ized use of a trim (and roll) capability is 

 found with the DSRV^s where a stricken sub- 

 marine may lay on the bottom at an angle 

 requiring the DSRV to attain both a down 

 angle on the bow and a starboard or port list 

 angle in order to mate with the submarine's 

 escape hatch. A similar case may be made for 

 lock-out submersibles mating with underwa- 

 ter habitats at other than a level attitude. 



The methods used to gain trim in submers- 

 ibles range from very simple to complex but 

 they all involve one of two procedures: 1) The 

 transferring of weight from one part of the 

 vehicle to the other; or 2) the taking aboard 

 or releasing weight at one location on the 

 submersible or another. The transfer of 

 weight, in the case of cylindrical, pressure- 

 hulled vehicles, need be nothing more com- 

 plicated than a passenger walking fore or aft 

 to produce an angle on the bow. In ALUMI- 

 NAUT, if one of the crew desired to walk fore 

 or aft, while the vehicle was underway, his 

 movement had to be countered with a con- 

 current movement of another member from 

 the opposite end to take his place. If this 

 procedure was not followed, the vehicle's 

 trim was substantially affected. The spheri- 

 cal-hulled DS-4000 (BG— 3 in.) experienced 

 the same effect if the aft crew member 

 leaned forward to join the operator and sci- 

 entist at the viewports. The "tenderness" of 

 submersibles toward such trim changes re- 

 sides in the small longitudinal metacentric 

 height (Table 6.7) which makes the vehicle 

 vulnerable to quite small weight shifts. BEN 

 FRAISKLIN is an exception to such vulnera- 

 bility. During the Gulf Stream Drift, the six 

 crew members ran fore and aft together in 

 an attempt to produce instability and could 

 do no more than produce a dive angle of 10 

 degrees from which the submersible immedi- 

 ately recovered although the crew remained 

 in the bow. 



While many submersibles do not have a 

 trim system per se, the manual placing of 

 lead or iron weights fore and aft during the 

 dive can, to a degree, produce the desired 

 results. On a mission off Vieques Island, 

 Puerto Rico, in 1968, ALUMIISAUT was able 

 to parallel a 30-degree sloping bottom by the 

 crew transferring ballast weights from for- 

 ward to aft. Such procedures are generally 



impractical in the single, spherical pressure 

 hulled submersibles as sufficient moment 

 cannot be gained in the small sphere, and 

 payload, which the lead takes up, is at a 

 premium. 



Trim systems may be located external to 

 the pressure hull or within it (Table 6.7). 

 Several factors affect the locating decision, 

 the most significant being limited internal 

 volume and effective moment. Submersibles 

 incorporating internal trim systems are 

 those with cylindrical pressure hulls where a 

 large moment arm can be attained and inter- 

 nal volume is available. Systems external to 

 the pressure hull are generally found in 

 spherical-hulled vehicles. The following is a 

 description of each type of trim system from 

 a selected vehicle in the groups shown in 

 Table 6.7. 



Internal Trim Systems 



Water Transfer Systems: 



The submersible BEN FRANIO^IN has one 



trim tank forward and one tank aft inside 

 the bottom of the end closures at each end of 

 the pressure hull (Fig. 6.9). Each tank has a 

 capacity of 50 ft^ (3,100 lb of fresh water), and 

 the base of each is formed by the inside 

 contour of the hull. Steel plate sections are 

 welded to form the upper surface of the trim 

 tanks. A vertical wall at the aft end of the 

 forward tank and forward of the aft tank is 

 made of one 3-mm steel plate welded flush to 

 the interior edge of the hull stiffeners; in 

 this wall is an inspection manhole. 



Enough water to fill one tank completely is 

 carried during a mission and is sufficient to 

 produce approximately a 10-degi'ee up/down 

 bow angle. 



The forward trim tank has a filling hole in 

 the top, closed by a threaded plug. Both 

 tanks are linked by a polyethylene pipe 

 joined to metal tubes located on the top of 

 each trim tank. Each metal tube has a valve 

 for air release. 



When water is pumped from either tank it 

 vents through the overhead pipe into the 

 opposite tank, and moves through two poly- 

 ethylene pipes, running fore and aft down 

 the port side of the boat, which are con- 

 nected to trim pumps. 



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