AFT 



FWD 



TRIM PUMP 



TRIM TANK 



Fig. 6.9 BEN FRANKLIN's trim system. 



console and connected by tubing to the tank. 

 A small plunger next to the gage is pulled 

 out to let air into a cylinder. The plunger is 

 then pushed in, forcing air into the manome- 

 ter and bottom of the ti"im tank. The amount 

 of back pressure forced into the manometer 

 depends on how deep the water level is in the 

 tank. 

 Mechanical Weight Shift: 



The simplest of all trim systems is dis- 

 played by submersibles which merely shift a 

 cast iron or lead weight fore and aft inside 

 the pressure hull. The Japanese HAKUYO 

 has adopted a lead weight moving system 

 which is described as the simplest, safest and 

 most accurate method of trim control (10). 

 The system (Fig. 6.10) consists of a lead 

 weight moved along a rail by means of a 

 hydraulic motor controlled by solenoid 

 valves. The trim may be changed ±10 de- 

 grees. 



External Trim Systems 



Mercury Transfer: 



To achieve relatively large trim angles, 

 several vehicles employ a fore and aft weight 

 shift in the form of mercury; ALVIN is one. 

 According to Mavor et al. (5), available space 

 on ALVIIS precluded the shifting of items 

 such as batteries for trim purposes, and fluid 

 was required for flexibility in geometric 

 shape. Several fluids were analyzed for their 

 suitability. Some of the less dense fluids 



would have added less to the gross weight of 

 the vehicle, but the space problem was so 

 critical that a fluid with a density approach- 

 ing mercury was required. 



Mercury, however, is a difficult fluid to use 

 and has a corrosive effect on many metals. 

 This problem was solved by using a hydro- 

 carbon fluid in the pumping system. The 

 fluid in the system thus became half mer- 

 cury and half oil. The two fluids were found 

 to separate adequately if a settling stand- 

 pipe was provided for this purpose — oil on 

 top, mercury on the bottom. 



The fluid is contained in three small fiber- 

 glass spheres, two forward and one aft. An 

 electrically driven pump is utilized to trans- 

 fer the mercury by displacing the oil in the 

 closed system. Two blocking valves and a 

 reversing valve are incorporated in the cir- 

 cuit to control fluid displacement. The valves 

 are electrically operated by solenoids. The 

 valves and pump motor are wired to a single 

 control for convenient operation. Filters and 

 relief valves are used for system protection 

 in the conventional manner. 



Angles of approximately ±25 degrees are 

 achieved by transferring 540 pounds of mer- 

 cury with the system as shown in Figure 

 6.11. 



VBT Differential Fill: 



The DEEP DIVER trim system can change 

 trim or overall buoyancy by using a seawater 

 medium which functions all the way from the 



304 



