Abstracts

ABSTRACTS IN ORDER OF PRESENTATION

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Prediction and Optimization of Aerodynamic Forces and Boat Speed of Foiling Catamaran with a Rig of a Rigid Wing and a Jib
K. Graf, H. Renzsch, J. Meyer - University of Applied Sciences Kiel, Germany

This paper describes a method to calculate the aerodynamic forces generated by a rigid two-element wing together with a jib. Additionally, investigations of hydrodynamic flow forces generated by water-piercing L-shaped foils are introduced. The aerodynamic and hydrodynamic flow force prediction methods are combined in a velocity prediction program featuring a constraint optimization method in order to predict boat speed and wing and foil trimming parameters for its maximization.

A velocity polar calculated by applying this method to a 50-foot catamaran is shown and the result of some studies are presented, varying design parameters of the catamaran.

 

A comparison of a RANS based VPP to on the water Sailing Performance
T. Doyle, B. Knight, D. Swain - Doyle CFD

This paper compares performance predictions from a Reynolds Averaged Navier Stokes (RANS) based Velocity Prediction Program (VPP) to on the water testing of a J70. The J70 has been outfitted with a system to determine sail flying shapes, apparent wind conditions and performance data. The on the water testing is conducted in both racing and controlled sailing conditions. Data taken during racing conditions is analyzed to determine optimal performance envelopes while data taken in controlled conditions is used to match exact sailing and VPP states. The data acquisition system combines a number of standard marine sensors including a sonic anemometer, a GPS, a digital compass, an accelerometer and a gyroscope with custom sensors that measure rudder and boom angles as well as a custom sail shape acquisition system. The RANS based VPP developed by Doyle CFD has three main components; an aerodynamic force model, a hydrodynamic force model and an algorithm to balance the forces. The force balance routine uses four degrees of freedom; boat speed, yaw, heel and rudder angle to balance the aerodynamic and hydrodynamic forces for a given true wind speed and angle. The force models are derived from RANS CFD data calculated using OpenFOAM. The aerodynamic forces are calculated using steady state RANS as a function of apparent wind angle, apparent wind speed and sail flying shape. The VPP force model is derived by fitting response surfaces to this data. The aerodynamic CFD is run with sail flying shapes recorded from on the water testing. Using accurate flying shapes is critical for picking out slight aerodynamic differences in sail and rig setup. The hydrodynamic CFD data points are calculated using RANS Volume of Fluid CFD (VOF) as a function of boat speed, rudder angle, yaw angle, heel angle and displacement. Response surfaces are generated from a 64 data point array of RANS VOF simulations.

Experimental and numerical trimming optimizations for a mainsail in upwind conditions
M. Sacher, F. Hauville, R. Duvigneau, O. Le Maitre, N. Aubin, M. Durand - IRENAV France


This paper investigates the use of meta-models for optimizing sails trimming. A Gaussian process is used to robustly approximate the dependence of the performance with the trimming parameters to be optimized. The Gaussian process construction uses a limited number of performance observations at carefully selected trimming points, potentially enabling the optimization of complex sail systems with multiple trimming parameters. We test the optimization procedure on the (two parameters) trimming of a scaled IMOCA mainsail in upwind conditions. To assess the robustness of the Gaussian process approach, in particular its sensitivity to error and noise in the performance estimation, we contrast the direct optimization of the physical system with the optimization of its numerical model. For the physical system, the optimization procedure was fed with wind tunnel measurements, while the numerical modeling relied on a fully non-linear Fluid-Structure Interaction solver. The results show a correct agreement of the optimized trimming parameters for the physical and numerical models, despite the inherent errors in the numerical model and the measurement uncertainties. In addition, the number of performance estimations was found to be affordable and comparable in the two cases, demonstrating the effectiveness of the approach.

 

Fully Integrated fluid-structural analysis for the design and performance optimization of fiber reinforced sails
S. Malpede, D. MacVicar, F. Nasato, P. Semeraro - SMAR Azure, UK


This paper presents an advanced and accurate integrated system for the design and performance optimization of fiber reinforced sails -commonly named string sails- developed by SMAR Azure Ltd. This integrated design system allows sail designers not only to design sail-shapes and the reinforcing fiber paths, but also to validate the performance of the flying sail-shape and have accurate production details including the overall sail weight, material used, which means costs, and length of the fiber paths, which means production time.

The SMAR Azure design and analysis method includes a validated and computationally efficient structural analysis method coupled with a modified vortex lattice method, with wake relaxation, to enable a proper aero-elastic simulation of sails in upwind conditions. The structural analysis method takes into account the geometric non-linearity and wrinkling behavior of membrane structures –such as sails-, the fiber layout, the influence of battens, trimming loads and interaction with rigging elements, e.g. luff sag calculation on a headstay, in a timely manner. This method has been extensively validated and used to optimize several racing and super-yachts sailing plans. Specifically, this paper presents a validated optimization of a real fiber reinforce membrane sail plan of 140’, 240 ton aluminium Super Yacht, carried out in collaboration with Paolo Semeraro (from Banks Sails Europe), who designs and produces the MEMBRANE™ and BFAST™ string sails, the latter with Marco Semeraro. Both BFAST and Bank Sails have been using the SMAR Azure technology for almost a decade and notwithstanding the long experience of Mr. Semeraro in using the technology, given the sailplan-size and detailed customer requirements, among which improved durability, strength and reliability and smooth use of in-boom furling, this project was carried out in-cooperation with the SMAR Azure technical team. A total of 1000 sqm of upwind sailing area was analyzed and optimized. A combination of Dyneema TM Sk 90 and black Twaron 2200 was chosen for the fibers and a triple step lamination under hi-pressure plus laminated patches utilizing the same fibers where added to prevent local deformation of the corners. A long term vacuumed post-curing period sealed the production phases. The final sail plan is -as anticipated by the analysis results- holds the desired shape and is stronger. The final fiber layout shows a reduction in maximum stress by 22% compared to the initial design; this was achieved with only 11% (4kg) gain in fiber weight.

Unsteady Sail Dynamics due to Bodyweight motions
R.Schutt, C.H.K. Williamson - Cornell University


In small sailboats, the bodyweight of the sailor is proportionately large enough to induce significant unsteady dynamics of the boat and sail. Sailors use a variety of techniques to create sail dynamics which can provide an increment in thrust, increasing the boatspeed. In this study, we experimentally investigate the unsteady aerodynamics associated with two such techniques, “upwind leech flicking" and “downwind S-turns". We employ a two-part approach:

First, on-the-water experiments are carried out using a Laser class sailboat sailed by Olympic and world championship level sailors. Data collected from an on-board GPS, IMU, anemometer, and camera array is used to generate characteristic motions of the boat and sail relative to the apparent wind.

Second, laboratory experiments using the characteristic motion of the sail are run in a computer-controlled 3 degree-of-freedom (X, Y, and θ) towing tank. We use water as the working fluid. Rather than directly experiment with three-dimensional sail shapes, we represent the primary effects of the sail dynamics using rapidly prototyped two-dimensional flexible sail geometries. Shapes are based on extruded draft stripes from the upper third of the sail. The laboratory experiments approximately match the key non-dimensional parameters of the on-the-water sailing conditions, including the reduced frequency and heave-to-chord ratio. Particle Image Velocimetry and force measurements are used to analyze the flow field and thrust generated by the model sail during the dynamic motions.

On-the-water testing shows that the characteristic sail motion in leech flicking is a combination of periodic heave caused by the actions of the sailor and a passive twisting of the sail due to rig flexibility. The heaving sail motions are due to rotation (roll) of the rig around the longitudinal axis of the hull. This is at an angle to the apparent wind, resulting in heave that has components both perpendicular and parallel to the oncoming wind flow. This is distinct from classical aerodynamic studies with heave purely perpendicular to the incoming flow.

In laboratory experiments, the characteristic flicking motion is applied to a NACA 0012 airfoil and a 2D sail, both angled at 15 deg to the flow. Lift increases and drag decreases, leading to an overall increase in resultant driving force of the boat. The beneficial effect of this dynamic motion becomes greater as the apparent wind angle increases. In the case of leech flicking, the experiments show that the formation of vortex pairs is fundamental to the augmented thrust due to heaving.

The presence of S-turns, whereby the sailor changes the boats direction simultaneous with rolling the boat, generally in the downwind direction, is also associated with vortex formation and pairing, which will be described at the conference. During downwind S-turns, large amplitude heaving motions are paired with substantial rotations of the sail caused by both adjustments of the main sheet and changes in heading. Increased velocity made good downwind is measured from the on-the-water experiments, and is associated with an increase of thrust during characteristic dynamics of the airfoil or sail shape in the laboratory.

 

A Comparison of RANS and LES for Upwind Sailing Aerodynamics
S. Nava, S. Norris, J. Cater – University of Auckland


Yacht sails experience complex aerodynamics that are challenging to reproduce with numerical methods. The experiments of Fluck on an idealized upwind sail plan [1] [2] showed areas of flow separation and vortex structures that subsequent Reynolds Averaged Navier-Stokes (RANS) calculations have struggled to correctly simulate [3] [4]. Therefore it is of interest to see if other methods, such as Large Eddy Simulation (LES), are able to more accurately predict this flow. To this end the experiments of Fluck have been reproduced in Fluent, using RANS and LES. Both RANS and LES accurately model the attached flow on the lower region of both sails, but RANS fails in capturing the separation bubble occurring at the top section of the mainsail, predicting it as either too short, non-existent, or extended for the entire chord length. However, LES correctly models the leading edge separation bubble with its reattachment point located approximately halfway along the sail chord. The LES calculations are sensitive to the free stream turbulence intensity with an increase in the turbulence intensity from 3% to 12% shortening the separation bubble by half. Accurately reproducing the experimental geometry also improves the numerical results, with modelling the computational domain as a simple box or as the real wind tunnel results in different flow, with the angle of attack varying by 3°. In conclusion, LES, even if it is very sensitive to the mesh and the choice of the solver, is more accurate than a steady RANS calculation, although the computational time increases by a factor of 50.

 

Pressure Measurements on Yacht Sails: Development of a new system for wind tunnel and full scale testing
F. Fossati, I. Bayati, S. Muggiasca, A. Vandone, G Campanardi, T Burch, M. Malandra -Politecnico di Milano, Italy

The paper presents an overview of a joint project developed among Politecnico di Milano, CSEM and North Sails, aiming at developing a new sail pressure measurement system based on MEMS sensors (an excellent compromise between size, performance, costs and operational conditions) and pressure strips and pads technology. These devices were designed and produced to give differential measurement between the leeward and windward side of the sails. The project has been developed within the Lecco Innovation Hub Sailing Yacht Lab, a 10 m length sailing dynamometer which intend to be the reference contemporary full scale measurement device in the sailing yacht engineering research field, to enhance the insight of sail steady and unsteady aerodynamics [1].

The pressure system is described in details as well as the data acquisition process and system metrological validation is provided; furthermore, some results obtained during a wind tunnel campaign carried out at Politecnico di Milano Wind Tunnel, as a benchmark of the whole measuring system for future full scale application, are reported and discussed in details.

Moreover, the system configuration for full scale testing, which is still under development, is also described.


Modal Analysis of Pressures on a full scale spinnaker
J. Deparday, P. Bot, F. Hauville, B. Augier, M. Rabaud, D. Motta, D. Le Pelley - Naval Academy Research Institute, France 

While sailing offwind, the trimmer typically adjusts the downwind sail "on the verge of luffing", letting occasionally the luff of the sail flapping. Due to the unsteadiness of the spinnaker itself, maintaining the luff on the verge of luffing needs continual adjustments. The propulsive force generated by the offwind sail depends on this trimming and is highly fluctuating. During a flapping sequence, the aerodynamic load can fluctuate by 50% of the average load. On a J/80 class yacht, we simultaneously measured time resolved pressures on the spinnaker, aerodynamic loads, boat and wind data. Significant spatio-temporal patterns are detected in the pressure distribution. In this paper we present averages and main fluctuations of pressure distributions and of load coefficients for different apparent wind angles as well as a refined analysis of pressure fluctuations, using the Proper Orthogonal Decomposition (POD) method. POD shows that pressure fluctuations due to luffing of the spinnaker can be well represented by only one proper mode related to a unique spatial pressure pattern and a dynamic behavior evolving with the Apparent Wind Angles. The time evolution of this proper mode is highly correlated with load fluctuations. Moreover, POD can be employed to filter the measured pressures more efficiently than basic filters. The reconstruction using the first few modes allows to restrict to the most energetic part of the signal and remove insignificant variations and noises. This might be helpful for comparison with other measurements and numerical simulations.



Effect of dynamic trimming on upwind sail aerodynamics in a wind tunnel
N. Aubin, B. Augier, P. Bot, F. Hauville, M.Sacher, R. G. J. Flay - IRENAV France, University of Auckland

An experiment was developed at the Yacht research Unit’s Twisted Flow Wind Tunnel (University of Auckland) to test the effect of dynamic trimming on three 60 IMOCA inspired main sails models in upwind configuration. This study presents dynamic fluid structure interaction results in well controlled conditions (wind, sheet length) with a dynamic trimming system. First the optimum optimization target CFobj coefficient with a steady trim for AWA = 60 degrees using the car traveler position and main sail sheet length is located. Oscillation are then done around this optimum value using the main sheet length Lsheet oscillation. Different oscillation amplitudes and frequencies of trimming are investigated. Measurements are done with a 6 components force balance and a load sensor giving access to the unsteady main sail sheet load. The driving CFx and optimization target CFobj coefficient first decrease at low reduced frequency fr for quasi-steady state then increase, becoming higher than the steady state situation. The driving force CFx and the optimization target coefficient CFobj show an optimum for the three different design sail shapes located at fr = 0:255. This optimum is linked to the power transmitted to the rig and sail system by the trimming device. The effect of the camber of the design shape is investigated too. The flat mainsail design benefits more than the other mainsail designs from the dynamic trimming compared to their respective steady situation. This study presents dynamic results that cannot be accurately predicted with a steady approach. These results are therefore valuable for future FSI numerical tools validation in unsteady conditions.

Towards a new mathematical model for investigating course stability and maneuvering motions of sailing yachts
E. Angelou, K. Spyrou - National Technical University, Athens, Greece

In order to create capability for analyzing course instabilities of sailing yachts in waves, the authors are at an advanced stage of development of a mathematical model comprised of two major components: an aerodynamic, focused on the calculation of the forces on the sails, taking into account the variation of their shape under wind flow; and a hydrodynamic one, handling the motion of the hull with its appendages in water.

Regarding the first part, sails provide the aerodynamic force necessary for propulsion. But being very thin, they have their shape adapted according to the locally developing pressures. Thus, the flying shape of a sail in real sailing conditions differs from its design shape and it is basically unknown. The authors have tackled the fluid-structure interaction problem of the sails using a 3d approach where the aerodynamic component of the model involves the application of the steady form of the Lifting Surface Theory, in order to obtain the force and moment coefficients, while the deformed shape of each sail is obtained using a relatively simple Shell Finite Element formulation. The hydrodynamic part consists of modeling hull reaction, hydrostatic and wave forces.

A Potential Flow Boundary Element Method is used to calculate the Side Forces and Added Mass of the hull and its appendages. The Side Forces are then incorporated into an approximation method to calculate Hull Reaction terms. The calculation of resistance is performed using a formulation available in the literature. The wave excitation is limited to the calculation of Froude - Krylov forces.


The SYRF Wide Light Project
M. Prince – Sailing Yacht Research Foundation/Wolfson Unit

Modern racing yacht semi-planing hull forms provide a number of complex challenges for designers and a minefield for those involved in yacht rating.

The SYRF Wide Light Project was initiated as a means of providing data with which to assess a range of alternative computation methodologies to analyze sailing yacht hydrodynamic forces and moments, making this data available to the entire sailing yacht research community and demonstrating how this type of study can be used to inform the rating process.

This paper presents a comprehensive set of tank test results in both canoe body only and appended configurations to be used as a benchmark for a defined geometry of a modern semi-planing hull.

Five different CFD stakeholders carried out ‘blind’ CFD analysis on the same test matrix using a range of different computational codes and approaches. The results are presented here along with feedback detailing the software, methods and resources used to generate the results.   

This project offers a comprehensive set of public domain data which researchers may use to validate and develop their numerical tools and highlights how successfully commercial CFD codes may be used to confidently predict the variation of the forces on a sailing yacht hull as speed, heel and leeway change.

Finally, discussion will be made on how this first phase of the project may be used to inform handicap rule makers. 

 

Numerical Simulations of a Surface Piercing A-Class Catamaran Hydrofoil and Comparison against model tests
T. Keller, J. Hendrichs, K. Hochkirch, A. C. Hochbaum - TU Berlin, DNV GL

Hydrofoil supported sailing vessels gained more and more importance with in the last years. Due to new processes of manufacturing it is possible to build slender section foils with low drag coefficients and heave stable hydrofoil geometries are becoming possible to construct. These surface piercing foils often tend to ventilate and cavitate at high speeds. The aim of this work is to define a setup to calculate the hydrodynamic forces on such foils with RANSE CFD and to investigate whether the onset of ventilation and cavitation can be predicted.

Therefore a surface piercing hydrofoil of an A-Class catamaran is simulated by using the RANSE software FineMarine with its volume of fluid method. The C-shaped hydrofoil is analysed for one speed at Froude Number 7.9 and various angles of attack (AoA). The rake was defined and a leeway angle was applied to simulate realistic set ups. It is presented how the rake and drift angle influence the lift to drag ratio. Over a wide range of AoA there is no ventilation predicted but cavitation may occur from AoA > 10°. Due to the very small aspect ratio (Λ=2.64), the maximum AoA before stall is increased. The simulations have been verified by extensive analyses, including domain size verification for unrestricted water, mesh refinement and y+ verification. The influence of the K27 (cavitation tunnel of the Technical University of Berlin) on the flow around the hydrofoil and the wave system is presented. It is shown how test section of the K27 influences the flow around the foil, the forces and the wave elevation. Finally the CFD results are compared against the experiments conducted in the K27.

 

Advanced CFD Simulations of free-surface flows around modern sailing yachts using a newly developed openFOAM solver
J. Meyer, H, Renzsch, K. Graf, T. Slawig - University of Applied Sciences Kiel, Germany


While plain vanilla OpenFOAM has strong capabilities with regards to quite a few typical CFD-tasks, some problems actually require additional bespoke solvers and numerics for efficient computation of high-quality results. One of the fields requiring these additions is the computation of large-scale free-surface flows as found e.g. in naval architecture. This holds especially for the flow around typical modern yacht hulls, often planing, sometimes with surface-piercing appendages. Particular challenges include, but are not limited to, breaking waves, sharpness of interface, numerical ventilation (aka streaking) and a wide range of flow phenomenon scales. A new OF-based application including newly implemented discretization schemes, gradient computation and rigid body motion computation is described. In the following the new code will be validated against published experimental data; the effect on accuracy, computational time and solver stability will be shown by comparison to standard OF-solvers (interFoam / interDyMFoam) and Star-CCM+. The code’s capabilities to simulate complex “real-world” flows are shown on a well-known racing yacht design.

 

Insights from the Load Monitoring Program for the 2014-2015 Volvo Ocean Race
S. Russell, G. Vanhollebeke, P. Manganelli – Gurit Composite Components

This paper describes insights into keel and rigging loads obtained through a data acquisition system fitted on the fleet of Volvo 65 yachts during the 2014-2015 Volvo Ocean Race. In the first part keel fin stress spectra are derived from traces of canting keel ram pressures and keel angle; these are reviewed and compared against equivalent spectra obtained by applying methods proposed by Det Norske Veritas - Germanischer Lloyd (“DNVGL”) guidelines and the ISO 12215 standard. The differences between stress spectra and their validity are discussed, considering two types of keel: milled from a monolithic cast of steel, and fabricated from welded metal sheets. The second part discusses predicted and actual rigging working loads for the Volvo 65 yachts, considering how safety factors vary between design loads proposed by DNVGL and actual recorded loads.

 

Influence of sailor position and motion on the performance prediction of racing dinghies
J. Taylor, J. Banks, M. Toward, D. Taunton, S. Turnock - University of Southampton, UK


The time-varying influence of a sailor’s position is typically neglected in dinghy velocity prediction programs. When applied to the assessment of dinghy race performance the position and motions of the crew become significant but are practically hard to measure as they interact with the motions of the sailboat. As the initial stage in developing a time accurate dinghy velocity prediction program this work develops an on-water system capably of measuring the applied hiking moment due to the sailor’s pose and compares this with the resultant dinghy motion. The sailor’s kinematics are captured using a network of inertial motion sensors (IMS) synchronized to a video camera and dinghy motion sensor. The hiking moment is evaluated using a ‘stick man’ body representation with the mass and inertial terms associated with the main body segments appropriately scaled for the representative sailor. The accuracy of the pose capture is validated using laboratory based pose measurements. The completed work will provide a platform to model how sailor generated forces interact with the sailboat to affect boat speed. This will be used alongside realistic modelling of the wind and wave loadings to extend an existing time-domain dynamic velocity prediction program (DVPP). The results are demonstrated using a single handed Laser and demonstrate an acceptable level of accuracy.

 

Development of a routing software for Inshore Match Races
F. Tagliaferri, I. M. Viola - Newcastle University UK

Yacht races are won by good sailors racing fast boats. A good skipper takes decisions at key moments of the race based on the anticipated wind behavior and on his position on the racing area and with respect to the competitors. His aim is generally to complete the race before all his opponents, or, when this is not possible, to perform better than some of them. In the past two decades some methods have been proposed to compute optimal strategies for a yacht race. Those strategies are aimed at minimizing the expected time needed to complete the race and are based on the assumption that the faster a yacht, the higher the number of races that it will win (and opponents that it will defeat). In a match race, however, only two yachts are competing. A skipper’s aim is therefore to complete the race before his opponent rather than completing the race in the shortest possible time. This means that being on average faster may not necessarily mean winning the majority of races. This papers present the development of software to compute a sailing strategy for a match race that can defeat an opponent who is following a fixed strategy that minimizes the expected time of completion of the race. The proposed method includes two novel aspects in the strategy computation:

  • A short-term wind forecast, based on an Artificial Neural Network (ANN) model, is performed in real time during the race using the wind measurements collected on board.
  • Depending on the relative position with respect to the opponent, decisions with different levels of risk aversion are computed. The risk attitude is modeled using Coherent Risk Measures.

The software is tested in a number of simulated races. The results confirm that maximizing the probability of winning a match race does not necessarily correspond to minimizing the expected time needed to complete the race.

 

Teamwork as Joint Activity in Sailing
F. Forsman, C. Finnsgard - Chalmers University of Technology, Sweden 

Sailing is a sport and activity that takes a long time both to learn and to master, as much of its competence-based knowledge is acquired through experience. Experience based learning is very important time-intensive, and the factors for success are often tacit and hidden. Should these success factors become explicit and salient, learning would occur faster and produce obvious competitive advantages.

This research was conducted by embedding on-going research results into two competitive sailing teams racing in different classes, one offshore keelboat racing with a crew of 8, and a one-design Star-class racing yacht with a crew of two. The data collection consisted of observations, interviews, and video recordings. The results were also verified with the crews to catch biases in the analysis process. A jibe, a specific but common maneuver was analyzed from the perspective of Common Ground within Joint Activity.

Maneuvering a competitive offshore sail racer or a previously Olympic Star-class yacht are tasks that fulfill the requirements for Joint Activity. A high level of Common Ground is required for the effective coordination needed in order to perform at a high level and maintain the safety of the crew and equipment.

Breakdowns in the coordination of maneuvers were observed, although they must be recorded on video for higher analysis reliability. To achieve greater validity, more and different maneuvers should be considered within the analysis.

By better understanding the factors for success, sail racing teams can more quickly gain competence and thus competitive advantages.

The research analyzes the teamwork found in sailing from the perspective of Joint Activity and Common Ground and provides insight into how to achieve performance improvements more efficiently.