Hi Myles.
Hard ballast and soft ballast have nothing to do
with hardness. Here's an article that may explain it to you.....
When a submarine is on the surface, its "soft" ballast tanks are full of air. These tanks are mounted high along each side of the hull, and are open to the sea at the bottom. Air is forced through valves at the top of the tanks, which displaces sea water and gives the submarine its surface stability and a minimum of 30" of freeboard between the water line and the top of the hatch combing. When the pilot is prepared to dive the submarine, he actuates exhaust valves in the top of the tanks which vent the air and allow the sea water to fill the tanks again. Differential ballast is provided by the "hard" ballast tanks which are designed to allow the submarine to achieve weightlessness or "neutral buoyancy" when it's submerged. The hard tanks must be filled with sea water if the submarine has just a few passengers, and they are empty if the vehicle is fully loaded. Water can be pumped back and forth between the bow hard tanks and the stern hard tanks to adjust the trim of the vessel. Some vessels also have a trim weight beneath the pressure hull that can be moved back and forth on a track to provide pitch trim. SafetyContemporary tourist submarines have a perfect safety record, having carried in excess of 7 million passengers without a single serious injury or fatality. Much of the credit for that safety record goes to the American Bureau of Shipping, an organization that "classifies" marine vessels. The ABS approves the submarine design and engineering, insures the safety of the materials used during construction, tests and inspects the various submarine systems, and attends the vessel's sea trials. Ongoing annual inspections are also required. In American waters the United States Coast Guard duplicates much of the work done by the ABS, and also stipulates requirements for the experience levels and licensing of the submersible crew, insures the existence of adequate documentation, and enforces compliance with a substantial body of safety regulations. In the event of an emergency, the pilot, who is in contact with the surface by underwater telephone, may employ a number of safety features. The submarine has an automated Halon fire extinguishing system, and each passenger has a closed system rebreather for use in case of a fire that is capable of providing oxygen for a minimum of two hours. An emergency ascent can be accomplished by 1) blowing high pressure air into the soft tanks, 2) dropping an emergency drop weight composed of several tons of lead ballast, 3) blowing the hard ballast tanks full of air, and 4) using the electric vertical thrusters. As an additional aid to safety, no tourist submarine may operate in waters deeper than its maximum design depth, typically between 150' and 328'.
You are correct Myles about the ballast tanks being as ambient when flooded or blown. One thing you can do is have the bottom of your ballast tanks open so the air can spill out if you overblow them like in the kittridge K350
sub. Some guys fill in the bottoms almost all the way but leave a hole for
excess air to spill out. That way they don't "burp" as
much from air spilling out of them when you don't
want it to like in choppy water. Remember, the air always goes up. So you can
put an overpressure valve in your tank to
vent the excess air or you can just make a hole in
the bottom. Does the same thing except the hole can't fail on you like the valve
can. Also you could use a sliding weight
under your keel in place of or along with hard
ballast tanks. For instance, you could use a long piece of stainless steel
threaded rod. Then attach a weight to it so that when
you crank the rod it moves the weight back and
forth under the sub for trimming it out. I have been thinking of making
something like this for my wetsub instead of using hard trim tanks,
but I haven't made up my mind on that yet. I would
suggest for your ambient design having both so in an emergency you have extra
tanks to blow if needed. In my wetsub it is not as
critical because I can easily bail or use my
personal Buoyancy control device (BCD) for extra lift if necessary. Hope
this helped you.
Bill.
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