The results of the comparative analysis of potential flow-based and measured pressure distributions over upwind sails were investigated. The sail models were supported by wires inside a frame, rather than being set on a yacht model that allowed trimming the models to several predefined positions very precisely and with good repeatability. The freestream dynamic pressure was q = 32.5 Pa, with a variation of ±1 Pa in the vicinity of the sails. The headsail and its image were considered to be one sail with a continuous deck-symmetrical circulation distribution. The vortex-lattice method (VLM) code uses mirror images of the sails to model the influence of the sea surface. The results indicated that the mast and hull, which are ignored in the theoretical analysis, lead to only a 3% reduction in lift of the headsail, but to a 7% reduction for the mainsail.

The introduction of the EEDI more than a decade ago, slow steaming, and the wish to reduce bunkering costs have resulted in a trend to install less powerful engines in ships. To avoid vessels becoming underpowered and thus unsafe, the International Maritime Organization (IMO) has published a guideline regarding the “Minimum Propulsion Power to Maintain the Manoeuvrability of Ships in Adverse Conditions”. This report presents a case study that follows the IMO-guideline step by step and works out the minimum engine size for the KVLCC2 tanker. Using a combination of Computational Fluid Dynamics and model tests, the parameters and assumptions behind the guideline are discussed in some detail. Results show that it is particularly important to determine the added resistance in waves correctly because it dominates the power prediction. It becomes clear, that the selection of the propulsive factors, particularly the “thrust deduction factor” has a significant influence on results. The work summarised here is part of a wider project that aims to provide experimental benchmarking data for added resistance predictions. It has been sponsored by the Swedish Transport Administration (Trafikverket) under grant number TRV 2021/53938.

Velocity Prediction Programs (VPPs) based on a steady-state equilibrium between aero- and hydrodynamic forces continue to be important tools when assessing the performance of yachts during the design process. Over the last decade a number of Dynamic Velocity Prediction Programs (DVPPs), which also allow study of the dynamic characteristics of the boat, have been developed. Most DVPPs are based on numerically solving the equations of motion of the yacht according to Newton's second law with the aerodynamic forces being calculated from quasi-steady theory. This paper discusses whether this assumption of quasi-steady aerodynamics can be justified and also analyses the error introduced by such a quasi-steady analysis. Unsteady potential flow theory is used to predict the pressure distribution on an aerofoil-like, two-dimensional "slice" of a mainsail carrying out harmonic oscillations both perpendicular to, and along the direction of the incident flow. Such types of motion occur when a yacht pitches or rolls in waves. Theoretical pressure distributions are compared to wind tunnel measurements on an oscillating, rigid mainsail model of 3.2 metre span and 0.447 metre chord length. Experiments were carried out at reduced frequencies ranging from k = 0 to k = 0.8, as the mainsail of an International America's Cup Class yacht sailing upwind in waves typically encounters reduced frequencies in this range. It is found that predictions based on unsteady theory match the measured pressure distributions much better then quasi-steady predictions. This leads to the conclusion that, if the performance of the yacht is to be predicted on a time-scale shorter then the pitching period, this can be achieved best with an unsteady aerodynamic model. In the paper no attempt is made to investigate the influence of the flexibility of the sails, sail interaction, three-dimensional effects or phenomena related to dynamic stall.

In this paper, unsteady thin aerofoil theory is extended to the case of two interacting aerofoils that oscillate harmonically perpendicular to the direction of the incident flow. The two aerofoils represent the headsail and mainsail of a yacht that sails upwind in waves. The developed theory is validated against theoretical data from the literature and results from wind tunnel tests with rigid, high-aspect ratio sail models oscillating at reduced frequencies from k = 0 to k = 0.68. Good agreement is found between the predicted and measured chordwise pressure distributions. An application of the theory to the case of an International America's Cup Class yacht reveals that the time-varying components of the aerodynamic forces are small and that the thrust gain is minimal, i.e. only very little energy can be extracted from the unsteady flow about the sails. No attempt is made to investigate the influence of the flexibility of the sails, three-dimensional effects or phenomena related to dynamic stall. 

The legislation on the Energy Efficiency Design Index (EEDI) requires determining a ‘weather factor’ fw that reflects how many percent of its calm water speed a ship can maintain in Beaufort 6 and corresponding waves. The higher the fw–value, the better the ship performs. In this paper we present and discuss a cost efficient way of experimentally finding the weather factor fw by means of wave tests in a seakeeping basin. To this end an evaluation software was developed to calculate fw from tests in both, regular and irregular waves in conjunction with calm water towing tank tests and wind tunnel experiments. Results show that the ‘fw standard curves’ from IMO Circular MEPC.1/Circ.796 are a useful tool to estimate an fw-value but are also very conservative i.e. the curves over-predict the speedloss in a seaway. Results from theoretical fw-calculations based on non-linear time domain seakeeping simulations are also presented and discussed

Assessing seakeeping performance at an early stage is even more important for wind powered vesselsthan for conventional ships, since there is little design experience to lean on. When the driving force comes from sails instead of a propeller, ship dynamics change considerably. Course keeping, turning ability, motions and acceleration in waves are just some of the properties that must be assessed. Including wind propulsion devices in a model test is however not straight forward. In this paper we present a methodology for model testing wind powered vessels. Rpm and azimuth-controlled fans/airscrews are used to mimic the aerodynamic forces from the sails. Results from model tests with a car carrier are presented and discussed while particular attention is paid to possible improvements of the test methodology.

In recent years a number of Dynamic Velocity Prediction Programs (DVPPs), which allow studying the behaviour of a yacht while tacking, have been developed. The aerodynamic models used in DVPPs usually suffer from a lack of available data on the behaviour of the sail forces at very low apparent wind angles where the sails are flogging. In this paper measured aerodynamic force and moment coefficients for apparent wind angles between 0° and 30° are presented. Tests were carried out in the University of Auckland's Twisted Flow Wind Tunnel in a quasi-steady manner for stepwise changes of the apparent wind angle. Test results for different tacking scenarios (genoa flogging or backed) are presented and discussed and it is found that a backed headsail does not necessarily produce more drag than a flogging headsail but increases the beneficial yawing moment significantly. The quasi-steady approach used in the wind tunnel tests does not account for unsteady effects like the aerodynamic inertia in roll due to the "added mass" of the sails. In the second part of paper the added mass moment of inertia of a mainsail is estimated by "strip theory" and found to be significant. Using expressions from the literature the order of magnitude of three-dimensional effects neglected in strip theory is then assessed. To further quantify the added inertia experiments with a mainsail model were carried out. Results from those tests are presented at the end of the paper and indicate that the added inertia is about 76 % of what strip theory predicts.

Drawing on our model basin's large database of experimental results this paper explores several options for reducing the EEDI. Starting from the equation that defines the index we present four of the many ways to achieve such a reduction: • Reducing the power demand of the hull by optimising main dimensions • Increasing the product of speed and deadweight • Classical hydrodynamic hull form optimisation • Tailoring the 'sea margin' of the engine power to the requirements of the ship on the intended route Using a practical example for each option it is estimated by how much the EEDI can be reduced. © 2014: The Royal Institution of Naval Architects.

Wind propulsion systems (WPS) are major investments and the decision to install them requires careful consideration of many complex questions. In this paper we present a systematic, scientific methodology to assess the benefits and drawbacks of such systems at the early concept stage of a vessel. The purpose is to provide guidance for shipowners and operators and help them make informed decisions. The proposed method was developed into a Software tool called ‘SEAMAN Winds’ and has been correlated to full scale results. The program draws on our large database of model tests, and CFD of hulls and wind propulsion technologies. It uses the intended trading routes of the vessel as an important input, typical output data are: a) performance values (ship speed, power requirements etc.) b) environmental parameters (CO2 avoided, EEDI and EEXI reduction, carbon intensity indicator) c) financial metrics (bunker savings, payback time for installation of WPS) Potential applications of the method include making the business case for one particular WPS or investigating in how far certain systems are more suited for a specific route than others.

Wind propulsion systems (WPS) are major investments and the decision to install them requires careful consideration of many complex questions. One of the recurring and challenging issues for ship owners is the choice of a suitable WPS for a specific ship and a specific operational pattern. Today most WPS providers offer on-demand case studies, but obviously the underlying performance prediction methodologies differ from provider to provider. This makes comparing different technologies from competing suppliers next to impossible. In this paper we present a numerical platform to compare different WPS of different makes, sizes, and costs in a fair way. The fundamental idea is to use aerodynamic WPS datasets that are independently verified by SSPA through wind tunnel test, sea trials or extensive CFD. This is combined with a hydrodynamic dataset from SSPAs database of tank tests. The same performance prediction method, identical routes and weather statistics are then used to determine Key Performance Indicators and financial metrics of the competing wind propulsion technologies. The purpose is to provide guidance for shipowners at the early concept stage of a vessel and help them select a system that suits their particular requirements.

This paper discusses how to maximize the drive force produced by an upwind sail. It aims to provide a better understanding of the behavior of this force as a function of the heel angle of the yacht and the wind speed. It also discusses the corresponding optimal spanwise loading distributions. An extended lifting line code, based on Weissinger's method, is developed to analyze the performance of an isolated mainsail in upwind conditions. It is extended to account for the heel angle of the yacht via effective angle theory, and an image sail is used to model the influence of the sea surface. Profile drag is modeled using experimental data. The extended lifting line code is validated against wind-tunnel measurements and data from the literature. A second code is then used to optimize the spanwise loading on a mainsail such that the drive force is maximized. Constraints are implemented to ensure positive circulation over the entire span and to limit the sectional loading to realistic values. Finally, the extended lifting line code is inverted to calculate the twist distribution necessary to produce the desired, optimized loading distribution for a given sail planform. The calculated twist distribution is found to be realistic and achievable. 

Before the background of the internationl Maritime Organization's 2050 emission reducation targets, the largest sailing ship in the world is currently being developed in Sweden. This wind powered car carrier, called Oceanbird, will have four 80-metre-high wing sails targeting CO2savings in the order of 90%. The prediction and analysis of the seakeeping performance of such a ship is of importance, not only in terms of sailing dynamics, but also when it comes to the structural design of the rig. To this end, a numerical method for predicting a ship's motions and loads on its rigid wing sails is described in this paper and a demonstration of how the method can be used to obtain such loads is presented. The numerical method is based on an unsteady 3D fully nonlinear potential flow hydrodynamic model coupled with a hybrid 2D RANS/3D lifting-line aerodynamic model. Simulations in a seaway with short-crested irregular waves and corresponding wind conditions are conducted, resulting in time histories of the aerodynamic and inertial forces acting on the rig. Possible applications of the method include fatigue analysis of the wing sails, where the accumulated fatigue damage over the lifespan of the rig structure depends on the sum of aerodynamic forces and motion induced inertial forces. Other potential applications include sail dynamics, parametric roll, sheeting strategies and appendage configuration studies. 

The need to reduce green-house gas emissions from shipping has reborn the interest in wind propulsion for commercial cargo vessels. This places new requirements on the tools used in ship design, as well as the methods usually applied in sailing yacht design. A range of design tools are used by designers at various stages in the design of wind-assisted ships and for different purposes. One important tool is the steady-state velocity prediction program (VPP) which is typically used to predict the speed of the vessel when sailing in a range of wind directions and wind speeds. Steady-state VPPs can be very efficient and fast and may be used to rapidly assess a large number of design alternatives. However, steady-state VPPs are not able to consider dynamic effects such as unsteady wave forces on the hull which may require the rudder to be active to control the heading and course of the vessel. This, in turn, leads to different mean forces than those predicted by a static VPP and therefore the sailing performance may be reduced compared to the predictions of a static VPP. Another effect of the ship’s motions in a seaway is that the angles of attack of the sails fluctuate, which can lead to different optimum sheeting angles and possibly a loss of performance. This study uses an unsteady 3D fully nonlinear potential flow hydrodynamic model coupled with an efficient lifting-line aerodynamic model to investigate the differences in sailing performance of a vessel sailing in steady conditions to the performance when sailing in a seaway and gusty wind based on a spatio-temporal wind model. The analysis shows clearly that the unsteady wind model affects the predicted performance. This is especially the case when sailing close-hauled and on a beam reach, where large changes in the local sail angles of attack can be observed.

The need to reduce green-house gas emissions has renewed the interest in wind propulsion for commercial cargo vessels. When designing such modern “sailing” ships, naval architects often lean on methods and tools originally developed for the design of sailing yachts. The most common tool today is the steady-state Performance Prediction Program (PPP), typically used to predict quantities like speed, leeway, heel of the vessel when sailing in a range of wind directions and wind speeds. Steady state PPPs are very efficient and can be used to rapidly assess a large number of design alternatives. PPPs are, however, not able to consider dynamic effects such as unsteady sail forces due to ship motions in waves or the turbulent structure of the natural wind. In this paper we present time-domain simulations with a Dynamic Performance Prediction Program (DPPP) that can take the “unsteadiness” of the natural environment into account. The program is based on coupling an unsteady 3D fully nonlinear potential flow hydrodynamic solver to an efficient lifting-line aerodynamic model. Particular attention is paid to a recently implemented unsteady aerodynamic model that employs an indicial response method based on Wagner’s function. The usefulness of such advanced simulations for performance prediction in moderate environmental conditions is investigated for a wind-powered cargo vessel with wing sails. Control system strategies such as sheeting of the wing sails close to stall are studied.

In this study, a comparative study on motion response and added resistance of a Supramax bulk carrier (K-Supramax Original) in regular waves was carried out to evaluate the reliabilities and accuracies of experimental and numerical simulation techniques. Two kinds of experiments were performed; one is the towing tank model testing for head wave conditions conducted at Seoul National University, and the other is the free-running model test for head and oblique wave conditions conducted at SSPA Sweden AB. Also, nine numerical computation results submitted by seven institutions were compared with the experimental data. The computation results were obtained by various seakeeping analysis methods such as the 2D strip theory, 3D Rankine panel method, and Computational Fluid Dynamics (CFD) based analysis. Based on the comparison, the characteristics of each numerical technique and resultant accuracies of seakeeping analyses were investigated. It was also confirmed that different results were obtained although the same program was used because of the user dependencies; setting for computation parameters, numerical schemes, and mesh generations, etc. Furthermore, the sensitivities of seakeeping quantities with respect to wave amplitudes were examined by conducting both model tests and nonlinear numerical simulations for different wave slopes. Lastly, the tendencies of ship motion and added resistance depending on the heading angle were identified, and the reliabilities of experiments and numerical computations for oblique waves were discussed.

This paper presents a comparative study of wave-induced motion responses and the added resistance of a ship. Four representative types of ships are adopted as test models: LNG carrier, tanker, containership, and bulk carrier. Two experimental techniques—captive and free-running model tests—are conducted under regular head and oblique wave conditions to create benchmark data. Several numerical computation methods (asymptotic formula, 2D strip theory, 3D panel method, and CFD) are applied to perform the seakeeping analysis. The comparison results indicate that the accuracy and reliability of each analysis technique are validated, and its characteristics and limitations are investigated with respect to the physical aspects of the added resistance caused by a wave. The analysis results are compared based on how steady flow-induced coupling effects are considered. Further, the sensitivities of seakeeping quantities with respect to wave steepness were examined based on the results of linear and fully nonlinear computations. The overall tendency of the added resistance in accordance with the incident direction of a wave is discussed. © 2021 The Author(s)

Hybrid testing is an experimental technique that can be used to test ships and marine structures when both hydrodynamic and aerodynamic effects are important, for example for wind powered or wind assisted ships and sailing vessels. SSPA is currently developing an experimental method using hybrid testing involving fan forces added to ship decks to simulate sails to assess the course keeping, seakeeping and manoeuvring performance of a wind powered ship. For conventional motor ships there are well established test methods and knowledge on how to scale the results from model to full-scale. For a wind propelled ship however, the driving force is no longer located at the propeller shaft but high above deck and at another longitudinal position that could vary with true wind angle and speed. Moreover, there is a large side force coming from the aerodynamic forces of the wingsails that needs to be counteracted with lifting surfaces underwater. The side-force and yaw moment are much more prominent than in conventional vessels. The combination of those factors will influence the manoeuvrability and course keeping, especially in waves. Having built up the research tools for predicting and simulating the behaviour of a full-scale vessel, making the model sail in a similar way as predicted for the full-scale vessel remains a challenge because of the difference between Froude scaling and Reynolds scaling applicable for the hull and lifting surfaces respectively. Using Computational Fluid Dynamics (CFD) to understand the scale effects in model tests for a wind powered ship and developing a methodology for determining the fan parameters that correctly model the ships behaviour and performance are the key objectives of the research study. The art of model testing encompasses the need to learn from different techniques to ultimately achieve the best agreement between model tests and full-scale results in terms of accuracy, repeatability, cost, and speed. Learning from preliminary experimental tests, through studies on CFD and ultimately paving the way to new testing methodologies is the main aim of the current paper.

In this paper, the added resistance of a large tanker is estimated experimentally and numerically in oblique sea. Experiments on ship motion response and added resistance in oblique sea are performed in the SSPA seakeeping basin. The experiments are conducted using the self-propulsion test for seven wave directions between 180° and 0°. In the self-propulsion test, the added resistance is estimated from the difference between the thrust of the propeller in calm water and waves. In the case of the head sea, the results are compared with those obtained from the captive test at the towing test of Seoul National University. As numerical method, two methods are selected: the strip method and the 3D Rankine panel method. The maximum value of the added resistance is observed between the incident wave directions of 180° and 150°. From 120°, the added resistance tends to decrease and the peak of the added resistance shifted to the short waves. Through the two numerical analysis methods, the tendency of added resistance and the cause of the change of the added resistance in the oblique sea are investigated.

This paper presents a comparative study on the motion responses and added resistance of a container ship. Eight institutions participated in the comparative study, and ten numerical results were compared with two experimental results. Two experimental results were obtained from Seoul National University towing tank and Sweden SSPA seakeeping basin. The results of two experimental institutions in head sea condition were compared and showed good agreement with each other. The difference in motion responses and added resistance according to the numerical analysis method were compared. Even though the same program was used, it was observed that different results were obtained depending on the users. The comparison of the motion response and the added resistance according to the change of wave slope showed that the added resistance greatly changed according to the wave slope. This tendency was the same in experimental results and CFD analysis results. From the comparative study, the influence of the experiment method on the added resistance, and the characteristics of numerical each code were identified.

A code which generates the camber shape of a sail from a desired sail plan-form, sail twist distribution and surface pressure map has been written. This is an iterative 3D inverse sail design code. The method initially uses inverse thin aerofoil theory, applies this to the desired pressure map and creates an initial sail shape. A theory which gives a relationship between the change in the pressure map and the change in the sail camber was developed and is described. The code applies that theory to the difference between the desired pressure map and the pressure map of the initial sail shape. The calculated camber difference is added to the initial shape to give an improved shape with a pressure distribution closer to the desired one. This process is repeated until the generated sail produces the desired pressure map. Validation tests were performed by generating a pressure distribution from a known sail shape using a VLM code, and then the method described in the paper was used to find the shape from the pressure distribution. The sail shape was successfully obtained in as few as five iterations, with a maximum error of only about 0.2 % of the sail chord, which is acceptable in sail design practice.

Today sail shapes are usually designed using analysis methods i.e. based on experience the designer specifies a certain sail shape and then proceeds to determine the aerodynamic characteristics of this sail. Finding optimum sail shapes using such a method can involve a lot of trial and error. A new approach in sail design is proposed in this paper, where an inverse method is considered. The inverse method involves specifying the aerodynamic characteristics, and working backwards to obtain the corresponding sail shape to produce those characteristics. The paper investigates a single sail in an upwind condition. Because the solution of the inverse process is not unique, some variables have to be fixed. The sail shape is defined by three parameters: the planform, the camber, and the twist. In the present work, the planform is assumed to be defined by the class-rules of the yacht and is thus known. The sail designer has to specify one of the two possible trims: the twist or the camber. Then the theory, described in the paper, shows that there is a unique solution of the inverse process. Thus two cases are considered. The first involves a fixed twist and planform. There, the code generates the camber of the sail which will produce a given pressure distribution. The second case considers a fixed camber and planform. Here the code trims the sail twist to match the desired pressure distribution. Validation tests have been performed and results are presented. To validate the current approach, the pressure map was first computed from a specified shape. Then the resulting pressure distribution was employed as an input to the inverse method. The shape of the sail obtained with the inverse method is compared to the shape initially used in the analysis. The agreement is good in both inverse computations.

This paper investigates an inverse process for the design of yacht sails. The method is described and then applied to the design of optimal sails for a specific yacht The proposed inverse method generates the three-dimensional shapes of a headsail and mainsail from prescribed loading (i.e. differential pressure) distributions, accounts for the effect of the sea surface, and also simulates the twist and shear of the incoming flow. The uncoupled iterative routine solves a sequence of analysis steps so that the sail shapes are deformed in such a way that their updated loading distributions converge to the specified target distributions. During each iteration equations derived from two-dimensional Thin Aerofoil Theory, calculate a geometry correction from the difference between the current and target loading distributions. This correction is applied to the sail geometry, and a vortex lattice method code calculates the updated three-dimensional differential pressure distributions, which are again compared to the target distributions. Usually only five iterations are required to converge to sail shapes that have the target loading distributions. The inverse method has been validated by inverting the traditional way of analysing sails, i.e. a set of sails with known geometry has been analysed and the loading distributions on the headsail and mainsail were calculated. These distributions were then used as an input for the inverse code. It was found that the difference in camber between the original sails and the calculated geometry is less than 0.01% of camber at the mid-span of the sails. The second part of the paper presents two methods for the design of optimal sails for a yacht One of the methods uses the more traditional analysis approach, while the other employs the inverse method described in this paper. The optimisation is performed for a Transpac 52 yacht in 12 knots (6.5 m/s) of true wind speed to obtain the best velocity made good. Results from both methods are presented and discussed and it is found that the results in terms of boat speed are similar although the trims differ slightly. However, the new inverse method is approximately nine times faster than the traditional analysis approach. 

Wind tunnel experiments using a Real-Time Velocity Prediction Programme to investigate the optimal trim of a VO70 model under various simulated true wind speeds are reported. The results illustrate that the decision made depend upon the particular apparent wind direction and true wind speed. It is suggested that these can be sub-divided into three broad bands: low wind speeds where the total drag is minimised and the trim that provides the maximum thrust coefficient is chosen, moderate wind speeds where the heel angle has a strong effect and the optimum choice includes a reduction in lift coefficient and centre of effort height and strong winds where the heel angle and hence heeling moment is limited to the maximum acceptable value and the optimum loading distribution is strongly constrained by this limit. Extended Lifting Line Theory is used to further investigate the detailed loading distribution on an AC90 mainsail. The result illustrate the way in which the optimal distribution changes with varying conditions.

Wind propulsion has emerged as one out of many possible solutions to reduce GHG emissions from ships. The industry for wind propulsion solutions develops rapidly. This calls for some industry standardisation. A committee under ITTC is currently working on recommended procedures for performance indicators, performance prediction methods and sea trial procedures for wind powered ships. This paper proposes indicators that can enable fair comparison and facilitate the investment decision. A new sea trial procedures for wind propulsion solution verification is also proposed. Finally, the application of performance models in cost-saving split agreements, monitoring and weather routing of wind powered ships are discussed.

Since the introduction of the EEDI almost a decade ago, slow steaming and the wish to reduce bunkering costs have resulted in a trend towards less powerful engines. To avoid vessels becoming underpowered and unsafe, IMO has published guidance on the 'Minimum Propulsion Power to Maintain the Manoeuvrability of Ships in Adverse Conditions'. A group of seakeeping experts from SSPA Sweden AB explains the rules.