be economically practicable, the ability to de- 

 ploy and recover the vehicle safely under av- 

 erage weather conditions is just as important 

 as pressure hull integrity. 



Bottom Conditions 



The ocean floor ranges in composition from 

 soft, fine muds to hard rock cliffs; a submers- 

 ible can be expected to operate within these 

 ranges. During operations requiring a vehicle 

 to transit near the bottom, search missions for 

 example, the pilot generally prefers to "fly" 

 just off the bottom, a few pounds negatively 

 buoyant. This procedure makes vertical con- 

 trol of the submersible much easier. Over a 

 rough, hard bottom, rugged skegs or other 

 devices (wheels, skids) are used to protect the 

 pressure hull and other components. On a soft 

 bottom the submersible may accumulate sedi- 

 ment, the weight of which can become great 

 enough to restrain the vehicle from surfacing. 

 ALUMINAUT, for example, accumulated ap- 

 proximately 4,000 pounds of sediment in this 

 manner during an operation off the coast of 

 Spain. 



tude 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 de- 

 sired: Positive, negative and neutral. Displace- 

 ment 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 condi- 

 tions 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 con- 

 tainers. 



Trim 



To correct unequal weight distribution 

 along the longitudinal axis which might cause 

 the vehicle to have an up or down angle from 

 the horizontal, or to intentionally obtain such 

 an up or 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. 



VEHICLE PERFORMANCE 

 REQUIREMENTS 



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 analyz- 

 ing past dives; these requirements are listed 

 below. 



Viewing 



Some means for external viewing is re- 

 quired. Viewports (windows) provide the easi- 

 est and most reliable solution, but their loca- 

 tion and quantity are arbitrary and fre- 

 quently dictated by other characteristics of 

 the hull configuration. Acrylic plastic pressure 

 hulls are available which can provide pano- 

 ramic viewing. Television cameras are an ad- 

 junct 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 magni- 



Stability 



Stability is that property of a body that 

 causes it, when disturbed from a condition of 

 equilibrium, to develop forces that tend to 

 restore it to its original condition. Equilib- 

 rium is a state of balance between opposing 

 forces which may exist in three states: Sta- 

 ble, 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. Sta- 

 bility in a submersible is intimately related 

 to center of buoyancy and center of gravity. 

 The center of buoyancy is the geometric cen- 

 ter 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 stable equilibrium, its cen- 

 ter of buoyancy and center of gravity are in 

 the same vertical line. Another term which 

 must be introduced to understand stability 

 is metacenter, which is the point of intersec- 



16 



