Catamaran Research Articles

Numerical investigation on seakeeping performance of SWATH with Three Dimensional Translating-pulsating Source Green Function

The seakeeping performance of Small Waterplane Area Twin Hull (SWATH) was investigated by the boundary element method based on the Three Dimensional Translating-pulsating Source Green Function (3DTP). Some factors that differ from conventional monohull ships, but may have great influence on SWATH were included in the equations of motion, namely the hydrodynamic interaction between the twin hulls, the viscous damping and the effect of stabilizing fins. Numerical simulations have been carried out to predict the wave loads and motions of several SWATH vehicles. The present method has been validated by comparison with both the strip theory formulation (STF) method and the experiment. The result indicates that the predictions of heave responses from both numerical methods agrees well with that of the experiment at different forward speed, while the discrepancies between numerical methods and the experiment are much more pronounced at pitch motions. Generally, the motions are better predicted by the present method based on 3DTP than by the STF method, particularly for pitch motion.

Station-keeping control of an unmanned surface vehicle exposed to current and wind disturbances

Field trials of a 4m long, 180kg, unmanned surface vehicle (USV) have been conducted to evaluate the performance of station-keeping heading and position controllers in an outdoor marine environment disturbed by wind and current. The USV has a twin hull configuration and a custom-designed propulsion system, which consists of two azimuthing thrusters, one for each hull. Nonlinear proportional derivative, backstepping and sliding mode feedback controllers were tested in winds of about 4–5 knots, with and without wind feedforward control. The controllers were tested when the longitudinal axis of the USV was aligned with the mean wind direction and when the longitudinal axis was perpendicular to the mean wind direction. It was found that the sliding mode controller performed best overall and that the addition of wind feedforward control did not significantly improve its effectiveness. However, wind feedforward control did substantially improve the performance of the proportional derivative and backstepping controllers when the mean wind direction was perpendicular to the longitudinal axis of the USV. An analysis of the length scales present in the power spectrum of the turbulent speed fluctuations in the wind suggests that a single anemometer is sufficient to characterize the speed and direction of the wind acting on the USV.

A modified BLISCO method and its combination with variable fidelity metamodel for engineering design

Purpose – The purpose of this paper is to present a modified bi-level integrated system collaborative optimization (BLISCO) to avoid the non-separability of the original BLISCO. Besides, to mitigate the computational burden caused by expensive simulation codes and employ both efficiently simplified and expensively detailed information in multidisciplinary design optimization (MDO), an effective framework combining variable fidelity metamodels (VFM) and modified BLISCO (MBLISCO) (VFM-MBLISCO) is proposed. Design/methodology/approach – The concept of the quasi-separable MDO problems is introduced to limit range of applicability about the BLISCO method and then based on the quasi-separable MDO form, the modification of BLISCO method without any derivatives is presented to solve the problems of BLISCO. Besides, an effective framework combining VFM-MBLISCO is presented. Findings – One mathematical problem conforms to the quasi-separable MDO form is tested and the overall results illustrate the feasibility and robustness of the MBLISCO. The design of a Small Waterplane Area Twin Hull catamaran demonstrates that the proposed VFM-MBLISCO framework is a feasible and efficient design methodology in support of design of engineering products. Practical implications – The proposed approach exhibits great capability for MDO problems with tremendous computational costs. Originality/value – A MBLISCO is proposed which can avoid the non-separability of the original BLISCO and an effective framework combining VFM-MBLISCO is presented to efficiently integrate the different fidelities information in MDO.

Farfield waves created by a catamaran in shallow water

The farfield waves created by a catamaran with identical twin hulls that advances at constant speed along a straight path in calm water of uniform finite depth are considered. The catamaran is represented as two point sources at the twin bows of the catamaran. This simple approximation of a catamaran as a 2-point wavemaker is realistic for the high-speed regime considered here. Constructive interferences between the divergent waves created by the two point sources result in an apparent wake angle, where the highest waves are found, that can be much narrower than the wake angle associated with the cusps or the asymptotes of the well-known wave patterns radiated from a single point. The farfield wave pattern of a high-speed catamaran can then differ markedly from the wave pattern obtained in the classical analysis of the waves created by a ship modeled as a 1-point wavemaker, in which interference effects are ignored. Moreover, the highest waves created by a catamaran can be much shorter than the waves at the cusps of the wave pattern, and significantly shorter than the ship length. Indeed, the wavelength of the highest waves is equal to the spacing between the twin hulls of the catamaran at high Froude numbers. Wave interferences between the twin bows of a catamaran (dominant at high Froude numbers) are compared to wave interferences between the bow and the stern of a monohull ship or a catamaran (significant at moderate Froude numbers).

The numerical simulation of a towing cylinder based on immersed body boundary method

An immersed body boundary method is adopted to track the motions of a towing cylinder, and a homogenous multiphase Eulerian-Eulerian fluid approach is used to capture the free surface. The Reynolds average Navier-Stockes (RANS) solver is applied to all gird nodes to deal with different velocities of the nodes that are in the body boundary, near the boundary and out of the boundary and their effect on the fluid. The towing cylinder resistance at different submerged depths in the tank is presented. The simulation results are compared with the experimental data, and the method is verified and validated. Finally, the hydrodynamic characters of the cylinder are discussed further. The numerical and experimental results show that at high speeds, the deeper the cylinder submerges, the lower resistance it suffers. The resistance coefficient trough is obtained at Froude number in the range of 0.3 < Fr < 0.4. These phenomena can provide some suggestions on the small waterplane area twin hull (SWATH) design.

Resistance and seakeeping numerical performance analyses of a semi-small waterplane area twin hull at medium to high speeds

The hydrodynamic analysis of a new semi-small waterplane area twin hull (SWATH) suitable for various applications such as small and medium size passenger ferries is presented. This may be an attractive crossover configuration resulting from the merging of two classical shapes: a conventional SWATH and a fast catamaran. The final hull design exhibits a wedge-like waterline shape with the maximum beam at the stern; the hull ends with a very narrow entrance angle, has a prominent bulbous bow typical of SWATH vessels, and features full stern to arrange waterjet propellers. Our analysis aims to perform a preliminary assessment of the hydrodynamic performance of a hull with such a complex shape both in terms of resistance of the hull in calm water and seakeeping capability in regular head waves and compare the performance with that of a conventional SWATH. The analysis is performed using a boundary element method that was preliminarily validated on a conventional SWATH vessel.

Preparing a wave adaptive modular vessel for automation

The ride control systems of high-speed vessels frequently use active stern tabs for both motion control and maintenance of correct trim at various speeds and sea conditions. This paper investigates the effect of water depth on the lift force provided by stern mounted trim tabs, of the type fitted to INCAT high speed wave-piercer catamaran vehicle ferries and similar vessels. This investigation was carried out at model scale with the use of a test apparatus in a flume tank in the University of Tasmania hydraulics laboratory. The lift force magnitude and location were measured over a range of tab angles and flow depths. This was used to calculate the lift coefficient of the tab and asses the performance of the tab over the range of flow depths. It was found that the lift force increased and the force location progressed further forward of the hinge as flow depth decreased. The lift curve slope of the stern tab increased by a factor of over 3 relative to the deep water value when the water depth below the hull was approximately equal to the tab chord. The deep water lift curve slope appears to be approached only when the water depth exceeded 4 or more tab chord lengths. The centre of pressure of the lift force was more than two chord lengths ahead of the tab hinge, showing that most of the lift produced by the tab was under the hull rather than on the surface of the tab itself.

Observations and computations of narrow Kelvin ship wakes

Computations of far-field ship waves, based on linear potential flow theory and the Hogner approximation, are reported for monohull ships and catamarans. Specifically, far-field ship waves are computed for six monohull ships at four Froude numbers F≡V/gL=0.58, 0.68, 0.86, 1.58 and for six catamarans with nondimensional hull spacing s≡S/L=0.25 at two Froude numbers Fs≡V/gS=1 and 2.5. Here, g is the gravitational acceleration, V and L denote the ship speed and length, and S is the separation distance between the twin hulls of a catamaran. The computations show that, although the amplitudes of the waves created by a ship are strongly influenced by the shape of the ship hull, as well known, the ray angles where the largest waves are found are only weakly influenced by the hull shape and indeed are mostly a kinematic feature of the flow around a ship hull. An important practical consequence of this flow feature is that the apparent wake angle of general monohull ships or catamarans (with arbitrarily-shaped hulls) can be estimated, without computations, by means of simple analytical relations; these relations, obtained elsewhere via parametric computations, are given here. Moreover, the influence of the two parameters Fs and s that largely determine the ray angles of the dominant waves created by a catamaran is illustrated via computations for three catamarans with hull spacings s=0.2, 0.35, 0.5 at four Froude numbers Fs=1, 1.5, 2, 2.5. These computations confirm that the largest waves created by wide and/or fast catamarans are found at ray angles that only depend on Fs (i.e. that do not depend on the hull spacing s) in agreement with an elementary analysis of lateral interference between the dominant waves created by the bows (or sterns) of the twin hulls of a catamaran. The dominant-waves ray angles predicted by the theory of wave-interference effects for monohull ships and catamarans are also compared with the observations of narrow Kelvin ship wakes reported by Rabaud and Moisy, and found to be consistent with these observations.

Hydrodynamic Characterization of USV Vessels with Innovative SWATH Configuration for Coastal Monitoring and Low Environmental Impact

The high costs associated with the use of research oceanographic vessels and the maturity of the unmanned surface vehicles (USV) makes now possible to develop systems for monitoring coastal areas based on networks of independent USVs. This type of vessels is a valid alternative to conventional vessels, which have a limited mission profile due to their high environmental impact (conventional propulsion systems based on polluting fossil fuels) inhibiting their access to protected coastal regions. Moreover, conventional vessels have high hydrodynamic resistance (limiting the autonomy) producing high levels of noise that can dramatically influence the monitoring equipment shipped: beside the environmental impact reduction, there is also the necessity of low-resistance/low-noise hydrodynamic specification. Consequently, the coastal monitoring (of also protected regions) needs unconventional vessels able to address both the issues related to the environmental impact and the hydrodynamic performance.In this framework, this work aims to characterize the hydrodynamic performance of a system based on USV units able to launch and recover autonomous vehicles of different nature (gliders, AUVs, motor-gliders, wire-guided ROVs), and able to acquire environmental data (in the column water from free-surface to the sea floor), in order to meet the requirements of civil and military applications. The cutting-edge aspects that characterize the USV studied are the hull SWATH type (Small Waterplane Area Twin Hulls) non-conventional, optimized so as to ensure a unique seakeeping and a reduced resistance, along with the propulsion system with propellers in mantle, developed to combine propulsive efficiency and low noise. In the present paper, a SWATH-shaped USV designed for monitoring of protected coastal regions is numerically studied solving the Navier-Stokes equations on the fully appended vessels with several environmental conditions. An accurate hydrodynamic characterization will presented in order to investigate its performances and eventual maneuverability issues.

Interference effects on the Kelvin wake of a catamaran represented via a hull-surface distribution of sources

Wave-interference effects on the far-field waves created by a catamaran, with identical twin hulls of length L at a lateral separation distance S, that advances at constant speed V along a straight path in calm water of large depth are considered. Systematic computations are performed for 125 Froude numbers Fs≡V/gS (where g denotes the acceleration of gravity) within the range 0.4≤Fs≤3.5, 25 hull spacings s≡S/L within the range 0.2≤s≤0.8, and seven simple mathematically-defined hulls that correspond to a broad range of main hull-shape parameters (beam/length, draft/length, beam/draft, waterline entrance angle), i.e. for 21,875 distinct cases in all. The two dominant wake angles ψi and ψo that correspond to the ray angles ψ=±ψi and ψ=±ψo where the largest divergent waves are found within the Kelvin wake −ψK≤ψ≤ψK with ψK≈19∘28′ are determined numerically via a realistic yet practical method. This practical method, used previously for monohull ships, is based on the numerical determination of the two major peaks of the amplitude function in the Fourier-Kochin representation of far-field ship waves, evaluated via the Hogner approximation and the stationary-phase approximation. The computations show that the hull shape only has a relatively small influence on the wake angles ψi and ψo associated with the dominant waves. A useful practical consequence of this numerical finding is that the two wake angles ψi and ψo can be estimated, without computations, for general catamarans in terms of the Froude number Fs and the hull spacing s via simple analytical approximations, obtained here via parametric computations. The computations also show that lateral interference effects are dominant if s and/or Fs are sufficiently large, i.e. for ‘wide’ and/or ‘fast’ catamarans. Moreover, the computations reported here for catamarans represented via hull-surface distributions of sources show that the basic two-point wavemaker model of interference between the waves created by the twin bows of a catamaran is realistic for wide and/or fast catamarans. However, wave-interference effects are more complicated if both s and Fs are small, i.e. for ‘narrow slow’ catamarans. The numerical analyses of dominant waves considered earlier for monohull ships and here for catamarans show that, although the amplitudes of the waves created by a ship are strongly influenced by the shape of the ship hull, the ray angles where the largest waves are found are mostly a kinematic feature that is only weakly influenced by the hull shape and the related wave amplitude.

Seakeeping Analysis of Two Medium-speed Twin-hull Models

Seakeeping Analysis of Two Medium-speed Twin-hull Models

Seakeeping Analysis of two Medium-speed Twin-hull Models

The use of twin-hull ships for high-speed passenger and car-passenger transportation is widespread, whereas their potential use for high-speed cargo transportation was estimated as limited. The present article discusses the seakeeping performance of two twin-hull models of an innovative medium-speed container ship design. Their hull form was the result of a thorough hydrodynamic optimization performed at the School of Naval Architecture and Marine Engineering of NTUA aiming to minimize the calm water resistance within the EU-funded project "EU-CargoXpress." The seakeeping analysis was performed by applying numerical tools and also by performing a series of experiments in the towing tank of NTUA and MARINTEK. The obtained results are presented and discussed.

SWATH- A new concept for the Safety and Security at Sea

SWATH means Small Waterplane Area Twin Hull and is guarantor for excellent sea keeping behaviour.The basis of this ship type was born in the 1960’s and 70’s when semi-submersible vessels were introduced to the offshore market. Naval architects developed these vessels further and it was used as navy test craft, pilot tender, yachts, passenger boats etc.The German Shipyard Abeking &amp; Rasmussen has more than 100 year of experience in innovative ships and designs. Since 1999 more than 20 SWATH vessels like pilot tender, pilot station vessel, explorer yacht, wind park maintenance tender, offshore patrol vessels and a hydrographical research vessel were built.This paper should give an insight to the SWATH technology and the implementation in the market. Different applications and ideas will be shown. Whatever you need - a navy ship, a workboat, a multipurpose platform a ferry or a yacht – there are many possibilities.

Viscosity and nonlinearity effects on the forces and waves generated by a floating twin hull under heave oscillation

Nonlinear hydrodynamics of a twin rectangular hull under heave oscillation is analyzed using numerical methods. Two-dimensional nonlinear time-domain solutions to both inviscid and viscous problems are obtained and the results are compared with linear, inviscid frequency-domain results obtained in [26] to quantify nonlinear and viscous effects. Finite-difference methods based on boundary-fitted coordinates are used for solving the governing equations in the time domain [2]. A primitive-variables based projection method [6] is used for the viscous analysis and a mixed Eulerian–Lagrangian formulation [11] for inviscid analysis. The algorithms are validated and the order of accuracy determined by comparing the results obtained from the present algorithm with the experimental results of Vugt [22] for a heaving rectangle in the free surface. The present study on the twin-hull hydrodynamics shows that at large and non-resonant regular frequencies, and small amplitude of body oscillation, the fluid viscosity does not significantly affect the wave motion and the radiation forces. At low frequencies however the viscosity effect is found to be significant even for small amplitude of body oscillation. In particular, the hydrodynamic force obtained from the nonlinear viscous analysis is found to be closer to the linear inviscid force than the nonlinear inviscid force to the linear inviscid force, the reason for which is attributed to the wave dampening effect of viscosity. Since the wave lengths generated at smaller frequencies of oscillation are longer and therefore the waves could have a more significant effect on the dynamic pressure on the bottom of the hulls which contribute to the heave force, the correlation between the heave force and the wave elevation is found to be larger at smaller frequencies. Because of nonlinearity, the wave radiation and wave damping force remained nonzero even at and around the resonant frequencies – with the resonant frequencies as determined in [26] using linear potential flow theory. As to be expected, the nonlinear effect on the wave force is found to be significant at all frequencies for large amplitude of oscillation compared to the hull draft. The effect of viscosity on the force, by flow separation, is also found to be significant for large amplitude of body oscillation.

Determination of the wind heeling moment for the small waterplane area twin hull ship

Determination of the wind heeling moment for the small waterplane area twin hull ship

Powering and Motion Analysis of Twin-Strut Swath Vessels

The hydrodynamic characteristics of three different underwater hull forms of the SWATH type are studied based on their powering and motion performance. A SWATH is a twin-hull ship design that holds various advantages over the conventional mono-hulls. The shape of the underwater hull and the struts determine the resistance and motion characteristics of the SWATH. It is important to arrive at an optimum hull shape for maximum performance. SHIPFLOW, a CFD tool developed especially for the marine industry, is used to estimate and compare the resistance and powering characteristics of the three designs. SEDOS, a motion analysis package based on strip theory, is used to estimate the seakeeping characteristics of the hulls. Comment is made on the actual performance on trials when compared against the tank test results, together with the results from various numerical prediction methods. Based on the results for the three hulls, the most optimum design is found out. Keywords: Resistance, Seakeeping, SEDOS, SHIPFLOW, SWATH

A new methodology for multi-objective multidisciplinary design optimization problems based on game theory

The design of engineering systems often involves multiple disciplines and competing objectives, which requires coordination, information exchange and share amongst the disciplines. However, in practical design environments, designers have to make decisions in isolation due to organization barriers, time schedules and geographical constraints. This paper will propose a new approach for the multi-objective multidisciplinary design optimization (MDO) problems in non-cooperative environments based on gene expression programming (GEP) and Nash equilibrium in the game theory. In this approach, the GEP method is used as a surrogate to construct the approximate rational reaction sets (RRSs) in the Nash model. The effectiveness of the proposed method is demonstrated by the design of a thin-walled pressure vessel and the hull form parameter design of a small waterplane area twin hull (SWATH) ship. The results show that this approach can fully explore and provide the explicit functional relationship between the strategy of an isolated player and the control variables of the other players, thus able to obtain a better Nash equilibrium solution.

Characterization and System Identification of an Unmanned Amphibious Tracked Vehicle

Experimental testing of an unmanned amphibious tracked vehicle on dry sand, in the surfzone and in water, has been performed to explore its maneuvering and performance characteristics for the planned future development of an automatic control system. The 2.69-m-long, 295-kg concept vehicle utilizes a small waterplane area twin hull (SWATH) configuration with integrated crawler tracks and twin propellers for propulsion. Maximum traversable incline tests on dry sand reveal that the vehicle has a drawbar pull (DP) of about 1000 N (about one third of its weight) and can operate on slopes of 15 ° briefly and 10 ° for extended periods of time. Maneuvering tests were performed on flat, dry sand, as well as into and out of a surfzone at a beach site with inclines ranging from about 3 ° to 6 ° . In each of the tests, the track forces, speed over ground, and both linear and rotational accelerations were measured. Also recorded were environmental conditions, such as wind speed, significant waveheight/period, and ground slope. The tests reveal that the vehicle has a maximum straight-line flat ground speed of about 2 m/s, a minimum flat ground turning radius of 2.4 m, an in-water minimum turning radius of 1.9 m, and a maximum straight-line waterborne speed of 1.2 m/s. It is also shown that system identification applied to the data recorded from the flat ground and waterborne maneuvering tests can be used to find a linear, parametric state-space model for the vehicle that adequately reproduces its motion. Land-to-sea transition tests in two different seas of one-third significant waveheights of 8 and 28 cm (both measured at a distance of 6 m from the mean waterline on the beach) show that the vehicle exhibits substantial track slip as it traverses into the surfzone and its weight becomes increasingly supported by the displacement of its hulls. The tractive force model developed by Wong (Theory of Ground Vehicles, New York, NY, USA: Wiley, 2001) is modified to account for the reduction in track-supported vehicle weight as the vehicle becomes waterborne. It is shown that this adapted model captures the main physical features of the measured track forces. The waveheights and periods recorded during surfzone transition tests are used to examine the seakeeping properties of the vehicle. It is found that the vehicle's natural frequency of roll is near the dominant wave frequencies measured. A Froude-Krylov strip theory simulation shows that the wave forces acting on the vehicle in 28-cm waves may be slightly larger than the force measured on each track when the DUKW-Ling is crossing the transition zone between the beach and surfzone and should not be ignored in modeling and simulation studies.

Numerical analysis of the interference effects on resistance, sinkage and trim of a fast catamaran

The purpose of the present paper is the numerical investigation of the interference phenomena between the waves generated by the individual hulls of a catamaran. The study focuses on the effects of both Froude number and demi-hull separation distance on resistance and on sinkage and trim. The numerical simulations are carried out by the URANS solver CFDShip-Iowa V.4 and, to assess the capability for prediction of resistance, sinkage and trim of the URANS code for twin-hull configuration vessels, a verification and validation study is performed for global as well as for local variables. A very good agreement between the numerical results and the experimental data is obtained, and the validation study demonstrates the high level of accuracy of the current predictions, which are used to have a better insight into the interference phenomena. In accordance with the experiments, within Fr = 0.45–0.65, the catamaran has a significantly higher resistance coefficient compared to the mono-hull; furthermore, the C T value increases with decreasing the separation distance between the twin hulls. On the contrary, at Fr lower than 0.45 and at Fr higher than 0.65, the effects of hull spacing on resistance as well as on sinkage and trim can be neglected. The flow field characteristics, wave pattern, wave cuts and pressure distributions are analyzed through the CFD analysis. Finally, the effects of the Reynolds number on resistance are also investigated and results show a small decrease in interference with increasing the Reynolds number.

Underwater image bidirectional matching for localization based on SIFT

For the purpose of identifying the stern of the SWATH (Small Waterplane Area Twin Hull) availably and perfecting the detection technique of the SWATH ship’s performance, this paper presents a novel bidirectional image registration strategy and mosaicing technique based on the scale invariant feature transform (SIFT) algorithm. The proposed method can help us observe the stern with a great visual angle for analyzing the performance of the control fins of the SWATH. SIFT is one of the most effective local features of the scale, rotation and illumination invariant. However, there are a few false match rates in this algorithm. In terms of underwater machine vision, only by acquiring an accurate match rate can we find an underwater robot rapidly and identify the location of the object. Therefore, firstly, the selection of the match ratio principle is put forward in this paper; secondly, some advantages of the bidirectional registration algorithm are concluded by analyzing the characteristics of the unidirectional matching method. Finally, an automatic underwater image splicing method is proposed on the basis of fixed dimension, and then the edge of the image’s overlapping section is merged by the principal components analysis algorithm. The experimental results achieve a better registration and smooth mosaicing effect, demonstrating that the proposed method is effective.