Stepped Hull Research Articles

Impact of Stepped Hull Design on Speed Boat Performance: A CFD Study

Impact of Stepped Hull Design on Speed Boat Performance: A CFD Study

Numerical Investigation of Single and Double Steps in Planing Hulls

Incorporating steps into a hull reduces the wetted surface, promoting improved hydrodynamic lift and reduced resistance at high speeds, provided that the step is designed appropriately. Traditional hydrodynamics studies rely on scaled model testing in towing tanks, but numerical tools offer a more efficient alternative. This study focused on investigating the hydrodynamic performance of stepped hulls by modifying the parent hull of the Naples Systematic Series (C1). The Computational Fluid Dynamics (CFD) code SIEMENS PLM STAR CCM+ version 2302 was used for simulations, including four different beam Froude numbers (FrB = 1.13, 2.22, 2.56, and 2.96) and a total of 15 hull configurations with single and double steps. By employing a three-dimensional computational analysis of multiphase flow using Dynamic Fluid–Body Interaction (DFBI) and overset mesh, various performance parameters such as resistance coefficient, dimensionless wetted surface, sinkage, and dynamic trim were analyzed. The accuracy of the CFD results was confirmed through comparison with experimental data and grid uncertainty assessment. The study demonstrated that placing a single step near the transom decreased trim and increased resistance and wetted surface. Conversely, positioning a step in the forward section reduced the trim angle at lower step heights but increased trim at higher step heights in single-stepped hulls. The application of these findings contributes to the design optimization of stepped hulls for enhanced performance in high-speed maritime applications.

A revisited verification and validation analysis for URANS simulation of planing hulls in calm water

Verification and Validation (V&V) is the foremost analysis which is carried out for evaluation of the accuracy level and dependability of computational fluid dynamic (CFD) simulations. The present study investigates the V&V of CFD models in predicting the dynamic trim and hull resistance of high-speed planing hulls with an aim to provide a deeper understanding of V&V analysis in this specific field of application. Two different planing hulls, namely the C05 stepped hull and the C1 interceptor hull, are analyzed with four different grids and time-steps using two mesh motion techniques, namely overset and morphing approach. The discretization (grid) and time-step uncertainties for each CFD simulation are estimated using the least squares method. The results indicate that the overset mesh approach performs better than the morphing grid method in terms of numerical uncertainty and validation achieved for both hulls. The error of both techniques in the prediction of resistance and trim angle of the boat shows an acceptable range of accuracy. The findings provide valuable insights for simulation-based designing and optimizing high-speed planing hulls, specifically by identifying the optimal mesh technique, cell number, and time-step for accurate prediction of wetted surface shape, ventilation formation, running attitude, and resistance.

Analysis of the double steps position effect on planing hull performances

Along with developing high-speed craft technology, the planing hull is growing with modifications for better performance. One such technology is stepped hull, both single and double. Planing hull with steps allows the boat to run at a relatively low drag-lift ratio with lower frictional resistance due to reduced wetted area. In this study, the hull was modified with variations in the position of the double steps, which aimed to determine the effect of the first and second step positions on the total resistance, dynamic trim, and dynamic sinkage generated by computational fluid dynamics (CFD). Based on the analysis results, variations in the position of the stepped can change the hull performance. Shortening the distance between the two steps and moving both rearwards toward the transom can lower the total resistance. The dynamic trim and dynamic sinkage decreased as the position of the two steps was shifted further forward. An equation created in a non-dimensional form relates the positions of two steps to the desired results of total resistance, dynamic trim, and dynamic sinkage, namely: {(x1-x2)/L + (x1x2)/(LB)} × Fr∇, where x1 is distance the first step from transom, x2 is the distance of the second step, L is the length of the boat, B is the beam of the boat, and Fr∇ is the volume Froude number.

A comprehensive stepped planing hull systematic series: Part 1 - Resistance test

This work addresses the experimental study of a new systematic series of stepped planing hulls. Indeed, the interest in the stepped planing hulls is constantly growing, both in the industrial/commercial and academic fields. Designers and boat builders have been orienting toward the multi-stepped hulls solution to ensure good dynamic stability, reliable seakeeping and operability at high speeds. However, there is a lack of a comprehensive stepped hull systematic series with various step configurations including a forward V-shaped step, as typically used on modern boats. For the abovementioned reasons, a systematic series of eight different models of stepped hulls have been developed and tested. The towing tank tests have been carried out at the naval basin of the Università degli Studi di Napoli “Federico II” Dipartimento di Ingegneria Industriale (DII) in calm water at different speeds (Fr∇ = 1.077–6.774) and for three different static trim conditions. All models are built with a transparent bottom to visualize the wetted surface and the eventual development of vortices generated behind the step. The eight models are defined by modifying three significant design parameters for stepped hulls (i.e. the number of steps, longitudinal step position, and step height).

Effect of Ventilation on Cavity Formation on Stepped Planing Hulls

_ A stepped planing hull is a type of high-speed boat with a transverse break in the bottom of the hull. The step allows the hull to run at a higher trim angle, and also reduces the wetted surface area at planing speeds because of a pocket of air that forms aft of the step, often reducing the frictional resistance. This paper summarizes model tests that were conducted on a prismatic planing hull, with two adjustable stern sections, to form either a single- or twin-step planing hull. Measurements were made of the resistance, heave, wetted lengths, and cavity pressures behind the steps for various speeds at fixed trim. The ventilation cases included natural ventilation, augmented ventilated by means of vent tubes located behind the step, and reduced ventilation, where the flow was blocked off from the step by closing the vent tubes and adding longitudinal fences to either chine to prevent air inflow. Blocking off the ventilation causes negative pressures to form and increases resistance. Tests conducted in waves showed that the size of the ventilation cavity behind a step is not affected significantly by small waves, indicating a shielding effect of the hull. Introduction Stepped hulls are becoming increasingly commonplace in high-speed craft for both military and recreational purposes. A stepped hull is a type of planing boat, in which there is a discontinuity in the bottom, resulting in flow separating from the step and reattaching father aft along the hull. This feature can reduce resistance in some cases. Although they’ve been in use for over a century, they remain a challenge to design and to physically model.

Experimental measurement of the nearfield longitudinal wake profiles of a high-speed prismatic planing hull

ABSTRACT This study details an extensive investigation into the nearfield longitudinal wake profiles of a high-speed planing hull. Developing understanding this flow is benaficial to the designers of stepped hulls in determining how the forebody’s wake intersects with the afterbody. As no experimental data was available, a novel small-scale model-testing programme was undertaken, measuring Centreline (CL) and Quarter Beam (QB) nearfield longitudinal wake profiles, as well as the forces. The nearfield wake profile was found to exhibit trends in agreement with the findings of studies investigating the mid and far-field wake. The results of this experimental work are provided in full as validation data. Savitsky’s Wake Equations and the Linear Wake Assumption are then assessed to determine the level of confidence with which they may be used to model this flow. Savitsky’s Wake Equations displayed a surprising level of accuracy whereas the Linear Wake Assumption was invalid for the conditions investigated.

A critical CAE analysis of the bottom shape of a multi stepped air cavity planing hull

One of the most important aspects, in the ACS and multi stepped hull design, is the choice of the geometrical shape of the cavity and the steps. In this article a complete experimental and numerical campaign on a multi stepped ACS has been carried out, varying the velocity and the air flow rate under the hull. The experimental tests have been conducted in an ITTC Towing Tank allowing to validate the numerical simulations obtained by means of a CFD U-RANSe (Unesteady Reynolds Averaged Navier-Stokes equations) code. The CFD setup is described in detail. From this campaign a critical analysis of the Froude number influence on the air cushion shape has been argued. The authors identified four different behaviours, from low to very high Froude numbers. The use of CFD has allowed to observe quantities of difficult evaluation by means of traditional experimental test, as e.g. the frictional component of the resistance, the airflow path lines and the volume of fraction in transversal and longitudinal sections. The results have been discussed.

Hydrodynamic study of a double-stepped planing craft through numerical simulations

This paper investigates the performance and hydrodynamic characteristics of a double-stepped planing hull and the effects of adding two steps to the bottom of a mono-hull. To study these effects, a non-stepped model with similar characteristics of a stepped hull is also modeled. The numerical simulations are conducted in different stages. First, a mesh study is performed and an optimum mesh size is adopted. Subsequently, the predicted resistances are compared against experimental data and good agreement is observed. Later, the targeted simulations are performed at five different Froude numbers and various characteristics are determined. The results of these studies indicate that frictional resistance of the double-stepped model is drastically smaller than that of the non-stepped model, while pressure drag of the stepped vessel is slightly larger than the non-stepped model. It is observed that adding steps to the hull does not reduce the wetted surface at lower Froude numbers, but its positive effect appears when Froude number exceeds 2.0. It is also seen that generated transom wave behind the double-stepped hull is larger than that of the non-stepped hull. In addition, the computed pressure distributions over the center line of both models indicate that the first maximum pressure of the double-stepped model is larger than that of the non-stepped hull. Ultimately, it is concluded that hydrostatic pressure has an essential role in producing the lift force of a non-stepped planing hull, but this pressure has very little contribution in generation of the lift force of the double-stepped hull.

A Validation of Symmetric 2D + T Model Based on Single-Stepped Planing Hull Towing Tank Tests

In the current article, the hydrodynamic forces of single-stepped planing hulls were evaluated by an analytical method and compared against towing tank tests. Using the 2D + T theory, the pressure distribution over the wedge section entering the water and the normal forces acting on the 2D sections have been computed. By integrating the 2D sectional normal forces over the entire wetted length of the vessel, the lift force acting on it has been obtained. Using lift forces as well as the consequence pitch moment, the equilibrium condition for the single-stepped planing hull is found and then resistance, dynamic trim, and the wetted surface are computed. The obtained hydrodynamic results have been compared against the experimental data and it has been observed that the presented mathematical model has reasonable accuracy, in particular, up to Froude number 2.0. Furthermore, this mathematical model can be a useful and fast tool for the stepped hull designers in the early design stage in order to compare the different hull configurations. It should also be noted that the mathematical model has been developed in such a way that it has the potential to model the sweep-back step and transverse the vertical motions of single-stepped planing hulls in future studies.

Numerical investigation of hydrodynamic forces acting on the non-stepped and double-stepped planing hulls during yawed steady motion

This article focuses on the steady motion of yawed planing hulls with emphasis on the effects of adding steps to the bottom of these vessels on the hydrodynamic forces and moments acting on the boat. To analyze the problem, the Ansys-CFX software is used and three different planing hulls are investigated in steady yawed condition. The main targeted results include hydrodynamic forces and moments acting on the boat at different yaw angles and beam Froude numbers which provide important insights regarding the effects of loading and adding step on these forces and moments. The numerically predicted sway forces are compared against experimental data, suggesting that the current numerical model predicts sway and surge forces with reasonable accuracy. Moreover, it is observed that surge force coefficient of the investigated prismatic planing hull with light loading condition does not change significantly when the hull is relocated in a yaw angle, while it is remarkably affected when the boat is heavy. Furthermore, it is observed that this prismatic planing hull has smaller rolling moment in a steady yawed motion, when it moves at larger beam Froude number. Meanwhile, the computed yawing moments of this hull indicate that an increase in speed does not change this moment notably, while an increase in its weight yields larger yawing moment. Comparison of the results of stepped and non-stepped planing hulls indicates that surge force coefficient of the stepped hull is larger, while its sway force and rolling moment are smaller. This is mainly caused by the shape of the interrupted wetted surface and larger number of maximum pressure area in the stepped planing hull. Finally, it is concluded that there is no significant difference between the yawing moment of the investigated stepped and non-stepped planing hulls.

A simplified method to calculate trim and resistance of a two-stepped planing hull

ABSTRACTIn a creative design process of planing craft, stepped hulls could fill a gap in the planing craft industries in response to low drag and high speed demands. However, there exists a need for new computational tools for performance prediction of such hulls. Therefore, in the current work, an attempt has been made to develop a mathematical model for performance prediction of two-stepped hulls. Savitsky's mathematical model has been modified, and in conjunction with linear wake theory, a new mathematical model has been proposed, which would enable prediction of trim, resistance and other parameters related to planing hulls with transverse steps. To validate the proposed model, existing experimental data have been used. The obtained results are in good agreement with experimental data. As such, developed mathematical model can be used in conceptual design phase of stepped hulls with transverse steps.

Experimental and numerical hydrodynamic analysis of a stepped planing hull

This work addresses the experimental and numerical study of a stepped planing hull and the related fluid dynamics phenomena typically occurring in the stepped hull in the unwetted aft body area behind the step. In the last few years, the interest in high-speed planing crafts, with low weight-to-power ratios, has been increasing significantly, and, in such context, naval architects have been orienting toward the stepped hull solution. Stepped planing hulls ensure good dynamic stability and seakeeping qualities at high speeds. This is mainly due to the reduction of the wetted area, which is caused by the flow separation occurring at the step. This paper presents the experimental results of towing tank tests in calm water on a single-step hull model, which is the first model of a new systematic series. The same flow conditions are analyzed via Reynolds Averaged Navier-Stokes (RANS) and Large Eddy Simulations (LES), with different moving mesh techniques (overset/chimera and morphing grid), performed at different model speeds. The numerical results are in accordance with experimental data, and overset/chimera grid is found to be the best approach between the analyzed ones. The flow patterns obtained numerically through LES on a refined grid appear similar to the ones observed in towing tank investigations through photographic acquisitions. These flow patterns are dominated by a rather complex 3D arrangement of vortices originating from air spillage at both sides of the step. The understanding of these phenomena is important for the effectiveness of stepped hull designs.

Effect of deadrise angles on hydrodynamic performance of a stepped hull

Stepped planning hulls are commonly used on fast planning boats and water-based aircraft, but numerical means for simulating their hydrodynamics are rather limited. In this study, a panel method involving hydrodynamic sources has been applied for modeling a stepped planning hull and accounting for water surface deformations in the air-ventilated cavity and surrounding water domain. Parametric calculations have been carried out for a selected hull with variable deadrise angles and at two speed regimes. Results are shown for the boat attitude, lift–drag ratio, and drag components. The variations of the wetted surfaces on two bodies of a stepped hull are also presented and related to variations in hydrodynamic characteristics of the hull.

Hydrodynamic modeling of semi-planing hulls with air cavities

ABSTRACT: High-speed heavy loaded monohull ships can benefit from application of drag-reducing air cavities under stepped hull bottoms. The subject of this paper is the steady hydrodynamic modeling of semi-planing air-cavity hulls. The current method is based on a linearized potential-flow theory for surface flows. The mathematical model description and parametric calculation results for a selected configuration with pressurized and open air cavities are presented.

Numerical investigation of a stepped planing hull in calm water

Stepped planing hulls enable the feasibility of running at relatively low Drag-Lift ratio by means of achieving more optimal trim angle at high speeds. Currently, there is no precise method to analyze these hulls over the full range of operating speeds. In this study, a three-dimensional computational fluid dynamics (CFD) model using volume of fluid (VOF) approach is presented for examining the characteristics of a planing hull having one transverse step. A procedure is presented to transform a series of fixed-position simulations into a free to heave and pitch simulation. Resistance, lift, running draft, dynamic trim angle, and wetted area are compared with available experimental data and those of a semi-empirical method at volumetric Froude number in the range of 2.41–7.12. Centerline wake profile and reattachment location are compared with those of empirical equations. Wetted area at forebody chines-dry mode is qualitatively compared with a typical under water photograph of a stepped hull. The quantitative and qualitative results are found in acceptable correlation with experimental data, hence they can be reliably used in the stepped hull hydrodynamic investigation.

EXPERIMENTS WITH STEPPED PLANING HULLS FOR SPECIAL OPERATIONS CRAFT

Many of today’s special operations craft and high-speed patrol boats can operate at volume Froude numbers high enough to justify the use of a stepped planing hull; however most of these boats are un-stepped. This paper describes experiments to determine the effect of adding a single step into the bottom of a planing hull with loading parameters consistent with modern special operations craft. In contrast to multi-step pleasure craft, which can operate at volumetric Froude numbers of 10, special operations craft often operate at volumetric Froude numbers of around 5, owing to the increased payload and decreased speed. Previous towing tests on a variety of stepped hull configurations indicated that a promising configuration for special operations craft may be a single step located near the transom (alternatively known as a hydrodynamic transom forward of the stern). The present experiments investigate this configuration further, by testing a single-step hull, with step located at 25% of the length forward of the transom. The step height is systematically varied to observe the effect on resistance, trim, wetted surface area and porpoising stability over a wide range of speeds. Of the configurations tested, the best reduced model resistance by 25% at the highest speeds tested, while increasing resistance by 10% in the hump speed regime. The stepped hulls tested had porpoising limits similar to conventional planing monohulls. A short method is provided to illustrate when a stepped hull may be advantageous for a given design.

The Systematic Variation of Step Configuration and Displacement for a Double-step Planing Craft

A model test was conducted to provide a systematic understanding of the effects that displacement and step location have on the performance of a stepped planing hull. Seven different step configurations were tested at three different displacements and over a range of four different speeds in calm water. Of all the configurations tested, the stepped hull configurations showed reduced resistance compared with the unstepped hull. The configuration with the smallest step height forward and the largest step height aft showed the least amount of drag over the speed range tested. The increased displacements had the same effect on craft performance for both the stepped and unstepped hulls.

The Systematic Variation of Step Configuration and Displacement for a Double-step Planing Craft

A model test was conducted to provide a systematic understanding of the effects that displacement and step location have on the performance of a stepped planing hull. Seven different step configurations were tested at three different displacements and over a range of four different speeds in calm water. Of all the configurations tested, the stepped hull configurations showed reduced resistance compared with the unstepped hull. The configuration with the smallest step height forward and the largest step height aft showed the least amount of drag over the speed range tested. The increased displacements had the same effect on craft performance for both the stepped and unstepped hulls.

Air cavity with variable length under a model hull

The application of air-ventilated cavities for marine vessels can result in substantial drag reduction. Experimental and modelling studies of air cavities formed under a model-scale hull are reported. Experiments were conducted with a simplified stepped hull on the water surface in a recirculating water channel. Stable cavities with different lengths and cavitation numbers were generated at fixed water flow velocity and model position. No air leakage from cavities was observed in a range of cavity lengths. The maximum length of a stable cavity with air leakage was identified. Significant enhancement of the maximum cavity length was achieved by placing an inclined plate under the cavity and supplying a large amount of air during a short period of time. This augmentation of the cavity presents an effective practical means for lengthening air-ventilated cavities and controlling cavity properties. Numerical studies simulating an air-cavity flow were conducted using a linearized potential flow method. Calculations show a qualitative agreement with trends of experimentally observed lengths of air cavities and confirm a possibility of increasing the cavity length by placing a vortex underneath the cavity. A quantitative agreement is obtained for maximum lengths of non-augmented cavities when non-linear and viscous effects are minimal.