The Hull
This is the most important part of our design. The hull is theheaviest and most costly aspect of a submersible and designing it is pivotal in coming up with the ultimate vehicle. When designing our Hull, we trained our eye on the end use of the vehicle (Research) and endeavored to achieve a tradeoff between a comfortable and spacious working environment and a strong hull for our modest depth of about four hundred feet.

The hull is the human container of the vehicle and a faulty hull could only lead to loss of life. It should also be able to withstand the enormous pressure of the sea environment. Hull geometry can be quite tricky because whilst we try to create as large a working environment as possible, a trade off has to be made between its length and shape. An elongated hull would experience terrific bending moments at the top and bottom of the support structure and would readily crumble under a few hundred feet of sea water. At 100 feet depth, for example, each square foot of the hull experiences a force equal to more than 6000 pounds pushing in on it.

The perfect shape does the trick of directing all the towards thecenter of the hull, and this is obviously the sphere. This results in each section of the hull being wedged against its neighbor, creating compression in the material rather than a large degree of bending torque.

The spherical shape however does a poor job of creating working space in the sub, an aspect which is crucial in a research submersible. Besides, at our depth it is pretty reasonable to trade the perfect< shape of the sphere with a more "user-friendly" shape that allows scientists more working space.

A suitable shape would be a cylinder with hemispherical ends, but this too has its limitations when the end of the pressure hull, as in our case, is not necessarily the end of the submersible. To avoid the wastage of space and awkwardness of a hemispherical back wall smack in the middle of our vehicle, we retained the cylindrical aspect of the hull, the hemispherical front enclosure and a flat rear enclosure. The front end of the hull will provide a 150 degree angle viewpoint providing a great view of the sea for the scientists. We plan to use acrylic plastic material for this viewpoint. The exostructure of the submersible will extend over the remaining part of the hemisphere in equal degree at the front and bottom.

The Exostructure
This is the framework of the submarine upon which everything else hangs. We decided to make our shape as streamlined as possible with an eye on the effect of the drag force on the power requirements for the vehicle. Our structure will take the form of a short cylinder that extends over the length of the hull and then tapers off in a a conical shape towards the end of the vehicle. The exostructure will extend to cover the cylindrical part of the hull and will also bend over the hemispherical viewpoint at the top. This part will be curved to reduce drag and will also serve as the support frame for the cameras and lights at the top and support the robot arm and basket at the bottom. Material considerations for the hull and the exostructure are discussed below.

Materials
The material in a submersible is assumed to obey Hookes law- that is the stress is proportional to strain and that the vehicle returns to its original shape when external loads are removed. Attachments and discontinuity points in the hull design should receive special attention for they signify areas of stress concentration. The material used here may yield locally and assume a new shape that leads to a more favorable stress distribution. Thermal stress also has to be taken into consideration. Luckily, we do not have to add ring stiffeners to our design given our modest depth, bet the thickness of the hull is also an important consideration that is determined by the choice of material.

The hallmark of a good design is of course that which always performs the required functions over a specified time at the lowest possible cost. Economic factors are therefore important as they have to scale with the extent of our operations. Other factors which we looked into include:

a) low temperature effects: This is, luckily for us, more heavily pronounced for deeper depths. Submersible chambers are generally heated, but their lowest operating temperatures should be carefully determined when making material selection.

b) corrosion and fouling: This is due to the corrosive nature of sea water. Marine growth on the surface is also common to nearly all nonmetallic materials, and sessile animals may attack protective coatings on the surfaces of metals. After careful consideration, we settled on steel which seems to fit our profile pretty well. Though a dense material and quite prone to corrosion, steel is relatively inexpensive and widely available when compared to titanium which would have made a lighter alternative. Steel is also very weldable, a crucial point considering the appendages we hope to add to our design, besides having a remarkable fracture toughness. We plan to use Steel HY-100, which has a higher compressive yield strength than Steel HY-80. We also plan to make the exostructure of the submarine using the same material.

Interior Structure Of The Hull
We have designed the Hull interior as follows:

1)The control panel will dominate the left and right walls of the cabin. We will use a state of the art computer integrated control panel to minimise on the usage of space, maximise efficiency and minimise the pilots workload. The display will be conveniently located towards the front of the hull and to the side to as not to obstruct the view offered by the viewport.

The following information will be displayed by the control panel:

-Navigation aids such as the orientation of the vehicle relative to the horizontal, the course, the submarines depth from the surface, altitude above the sea floor, global positioning system and, of course, the sub's velocity.
-Date, time hatch was closed and elapsed dive time.
-Trim and main tank indicators
-Oxygen content in the cabin and in the reserve tanks, CO₂ and humidity content in the cabin, and cabin pressure.
-Battery power consumption and reserve

2)The seats will be placed next to each other between the front of the cylinder and the hatch. Some space will be left over between the two seats to enable the scientists access the back of the hull.

3)A 75 cm diameter circular hatch will be located just behind the seats and at the bottom of the cabin. The Hatch will be located across the center of the floor of the cabin. The specifics of the hatch will be discussed later.

4)Life Support Systems: These are discussed in detail in a separate section of our report.

5)The submarine will be maneuvered using a joystick located at the base of the control panel for upward, downward and sideways movement, and pedals located at the front of the hull (In front of the seats) for acceleration, deceleration and braking. The power will be transmitted to the thrusters through a hydraulic system.

6)There will be a pop down control panel next to the left seat for manipulating the robot arm. Details of the robot arm are also covered in detail in a separate section of our report.

7)Emergency provisions and their locations in the cabin are discussed together with the life support systems.

The Hatch
One aspect of the design which posed a challenging problem for us was the hatch design. A hatch is the pressure tight container that connects the pressure hull to the exterior of the sub, providing passage for men and equipment into and out of the hull.
The size is obviously the first consideration, and typical sizes range between 1.5 and 0.75 meters in diameter. It is prudent to keep the hatch as small as possible for large hatches are clumsy and complicate reinforcement required around them.

Our hatch will be 0.75 meters in diameter for reasons we shall shortly see. The hatch will also be located at the bottom, contrary to the widely held norm.

We decided to locate our hatch at the bottom because it must possess mating capabilities with the entrance to the research station, which is located at the top of the station. Our hatch will therefore be more of a mating device, and we had to compromise on the size to include the intricacies such a device demands.

The trickiness ensues from the fact that we will be transferring men and equipment from one chamber to another chamber at a pressure differing from that inside the chamber.

The mating flange (point of contact with the station) will consist of screws at projecting from the bottom of the sub around the hatch, and these will fit into holes drilled into the station and the two(Sub and station) joined firmly by the screws. The screws will be made of a very strong material which can withstand inertial loads due to wave motion. The screws will be retractable and will only be exposed when the vehicle is about to dock.

To vehicle will use navigation lights to position itself over the entrance after which the screws will be lowered into the slots in the opening to the station.

The mating flange has to be water tight and the pressure difference should press the two openings together so that they are watertight. The hatch will be a sealed in enclosure between the bottom of the pressure hull and the exterior of the vehicle. It will have a small 10 cm extension outside the submersible to facilitate the mating process and the interior and external openings shall be electrically operated so as to slide back and open the hatch. The hatch shall be operated electronically.