Hydrodynamic testing of wind-assisted cargo ships using a cyber- physical method Thomas Sauder, Sverre A. Alterskjr Highlights Hydrodynamic testing of wind-assisted cargo ships using a cyber- physical method Thomas Sauder, Sverre A. Alterskjr •A novel method to test wind-assisted ships in hydrodynamic labora- tories is presented. •The physical ship model, including propulsion units, interacts in real- time with a numerical sail model during free-running tests. •The method is applied to investigate the bene ts and limitations of wind-assistance on a 190m bulk-carrier, equipped with four rotor sails. •Key performance indicators are presented and analyzed for steady wind pro les with various directions and velocities. Hydrodynamic testing of wind-assisted cargo ships using a cyber-physical method Thomas Saudera,b, Sverre A. Alterskjra aSINTEF Ocean AS, P.O. Box 4762 Torgarden, Trondheim, 7465, Norway bNorwegian University of Science and Technology, Dept. of Marine Technology, Trondheim, 7491, Norway Abstract A novel empirical method to study wind-assisted cargo ships is presented. The physical ship model, including propulsion units, interacts in real-time with a numerical sail model during free-running tests. Loads from the (vir- tual) sails are applied on the physical model using a cable-driven robot. All loads components except heave are applied with high accuracy and re- peatability. The method is described thoroughly, and applied to investigate the bene ts of wind assistance on a 190m bulk-carrier, equipped with four rotor sails. Key performance indicators for wind assistance are established when sailing in steady wind pro les of various directions and velocities, and a propulsion analysis is performed. An important conclusion is that the increase of hydrodynamic resistance due to heel, leeway and rudder (that balance transverse sail loads) is rather limited for this ship, except when sail- ing close-hauled in strong winds. It is also demonstrated that experiments in a unsteady (virtual) wind environment can be successfully conducted. The e ect of turbulence on the motions of this ship are found to be very limited. The conclusion discusses the other types of studies that can be enabled by this novel cyber-physical empirical method. Keywords: sail-assisted propulsion, hydrodynamic testing, cyber-physical empirical method, bulk carrier, rotor sail Preprint submitted to Ocean Engineering November 29, 2021 Table 1: Nomenclature.See also Table 2 for quantities related to vessel main particulars. y: see details in Section 3. Symbol Description Unit Support AWA Apparent wind angle (relative to bow)o[180;180] AWS Apparent wind speed m/s R+ TWA True wind angle (relative to bow)o[180;180] TWS,Uw True wind speed m/s R+ TWD True wind direction (relative to North)o[0;360] COG Course over groundo[0;360] h Height above deck m R+ CD Drag coecient - R+ CL Lift coecient - R+ AR Aspect ratio - R+ H Rotor height m R+ SR Spin ratio - R+ d Rotor diameter m R+ de Rotor end-plate diameter m R+ Rotor rotational velocity rad/s R air Air density kg/m3R+  Inverse Bellman coecient - R+ fbg Body- xed coordinate system - - fng Earth- xed coordinate system - - = (p;) Vessel posey p Vessel positionyinfng m R3  = ( ;; )>Vessel attitude (roll, pitch, yaw)yinfng rad SO(3) = (vb;!b) Vessel velocityyinfbg != (p;q;r )>Vessel angular velocity (roll, pitch, yaw)yinfbg rad/s R3 vb= (u;v;w )>Vessel linear velocity (surge, sway, heave)yinfbg m/s R3 Load vector expressed in fbg N R6 T tuple gathering line tensions N (R+)6 J Jacobian matrix - R65orR66 rb iPosition of ithactuators's attachement point in fbg m R3 pn a;iPosition of ithactuators infng m R3 rb r;sPosition of the sthstrip of the rthrotor infbg m R3  Water density kg/m3R+ n Propeller rate of revolution Hz R+ D Propeller diameter m R+ Tp Propeller thrust N R+ Qp Propeller torque Nm R+ Ts Thrust force from sails N R+ J;J0 Advance coecient - R+ KTpThrust coecient - R+ KQp;KQ0Torque coecient - R+ t;tThrust reduction fraction - R+ w Wake fraction - R+ t;tThrust reduction fraction - R+ R Relative rotative eciency - R+ 0 Propeller eciency in open water - R+  HHull eciency - R+ RTm Hydrodynamic resistance without rotors N R+ R TmHydrodynamic resistance with rotors N R+ 2 Figure 1: Very large ore carrier (VLOC) M/V Sea Zhoushan of the Brazilian mining company Vale, equipped with ve rotor sails. Pictured in Zhoushan, China April 29, 2021. Credit photo: Vale. Nomenclature 1. Introduction While the idea of using wind assistance for the propulsion of modern ships dates back to the early 1900s [1], the wind-assisted propulsion de- vices have been receiving increased attention over the recent years [2]. This is mainly due to the increasing pressure for emission reduction and decar- bonization of maritime sector [3]. Figure 1 presents the most recent, at the time of writing, new-built vessel equipped with 24-meter tall by 4-meter diameter rotor sails. A vast number of research and development projects deals with tradi- tional propelled vessels, aimed at hull and propulsion design optimisation, friction reduction, alternative fuels, operation optimization, among others. In contrast, there is a knowledge gap in design, integration and operation of wind assisted propulsion in shipping. The physical complexity of the aerody- namics and hydrodynamics interaction of wind assisted ships are generally oversimpli ed, or neglected, to provide a performance prediction of a wind- assisted ship [4]. Indeed, the application of wind propulsion a ects many aspects of the ship system as whole. (1) Wave patterns and hull resistance are modi ed and due to drift and heel angles, (2) Considerable hydrody- namic lift force and induced moment are generated by the hull due to drift (3) Appendage forces and rudder/propeller interaction are modi ed too, (4) Email address: thomas.sauder@sintef.no (Thomas Sauder) 3 Propulsion forces vary due to change of propeller loading and in ow angles, (5) The machinery system are a ected through change of propulsion point (6) Roll and pitch damping increases due to the presence of sails which a ects the added resistance in waves. (7) In ow on bilge keels is consider- ably altered. Modeling each of these e ects individually poses a challenge in itself, and so does the treatment of the coupling between the interacting e ects [5]. Standards and procedures for performance prediction of vessels with- out wind propulsion through model tests are well established for a range of operational aspects. They are appropriately adopted for classi cation and veri cation of, for instance, EEDI-rating [6]. However, there is a need for tailor-made empirical validation methods both as part of design optimiza- tion and nal validation of new concepts comprising wind propulsion. In addition to classical ship model tests to study isolated e ects such as hull, appendage and steering forces for a range of operating points relevant for a wind propelled vessel, integrated performance tests with free running ship models will necessitate the application of wind forces on the model. Ship model testing follows the Froude scaling laws to ensure a correct representa- tion of wave systems. It is in practice impossible to ful ll Reynolds number similarity for wind propulsion systems at the same time, and this is a sig- ni cant issue for e.g. Flettner rotors and wing sails [7] Furthermore, there are technical challenges related to generation of a wind eld (which ideally also could be dynamically changed during a test) around a moving object of appropriate quality in a hydrodynamic laboratory. These issues are very similar to the ones that have been encountered in the early experimental studies of oating wind turbines, discussed in [8, 9]. For this application, poor wind eld generation capabilities and Froude-Reynolds con ict have been solved by using a cyber-physical em- pirical method approach named "Real-Time Hybrid Model testing"1[10], in- stead of classical "fully-physical" experiments. In such a method, one part of the system is modelled physically, while the other part, whose behaviour is assumed to be well described theoretically, is modelled numerically. Both physical and numerical substructures interact, in real-time, through a con- trol system, composed of sensors, actuators, observers, allocation routines, etc. For oating wind turbines, ReaTHM testing enabled to study the dy- namical behaviour of large wind oating wind turbines [11, 12], and put in evidence the e ect of velocity-dependent eigenmodes that was then studied 1a.k.a. ReaTHM®testing, a registered trademark of SINTEF Ocean 4 theoretically [13]. Back to the eld of sail-assisted propulsion, advances have been made in the recent years in the numerical modeling of sails, based on analytical approaches [5], numerical studies [14] and wind tunnel tests [7]. See the review in [5] for details. In the present paper, we present the rst empirical study on sail-assisted ships using a cyber-physical approach, where wind loads, among others, are modelled numerically. We provide a detailed description of the proposed method, and present experimental results obtained during a pilot study performed in Mai 2021 in SINTEF Ocean's towing tank. In that study, a bulk carrier model retro tted with four (virtual) Flettner rotor sails was freely running in the tank. Aerodynamic loads on the the sails were com- puted in real-time based on the ship motions and on computational uid dynamics results. These loads - all components except the heave force - were applied to the ship with high precision, closing the feedback loop. It is, to the authors' knowledge, an unprecedented contribution to the eld of wind-assisted propulsion. The present paper is structured as follows. In Section 2, we rst present the full-scale system that has been investigated, namely the vessel and the rotor sails and the wind environment. In Section 3, we detail the cyber-physical test setup that has been developed. The results of the test campaign are presented and analyzed in details in Section 4. Conclusions are drawn in Section 5. 2. System description 2.1. SINTEF Ocean Bulk Carrier (SOBC-1) The concept of this ship is based on the idea of designing a benchmark vessel which is representative of the class of medium-size, medium range, single-screw tank/bulk carriers. The ship design features unconventional main dimensions originating from feasibility studies which have been con- ducted in the Centre for Research based Innovation (SFI) Smart Maritime [15] in 2019, and it implements state-of-the-art solutions regarding the hull lines, controllable pitch 4-bladed propeller and high-eciency rudder de- signs. SOBC-1 is intended to serve as a testbed { physical and numerical { for various energy saving solutions and innovative "low-"/"zero-" emis- sion technologies. In autumn 2020, the rst hydrodynamic designs of the ship hull, propeller and rudder have been the result of joint e ort between three research groups at SINTEF Ocean. This design version is based on the experience and current best practices, i.e. without heavy involvement of advanced numerical simulations. This ship type is highly relevant with respect to application of wind propulsors, and was therefore a natural choice 5 Table 2: SOBC-1 main particulars at design waterline (DWL). Parameter and symbol Value Length overall, LOA 200 m Length betw. perp., LPP 190 m Length on waterline, LWL 196.942 m Breadth moulded, B 32.295 m Breadth waterline, BWL 32.201 m Draught at FP/AP and LPP/2, T 11.000 m Bilge radius 2 m Wetted surface 8485.24 m2 Wetted surf. of transom stern AT 2.91 m2 Volume displacement r 48957 m3 Prismatic coecient (based on LPP), Cp 0.7308 Block coecient (based on LWL), CBLW 0.7018 Propeller diameter, D 6.75 m Pitch ratio at r/R = 0.7, P=D 0:7 0.970 - Blade area ratio, AE/A0 0.483 - Number of blades, Z 4 - Chord/Diameter ratio, c/ D0:7R 0.3209 - Thickness/Diameter ratio, t/ D0:7R 0.0324 - Hub diameter ratio, d/D 0.252 - Number of rotor sails, nrotors 4 Rotor sail height, H 35 m Rotor sail diameter, d 5 m Rotor sail end-plate diameter, de 6 m Rotor sail rotation velocity 180 rpm of case vessel for this study. Main particulars are given in Table 2 and hull lines are presented in Figure 2. Geometries of the hull, propeller, rudder and headbox will be made publicly available however are not published at the time of writing. 2.2. Flettner rotor sails arrangment Rotor sails, commonly named Flettner Rotors after its 1920's inventor German aviation engineer Anton Flettner, are rapidly gaining interest. A number of installations are completed (see Figure 1), and more are planned for RoRo/RoPax, product tankers, bulk carrier, among others. The Flet- tner rotor is a spinning vertical cylinder installed on the ship deck. The cylinder's rotational speed, provided by an electrical motor, combined with an incoming wind speed generates a lift force through the Magnus e ect [16], see Figure 4. The lift force is perpendicular to the air ow, thus the maximum exploitation of the wind happens in apparent beam wind. When placing rotor sails on deck, care should be taken to avoid unfavorable interac- tion e ects between rotors, as with all wind propulsion systems. Interaction e ects between Flettner rotors in particular can according to [5] partly be compensated for by individual control of rotor RPM. Some comments and