Just to expand on the point about salinity changes - if you are transiting from salt water (~1025 kg/m^3) through a region of fresh water (~1000 kg/m^3), your boat is going to lose about 2.5% of its buoyancy. Depending on your overall displacement, that could correspond to a significant volume of ballast water, but for any sub operating in salt water I would consider this to be an absolute minimum VBT volume. Other factors to consider include payload (passenger and cargo, fuel, water, etc) compensation - where you do not wish to spend time adding or subtracting hard ballast (lead) as your manifest changes, and hull volume changes - regardless of your hull design, pressure at depth will cause small scale deflections which will change its shape and consequently reduce its buoyancy by a small amount. Consider too that a sub is not an entirely closed system, and that the loss of gases (from HP gas flasks, MBTs, etc.) to the sea is weight that needs to be accounted for. For example, a standard 80 cu. ft. aluminium SCUBA tank loses about six pounds full to empty.
With respect to blowing MBTs at depth, hopefully a scenario in which this action is necessary will never arise, but in consideration of the possibility, a few things to think about:
1) That the release of high pressure air into the MBTs will not create an overpressure in excess of their design strength in consideration of the maximum rate that water can be blown out, and the presence / effectiveness of any incorporated pressure relief mechanisms in MBTs that are not normally open to sea,
2) That in the event of an emergency MBT blow at depth, the sub is designed to maintain both adequate stability and attitude control throughout the resultant ascent and when breaking the surface, and
3) Under circumstances where a MBT blow at depth is desireable, but there is not an immediate threat to life support or pressure hull integrity which demands surfacing at the maximum possible ascent rate, and that a greater volume of air is not required to initiate ascent, that a means of controlling the volume of HP air admitted to the MBTs exist such that the volume of gas admitted to the MBT at depth is not greater than the MBT capacity when expanded at the surfaced pressure (so ballast air is not wasted).
-Sean
ShellyDalg@aol.com wrote:
> Hi Brian. That's how it COULD work. I don't think that's how best to > use it though.
> As I understand the variable ballast tank ( VBT ) it's used as a fine > adjustment for buoyancy. Changes in buoyancy occur on several > occasions. The salinity of the water, like if you go from a fresh > water harbor to salt water, or even pass through a salty layer in the > ocean, or a fresh water current in the ocean near a river mouth. These > things will have an effect on how buoyant a sub is.
> Water temperature can make a difference too. Although not much, it > gets a little more dense when the temp goes down.
> I think the most common way to handle the VBT is to adjust the volume > of water in the tank while submerged at a shallow depth, to achieve > neutral in the water column. Once the sub is neutral, the valves are > closed and you're good to go. Now, a "hard tank" is the same, but is > stronger. Let's say you are adjusted to neutral, and driving along > submerged at 300 feet. There's not much pressure in the tank, and it's > half full, with the valves closed.
> You come to an area where the buoyancy changes for what ever reason, > and the sub starts to float up. You want to add more water to the > tank, so you crack the water-in valve and add a little. The pressure > of the surrounding water compresses the bubble in the tank, and the > sub gains a little weight. Again you are buoyant neutral. You close > the tank valves again to maintain that level of buoyancy.
> You're moving along and the sub starts to sink, so you want less water > in the tank. Read the surrounding depth/pressure on the depth gage, > add some high pressure air to the tank to just overcome the ambient > pressure, then dump a little water. Again, you keep the valves closed > when not actively adjusting the tank volume.
> Now you've got some serious pressure in the tank, but it's > water volume is constant. OK, now it's time to end the dive and > surface. Use the motors to go up, not the tanks. Once you get near the > surface, dump the water from the VBT as it's already full of high > pressure, and the water will blow right out. now you're buoyant > positive and the sub should float at the surface. At this point, you > blow the main ballast which raises the tower/hatch as far out of the > water as possible. If you just want to look around and go back down, > you wouldn't blow the MBT's. Save that air until you're ready to open > the hatch.
> You've gone down and up, and still have most of your HP air in the > scuba tanks.
> Now, if a situation develops where you want to go up really fast, then > blow the VBT at depth and still use the motors. Maybe give the MBT's a > little squirt of air to help, but be careful. A fast ascent can be > very dangerous. If it's not an emergency, use the motors to go up and > down. Save the air as much as possible. Blowing either tanks at depth > and rising in the water column, the bubble expands unless it's valved > off.
> This can lead to unsafe control issues. Frank D.
>
> ------------------------------------------------------------------------
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