Take Her Down!
The essential ingredient for any successful static dive boat is the Ballast System. A good ballast system will not only cause a boat to submerge and surface on command, but will do so reliably. Ideally, it is not overly complicated to build and simple to maintain.
Through years of experimentation, four major categories of successful ballast systems have emerged: Pump System, Piston Tank, Gas System, and RCABS. The first to achieve widespread acceptance among the R/C submarine community was the Pump System and its two main types; low pressure, and high pressure.
Low Pressure Pump Ballast System
The Low Pressure Pump Ballast System
is not a true static dive system, but rather
a hybrid of the static and dynamic dive systems.
In this system, a flood valve is opened and air leaves the ballast
tank through a vent. The vent typically exits
very high on the model, hidden as a periscope or
mast. Water enters the ballast tank from the flood
valve until the tank is fully vented, at which
time its buoyancy will be slightly positive and the
boat rides “decks awash”. With the reduced
buoyancy, the boat’s dive planes can now be used
to pull the boat under the water with
very little speed.
To surface, the flood valve is closed and the positive buoyancy of the model brings it back up to the “decks awash” state on the surface. A small low pressure pump then pumps the water out of the ballast tank, drawing in air from the surface through the vent mast.
The low pressure pump system is simple, very reliable and draws little current from the battery. The slightly positive trim of the boat is inherently safer. However, the vent must be above the surface to empty the ballast tank of water. And though it is closer than a dynamic dive boat, it is not a truly static diving system.
High Pressure Pump Ballast System
In this system, the air in the ballast tank begins at ambient pressure.
A valve is opened and water is pumped into the ballast tank through a high
pressure pump. As the water rises, it compresses the air in the tank reducing
its volume and making the model heavier. Some systems allow the air to escape
into the larger dry space in order to minimize the overall pressure.
When the pump can no longer move any more water into the tank, or when the
desired buoyancy is achieved, the valve is shut and the pump turned off.
To surface, the valve is simply opened and the resulting avenue of escape for the pressurized air forces water back through the pump mechanism and out of the model. The air expands causing the model to become positively buoyant again. With a two-way pump installed the pump itself can pump the water back out of the tank.
Like the low pressure system, it is simple to set up and operate and has proven to be very reliable. Further, if a leak develops in the valve system while submerged, onboard pressure will inherently blow the ballast, causing the model to surface. Thus, the system has a built-in failsafe. Commercially available systems install the flood valve so it automatically opens when power is lost.
Utilizing the pump to force water into the ballast tank slightly increases the current draw on the battery. The system also fails to make maximum use of internal space. Factoring in the strength of the pump and compressibility of air, a modeler can expect to only utilize approximately 70-80% of the volume of the ballast tank.
Contributor: Bob Martin
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