Journal of Sailing Technology

The Journal of Sailing Technology will be available free of charge to all members of SNAME. For a limited time articles will be free to the public.


Towards unsteady approach for future flutter calculations

By A. Luc Mouton, Alban Leroyer, Gan Bo Deng, Patrick Queutey, Thibaut Soler, Britt Ward

Abstract: In prior work to define an improved hydrodynamic approach to flutter calculations, Centrale Nantes, Bureau Veritas Marine & Offshore and Farr Yacht design investigated the possibility of defining a linearized unsteady hydrodynamic model using a fluid response database coming from a series of 2D unsteady RANSE computations. The approach is compared to the Theodorsen theory. The linearized hydrodynamic model was used in a strip theory model for frequency domain flutter analysis. In this latest work, the IMOCA 2006 keel which has been used previously in frequential domain flutter calculation is also analyzed using an alternative and more accurate solution, featuring a fully coupled FSI modal approach with CFD.
As described in the literature, the results present a large impact of the unsteadiness on the phase and module for both lift and moment with a fairly good match compared to Theodorsen theory. The implementation of the results on a frequency domain flutter analysis tool reduces the critical speed for the studied model. Thus the results are closer to the 3D modal CFD approach which gave an lower critical speed.



Design of a Foiling Optimist

By A. Andersson, A. Barreng, E. Bohnsack, L. Larsson, L. Lundin, R. Sahlberg, E. Werner, C. Finnsgård, A. Persson, M. Brown, J McVeagh

Abstract: Because of the successful application of hydrofoils on the America's Cup catamarans in the past two campaigns the interest in foiling sailing craft has boosted. Foils have been fitted to a large number of yachts with great success, ranging from dinghies to ocean racers. An interesting question is whether one of the slowest racing boats in the world, the Optimist dinghy, can foil, and if so, at what minimum wind speed. The present paper presents a comprehensive design campaign to answer the two questions. The campaign includes a newly developed Velocity Prediction Program (VPP) for foiling/non-foiling conditions, a wind tunnel test of sail aerodynamics, a towing tank test of hull hydrodynamics and a large number of numerical predictions of foil characteristics. An optimum foil configuration is developed and towing tank tested with satisfactory results. The final proof of the concept is a successful on the water test with stable foiling at a speed of 12 knots.



Application of phase averaging method for measuring kites performance: onshore results

By Morgan Behrel, Kostia Roncin, Jean-Baptiste Leroux, Frederic Montel, Romain Hascoet, Alain Neme, Christian Jochum and Yves Parlier

Abstract: This paper describes an experimental set-up aiming to control and measure performances of small leading edge inflatable tube kites, with surface area of less than 35 m2. This set-up can be deployed either onshore or on a dedicated boat. This article focuses on the onshore results. A 3D load cell is used to obtain the position of the kite under a straight-line assumption. A wind profiler (SODAR) is deployed to determine the wind speed and direction at the kite position. A specific post-processing of the data is presented, including phase averaging. The guideline of this work is to estimate the variation of the aerodynamic lift coefficient and lift to drag ratio along figure-of-eight trajectories. Results, for a chosen particular case, show a decrease of lift coefficient of about 20% of the maximum value during turning maneuvers of the kite. The lift to drag ratio evolution along a trajectory is also observed to go through a local minimum during turning maneuvers of the kite in this case. Influence of weight and inertia is highlighted introducing a point mass model of the kite and tethers. Buckling phenomena of the kite inflatable leading edge tube are highlighted and it is shown that they cause control problems.



Performance evaluation and ranking of 7 rudders for the finn dinghy

By Adam Persson, Lars Larsson, Matz Brown and Christian Finnsgård

Abstract: As a follow up to the Olympic Games, commercially available Finn dinghy rudders were tested to determine their hydrodynamic performance. Seven rudders were tested, out of the nine different rudder models that were measured for competition at the 2016 Olympic Games, thus representing a large portion of the rudders used by sailors. The remaining two rudder models could not be tested, since they are of semi-custom or custom design or manufacture. Each rudder was tested in seven different conditions, selected to cover a wide range of sailing conditions. The testing revealed considerable differences, both in performance and handling.



Modelling and control of a tethered kite in dynamic flight

By Baptiste Cadalen, Fabien Griffon, Patrick Lanusse, Jocelyn Sabatier and Yves Parlier

Abstract: The need to reduce CO2 emissions and the increase in oil prices affect all transportation industries and especially the maritime industry. This has led to the search for more energy-saving propulsion systems for ships. Taking advantage of wind energy by using tethered airfoils, or kites, as an alternative propulsion source can be an effective solution. The "Beyond The Sea" project, led by Yves Parlier, aims to provide ships with an alternative green energy source. An automatic pilot was designed in order to control a kite that can generate enough power to tow a ship. To this end, a point mass model was first used to describe the kite dynamics. The model parameters were deduced from experimental data and the aerodynamic coefficients were identified using data from a quasi-static flight. Open loop simulations were conducted to verify the kite behaviour and the overall coherence of the model. An eight-shaped trajectory was defined on the wind window. Its position, size, orientation and direction are all adjustable parameters. A path-following strategy was then designed. It computes an error signal between the kite velocity orientation and a desired velocity orientation ensuring that the kite is steered towards and along the trajectory. Closed-loop simulations are presented to show the efficiency of the path-following algorithm, and the various theoretical performances obtained depending on the chosen trajectory are discussed.



Structural design of high performance composite sailing yachts under the new BS EN ISO 12215-5

By Jean-Baptiste Souppez

Abstract: The forthcoming publication of the revised BS EN ISO 12215-5 is set to transform the structural design of most small crafts for the next decade, with the implementation of significant changes to the scope and underpinning theory, such as an applicability extended up to 24 m Load Line, and the use of finite element methods as part of the compliance assessment process. This paper represents the first public release of the major changes and novelty in the standard, with a strong emphasis on high performance composite sailing yachts. The aim is to provide designers and builders with an insight into the technical background and practical applications of the new regulation for structural optimization, and how the marine industry will be impacted.



Art amid science: retaining the role of 'the designer's eye' in innovative performance yacht design

By Robert Shaw

Abstract: Increasingly, the dominance of engineering processes in yacht design development imposes limitations on the possibilities and scope for creative design. This paper explores an alternative methodological framework for yacht design that has innovation as its central concern. The method presented encourages innovation by blending the empirical knowledge obtained through scientific techniques with experiential wisdom and artistic input. It focuses on meeting the key criteria of user experience, performance and handling characteristics, and argues that in a field increasingly dominated by mathematical models and computer-based analysis, there remains a powerful role for the harder-to-quantify role of art in the creation of high-performance yachts. Finally, a case study of the development of the Shaw 11 metre Little Nico provides a platform to both explore and evaluate this alternative methodology in practice.



A new mathematical model for investigating course stability and maneuvering motions of sailing yachts

By Manolis Angelou and Kostas J. Spyrou

Abstract: In order to create capability for analyzing course instabilities of sailing yachts in waves, the authors have set up a mathematical model comprised of two major components. The first is an aerodynamic model focused on the calculation of the forces on the sails, taking into account the variation of their shape under wind flow. As the sails are very thin, their shape is 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 should be considered as initially 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. The deformed shape of each sail is obtained using a shell finite element formulation. The other component of the presented mathematical model refers to the hydrodynamic part and it is focused on handling the motions of the hull, with her appendages, in water. The hydrodynamic part is comprised of sub-models for hull reaction, hydrostatic and wave forces. A potential flow boundary element method is applied for the calculation of the side forces and added masses of the hull and the appendages. The calculated side forces are then incorporated into an approximate scheme for identifying the hull reaction terms. The wave excitation involves the calculation of Froude - Krylov forces while radiation terms are found using a strip theory formulation.



Modal analysis of pressures on a full-scale spinnaker

By Julien Deparday, Patrick Bot, Frédéric Hauville, Benoît Augier, Marc Rabaud, Dario Motta and David Le Pelley

Abstract: While sailing offwind, the trimmer typically adjusts the downwind sail “on the verge of luffing”, occasionally letting the luff of the sail flap. Due to the unsteadiness of the spinnaker itself, maintaining the luff on the verge of luffing requires 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 data and wind data. Significant spatio-temporal patterns were 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 makes it possible to restrict the flapping analysis 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.



Numerical simulations of a surface piercing A- class catamaran hydrofoil and comparison against model tests

By Thilo Keller, Juryk Henrichs, Dr. Karsten Hochkirch and Dr. Andrés Cura Hochbaum

Abstract: Hydrofoil supported sailing vessels gained more and more importance within 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 cause cavitation at high speeds. The aim of this work is to define a setup to calculate the hydrodynamic forces on such foils with RANS CFD and to investigate whether the onset of ventilation and cavitation can be predicted with sufficient accuracy.
Therefore, a surface piercing hydrofoil of an A-Class catamaran is simulated by using the RANS software FineMarine with its volume of fluid method. The C-shaped hydrofoil is analyzed for one speed at Froude Number 7.9 and various angles of attack (AoA) by varying rake and leeway angle in ranges actually used while sailing. In addition, model tests were carried out in the K27 cavitation tunnel of TU-Berlin, for the given hydrofoil and in the same conditions as simulated with CFD to provide data for validation. Based on the CFD calculations this paper presents how the rake and leeway angles influence the foil’s lift to drag ratio.
The simulations have been verified by extensive analyses, including domain size verification for unrestricted water, mesh refinement and y+ verification. The influence of the dimensions of the K27 (cavitation tunnel of the Technical University of Berlin) on the flow around the hydrofoil and the wave system is also considered, as the test section of the K27 significantly 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.



Pressure measurements on yacht sails: development of a new system for wind tunnel and full scale testing

By Fabio Fossati, Ilmas Bayati, Sara Muggiasca, Ambra Vandone, Gabriele Campanardi, Thomas Burch and Michele Malandra

Abstract: The paper presents an overview of a joint project including 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 research has been developed for the final employment on the Lecco Innovation Hub Sailing Yacht Lab, a 10 m length sailing dynamometer which aims at being the reference contemporary full scale measurement device in the sailing yacht engineering research field, to enhance the insight of sail steady and unsteady aerodynamics.
The pressure system is described in details, the data acquisition process and system metrological validation are reported; 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 being finalized at the time of this paper, is also described

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Development of a sailing-specific pose capture method to measure dynamic sailor loadings

By J. C. Taylor, J. Banks, D. Taunton, S. R. Turnock and D. Hudson

Abstract: Research into the dynamics of sailing vessels, most notably yachts, has led to the development of sophisticated models including the unsteady aero and hydrodynamics and even sailor's tactics. However, the time-varying loadings caused by a sailor’s motions have typically been neglected in velocity prediction programs (VPPs). When applied to the assessment of sailing dinghy performance, the position and motions of the crew are significant but impractical to measure mechanically. A sailing-specific pose capture method to determine the sailor loadings using orientation sensors and a model of the sailor’s mass distribution is presented. The accuracy of the hiking moment estimate was evaluated using laboratory-based measurements. The estimated hiking moment exhibits excellent dynamic tracking of the measured moment. The method is used to measure on-water hiking moment for the first time and the results are discussed. The proposed method provides a platform to analyze and model how sailor-generated forces interact with the sailboat to affect boat speed. This can be used alongside realistic modelling of the wind and wave loadings to extend existing time-domain dynamic velocity prediction programs (DVPPs). This opens a new area of sailing research as the human can now be modelled to respond to force perturbations.



Wind tunnel investigation of dynamic trimming on upwind sail aerodynamics

By Nicolas Aubin, Benoit Augier, Matthieu Sacher, Patrick Bot, Frédéric Hauville and Richard G. J. Flay

Abstract: An experiment was performed in the Yacht Research Unit’s Twisted Flow Wind Tunnel (University of Auckland) to test the effect of dynamic trimming on three IMOCA 60 inspired mainsail models in an upwind (apparent wind angle βAW = 60°) unheeled configuration. This study presents dynamic fluid structure interaction results in well controlled conditions (wind, sheet length) with a dynamic trimming system. Trimming oscillations are done around an optimum value of the optimization target coefficient CFobj previously found with a static trim. Different oscillation amplitudes and frequencies of trimming are investigated. Measurements are done with a 6 component force balance and a load sensor giving access to the unsteady mainsail sheet load. The driving force coefficient CFx and CFobj first decrease at low reduced frequency fr for quasi-steady state then increase, becoming higher than the static state situation. CFx and 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 also investigated. The flat mainsail design benefits more than the other mainsail designs from the dynamic trimming compared to their respective static situation. This study presents dynamic results that cannot be accurately predicted with a quasi-static approach. These results are therefore valuable for future fluid-structure interaction numerical tools validations in unsteady conditions.



RANS Based CFD Investigation of the Interference Between Two Boats Sailing Upwind

By Christoph Boehm and Kai Graf

Abstract: Two sailboats sailing in close proximity will influence each other’s wind field. Depending on their relative positions, positive or negative aerodynamic interference may occur. This phenomenon is generally known to sailors and well understood in a qualitative manner. However, it is of great value for aerodynamicists, performance analysts and tacticians to gain a quantitative understanding of this phenomenon. Modern CFD methods allow extensive flow visualization and are therefore helpful to improve our understanding of flow interference. 


This paper describes the results of numerical simulations of the aerodynamic interference of two Elliot 6m sailboats sailing upwind. The simulations were performed using methods based on the solution of the Reynolds Averaged Navier Stokes Equations, so called RANSE-solvers. A broad range of relative positions of the two sailboats were investigated and the differences in boat speed were quantified. The results show that, for a situation where the two boats sail directly abeam of each other, neither of them has an advantage. However, for a situation where the boats sail even with respect to a regatta start line, the leeward boat is able to achieve a quite large speed advantage of 0.2 knots.


Journal of Sailboat Technology (2010-2015)

The Journal of Sailboat Technology was launched in 2010 and run until 2016, when it was renamed into Journal of Sailing Technology. Articles will be available free of charge to all members of SNAME. For a limited time articles will be free to the public.


Bifilar Suspension Measurement of Boat Inertia Parameters 

By Peter F. Hinrichsen

Abstract:Measurements of the inertia parameters (Gregory, 2006) of a keelboat hull using a bifilar suspension (Newman and Searle, 1951) are described. Bifilar yaw moment measurement normally entails accurate measurement of the length l and spacing 2d of the suspension, and of Ty the period of pure yaw oscillation (Miller, 1930). The primary difficulty with a bifilar suspension is avoiding unwanted modes of oscillation, specifically sway when measuring yaw. However, for an athwartships suspension, the sway motion is that of a simple pendulum of period Ts and observation of the combined motion allows the yaw gyradius ky ≡ kzz to be determined as ky = (Ty/Ts)d. Thus only the ratio of the periods and the suspension spacing need to be measured.  Measurements of the normal mode periods of the double pendulum motion (Rafat, Wheatland et al., 2009) when the hull is displaced in surge allow for the pitch gyradius kp ≡ kyy and the height l2 of the center of mass to be determined. The latter can be confirmed by measuring the incline angle of the hull when a weight is suspended from the stern and/or the bow.  Repeating yaw measurements with the hull tilted, and then with the bifilar suspension fore and aft to measure the roll gyradius, kr ≡ kxx, allows for the angle of the inertia ellipsoid (Wells 1967) principal x axis to the hull x axis to be calculated.  Although the present keelboat measurements were made using ultrasonics (Daedalon, 1991) and photogates (Pasco, 2000), such measurements can now be more easily made using MEMs gyros, such as that in the iPhone (xSensor, 2010). This is illustrated by the measurements on a model keelboat.


Experimental Investigation of Sail Aerodynamic Behavior in Dynamic Conditions
By Fabio Fossati and Sara Muggiasca

Abstract: The aerodynamic behavior of a 48’ yacht rig 3D scale model was characterized by means of wind tunnel tests. The experimental program, performed in the Politecnico di Milano Twisted Flow Wind Tunnel, allowed the aerodynamic forces to be characterized using forced motion tests and the aeroelastic behavior of the sail plan to be studied. Preliminary results are presented with reference to the typical encounter frequency range and typical pitch amplitudes corresponding to best yacht Velocity Made Good (VMG) condition for different true wind speeds and related sea states. A new representation of the aeroelastic effects is proposed in the form of aerodynamic hysteresis loops, obtained by plotting the sail plan aerodynamic coefficients against instantaneous apparent wind angle, varied using a harmonic law combining wind tunnel constant wind speed with sail plan center of effort velocity induced by yacht pitch motion. A “sail plan reduced velocity” concept is defined, and experimental results show that this reduced velocity play a fundamental role in sail plan aerodynamics. Finally, an estimate of the aerodynamic added mass effects is provided based on the comparison between the measured pitch moment during the “wind off” tests and the experimental test rig pitch moment of inertia.


Tacking Simulation of Sailing Yachts With New Model of Aerodynamic Force Variation During Tacking Maneuver
By Yutaka Masuyama and Toichi Fukasawa

Abstract: A mathematical model for the tacking maneuver of a sailing yacht is presented as an extension of research by the same authors. The authors have proposed the equations of motion for the tacking maneuver expressed in the horizontal body axis system. The calculation method was applied to a 34-foot sailing cruiser and the simulated result showed good agreement with the measured data from full-scale tests; however, the modeling of aerodynamic force variation during tacking was insufficient due to lack of information about the sail forces. In this report, the authors performed full-scale measurement of sail forces during tacking maneuvers using a sail dynamometer boat Fujin. The Fujin is a 34-foot sailing cruiser which has a measurement system to obtain simultaneously sail forces, sail shapes, and boat attitude. Based on the results of full-scale measurements, a new model of aerodynamic force variation for the tacking maneuver was proposed. The equations of motion were also simplified to more easily perform the numerical simulation. Using this calculation method, the tacking simulations were performed and compared with the measured data from three full-scale boats. The simulated results showed good agreement with the measured data. This simulation method provides an effective means for assessment of tacking performance of general sailing yachts.


Experimental Study of the Hydro-Impact of Slamming in a Modern Racing Sailboat
June Lee and Philip A. Wilson

Abstract: The hydrodynamic impact, or hydro-impact, phenomenon caused by slamming on racing yachts and the local structure’s response is studied experimentally. Pressure transducers and a special measurement system named ‘Slam Patch’ have been designed and implemented to measure the hydro-impact pressure and/or the local structure’s response. The measurement systems were installed on a 1/7-scale model of an Open 60 yacht. Modal, rotational drop, and seakeeping-slamming tests are carried out. The measured hydro-impact pressure was processed statistically. A methodology to scale up the test results to prototype is mentioned. At the same time, the transient response of a simple structure under half-sine impulse is calculated using a commercial finite element analysis program to study the effect of the relationship between impulse duration and natural frequency of the structure.