In the racing boat design world, the most important aim is researching the best compromise between performances and safety. Nowadays, countless structural design tools exist, such as Finite Element Analysis (FEA), however every numerical model needs a controllable, repeatable, reliable experimental validation. The present work focuses on the structural design cycle adopted by Polito Sailing Team during the design and building of their own new skiff, a high-performance sailing dinghy, built mainly with natural composite material like balsa wood and flax fiber. The whole boat was completely designed by students, according to eco-sustainable principles, in order to participate in a university competition called 1001VelaCup

Fontana, M.

Glorioso, M.

Mattiazzo, G.

Moscoloni, C.

Pilagatti, A.

English

Fontana M.

Glorioso M.

Pilagatti A.

Moscoloni C.

Mattiazzo G.

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

04 August 2020POLITECNICO DI TORINORepository ISTITUZIONALEExperimental validation of a FEM-aided structural design of an eco-sustainable high performance made skiff / Fontana,M.; Glorioso, M.; Pilagatti, A.; Moscoloni, C.; Mattiazzo, G.. - ELETTRONICO. - (2020), pp. 233-237. ((Interventopresentato al convegno 2019 IMEKO TC19 International Workshop on Metrology for the Sea: Learning to Measure SeaHealth Parameters, MetroSea 2019 tenutosi a Genova (ITA) nel 2019.OriginalExperimental validation of a FEM-aided structural design of an eco-sustainable high performance madeskiffPublisher:PublishedDOI:Terms of use:openAccessPublisher copyright(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2837102 since: 2020-06-23T12:50:13ZIMEKO-International Measurement Federation Secretariat2019 IMEKO TC-19 International Workshop onMetrology for the SeaGenoa, Italy, October 3-5, 2019Experimental validation of a FEM-aidedstructural design of an eco-sustainable highperformance made skiffM. Fontana1, M. Glorioso1, A. Pilagatti1, C. Moscoloni1, G. Mattiazzo11Politecnico di Torino, Department of Mechanical and Aerospace EngineeringAbstract – In the racing boat design world, the mostimportant aim is researching the best compromise be-tween performances and safety. Nowadays, count-less structural design tools exist, such as Finite Ele-ment Analysis (FEA), however every numerical modelneeds a controllable, repeatable, reliable experimen-tal validation. The present work focuses on the struc-tural design cycle adopted by Polito Sailing Team dur-ing the design and building of their own new skiff,a high-performance sailing dinghy, built mainly withnatural composite material like balsa wood and flaxfiber. The whole boat was completely designed by stu-dents, according to eco-sustainable principles, in or-der to participate in a university competition called1001VelaCup.I. INTRODUCTION AND STATE OF THE ARTIn order to validate a sail skiff structural model based onfinite element method (FEM) [1], it is needed to evaluatethe strains in specific positions at certain boundary con-ditions. In fact, in a cruising regime is possible to studythe skiff behavior through a static approach identifying themost impactful loads as rig and rigging due to sails, right-ing moment due to the crew and hydrodynamic pressuresacting on the hull at specific speed and roll angle. Uncount-able works focus on this working pattern, like [2] and manyothers.II. NUMERICAL MODELThe load condition to which a skiff is exposed consid-ers countless contributions represented by many variablesnot easily correlated [3], the aim of the numerical modelis divide these contributions in order to approximating themost important and then to obtain the strain and the stressfield verifying the compliance with the allowable stress ofthe material. In fact, regarding the deck’s loads, a 1D FEMmodel has been implemented in a Nastran-Patran environ-ment, while the hydrodynamic pressures have been ob-tained through a CFD analysis from StarCCM+ software.These loads have been reported in the full model, on theside of rig and rigging loads through the reaction forces,on the hydrodynamic pressure sides, a transfer algorithmhas been designed.Fig. 1. Load Condition.At least, the equilibrium has been imposed varying the po-sition of the crew respect the mast foot.A. Load ConditionThe most contribution in the skiff load situation, con-sidering a static approach, is the eccentric load located atthe mast, but also the loads due to both chainplates andshroud as shown in Fig.1. The compressive load has beenobtained through the balance between the righting momentdue to the crew and the interaction with the sails plane[4].F =1.5 ·RM(30◦)HCPB+12 ·RM(30◦)PW+2.5 ·RM(30◦)J(1)In which:• PW = height of mainsail tip from the water line;• J = foretriangle base length;• HCPB = half distance of the chainplates;• RM = righthin moment at 30◦ due to the crew.Sizes PW, J, HCPB, RM are defined in Fig.2B. Shroud and Chainplates DesignThe shroud and chainplates loads have been obtainedthrough experimental trials, in fact a couple of load cellshave been assembled. In order to considering the preten-sion loads, it was necessary to model these lasts through233Fig. 2. Characteristic Dimentions.an imposed displacement located at the ends of the ca-bles. Anyway, the physical behaviour of a steel cableshould consider that the compressive stresses are negligi-ble, however the ROD element doesn’t contemplate thisphenomenon. In order to fix this problem, it was neces-sary acting on the material behaviour designing a bi-linearsteel in which for negative strains the Young’s modulus isnegligible, as shown in Fig 3.The imposed displacemets have been obtained consider-ing the first setup adopted before the starting. The loadsmeasured in the harbour are equal to:• Shroud: 1128 N;• Chainplates: 1569.6 NThese force values have been converted in displacementthrough the following equation:δL =F · LE ·A (2)Where:• F = force recorded;• L = cable length;• E = Young’s modulus;• A = cable section area.C. CFD to FEM Tranfer AlgorithmThe hull pressures have been obtained in a CFD envi-ronment in which it is considered a speed of 6 knt with-out waves. The most important problem is to transfer thepressures calculated at the forequarter of the element tothe FEM environment, in which the load is applied at themiddle of the element. To overcome this limit, a simpletransfer algorithm has been designed. In fact, the samedivision pattern between CFD and FEM hull model hasFig. 3. 1D Model and Material Design.Fig. 4. Hull’s Pressuresbeen obtained, through a simple index system that identi-fies the corner nodes of each panel. This panelling is theson of a previous work in which the panel partition is basedon a criterion provided by regulations. The average of thepressures obtained by CFD has been calculated, as shownin Fig.4 and then applied to the same FEM model panelthrough a biunique relationship.D. Numerical ResultsThe predictive FE Model integrates the steps previouslydiscussed in an embedded system that takes into accountboth the loads on the deck, in the form of reaction forces,both the hull’s pressure. The results obtained are consistentwith the physic’s of the problem and suggest that the regionsubjected to the highest strain field is under the mast foot,as shown in Fig5.III. EXPERIMENTAL SETUPThe aim of this work is to validate the structural FEMmodel of the Polito Sailing Team skiff, in order to do thisconfirmation, an experimental campaign has been needed,as shown in Fig6 and in Fig7. The experimental setup con-sists in:1. Strain acquisition system;2. Load Cells;3. GPS, pitch angle, roll angle acquisition system basedon Arduino.The sample frequency chosen for the strain gauges sys-234Fig. 5. Strain FieldFig. 6. Acquisition Systemtem si about 100 Hz, this value appears reasonable forstatic phenomena. Moreover, the saving setup has been de-signed considering a multiple redundancy in order to avoida data loss due to failures depending on the adverse envi-ronment in which the experiments have been conducted.In order to obtain the best strain gauge location pattern,a first predictive FEM model has been considered. Afterthis, the whole strains acquisition system is composed bysix three-grid rosettes (RY10-3/120) and four linear straingauges (LY11-6/350) [5]. The link with the acquisition de-vice involves a half-bridge Wheatstone configuration with0.1% of precision resistors. To understand the strain fieldof the whole structure in according to with the FE modelthe best measure points chosen are in the joint between theinner structure and the hull in a specular way, while the lin-ear strain gauges have been set on the inner structure beamin the closeness of the chainplates, as is shown in Fif9. Dueto the aggressive environment, a way to protect the sensorshas been needed the whole sensors located both on the hullboth on the inner structure have been treated with a coatingcomposed by a polyurethane skin and a layer of silicon.IV. BOUNDARY CONDITIONSThe environment in which a sail skiff works is affect bycountless noises like waves, wind gusts and unintentionalbumping by the crew during the maneuvers. This workfocuses on ranges in which the dynamic effects are negli-gible only. In compliance with the assumptions, this workFig. 7. Arduino acquisition system schemeFig. 8. Strain gauges locationconsiders a range about 3 minutes in which the hypotheseshave been respected, in particular, the speed range consid-ered is about 6± 0.5 ktn as shown in figure.The whole experimental campaign has been conductedin the lake of Como, in the following, the GPS trackingwill be presented, in Fig.10, with the aim to describe thestraight trajectory at most of capabilities of the crew.V. EXPERIMENTAL RESULTSThe results outcome from the acquisition system areconsistent with the first predictive FEM model. In partic-ular, the most significant strains gauge and the load cellsdata are reported in the following figures for the time rangeconsidered.The maximum loads recorded are about 1650 N for theshroud and 1745 N for the chainplates, as is shown inFig.11 and in Fig.12 respectlively.Moreover, the strains time histories, in the range se-lected will be presented how you can see in Fig.13 andin Fig. 14.In order to validate the numerical model, a comparisonbetween experimental results and FEM model has beenconducted. In particular, due to the huge numbers of dy-namic phenomena involved, the error between Numericaloutcomes and Experimental ones has been considered asthe difference between the FEM value and the average ofthe experimental signal. This is possible due to the low val-ues of standard deviation. The results have been reportedin Tab.1 and Tab.2.23518:43:0018:44:0018:45:0018:46:0018:47:0018:48:00Time0510Speed [knt]Velocity RangeVelocity acquiredAverage = 6 [knt]18:4318:4418:4418:4518:4518:46Time5.866.26.4Speed [knt]Velocity between 5.5 and 6.5 [knt]Fig. 9. Velocity RangeFig. 10. GPS TrajectoryTable 1. Experimental ResultsSensor Numerical Experimental Error STDRY 1 45◦ 1.61e− 05 1.22e− 05 14.98% 8.794e− 06RY 4 90◦ 1.608e− 06 1.536e− 06 4.48% 5.201e− 07Clearly, the discussion about the strain values and thecorresponding comparison with to the FEA results deservea clarification. In fact, the sensors are located in the innerside of the hull, but also at the top layer of the sandwichstratification, in order to compare correctly the equivalentvalues between experimental data and numerical ones, thecomposite strain at the top layer has been considered. Ithas been possibile thanks to a reverse assignment of theproperties. In this way it is possible to consider each layerindividually.Moreover, a further comparison has been done. In fact,the predictive FE model is characterized by a flexible con-strain in order to modelling the interaction with the fluid(sea). An important result is verifying the gap between aflexible model and a rigid one with the experimental re-sults. The results are reported in Tab.2.Considering the RY 1 45◦ strain gauge it’s impossible to4760 4770 4780 4790 4800 4810 4820Time (s)90010001100120013001400150016001700Load (N)ShroudShroud loadPrestress loadFig. 11. Shroud load history3900 3950 4000 4050 4100 4150 4200Time (s)60080010001200140016001800Load (N)ChainplatesChainplates loadPrestress loadFig. 12. Chainplates load historyTable 2. Rigid Model ComparisonSensor Numerical Experimental ErrorRY 1 45◦ 3.151e− 05 1.22e− 05 /RY 4 90◦ 9.529e− 05 1.536e− 06 182%estimate the error properly, in fact, the sensor is too closeto the symmetry constraint and it’s affected by it. It is pos-sible to affirm that the flexible model is more reliable thanthe rigid one.VI. CONCLUSION AND FUTURE WORKSIn the end, we can affirm that the FEM model is ver-ified by experimental results. This work represents a firststep in the structural design cycle adopted by Polito SailingTeam. Clearly, the future steps will have to involve a VPP(Velocity Prediction Program) model in order to integratethe skiff system with the boundary conditions as much aspossible. Further, another aim is obtaining the Tsai-Wu, orHill, coefficients by experimental benchmarking in orderto apply failure criteria in the design process that involves2363.2234 3.2236 3.2238 3.224 3.2242 3.2244 3.2246 3.2248 3.225 3.2252105-1.5-1-0.500.511.5 10-5 RY 1 45°Experimental dataFEM dataFig. 13. RY 1 45◦3.3158 3.316 3.3162 3.3164 3.3166 3.3168 3.317 3.3172 3.3174 3.317610500.511.522.533.54 10-6 RY 4 90°Experimental dataFEM dataFig. 14. RY 4 90◦eco-sustainable composite materials.VII. ACKNOLEDGMENTThe authors want to acknowledge all the members ofPolito Sailing Team who worked at this project and thefacilities of the Department of Mechanical and AerospaceEngineering of Politecnico di Torino.REFERENCES[1] C. Moscoloni, M. Fontana, B. Passione, M. Cinefra, G.Mattiazzo, “FEM-Aided Structural Design of a Natu-ral Fiber Composite Made Skiff”, NAV 2018 19th In-ternational conference on ship and maritime research,IOS Press, 2018.[2] A. Mancuso, G. Pitarresi, G. B. Trinca, D. Tumino,“Refitting of an eco-friendly sailing yacht: numeri-cal prediction and experimental validation”, LectureNotes in Mechanical Engineering, Springer Interna-tional Publishing, 2016.[3] A. Quarteroni, N. Parolini, “Modelling and numericalsimulation for yacht engineering”, Proc. of 26th Int.Symp. On Naval Hydrodynamics, 2007.[4] C. J. Mitchell, “Rigging loads on the yacht ’NewZealand’ and rig design formulae. RINA Transaction,1993.[5] K. Hoffman, “An Introduction to Stress Analysis andTransducer Design using Strain Gauges", HBM Testsand Measurement, 2012.237

Experimental validation of a FEM-aided structural design of an eco-sustainable high performance made skiff

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Experimental validation of a FEM-aided structural design of an eco-sustainable high performance made skiff

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