Design Justification – Cornell Autoboat

Cad of Assembly

Trimaran

This year we opted to go for a trimaran design to maximize stability. Stability is crucial for our mission since the computer vision model works best when viewing a clear image from the camera. Trimarans are very stable due to the two hulls on the side (called amas). If the boat were to roll into the water, one of the amas would be pushed down, causing the ama to displace water. This then causes a buoyant force to act upwards, which imposes an upward moment on the boat, correcting the roll. The further away the amas are from the main hull, the stronger the moment is, meaning it is more stable.

Main Hull

From testing, one of issue we are trying to fix with this new design is to reduce the trim angle of the boat while moving near top speed. The trim angle is the angle that the boat makes with the water – a larger trim angle means that the front of the boat is sticking out of the water more. In order to reduce this the hull shape was changed to displace more water. By increasing the length of the waterline and ensuring that the entire boat stays under the waterline, decreases the amount of lift that the boat generates, keeping it more in the water. Another good way to decrease the trim angle is to add more weight closer to the front of the boat. This is where the skeeball launcher and watergun shooter will be, which will help to keep the trim angle lower. The main downside to this design change is that it is not as hydrodynamic as it was previously. However, the boat will not be traveling very fast due to the limitations of the computer vision model, so hydrodynamics is not as important to us as maintaining a low trim angle.

Dimensions:

54in x 10.5in x 8in (LxWxH)

Amas

The reason why the amas are long and skinny is to optimize the volume of water displaced while keeping them hydrodynamic. Ideally, they should barely be in the water until the boat starts to roll. This maximizes the amount of extra volume that could be displaced while not inducing too much extra drag. By having the amas skinny, we can also put them further away from the boat. The maximum total width allowed is 3ft. Some of the navigation tasks have gaps between the buoys of exactly 3ft, so we aimed for a total width of 2.5ft. The more narrow the boat, the easier it is to navigate between the buoys, but the wider it is, the more stable it will be.

Dimensions:

30in x 3in x 6in (LxWxH)

ANSYS Simulations

Mesh

This mesh is good because it is more complicated near the areas of interest (hull, waterline) and less complicated over the domain (further away from the areas of interest). The mesh is also very even over the hull.

Mesh of hull curvature:

Close up on hull curvature mesh with 3 boundary layers:

Cool inside view of the main hull and the ama:

Ama and main hull beside each other:

Also one important note the mesh is only computed on half of the boat – so half of the main hull and one ama. This is to save the number of computations needed, as the boat can simply be reflected later to get the full thing.