When designing an ROV, it is usual to attempt to use light weight components to keep the overall vehicle weight within practical limits, thus the reason for using aluminum and other light weight materials. The weight of the vehicle consists of:

• Subsystem components
• Lead margin/payload
• Buoyancy required to establish the desired operational specific gravity

It is conventional operating procedure to have vehicles positively buoyant when operating so they can be operated anywhere in the water column, and to ensure they will return to the surface if a power failure occurs. This positive buoyancy would be in the range of 5 lb (2.3 kg) for small vehicles and 11 to 15 lb (5.0 to 6.8 kg) for larger vehicles, and in some cases, vehicles will be as much as 50 lb (22.7 kg) positive. Another reason for this is to allow for near-bottom maneuvering without thrusting up, forcing water down, thus stirring up sediment. It also obviates the need for continual thrust reversal. Very large vehicles with air-blown ballast tanks that allow for subsurface buoyancy adjustments are an exception.

The measure of stability of a vehicle is conveyed by the assessment of the moment required to change the pitch angle of the vehicle. It is characterized by the equation:

Obviously, the selection of units must be consistent. That is, if "W" is in pounds and "BG" is in inches, "m" will have to be in inch-pounds. By inspection, it is clear that a large BG, which can be readily produced by having weight low and buoyancy high, produces an intrinsically stable vehicle. External forces do, however, act on the vehicle when it is in the water, which can produce apparent reductions in the BG. For example, the force of the vertical thruster when thrusting down appears to the vehicle as an added weight high on the vehicle and, in turn, makes the center of gravity appear to rise and hence destabilizes the vehicle in pitch and roll. The center of buoyancy and center of gravity can be calculated by taking moments about some arbitrarily selected point.

Most ROVs are designed to be as stable as practical (i.e., stiff in roll and pitch). When designing an ROV, stability may be kept high by placing heavy weight components such as electric motors low on the vehicle and buoyant components (GRP chambers and syntactic foam) high on the vehicle.

Ballast may be classified as fixed ballast or variable ballast (VB). Fixed ballast may be syntactic foam, closed chambers, and lead. Variable ballast may be provided by open, air-blown tanks called "soft tanks" or pumped or blown sealed tanks that can take full diving pressure called "hard tanks".

Fixed ballast (positive fixed buoyancy) of a vehicle is achieved by pressure resistant buoyancy chambers, syntactic foam and lead to bring the vehicle to the desired specific gravity. Most vehicles use a syntactic foam block near the top of the vehicle to gain positive buoyancy.

There are currently two types of syntactic foam. One is a matrix of plastic macrospheres and glass microspheres in a binder, the other has microspheres only. In general, the micro/macro material is used for shallower water depth capability and microsphere material for greater depths. Obviously, the smaller the microsphere, the higher pressure it can take, thus the density of the foam increases, along with the cost, as the depth of application increases. The trade off is based on cost, weight and pressure rating.

Vehicles that use sealed tubular frame members to gain buoyancy may be subject to operational damage. Therefore, it is conventional to use multiple compartments in the frames to prevent significant loss of buoyancy in the event of impact damage. Filling the frame with foam can also maintain buoyancy in the event of impact damage.

Depending on the depth requirement, it may be desirable to use a pressure vessel as buoyancy, however, this technique has found limited use in commercial ROVs. It is more common in AUVs where the primary structure is often a large pressure vessel.

Fixed payload on the vehicle is usually in the form of several lead blocks. This lead may be exchanged for equipment without adjusting the vehicle's foam package.

Variable ballast permits picking objects up from the sea floor and maneuvering them without thrusting downward. It also allows the ROV to be heavy when diving in high current situations. A typical soft ballast subsystem could include one or more 3000 psi scuba bottles, a pressure reduction regulator, a surface controlled solenoid valve, and a thin wall tank with a large opening at the bottom. The soft tank approach has the disadvantage that air in the tank changes volume as the vehicle changes depth.

Variable payload may also be obtained by flooding or deballasting hard (i.e., pressure resistant) buoyancy chambers. Flooding a hard buoyancy chamber when a weight is released from a submerged vehicle is a simple, effective technique. Deballasting the hard chamber may be accomplished by forcing the water out with air when valves are opened or by pumping.

Variable buoyancy is uncommon in most ROVs but is widely used in hybrid vehicles where the vehicle must be neutrally buoyant for some operations and then become heavy for operations on the seafloor (e.g. cable and pipeline burial, repair, etc.).