Multihull ships create engineering challenges that are “out of range” of conventional ship design techniques. They require complex, CFD analysis to optimize multiple performance variables like resistance, endurance, stability, seakeeping, etc. In this article, we take you behind the scenes at KUASAR MARIN Engineering Inc., where we leveraged ANSYS Fluent to explore design iterations for a three-hulled, high-speed passenger ferry that could compete with existing two-hull catamarans.
Baseline Design Encounters Problems
Initial design efforts yielded a solution comparable to a common catamaran in size and displacement. Using ANSYS Fluent obtained through the ANSYS Startup Program, we analysed this baseline design for resistance and flow characteristics at 27 knots.
The results (Figure 1) revealed three major design problems. The first was the ventilation of the propeller tunnels (indicated by the red circle). The second problem was the insufficient submergence of the side hulls due to depression of free surface in the aft region. While this resulted in reduced frictional resistance, it also reduced stabilization and increased sensitivity to roll motion. The third issue was the large amplitude of the bow divergent wave and its interaction with the side hulls.
Resolving Ventilation and Bow Wave Problems
To correct these problems, we used CFD analysis to evaluate iterative changes in hull form geometry. And, to speed the modification-analysis processes, we designed the first Variants without side hulls and superstructure, since central hull geometrical characteristics played a role in both the ventilation and bow wave problems.
We developed the Variant 1 center hull by modifying the tunnel side chine and bow entrance. After CFD analysis was complete, the resulting free surface and trimmed hull geometries were exported from ANSYS Fluent to CAD software using ANSYS CFD-Post post-processor. This enabled a 3-D evaluation and measurement of free surface characteristics at all regions.
In Figure 2, we see that the ventilation problem is solved by the Variant 1 hull aft design. Figure 3 shows both the baseline (green) and Variant 1 (magenta) free surfaces, and the desired reduction in bow wave amplitude. However, because the crest of this modified wave followed a similar lateral curve, we needed additional amendments to the geometry to push the wave further away from the side hull bow.
The Variant 2 hull form resulted from changing the sectional geometries to push the bow wave forward. Figure 4 compares the Variant 2 with the Variant 1 simulation results. The magenta color represents the mesh surface of Variant 1, while the green represents the mesh surface of Variant 2. The bow wave amplitude of Variant 2 was not changed significantly. There was, however, a slight forward shift of the wave crest as targeted.
Solving Side Hull Submergence Problem
After solving the ventilation problem and achieving improvements in bow wave formation, we focused on solving the side hull submergence issue. We generated a new, full model based on the Variant 2 center hull, and extended the side hulls below the water surface depression. Figure 5 shows the results of the subsequent, full-model analysis with new side hulls properly submerged.
Reducing Resistance by Eliminating Excessive Spray
By solving the three major problems of the baseline design, we were now able to focus on the spray formation problem, which was increasing the resistance. Figure 6 depicts the problem areas — where the high bow wave amplitudes were occurring on the central hull and side hulls due to spray sheet formation. Adding spray deflector chines to the hull geometry is a common fix for this issue.
Using the mesh surface export capability of ANSYS Fluent, we were able to work in a 3D CAD environment to develop suitable spray deflector chine geometries. Figure 7 shows the new CAD model of the hull form after the deflector chines were added. This new hull form was designated as “Variant 3.”
Figure 8 shows the improvements achieved on spray formation by the deflector chines. As seen, chines are now limiting sprays and pushing them away from the hulls to decrease frictional resistance.
Comparison of Final Design to Baseline Design
Figure 9 compares the wave formations and hull interactions of the Variant 3 hull (final design) and the baseline hull. Propeller tunnel ventilation has been eliminated. The side hulls are now properly submerged and contribute to the transverse stabilization as desired. The contribution of the side hulls to the transverse moment of inertia is now 97 percent higher. The bow divergent wave of the central hull has reduced amplitude and its crest is now at a greater distance from the side hull bows. The amplitude reduction with respect to still waterline of each hull is about 18 percent.
Figure 10 shows the wetted surface distribution of the final design. On the central hull, the effect of the chines can be seen at the forward sector (there is no extension above them). The propeller tunnels are fully wet (no ventilation) to provide suitable flow . The chines on the inner surfaces of the side hulls limit the extension of flow successfully at the chine contour. There is a minor ventilation at the outer surface of side hulls, but this has no adverse effect on stabilization and/or increased resistance. Due to the full submerge of the side hulls, the final design wetted surface is about 1 percent higher compared to the baseline. Additionally, the spray deflector application has yielded a 3 percent reduction in wetted surface over Variant 2.
ANSYS Fluent has once again proven to be an effective tool for developing unconventional vessels that cannot be described by classical engineering techniques. For multihull ships, designers need to investigate not only the resistance but also the flow characteristics to validate and improve their designs. Advanced engineering techniques have enabled designers to incorporate this kind of investigation into early-stage development to save resources and gain faster feedback for design improvements. These techniques also provide information about the magnitude of improvements that are possible, which is so critical when deciding whether or not to pursue a particular solution. Utilization of ANSYS Fluent through the ANSYS Startup Program in our development work reduced resistance at least 10 percent in comparison with the catamaran hull form. This three-hulled concept can now be further developed to meet our customers’ needs.