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RE: <SPAM> RE: <SPAM> [PSUBS-MAILIST] Ballast

Errata
 
The last sentence on my las post that reads as follows:
 
 Or worse yet, if your Hard ballast system floods you will be able to surface if you don't have a soft ballast system.
 
 
Should read as corrected:
 
 Or worse yet, if your Hard ballast system floods you will NOT be able to surface if you don't have a soft ballast system.
 
Hugo
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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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