den anchor stops. Although a variety of an- 

 chors may be used, a 380-pound clump was 

 employed. A manually operated hydraulic 

 system located inside the pressure hull 

 serves as a means of cutting the anchor 

 cable and controlling the mechanical winch 

 brake. 



Cables: 



Function: To provide a means of maintaining 

 constant altitude within range of bottom 

 viewing. 



Operation: Consists merely of a cable (wire 

 rope or chain) of desired length and weight 

 which is attached to the vehicle on or near 

 the keel. During descent, the submersible 

 slowly nears the bottom with the cable arriv- 

 ing first and accumulating there until its 

 weight loss puts the vehicle at neutral buoy- 

 ancy. At this point, the vehicle is essentially 

 anchored in the vertical, but is free to move 

 in the horizontal. 

 Ex€iniple: 



All of the bathyscaphs employed a ca- 

 ble for altitude control at one time or an- 

 other. The only submersible to extensively 

 use such a device was BEN FRANIH,IN on its 

 30-day drift in the Gulf Stream. This system 

 is described below. 



A 40-foot-long 100-pound chain, housed 

 in a flexible polyvinyl chloride (PVC) hose, 

 was attached to BEN FRANKLIN's stern. A 

 weak link stood between the chain and vessel 

 which would break before the vehicle 

 reached its capacity to attain full positive 

 buoyancy in the event of the chain fouling. 



In operation, the submersible bottomed 

 and then blew its VBT's enough to ascend 

 slightly to a point some 20 feet off of the 

 bottom where the chain restrained it. The 

 current then caused the vehicle to move and 

 drag the chain behind. With the chain at the 

 stern providing drag, the submersible was 

 oriented with its bow pointed downstream 

 and it proceeded along as if under power. 

 When a mound or other minor relief feature 

 was encountered, the excess chain accumu- 

 lating on the feature decreased the vehicle's 

 weight and it ascended accordingly; in the 

 event of a depression, the excess chain hang- 

 ing suspended caused the vehicle to descend. 

 While there are drawbacks to this method of 

 buoyancy control, for near-bottom cruising 



at a specific altitude and over relatively 

 smooth terrain, the system is virtually un- 

 beatable. According to the elder Piccard in 

 In Bdlloon antl BathYsc€iphe, this technique 

 was first used by the balloonist for cruising 

 at low altitudes over land. 



Irreversible Systems 



Pressure Hulls: 



In most submersibles (excluding the bathy- 

 scaphs) the pressure hull exerts a positively 

 buoyant force, the extent of which depends 

 on the W/D ratio. This force is not constant 

 at all depths owing to the compressibility of 

 the hull which reduces its displacement. The 

 SP-3000, for example, carries two jettisona- 

 ble 33-pound weights which are released to 

 compensate for loss of positive buoyancy 

 through hull shrinkage. The bathyscaphs are 

 a different situation wherein the pressure 

 hull is negatively buoyant and the problem is 

 one of getting it to ascend rather than de- 

 scend. 



Syntactic Foam: 



For great depths syntactic foam is one of 

 the most promising positive buoyancy mate- 

 rials. The foam consists of a mixture of hol- 

 low microspheres embedded in a resin ma- 

 trix. Some foams use plastic or glass micro- 

 spheres, and matrix materials of polyesters, 

 phenolics, polyethylenes or vinyls (7). Sev- 

 eral factors make syntactic foam an attrac- 

 tive buoyancy material: 



Low density 



High hydrostatic strength 



Low water absorption 



Immunity to catastrophic failure 



Bulk modulus equal to or slightly higher 

 than seawater 



Fabricability to irregular shapes by cast- 

 ing or machining 



One of its greatest features is the relative 

 ease with which it can be machined or cast to 

 fill small, large, and irregularly shaped voids 

 within the exostructure. In some cases, it is 

 merely attached as blocks to the vehicle or 

 strapped into an existing cavity to provide 

 greater payload for a specific dive. At times, 

 syntactic foam performs more than one func- 

 tion. Figure 6.6 demonstrates its uniqueness 

 by serving both as positive buoyancy mate- 

 rial and a stabilizing rudder on TURTLE. 



296 



