The main thing that makes submarines different then "normal" boats is that they have the ability to go underwater. There are two methods for accomplishing this. The first method is called Dynamic Diving, which requires the submarine to be moving to dive. The second method is called Static Diving and does not require forward movement of the submarine to dive. Each of these two methods can be accomplished by using several different ballast systems. A ballast system is simple a way to change the weight of the submarine to raise or lower it in the water. Let's take a closer look at Dynamic diving submarines first since these are usually the simplest systems.

Dynamic divers are set up with positive buoyancy, which means that they will float to the surface if they are standing still even if the ballast tank is filled. The advantage of this method is that if something goes wrong with the submarine or you lose control of it, just stop the motor and it will float to the surface. The disadvantage is that you must be moving to dive.

NO BALLAST SYSTEM -The simplest subs do not have any type of adjustable ballast. They simply rely on forward movement to dive. This requires the dive planes (rudders that control up/down movement) to be under water at all times. If the submarine is modeled after an actual submarine, this usually forces the model to sit lower in the water then the real boat would when surfaced. An example of this type of submarine is the Graupner Shark.

PUMP BALLAST SYSTEM - This is the next level up in Dynamic diving subs.  A water pump is fills a tank to partially submerge the sub and empty the tank to raise the sub to normal surfaced height.  This system requires that the submarine be at the surface when empty the tank of water so the submarine must be set up with slight positive buoyancy when the ballast tank is full.  This setup allows the submarine to dive at slower speeds then a similar no ballast system because with the ballast tank full, it sits much deeper in the water and more of the diving control  planes are in the water.   It also allows the submarine to be raised to a realistic surfaced height.  An example of this type of submarine is the SubTech Albacore.

HYBRID SYSTEM - An interesting option to the Albacore mentioned above is a gas ballast backup system.  This system is used for emergencies to completely push all of the water out of the ballast tank providing maximum positive buoyancy for the submarine in case of problems.

GAS BALLAST SYSTEM - This is probably the most common system used on static divers.  It operates by having a tank filled with compressed gas that is filled before the boat is put into the water.  A valve in the ballast tank allows water to flow in the tank to dive.  To surface, a charge of air from the compressed air tank is released into the ballast tank to push the water out.  The advantage of this method is that it is a simple design and works well.  The disadvantage is that the compressed air tank will only hold a limited amount of air so the number of times that one can surface and dive before refilling the tank is limited as well.  Also, there is a possible danger of the compressed air tank exploding when filling because of to much pressure.  Finally,  a supply of compressed air must be available.   Typically, air brush propellant is used.

COMPRESSOR BALLAST SYSTEM - This system works very similarly to the Gas Ballast system described above with one important difference.  Instead of relying on having a compressed air tank filled by the operator before sailing, the boat outfitted with a compressor fills it own air tank by compressing the available air when surfaced or by pumping water into the ballast tank there by compressing the air that was originally stored there and using it later to push the water out.  The advantage of this system is that no external source of compressed gas is needed.  The disadvantage is the cost, complexity and possibly the increased battery drain.

PISTON BALLAST SYSTEM - This system uses a movable piston to increase or decrease the displacement of the ballast tank.  To submerge the piston is moved farther out of the ballast cylinder, letting more water in.  To surface the piston moves inward in the ballast cylinder, pushing the water out.  The advantage of this system is that no high pressure air is used.  The disadvantage is that the control mechanism for the piston can be somewhat complex to design and more room must be allowed for it.