Re: [PSUBS-MAILIST] Main ballast.

[VBra676539@aol.com]

Pierre,

Don't confuse surface buoyancy tanks with variable ballast. They may be used interchangeably on occasion, but are, in fact, two different things.

MBTs are designed to hold the submarine on the surface. Nothing else. They change the displacement of the submarine without changing its weight, making it more buoyant than it is in ballast. Once the mains are vented, your vehicle should be neutrally buoyant with the variable ballast half full (if you have not lied about your weight, that is).

The variable system, however it is designed, does the opposite. It adjusts the weight of the submarine without changing its displacement. This gives you fine adjustment at depth for changes in water density or temperature, or payload adjustments (after you retrieve the gold from the sunken pirate ship, for instance).

You can always add buoyancy by squirting extra air into the mbt, but you run the risk of runaway expansion as you rise through the water column, which would result in a loss of depth control. Remember, air volume halves at 33 feet, 66 feet, 132 feet and so on. If you depend on your mbt, you will have to adjust constantly (which I have done on many occasions, but carefully). On the way up, you have to vent mbts like a crazy man, and the nearer you get to the surface, the larger vents you must perform.

Best Regards,
Vance