This section outlines the structural design of the SAVER device, including the hull shape, materials, design, construction, and waterproofing. Generally, the structural portion consists of a frame mounted inside a fiberglass hull, which is then filled with foam and sealed with a sealing plate. Each of these components are discussed in the following sections. For a complete drawing package, see Appendix J.
5.2.1 Frame Shape
MDF (medium density fiberboard) was chosen as the internal frame material due to ease of manufacturing and ability to iterate on the design. Note that this material is great for the proof of concept design but would not be ideal for a final product, as MDF does not perform well in water. The frame should help maintain the rigidity of the hull and allow for easy mounting of critical components on the interior of the hull. More details on the frame, including manufacturing and assembly instructions, can be found in section 8.2.1 and in Appendix J.
Figure 13. Frame Shape
5.2.2 Hull Shape
Watercraft can be designed either as non-planing or planing craft – that is, with hull shapes that cut through the water versus those that skim across it. As planing hulls tend to have less drag and thus move much faster, the SAVER’s hull has been designed as a planing hull, with a gradually rounded front, center ridge, and sharp back edge. These features should give the hull the proper planing shape to allow the device to plane on top of the water at 5mph (the design operating speed) and minimize drag.
Figure 14. Isometric View of Hull Figure 15. Side View of Hull
5.2.3 Materials
The internal frame should be made of MDF. This material was chosen because it is resilient against compressive forces, while also having some flexibility in the shear direction. This flexibility allows the frame to be bent and “snapped” into place to allow for the desired shape and rigidity of the frame. While lots of different materials have similar properties, MDF is inexpensive and can be manufactured into the desired shape using resources available on Cal Poly’s campus, which is ideal for prototyping.
For initial prototyping, the bottom shell should be made of fiberglass. The bottom shell needs to be sturdy and hydrodynamic, as it is the surface of the SAVER device that should be in contact with the water. Fiberglass was chosen, as it can be manufactured, by laying it up into a mold, to have curved irregular shapes. In order for the SAVER device to plane, the design needs this sort of curved shape. The full process for manufacturing the hull is outlined in section 8.2.2. In anticipation of higher stress regions in the hull, the SAVER team has integrated additional strengthening members. These additional members should be composed of aluminum plates to distribute the load.
The top shell should be made of pourable polyurethane foam. The foam shape shown in Appendix J contains the final design with ergonomic requirements taken into consideration. The chosen pourable polyurethane foam should be an easy to use foam that provides the necessary 15 pounds of buoyancy force [29].
5.2.4 Design and Construction
Initial designs for the SAVER device have been documented in SOLIDWORKS. Iterations of the frame have been tested using simple prototypes laser-cut from MDF. Final pieces for the frame should be made using a jig saw. The fiberglass shell should be laid up in a mold made of foam using a CNC mill in the Cal Poly shop. Excluding the molding process, all the construction should be done manually. More about this should be covered section 8.2 on manufacturing. For a complete drawing package including dimensions see Appendix J.
5.2.5 Waterproofing
Since the bottom shell is to be made from a fiberglass layup, it should be inherently waterproof. The top foam should be made from the pourable polyurethane foam, which is not inherently waterproof, but is resistant to the absorption of water. If the polyurethane is continuously submerged for months at a time, this can lead to losses in buoyancy, but continuous submersion is not intended as part of the use for the SAVER device. This means that if the SAVER device is stored properly, the polyurethane should function as if it is waterproof for these purposes. To include an additional layer of waterproofing, the sealing plate was added between the top foam and the hull. The sealing plate will contain a rubber gasket inside of the ring of clamping bolts to prevent water from seeping in between the sealing plate and the hull.
With such a high risk of ruining the electronics inside, the internal electronic components and batteries should have an extra layer of waterproofing on the inside of the hull. The initial prototype was designed to have electronics stored inside waterproof enclosures made from ABS pipe, although future design iterations may encase the electronics in potting compound. To ensure the safety of the team as well as the victim in the water, the team has consulted with an electrician to get advice on risks and mitigations for electrical hazards – this is discussed in section 5.7.1.