• weather routing
• powertrain design
• 2D+t model
• planing hull
• digital twin

In recent years, the reduction of CO2 emission has become a crucial target for the European Union member States. If on the one hand many developments have been carried out in the automotive industry on this topic, on the other several industrial sectors need further improvements.
Focusing on the marine sector, the International Maritime Organization (IMO) adopted in 2018 the GHG Reduction Strategy, outlining a 50% reduction target by 2050 compared to 2008 levels. Leisure crafts (up to 24-meter length) are not covered by the IMO regulation and the European Recreational Craft Directive 2013/53/EC, in force since 2016, does not concern CO2 emissions. However, times are changing, and the reduction of CO2 emissions in case of leisure crafts, strictly related to the consumption of fossil fuel, will involve not only the design of the powertrain, but also the navigation methodology.
This manuscript originates from this need and concerns the development of a computationally efficient digital twin of a 10-meter leisure planing boat aimed at the design of innovative hybrid powertrains and smart weather routing devices. The digital twin developed in the MATLAB® Simulink® simulation framework can thus be considered the core of two distinct research topics.
On the one side, it provides the possibility to design, size, and virtually test innovative hybrid powertrains, focusing on energy aspects, in order to find the best powertrain set-up. From this perspective, the digital twin should be intended as a cost- and time-efficient tool for boating manufacturers who desire to introduce new products in the market.
On the other side, it behaves as a virtual replica of the investigated boat during navigation, predicting energy flows and seakeeping features. From this point of view instead, it is suited to be used in innovative weather routing systems, ensuring seaworthiness and comfort as well as minimizing fuel consumption.
The digital twin was designed to become a flexible instrument, so that different hull geometries and different powertrain components (such as internal combustion engine, electrical machines, propeller) can be virtually represented by providing in input few plate data.
In the manuscript, the background of the development of the digital twin is presented, starting from the motivation, the regulatory aspects, the Literature overview concerning the modelling methodologies, and the state-of-the-art of weather routing systems and hybrid powertrains.
The dynamic model of the digital twin is based on a 3-DOF (Degree-of-Freedom) 2D+t approach. Many challenges have been faced to make the original 2D+t theory suited for the purpose. An approach to include the surge motion was developed to predict the thrust request. An empirical corrective factor to be applied to the Faltinsen’s asymptotic formula was studied to account for diffraction forces on planing hulls. Moreover, a novel methodology to consider the non-vertical component of the buoyant force at the stern was developed to extend the applicability to the pre-planing area. Finally, a novel methodology to account for the aerodynamic forces in the 2D+t model was developed by means of the results of a CFD campaign carried out by the CFD experts of the REASE Research Group.
The proposed 2D+t model, predicting the resistance and the behaviour of the dynamic variables (pitch, heave, and surge) in calm sea and regular waves, is presented in detail, and validated by means of Literature data.
The powertrain framework of the digital twin, based on the computationally efficient mapping approach, is then depicted. The methodology that was followed to provide a flexible and reliable tool is presented. A parallel hybrid architecture, also exploiting renewable energies, is proposed to improve the efficiency of the powertrain. The adoption of a high-speed generator on the turbocharger is also proposed to further improve the technological level. Several standalone models were required to supply the characteristic maps of powertrain components. In detail, two GT-Suite® models were required to provide the data of the internal combustion engine (with and without the high-speed generator). Moreover, a MATLAB® Simulink® electromechanical model was required to assess the behaviour of the high-speed generator, which data were missing in the Literature.
Some examples of powertrain study concerning the proposed architectures, applied to the 10-meter test case boat, are then discussed. The benefits of the adoption of the hybrid powertrain for the end customer are presented in detail in different sea conditions and mission profiles. The analysis includes the evaluation of the most suited energy management strategies and the size of the battery pack. The contribute of exploiting photovoltaic modules during docking in port is also assessed.
The last part of the manuscript finally presents the first steps towards the development of a smart weather routing system, based on the dynamic and powertrain models of the digital twin and adopting the Dijkstra’s minimum cost path finding algorithm. The image processing technique developed for the acquisition of open-source real-time data from the “LaMMA meteo Mare e Vento” webpage is delineated.
Some examples of application of the smart navigation tool to the test case boat, powered by the conventional diesel engine, on a typical route of the Tuscan Archipelago is then shown, proving the potential of contemporary reducing fuel consumption and improving comfort of passengers.