The new Fleming 85 from the Taiwan-based shipyard has been described as the ‘ultimate cruising yacht’. From the renowned naval architects, Norman R Wright and Sons, the semi-displacement hull has been designed and engineered to be highly efficient. The optimum strength/weight ratio of the structure has been achieved, whilst retaining the brand’s traditional robust construction, but with less weight, verified by DNV (Det Norske Veritas) standards.
From a long established relationship and following previous CFD analysis and studies of the Fleming 58, Cape Horn Engineering were delighted to once again be commissioned for their CFD services to fine-tune the Fleming 85 hull design. A multitude of CFD reports and analysis were carried out to ensure the most efficient hull possible and good seakeeping characteristics were achieved in all sea states.
Fleming anticipate that the 85’s hull design and engineering will result in the best fuel economy and range of any semi-displacement boat in the world. “Our aim with the Fleming 85 is simply to build the finest possible ocean-going pilothouse motor yacht, in every respect,” said Adi Shard, Fleming Director and Design Engineer.
Cape Horn Engineering presented a CFD analysis of the resistance/propulsion performance of the Fleming 85 motor yacht in calm water, with all appendages. The full-scale CFD simulations were completed for diﬀerent loading conditions and speeds ranging from 5 kn to 26 kn. The primary objectives of the simulations were to determine the resistance, running trim and propeller eﬃciencies of the motor yacht.
All CFD simulations are processed on an HPC cluster running the latest general-purpose commercial code STAR-CCM+ in it latest version.
A virtual disk model was used to simulate the eﬀect of the propeller and model the ﬂow ﬁeld interaction of the hull and the propeller. The ﬂow that is induced by the propeller depends on the ﬂow around the ship hull. Similarly, the hull ﬂow is inﬂuenced by the propeller. From the deﬁnition of the virtual disk, the distribution of the axial and tangential forces of the modelled propeller and its eﬀect on the ﬂow is calculated. The integration of these forces over the disk gives the thrust and torque of the propeller, which are then available for coupling with the rigid body dynamics.
The mesh contains volume refinement zones at the free surface that are needed to capture the waves generated by the hull, with additional refinement around the stem, transom, chines and other hull features, and all appendages including fins, rudders, shafts, the bow and stern thrusters and the skeg. The virtual disk also has its own very fine refinement zone. Refinements are made where flow gradients are the largest and a very good representation of the geometry is needed.
The resulting forces and moments were given for the yacht as a whole and for each individual component or appendage. Sinkage, trim, and wetted areas were also tabulated, along with the propeller performance results. Some typical results are shown in the images below (the values have been deleted due to confidentiality).
The deployment of the ﬂaps (interceptors) greatly changed the trim angle of the vessel, with a slight improvement in total drag for the mid-range speeds, but with a cost of increased total drag for the higher speeds. Thus at higher speeds the interceptors should be retracted.
Using the virtual disk approach, the propeller-hull interactions could be determined and results for propeller RPM, torque and thrust as a function of speed were delivered. Moreover, the propulsive coefficient defined as the quotient between the effective power and the delivered power was presented. The delivered power is defined as the propeller torque calculated by the virtual disk model multiplied by the propeller revolutions in radians. The effective (towing) power is the total vessel resistance multiplied by the vessel speed.