A Study of the Twin Fin Concept for Cruise Ship Applications

Abstract: The aim with this thesis is to investigate if the Twin Fin concept can be a beneficial propulsion system for large cruise ships, about 300 m long. The Twin Fin concept is a new propulsion system, launched in 2014 by Caterpillar Propulsion [1]. The concept is diesel-electric and has two fins, containing a gearbox and an electric motor, immersed in water [2]. Previous investigations have shown the concept to have several advantages compared to other propulsion systems . A seismic vessel, Polarcus, has been retrofitted with the Twin Fin concept and it has been proved to have both operating and cost benefits compared to its previous arrangement with azimuth thrusters [3]. Diesel-electric propulsion is common for cruise ships, which would make the Twin Fin concept a possible propulsion solution for them. It’s of interest to investigate if the concept can be as beneficial for large cruise ship as it has shown to be for other vessel types. To investigate this the whole concept is considered. A cruise ship hull and fins are modeled with computer-aided design (CAD) using CAESES/FRIENDSHIP-Framework (CAESES/FFW), starting building up a procedure for customization of fin design into ship layout. Tracking of the operation of similar cruise ships is performed with automatic identification system (AIS) in order to create an operational profile for the model cruise ship. A propeller is designed for the model cruise ship, using a Caterpillar Propulsion in-house software. A conceptual drawing of the electric power plant is also created. Computational fluid dynamics (CFD) simulations are performed on CAD model hull with and without fins, in order to find out how much resistance is added due to the presence of the fins. These CFD analyses are performed with the open source CFD toolbox OpenFOAM, using the volume of fluid (VOF) method for free surface modeling. Reynolds-averaged Navier-Stokes (RANS) is used for modeling turbulent flow, using the turbulence model SST together with wall functions. Also a coupling between RANS and the boundary element method (BEM) software PROCAL is used for an active propeller behind the ship, computing the effective wake fraction and thrust deduction.  k Finally the Twin Fin concept is compared to other propulsion systems, conventional shafting and Azipod, finding its advantages and disadvantages. The CFD simulations results in an added resistance of 18% in 20 knots due to the presence of the fins. A larger propeller can be fitted compared to the other propulsion systems, especially compared to the Azipod system, resulting in an increase of thrust by about 5.6% for the Twin Fin concept. The comparison between the systems shows that the Twin Fin concept have several advantages compared to the other systems, e.g. increased payload and increased reliability. One main conclusions drawn from this investigation is that the added resistance is very dependent on hull form and it's important to customize the fin to hull form and operational profile. Another conclusion drawn is that the larger propeller can't fully compensate for the added resistance due to the fins. Since the Twin Fin concept have other advantages it could still be beneficial for cruise ship applications, especially if further optimizing the fin shape and position and by this lowering the added resistance.