Prediction methods for sailing yachts in the early design phase
ecent sailing yacht projects have highlighted the importance of getting involved in the design
process as early as possible. And given the ever-shrinking amount of time available for design development, it is vital to be as efficient as possible. Here we describe some of the studies that can be done in an early phase, including tools available to conduct those studies.
Design challenges Over the years design challenges for just about any sailing yacht has remained the same: • Wide versus narrow – Wide hulls exhibit a large righting moment and thus can carry a large sail plan, whereas narrower hulls exhibit lower resistance. The trade- off is a classic one. Picking the best point on this trade-off involves the operational profile, overall speed potential (planing or displacement sailing) and displacement of the yacht;
• The volume distribution and profile outline over the length of the vessel - a more flat outline and volume distribution is generally favourable to achieve higher speeds. However, when such a high speed cannot be achieved (in low wind speeds or because of a small sail plan) then more rocker is preferred;
• Proper distribution of the side force over the length to get the right yaw balance and loading of the appendages and hull, at various heel and trim conditions;
• Various other shape variations, such as bow and stern sections; • Design rule restrictions.
Luigi Francesco Minerva & Rogier Eggers
Then there are those custom challenges. A particular one is how to efficiently generate side force on very large draught restricted designs. This is a challenge because, when the allowed draught reduces, the lift
production of the hull becomes less efficient due to a lower aspect ratio. At the same time, these very large yachts also typically have strict requirements for comfort such that the alignment and positioning of appendages becomes important to avoid trailing vortices passing through the propeller disc as much as possible.
Calculation tools Recent projects allowed us to better test our in-house CFD codes for specific sailing yacht applications. The tools used are: • ReFRESCO, a viscous-flow code that solves multiphase (unsteady) flows using the incompressible Navier-Stokes equations, complemented with turbulence and cavitation models.
• PANSHIP, a semi non-linear time domain panel code complemented with viscous effects.
ReFRESCO, with its more accurate description of the flow, is well suited to evaluate performance in calm water, whereas PANSHIP, with its faster calculation time is suitable for larger calculation matrices or when dynamics become relevant.
Wide versus narrow
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