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RE: <SPAM> [PSUBS-MAILIST] Ballast
Cliff,
I am
not sure if you got all the answers you needed for the ballast, but for the
benefit of all I am going to cover briefly the most commonly used ballast
systems on a submersible.
Soft Ballast is also
referred as Air Ballast- The Soft Ballast system is an open
system, meaning that is open to the atmospheric pressure of the
vessel. The purpose for this system is to provide sufficient buoyancy and
stability on the surface and enough freeboard to open your hatches safely.
The tanks are normally open at the bottom to the sea and have valves on the top
to vent the air out from them. These tanks are normally operated full of
air (on the surface) or full of water (during a dive). Thus the Soft
Ballast system provides gross changes in buoyancy to the vessel, and provides
surfaced stability.
Hard Ballast on the
contrary is a closed system to its surrounding
atmosphere. The Hard Ballast Tanks shoud therefore be treated as pressure
vessels. This system provides small changes in buoyancy to adjust for the
changes in payload from dive to dive by changing the amount of fluid in the
tanks. Depending on the design, the Hard Ballast System may also be used to
adjust the submerged trim and attitude of the vessel. A properly designed hard
ballast system is operated between 20% and 80% of it's rated
capacity.
There
is another less known component to the ballast systems of a submersible, and
this is the Fixed Ballast
System. This is basically a set of lead weights positioned throughout the
submersible at permanent (Fixed) locations and it serves two purposes. The first
is to compensate for any excess buoyancy that the
submersible may have by design. Without these, the submersible will
not submerge. Second, it allows you to manipulate the Center of
Gravity of the vessel to optimize its submerged stability. Another component of
the Fixed Ballast System is Flotation. This is normally achieved by the use of
Syntactic Foam. The Syntactic Foam can be used to compensate for buoyancy
deficiencies of the vessel, and for correcting attitude or submerged stability
issues that can not be achieved with fixed ballast alone.
I
noticed in your note that you desire to use only Hard Ballast Tanks on your
design. This is the equivalent to going out SCUBA diving without a
BC. Regardless of how small your submersible is, the Soft
Ballast system is a primary component of your
submersible. The Hard Ballast is more likely to fail than the soft ballast as it
is more complex in design and it has more components. Also the reserve buoyancy
of a Hard Ballast System is a fraction of that of the Soft ballast System. In
the event of entanglement at the bottom, you may be able to free yourself up
using the reserve buoyancy of your air ballast alone. Or worse yet, if your Hard
ballast system floods you will be able to surface if you don't have a soft
ballast system.
I hope
this gives you enough information to chew on and to do a more thorough research
on your ballast design.
regards,
Hugo
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What is the
difference between soft and hard ballast?
I’m
planning to use only sealed ballast tanks (valves at top and bottom) to help
maintain neutral buoyancy at varying depths. I have assumed that soft ballast would be an open bottom
ballast tank that would allow buoyancy to change with depth, but have talked
myself into the possiblity that hard ballast might actually refer to hard
weight (trim, drop). Also, the
term variable ballast would mean
any ballast tank that air could be added to or subtracted from, right?
Thaks,
Cliff
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