Ohio Class, Ballistic Missile Nuclear Powered Submarine, USS Kentucky SSBN737
Based on the previous chapters, we can decide on the type of diving system that we want. I have chosen the compressed air type, where we can use either compressed air or CO2 / similar. This offers the most realistic dive system, and a wide range of propellant.
Comments on this system:
The function of the ballast tanks is to control the buoyancy of the sub. Positive buoyancy brings the sub to the surface, neutral buoyancy keeps it steady at the present level, and negative buoyancy causes the sub to dive. All though the tank just need to be large enough to offer the needed variation in buoyancy, there are a few considerations:
The minimum of required buoyancy is equal to the weight of the part of the sub, that you want out of the water when surfaced.
Too small tank: Your sub will not be able to produce positive buoyancy, and will sink, or at the very least stay at some level below.
Too big tank: You would use a lot of valuable space inside your sub. On the other hand, big tanks would leave room for future additions to go in the sub without redesigning the tank too, so something in between is what we want.
How do we calculate this?
If you imagine an air filled container, and disregard the weight and buoyancy of the container itself, and imagines that the container displaces 1 liter of water, then this container would give you ~1 kg. of positive buoyancy. (1l. water weights ~1 kg.) Now fill it half full. The positive buoyancy is now only Ĺ kg. Fill it completely, and youíve got neutral buoyancy (as we disregard the weight etc. of container it self). Had the container had a weight of itís own higher than water, it would have sunk, caused by the negative buoyancy for the system in total. (We also disregard the fact that salty waters and fresh waters are different when it comes to the calculation of buoyancy.)
Our sub will have a fixed mass (negative buoyancy), but itíll also hold air in the watertight compartments (WTC), giving us some positive buoyancy. The hull it self will also give some positive buoyancy as itís made of plastic. We want to be able to vary the buoyancy just enough for the system in total, so that we can dive, and re-surface. Just like a real sub marine, using static diving.
* Put the electronics, motor, ALT mechanism, servos and the receiver etc. into the watertight compartment (WTC) assigned for this. Put the valves and the pump into their watertight compartment (WTC). (Notice: It is vital that the WTC's are included in this process as they offer quite a bit of positive buoyancy.) Now place the batteries inside a sealed plastic back and remove as much air as possible, then place it inside the hull along with the two WTC's and the air tank (Filled with air if possible) including associated manual valves etc. With everything now inside the hull fit the top, and hang the entire sub from a scale (Same type as used for fishing), then lower the hull fully into the water. (Do this in shallow water only!) This simulates a full diving session, and we expect the sub to sink as there are no air filled ballast tank inside the completely flooded hull.
* If it sinks: Great! You have negative buoyancy, and you should now read the scale. Let's say it reads 2 kg. We now know that it would take at least 2 kg. of positive buoyancy to create neutral buoyancy, and that the ballast tank therefore, in total, must displace more than 2 liters of water in order to create positive buoyancy, and surface. Now lift the sub to the expected surfaced waterline, and read the scales again. It now shows the minimum positive buoyancy required to keep the sub at the surface. Add a little for future expansions. This brings you to a total of 3 liters. It is possible to fit small permanently air filled containers inside the hull to give you permanent positive buoyancy. If you do that, place them high so the sub will not roll over. You should only do this if the needed ballast tanks prove to be way too big caused by a high weight of the sub it self.
* If it floats: Hmm... we need to increase the onboard weight, and thus create negative buoyancy, by adding weight bow and aft. Put rocks or similar with known weight inside the hull until it sinks. You now have negative buoyancy, and you should now read the scale.
Let's say it reads 2 kg. We now know that it would take at least 2 kg. of positive buoyancy to create neutral buoyancy, and that the ballast tanks therefore, in total, must displace more than 2 liters of water in order to create positive buoyancy, and surface. Now lift the sub to the expected surfaced waterline, and read the scales again. It now shows the minimum positive buoyancy required to keep the sub at the surface. Add a little for future expansions. This brings you to a total of 3 liters. Remember to collect all of the rocks or what ever you used, and note the total weight of these. Later you will have to place metal weights inside the hull with the same weight.
How do we make the tank?
The tank is fitted after the hull is completed, and is located slightly above, but in line with, the very center of gravity.
Although it might not be applicable with ballast tanks this small, you might want to include dividers going from end to end inside the tank so that the water can not run back and forth uncontrolled, as this would endanger the stability of the sub. These dividers do not fully prevent the water from flowing forward or backwards within the tank, but they do slow down the motion. Without these the body of water could rapidly change the center of gravity, thus listing the sub end over end, as the operator could not react fast enough. It might be a good idea to place a few dividers that goes from side to side as well in order to prevent the same problem, only side-to-side ways. These dividers are also found in tank trucks.
Building the tank: You might want to look around for plastic bottles or similar. Test the volume by filling them with water, and then pour the water into a measuring cup, or use a scale subtracting the weight of the container. The idea of using a collapsible container might pop up, but donít. It is not an option because our hull is a so-called "wet hull". You might also consider using the hull as the container, but remember that this approach is less flexible and might make later additions to the sub impossible, without a great deal of foam and contra weights to keep the center of gravity in the same place. If you want to construct the tank your self, you'll need to calculate the volume of the cylinder you are about to build. The formula for this is the following: Volume = Circle Area x Height, or V = ( 3.14 x r2 ) x h I have seen other builders on the Internet using transparent plastic tubes for this as well as for the WTC's to the electronics etc. This is a highly recommended, as this enables you to visually inspect the systems.
Water / air Inlet / outlet:
The air and the water inlets / outlets of the ballast tank deserve a comment, as it's not without importance how you design where and how the ports are located. The lower ports are open to the sea at all times, just like a real boat. There are several holes, so that the rate of flowing water is not too low. (slowing down dive and surface commands.)
The air vents must let the air out on the outside of the hull. Venting inside the hull breaches stability, and is a huge no-no in wet hull subs.