O BJETIVOS E STRATÉGICOS

Orca3D is a suite of tools used for naval architectural design and analysis that were written as a plug-in for the Rhinoceros 3-D modeling software. For this research, Orca3D was used to determine if the amphibious aircraft that was output by the sizing code had satisfactory longitudinal and transverse stability in the water. The stability of the aircraft in water is determined by finding the location of the longitudinal and transverse metacenters.

Classically, Simpson’s Rule was the primary tool used in designing boat hulls. Simpson’s Rule is a method of numerical integration and is governed by

Using this approximation, a designer could calculate the location of the longitudinal and transverse metacenters, as well as the center of buoyancy and center of floatation, of a boat hull being designed.

Computer aided hull design has been becoming more and more prominent in recent years, as it offers advantages over traditional design methods. A majority of CAD representations of a boat hull are curve-surfaces. Orca3D generates a mesh from this surface to calculate most of its hydrostatic and stability parameters. With a mesh created, Orca3D then uses Simpson’s Rule to numerically integrate between the mesh nodes. This method of computer aided mesh generation and numerical analysis leads to more accurate results, as it doesn’t rely on a manually calculated station model, which has a tendency to miss local hull features, such as discontinuities and curvature changes.

According to Sederberg [69], the de-facto CAD standard for representing curved surfaces since the 1970’s has been the non-uniform rational B-splines (NURBS). NURBS are the only free-form surface type supported in the IGES file format, which is the most popular format for transferring data between CAD software. Unfortunately, NURBS have two weaknesses that impact hull design and analysis. First, to make hull optimization and analysis easier, it is helpful to have the entire hull represented with a single surface. Unfortunately, due to NURBS needing rectangular topography when making its mesh, a complicated hull cannot always be represented by a single surface. Secondly, control node points for a NURBS surface must lie in a rectangular grid, which leads to there being a large number of control points that carry no significant geometric information and only serve to increase calculation time. Fig. 62 illustrates the drawbacks of the NURBS model. The personal watercraft depicted is divided into 13 surfaces in order to maintain the required rectangular topography. Also, many of the nodal points are unnecessary and are merely included to satisfy rectangular shape constraints.

To address the limitations of using NURBS to model complex boat hulls, T-Splines were introduced. T-Splines are not limited to a rectangular geometry, which significantly reduces the number of superfluous points needed to complete the mesh as well as make the modeling of a complex body with a single surface possible. In the NURBS mesh, all the interior control points have a valence of four, which means that all the interior control points touch four edges. A T- Spline mesh (T-Mesh) allows control points to have valence other than four. These points are shown in Fig. 62 as yellow dots. The ability to have nodal points with different valence values gives T-Splines the ability to model any surface using a single T-Mesh. T-Splines were integrated into Orca3D to decrease calculation time as well as increase the accuracy of the calculations by removing discontinuities between surface meshes.

The 3-D model of the full scale optimized design was transferred from SolidWorks to Rhinoceros in the form of an IGES file. While Rhinoceros only accepts NURBS meshes, the Orca3D add-on allows for the creation of a single surface T-Spline mesh when performing stability and hydrostatic parameter calculations.

4.3.2.1. Results

Orca3D requires a number of parameters in order to output meaningful hydrostatic and stability data. The hull was designed so that the load water line (LWL) would have the aircraft sitting at a 1.5 degree trim angle in the water. During the design of the hull shape, the load water line was specified and the hull shaped to provide that attitude while in the water. Considerations were taken to keep the vertical tails out of the water during water operations. Next, Orca3D needs an input weight (GW = 6,600 kg) and heel angle. The heel angle was set to 0 degrees. Finally, a vertical center of gravity position was needed, which directly affects the metacentric height of the craft, as that is the distance from the metacenter to the center of gravity. From this data and given aircraft geometry, Orca3D outputs the location of the center of buoyancy and center of flotation. It also gives the lateral and longitudinal metacentric heights, and draws in a waterline on the model to give a visual representation of its attitude on water (Fig. 63, Fig. 64).

Fig. 63: Orca3D Results with Virtual Waterline of Trimaran Seaplane (Longitudinal)

Fig. 64: Orca3D Results with Virtual Waterline of Wing Tip Floats Seaplane (Transverse)

The longitudinal and transverse metacentric heights of the trimaran seaplane were calculated to be 33.42 and 2.15 m. respectively. The values obtained by Orca3D are related to those shown from the theoretical calculations done by the FOTS code, shown in Table 12.

Another important aspect that was analyzed by Orca 3D is the overturn of the seaplane. This tool is useful in analyzing the conditions in which the seaplane can operate on rough, high wave water conditions. The overturn acts predominantly at the transverse stability. The seaplane is able to obtain a maximum heel angle before it overturns. Using the same criteria to calculate the metacentric height which needs input weight but now adding a heel angle, this analysis was performed. The maximum heel angle the trimaran seaplane could obtain is 12o before overturning as shown in Fig. 65.

Fig. 65 shows the trimaran seaplane when overturning. The outriggers turn in line with the seaplane, where the right outrigger is sunk inside the water and the left outrigger is out in the air. However, with the retractable float system, the outriggers are automated to stay in the water line, giving the seaplane more stability. Applying this system into the Orca 3D analysis, the maximum heel angle the seaplane trimaran can obtain increased to 18o.

Fig. 65: Trimaran Seaplane at 12o Heel Angle before Overturning

Furthermore, the idea to obtain a seaplane that has outstanding water capability added the solution to fold the wings to increase the heel angle of the seaplane. Folding the wings gives the seaplane a better clearance when it is maneuvering in the water, especially in bodies of water with too congested boats or ships. Using the retractable float system and folded wings, the heel angle increased to 20o, shown in Fig. 66.

Fig. 66: Trimaran Seaplane at 20o Heel Angle before Overturning