Fathom’s Insight

No one submersible is designed to perform all the underwater tasks that may arise, but there is a commonality of vehicle performance requirements which may be found by analyzing past dives; these requirements are listed below.

VIEWING

Some means for external viewing is required. Viewports (windows) provide the easiest and most reliable solution, but their location and quantity are arbitrary and frequently dictated by other characteristics of the hull configuration. Acrylic plastic pressure hulls are available which can provide panoramic viewing. Television cameras are an adjunct to direct viewing and, with low light level amplification, may provide greater range and resolution. Optical viewing systems, e.g., periscope-type, have also been employed.

BUOYANCY

Archimedes’ principle defines the magnitude of upward buoyant force: any object immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced. Three states of submersible buoyancy are desired: Positive, negative, and neutral. Displacement volume (D) determines the buoyant force, and buoyancy is expressed by the ratio W/D, i.e., weight of vehicle (W) to weight of displaced water. Buoyancy regulation under different vehicle load and water density conditions requires variable ballast systems which may include one or more of the following: Water ballast tanks, steel shot, gasoline filled tanks, or interconnected hard and soft containers.

TRIM

To correct unequal weight distribution along the longitudinal axis which might cause the vehicle to have an up and down angle from the horizontal, or to intentionally obtain such an up and down angle for the dive mission, a trim system is required. This system, through a variety of methods, acts to transfer weight or ballast forward or aft.

STABILITY

Stability is that property of a body that causes it, when disturbed from a conditional equilibrium, to develop forces that tend to restore it to its original condition. Equilibrium is a state of balance between opposing forces which may exists in three states: Stable, neutral, and unstable. For example, if when an angle is put on a ship forces are set up which act to reduce the angle, the ship is stable. Neutral equilibrium exists when a body remains in its displaced position after a force that displaced it is removed; unstable equilibrium exists when a body continues movement after a slight displacement. Stability in a submersible is intimately related to center of buoyancy and center of gravity. The center of buoyancy is the geometric center of volume of the displaced water. The center of gravity is the effective center of mass. These two centers are indicated as B and G, respectively, in Figure 2.3a. When a floating body is in a state of equilibrium, its center of buoyancy and center of gravity are in the same vertical line.

Fig. 2.3. Change of center of buoyancy metacenter during submergence.

of a vertical line through the center of buoyancy of a body floating upright and a vertical line through the new center of buoyancy when it is inclined a small amount as indicated by the letter M in Figure 2.3b.

When a surfaced submersible is tipped as shown in Figure 2.3b, the center of buoyancy moves from B to B1 because the volume of displaced water at the left of G has been decreased while the volume of displaced water to the right is increased. The center of buoyancy, being at the center of gravity of the displaced water, moves to point B1 and a vertical line through this point passes G and intersects the original vertical at M. The distance GM is known as the metacentric height. This illustrates a fundamental law of stability. When M is above G, the metacentric height is positive and the vessel is stable because a moment arm, GZ, has been set up which tends to return the vessel to its original position. It is obvious that if M is located below G, the moment arm would tend to increase the inclination. In this case, the metacentric height is negative and the vessel is unstable.

When on the surface, a submarine presents much the same problem in stability as a surface ship. However, differences are apparent as may be seen in the diagrams in Figure 2.3c, where the three points B, G, and M, though in the same relative position, are much closer together than is the case with surface ships.

As noted above, when a ship on the surface heels over, there is a shift in the position of the buoyancy center because of the volume shape change below the waterline. In the case of a submerged submarine, no such change takes place because all the volume of the submarine is below the surface of the water. Thus, for submerged stability, the center of gravity must be below the center of buoyancy.

During the process of going from the surfaced condition to the submerged condition, the center of gravity of the submarine, G, remains fixed slightly below the centerline of the boat while B and M approach each other. At complete submergence, G is below B, and M and B are at common point. These changes are shown diagrammatically in Figure 2.3c.

As the ballast tanks fill, the displacement becomes less with the consequent rising of B and lowering of M. There is a point during submergence when B coincides with G. Due to the configuration of the upper part of the hull, B would only move a short distance from G if a list were taken at this point. In this condition, the stability is least; and the time spent at this low-righting stage must be minimal. When the ballast tanks are fully flooded, B rises to the normal center of buoyancy of the pressure hull, and stability is attained with G below B.

To keep the center of gravity low, batteries and other heavy items are carried as low as possible where they have the greatest effects on stability. Submersible transverse metacentric heights (submerged) are quite small and range from 3 to 12 inches.

POWER

Electric power is compatible with all propulsion, lighting, hotel, and virtually all instrument requirements and is the exclusive ultimate power source in all deep submersibles. Long duration power can be supplied from the surface through a cable, but at the expense of maneuverability; conversely, maneuverability is retained using self-contained batteries, with a corresponding limitation in operating time. Two power options predominate in shallow (less than 1,000-ft) submersibles: Manual power through the pressure hull

can be by direct mechanical linkage (limited to shallow depths owing to compression on the hull with consequent size reduction of thru-hull penetrations) or by hydraulics.

MANEUVERABILITY

The requirements for maneuverability vary considerably in speed and degree, but generally the vehicle is expected to be capable of controlled movement in the vertical and horizontal. For many if not all missions, the vehicle must be able to “hover” (dynamically or statically) at a given depth or distance above the bottom.

EXTERNAL ATTACHMENTS

Fro maximum mission adaptability, the vehicle should have external attachment points for installation of various instruments or devices to conduct undersea tasks. Since few, if any, of these instruments are standard in weight, size, shape, or mode of operation, a degree of flexibility in such attachment points is desirable. In the probable event that such devices will require electrical power and/or control, provisions must be made to spare electrical connectors and thru-hull penetrators..

LOCK-OUT/LOCK-IN

If the submersible is designed for transporting and supporting divers, provisions must be made for ballasting the vehicle when they leave (to restrain it from ascending) and deballasting when they return. Hatches and viewports in the diver’s compartment must be double-acting to resist not only external pressures, but internal pressures as well. Communications must be arranged between the diving compartment and the unpressurized part of the pressure hull; and, when surfaced, a means of providing food or medical aid must be incorporated in the design if decompression is required. Whereas the egress/ingress hatch will be on the keel of the submersible, and the vehicle might be bottomed during diver operations, space between the hatch and bottom must be sufficient to allow easy access to the hatch. Consideration must likewise be given to personnel transfer to a decompression chamber.

WEIGHT AND SIZE

The submersible’s dry-weight (in air) and physical dimensions will govern the methods of launch and retrieval as will as the size of its support ship and the methods available to it for land and air transport.

PAYLOAD

There are no minimum or maximum, payload standards, and they range from less than 100 pounds to several tons. The larger the payload requirements, the larger the vehicle size and, correspondingly, the greater the necessary support efforts become, with resultant lowered mobility. Trade-offs are possible whereby a non-essential manipulator, for example, might be replaced with another instrument or5a lock-out chamber replaced with a different module for a particular dive. Distribution of payload weight and balance must be considered to assure that vehicle trim and control are not jeopardized.