Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014 EXPERIMENTAL ASSESSMENT OF MEGA-YACHT AERODYNAMIC PERFORMANCE


AND CHARACTERISTICS (DOI No: 10.3940/rina.ijsct.2014.b2.157) F Fossati, F Robustelli and M Belloli, Politecnico di Milano – Department of Mechanics,


University of Naples Federico II, Italy, S Dellepiane Azimut - Benetti, Italy SUMMARY


Larger main dimensions, superior comfort requests and higher manoeuvrability characteristics focus mega-yacht designer attention towards hull and superstructure aerodynamic performance. Windage evaluation of very high superstructure, aerodynamic load assessment, and accurate air flow simulation are considered very important in the design procedure to get optimal layout and opening position. Moreover they are used to precisely evaluate thrusters and Dynamic Positioning Systems horsepower. The lack of data concerning the peculiar mega-yacht forms and the result requested accuracy lead to the requesting for scale model wind tunnel tests. These tests can asked more and more frequently by designers and surveyors and considered a fundamental reference for any further CFD simulation. In this paper experimental methodologies developed at Politecnico di Milano Wind Tunnel in order to assess mega-yacht aerodynamics in terms of aerodynamic loads, comfort analysis, pollutants dilution and re-ingestion analysis using wind tunnel tests are described.


1. INTRODUCTION Larger main dimensions, superior comfort


higher manoeuvrability characteristics focus mega-yacht designer attention towards hull and


requests and superstructure


aerodynamic performance. Windage evaluation of very high superstructure, aerodynamic load assessment, and accurate air flow simulation are considered very important in the design procedures to get optimal layout and opening position, to support Dynamic Positioning Systems design and in general to evaluate on board horsepower.


Aerodynamic resistance and wind effects have been considered in naval architecture as regards large ship superstructure influence on propulsion and manoeuvring. In [1] extensive experimental research performed in the early 30s by Hughes [2], [3], are reported.


In 1942 G.S Baker [4] presented a formula for the assessment of aerodynamic resistance of merchant ships considering the different


shapes of hull and


superstructure. The topic was described by Saunders [5] that considers air resistance due to pressure effects only and by Isherwood [6] who developed a multiple linear regression analysis of experimental tests to get non dimensional coefficients for longitudinal and transverse aerodynamic components as well as for yaw moment.


Similar analysis based on a wider and updated set of data was presented by Fujiwara et al. [7],


[8]. Further


reference data relative to aerodynamic forces on marine structures and ship superstructures are available [9],[10], but relative to shapes very different to mega yachts.


Only recently aerodynamic specific matters related to maxi yacht design have considered. They are extensively reported in [11], and in [12].


Dynamic Positioning System (DPS) is a computer controlled system to automatically maintain the ship


© 2014: The Royal Institution of Naval Architects


position and heading by using her own propellers and thrusters. Position reference sensors, combined with wind sensors, motion sensors and gyro compasses, provide information to the system about the ship position and the magnitude and direction of the acting external forces [13]. The system software contains a mathematical model of the ship that includes information relative to its hydrodynamic and aerodynamic characteristics and to the location and propulsive characteristics of the thrusters. This knowledge, combined with the sensor information, allows the computer to assess the steering angle and thruster output for each thruster. In Balchen et al. a mathematical model of a Dynamic Positioning System based on Kalmar filtering is described [14].


This allows operations at sea where mooring or


anchoring is not feasible due to deep water, congestion on the sea bottom (pipelines, templates) or


other


problems. Dynamic positioning may either be absolute in which case the position is locked to a fixed point over the bottom, or relative to a moving object like another ship or an underwater vehicle. It’s also possible to position the ship at a favourable angle towards the wind, waves and current (weathervaning).


While a detailed description of the system is outside the aim of this paper, any DPS contains an aerodynamic - as well as hydrodynamic model of the ship. DPSs have been applied since the early seventies when drilling has become interesting also in deep sea where no anchoring system could be considered. In this case, and more in general in the offshore drilling field, DPS development has been focused mainly on the hydrodynamic aspects as wave loads and current were


predominant. For maxi and mega-yacht


applications the scenario is quite different. DPS is used to get and maintain a fixed point or to maintain a defined course at very low speed, mostly in good weather conditions or in any case in low Beaufort


sea states. Typical


applications are maintaining ship position and orientation in sheltered waters without anchoring, or course keeping in


B-71 Italy, C Bertorello,


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