Gas Ballast System
One of the more popular ballast systems employed in North American model submarines is the Gas Ballast System. In this system, a low pressure gas is stored in a liquefied state in an onboard pressure vessel. It is used to purge, or "blow", water from a filled ballast tank. Air brush propellant, such as Badger's Propel, is typically used to charge the pressure vessel. As a result, the system is sometimes called the "Propel System".
Operation is straightforward and requires only a single servo. In one direction the servo opens a vent in the top of the ballast tank, releasing air and allowing water to enter through open holes in the bottom of the tank. When moved in the opposite direction, the servo opens a valve in the pressure vessel, releasing the gas into the ballast tank and displacing the water back out through the holes in the bottom.
As with any liquefied gas, care must be employed when filling the pressure vessel, as the surrounding area gets extremely cold when it reverts to a gaseous state. A touch of frostbite can occur if flesh is exposed to it. Due to the type of gas employed, pressure in the vessel is generally low and does not require a dedicated pressure relief valve. Commercially available systems utilize soft connections that will fail in overpressure conditions to ensure user safety.
Operators of gas ballast systems must count their blow/vent cycles to guard against emptying the pressure vessel prior to the final surface. Most models can achieve between 8 and 12 cycles before exhausting the onboard supply of gas.
A consequence of the gradual depletion of the gas is a slow change in the overall trim of the model. However, it is small and typically centered on the ballast tank. The overall effect is that the boat is slightly more positively buoyant when submerged. Because the onboard gas is expended during operations, the gas system has an ongoing expense that other systems don't have.
If vented too quickly, gas systems can rapidly empty a ballast tank causing the boat to "pop" to the surface. However, most proponents of the gas system feel the hiss of gas and trail of bubbles adds a pleasing realism to model operations.
Due to its relative simplicity, the gas ballast system can provide many seasons of operation with little maintenance.
Contributor: Bob Martin, Paul Crozier
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