition in the most important route within The Canary Islands (Spain). We estimate different Nested Logit (NL) models that allow us to know the main factors that determine modal choice among a set of three existing alternatives: plane and two fast ferries. We obtained passengers' Willingness To Pay (WTP) for improvements in the main level-of-service attributes. Direct demand elasticities for the different competing modes are also derived. The results of our analysis can be used as an interesting policy instrument for operators' managers and policy makers.

Field observations of waves generated by passing ships: A note

This short contribution reports the results of a field study on the nearshore characteristics of waves generated by both conventional and high speed passenger ferries. The field observations took place in the late summer of 2005, at a beach close to the port of Mytilene (Island of Lesbos, Greece), and involved the visual observation of ship waves, using digital video recordings and image processing techniques. The results showed that passage of the fast ferry was associated with a longer, more complex and energetic nearshore event; this event not only did include higher nearshore waves (up to 0.74 m) and was organised in different wave packets, but it was also an order of magnitude longer (∼ 680 s) than the conventional ferry event. Regarding the effects on beach sediment dynamics, the fast ferry waves were estimated to be very efficient in mobilising the nearshore sediments in contrast to those of the conventional ferry. The fast ferry service appears to generate daily prolonged nearshore events, which contain waves with higher energy than those expected from the normal summer wind wave regime of the area; these events also include some high and very steep waves, which can be particularly erosive. Therefore, fast ferry wakes may have considerable impacts on the seasonal beach sediment dynamics/morphodynamics and the nearshore benthic ecology, as well as they may pose significant risks to bathers, affecting the recreational use of the beaches exposed to fast ferry traffic. Finally, the study has shown that satisfactory field observations of the nearshore characteristics of ship-generated (and wind) waves can be obtained using inshore deployments of calibrated poles, digital video cameras and appropriate image processing algorithms.

Aerodynamic characteristics of a hybrid trimaran model

Ultra fast marine vehicles can substantially benefit from aerodynamic unloading. The subject of this study is aerodynamics of a hybrid trimaran that comprises three wave-piercing planing hulls and a wing-shaped superstructure. This configuration is characterized by high efficiency and good seaworthiness at speeds about twice those of contemporary fast ferries and combat ships. Aerodynamic coefficients of the above-water structure of the hybrid trimaran were measured in a wind tunnel. A computer code based on the vortex lattice method is applied for modeling steady aerodynamics of this structure in a range of attack angles. The application of an interceptor at the pressure side of the wing is shown to produce significant increase in aerodynamic lift.

On the performance of a large high-speed trimaran

Trials and measurements conducted on an aluminium 127 metre long trimaran fast ferry are presented, covering the period of builder’s trials, delivery voyage, and during operation. Measurements were taken of strains at strategic locations in the structure, together with accelerations in all principal directions and the wave environment. Strain correlation factors were derived to be applied to the Finite Element models for future trimaran structural design. Comparisons are made between the trimaran and a range of equivalent catamarans, and the superior speed of the trimaran over the catamaran in both calm water and in waves is demonstrated. The measured speed loss in waves for the trimaran is also compared against a numerical prediction. Improved roll and pitch characteristics are presented for the trimaran and generally improved levels of comfort, with a demonstrated 50% reduction in passenger sea sickness for the trimaran compared with the catamaran.

The integration of lifting foils into ride control systems for fast ferries

The integration of lifting foils with a Ride Control System (RCS) can provide a significant improvement of both the efficiency and the seakeeping of high-speed ferries. Lifting foil RCS solutions extend the overall performance of the vessel beyond that of a displacement hullform without resorting to expensive, fully foil supported, hydrofoil solutions. VT Maritime Dynamics, Inc (VTMDI) has developed simulation and integration tools that permit optimisation of vessel performance and efficiency with realistic lifting foil design constraints. Design tools range from finite element analysis and performance simulations, to seakeeping simulations and computational fluid dynamic models. This paper discusses the lifting foil design process and its application to the trimaran “Dolphin Ulsan” designed and built by North West Bay Ships (NWBS) of Australia.

Co-operative investigation into resistance of different trimaran hull forms and configurations

This paper presents the results of a research program carried out on different trimaran hull forms and configurations suitable for medium size fast ferries. Resistance tests on large size scale models of the two different hull forms of main hull and outriggers, both isolated and combined in different configurations have been carried out at the towing tank of Naples University. For each isolated hull and trimaran configuration the total resistance and running trim have been assessed. Wave pattern analysis was also performed by experimental tests and by numerical computations, so that the influence of each resistance component has been highlighted. The results of this work could be used as a reference in trimaran hull design and optimization.

A neuro-fuzzy approach to fast ferry vertical motion modelling

In order to stabilise the motion of a high speed craft, and so to improve the comfort of the passengers and the crew while maintaining the speed, control-oriented model are needed. For this purpose, neuro-fuzzy systems have been used to obtain general models of the non-linear behaviour of a fast ferry. The sources of the available knowledge are the physical laws of the vertical dynamics of the craft, and some experimental and simulated data of the ship performance. Two non-linear models focused on the vertical motion of the craft, both heave and pitch, are proposed: an academic one and a predictive one. The modelling task is complex and the results are original as the problem has not been previously solved in a general way neither by applying artificial intelligence techniques. The models have been proved satisfactory with regular and also irregular waves, and they have been used for ship control purposes.

Ultimate compressive strength design methods of aluminum welded stiffened panel structures for aerospace, marine and land-based applications: A benchmark study

The high strength-to-weight advantage of aluminum alloys has made it the material of choice for building airplanes and sometimes for the construction of land-based structures. For marine applications, the use of high-strength, weldable and corrosion resistant aluminum alloys have made it the material of choice for weight sensitive applications such as fast ferries, military patrol craft, luxury yachts and to lighten the top-sides of offshore structures and cruise ships. And while, over the last two decades, the ultimate limit state (ULS) design approach has been widely adopted in the design of aerospace and land-based (steel) structures, it is just recently being considered as a basis for the structural design and strength assessment of ships and offshore structures. Practical ULS methods or design codes are available in the aerospace and civil engineering industries, but they are just now being developed for use by the marine industry. The present paper compares some useful ULS methods adopted for the design of aerospace, marine and land-based aluminum structures. A common practice for aerospace, marine and civil engineering welded stiffened panel applications is discussed.

Fast Ferry Traffic as a Qualitatively New Forcing Factor of Environmental Processes in Non-Tidal Sea Areas: A Case Study in Tallinn Bay, Baltic Sea

The impact of wake wash from high-speed ferries on the coastal environment in non-tidal seas is analysed in terms of wave energy and power, and properties of the largest waves. Shown is that hydrodynamic loads caused by heavy high-speed traffic may play a decisive role not only in low-energy coasts but also in certain areas with high wind wave activity. For example, ship-generated waves form, at least, about 5–8% from the total wave energy and about 18–35% from the wave power in the coastal areas of Tallinn Bay exposed to dominating winds. The periods of wake waves from high-speed ships frequently are much larger than dominating periods of wind waves. The leading waves typically have a height of about 1 m and a period of 10–15 s. Such waves extremely seldom occur in natural conditions in many regions of semi-enclosed seas. They cause unusually high hydrodynamic loads in the deeper part of the nearshore. The fast ferry traffic thus is a qualitatively new forcing component of vital impact on the local ecosystem. It is demonstrated that wakes from high-speed ferries may trigger considerable changes of the existing balance of coastal processes. Owing to their low decay rates combined with their exceptional compactness after crossing many kilometres of the sea surface, such wakes may cause considerable remote impact of the ship traffic. This feature has to be addressed in the analysis of the impact of harbours and associated ship traffic in the neighbourhood of vulnerable areas.

OVERVIEW OF A RESEARCH ON ACTUATORS CONTROL FOR BETTER SEAKEEPING IN FAST SHIPS

The paper is an overview of a research initiated seven years ago by three groups of three universities, under the auspices of a Spanish shipbuilder. The research aims to improve the seakeeping performances of fast ferries, by using moving appendages such transom flaps and T-foil. There is a problem of control design to move the actuators in adequate way, to counteract the effect of encountered waves. The research focuses on alleviation of seasickness. Several aspects have been covered along the research, motivating a long series of papers. The overview gives an ordered account of the main results, with the corresponding references. Main results concerning seasickness and navigation, prediction of seasickness during ship design, experimental modelling, control design and experimental verification, are presented.

Design of a multivariable robust controller to decrease the motion sickness incidence in fast ferries

This work serves as a pioneering contribution in the use of quantitative feedback theory methodology to the design of a controller for a high speed ship. We have improved the design of a multivariable robust controller so that it will be able to reduce incidences of motion sickness on high speed ferries. Motion sickness is caused by vertical accelerations associated with the heave and pitch motions induced by waves. Therefore we are dealing with regulation problems of a highly perturbed system. The design regulator has been validated in sea behaviour trials, using a scaled down replica 1 25 the size of a high-speed ferry.

Development of a Control-Oriented Model of the Vertical Motions of a Fast Ferry

As a main part of a research study on the control of active flaps and a T-foil of a high-speed ferry, a control-oriented model of vertical motions of the ship has been developed. The objective of the control is to improve comfort, decreasing the impact of heave and pitch motions. We have experimental data from a towing tank institution and simulations with PRECAL. The model is based on a decomposition of the physic phenomena into two main aspects: the coupling of the ship with distance between waves and the dynamics of a semisubmerged mass. The model can be handled with MATLAB-SIMULINK, which is useful for studying control strategies. The model shows good agreement (model validation) with the experimental and simulated data for regular and irregular waves. The article shows a methodology, based on MATLAB tools, for obtaining control-oriented models from computer-aided design (CAD)–based programs. That means that the control-oriented model can be derived from the ship design, even before the ship is built.

Some Notes on the Influence of Manufacturing on the Fatigue Life of Welded Aluminum Marine Structures

Designers are constantly looking for ways to reduce the structure weight to lower the overall displacement and hence the cost of fast ferries and other high-speed vessels. The easiest option for the designer is to choose a lightweight material. Aluminum has become the adopted choice of material for high-speed vessels owing to its high strength to weight characteristics. Unlike steel, aluminum is more prone to fatigue cracking and has no fatigue limit. In order to minimize weight, the designer will make use of finite element methods to optimize the scantlings and perform fatigue checks against established codes. This can lead to a structure that has the empirical margins of safety reduced owing to the accuracy of mathematical modeling. However, what is often overlooked is the effect the manufacturing process has on the fatigue life of the fabricated structure. This aspect is excluded from the designer's fatigue calculations, which assist in reducing the scantlings. Currently, there is no guidance for fatigue life reduction for the designer that establishes good and bad workshop practice, other than experience, or the implications of basic shipyard fabrication. It is shown that whereas strain-hardened alloys improve mechanical strength, they reduce ductility. This has consequences when forming the hull plate by potentially introducing crack like flaws into the alloy matrix if the plater overrolls the plate. If there is misalignment or there is too much gap between the plates, the weld will create localized stress concentrations. If the welder has poor joint preparation or gas shielding, porosity can be introduced into the weld. Porosity has a significant effect on the fatigue life of the weldment. This paper brings together a collection of data on such issues that the designer needs to be aware of to prevent an unwanted fatigue failure in the fabrication process.

New results about model based study of seasickness in fast ferries

As part of a research on increasing the passengers comfort in fast ferries by using moving actuators, some study was devoted to the application of control-oriented models to predict seasickness. The key idea is to apply an analogy of three filters. In a previous paper, some criteria were presented concerning the captain, on how to avoid navigation parameters inducing seasickness, and also criteria were given for the ship designer. In this paper new results are presented, linking two seasickness indexes, and offering a frequency domain method to estimate the comfort of a ship from the beginning of the ship design.