RE: [PSUBS-MAILIST] Ballast Question

["Alec Smyth" <Asmyth@changepoint.com>]

Pierre,
 
I'm building a sub with NO tankage so I'm no expert. However, my understanding in the simplest terms is that you would normally have the following two, independent, ballast systems:
 
1) Main ballast
To be used basically in all-or-nothing mode. You flood these (completely) to submerge, and blow them (completely) to surface. You would not normally try and submerge with ballast partially blown as you describe. Soft tanks make sense for main ballast, as they are large.
 
2) Variable ballast
This is for fine trimming the overall buoyancy of the boat. Generally a hard tank, to prevent the kind of feedback loop you mention -- getting heavier the deeper you go. It would normally be placed to be vertically aligned with the submerged center of buoyancy. Some subs use a soft variable ballast tank inside the hull, but with high pressure tubing and pumps to dump water overboard. Whether you go with hard or soft, as with all hull penetrations do put shutoff valves immediately inside the hull.
 
That said, some small subs do combine both systems in one. The Deep Workers I believe are a nice example of that, with a single, soft (plastic), open-bottom tank.
 
In addition, there are two more ballast systems often mentioned, although frequently dispensed with in psubs:
 
3) Trim tanks
For trimming fore-and aft or side to side. You might keep the volume of water constant, but pump some from bow to stern or vice versa, to get the boat horizontal. There are all sorts of options, such as moveable weights or moveable batteries. 
 
4) Dive tanks
Small tanks that are calculated to put the boat in approximate trim when fully flooded. This is a simple way to reach a trimmed condition fast, and I think is mostly of interest to military subs.
 
I could not agree more with Herve about the desirability of a scuba course for anyone building a sub, and especially if its an ambient sub. In an ambient sub you will be scuba diving, although your scuba apparatus looks just like a submarine -- there's absolutely no difference from a physiological perspective. So you better know about decompression, safe rates of ascent, nitrogen narcosis, etc. as well as basic high school physics.
 
rgds,
 
Alec

	-----Original Message----- 
	From: Pierre Poulin [mailto:pipo305@hotmail.com] 
	Sent: Mon 12/30/2002 9:30 AM 
	To: personal_submersibles@psubs.org 
	Cc: 
	Subject: [PSUBS-MAILIST] Ballast Question
	
	

	Hi there every body! It's the newbi again.
	
	I'm working on an a dry ambient sub. The hull is 60% finish.
	
	I tought that the ballast system was pretty simple but then I figured that
	if I have a main ballast system witch is opened at the bottom and I ajust
	the buyancy at 33 feet. Then, I go at 66 feet using electric motors. The
	buyancy will need to be ajusted again since the air in the ballast will be 2
	times smaller. Right? So the deeper I go, the faster I sink?
	
	Would it be better if the ballast was able to support the pressure like an 1
	atm design and you close the bottom of the ballast witch is normaly open? So
	the volume of air will remain the same? I do realize that when I will try to
	put more air in the ballast, I will have to play with 2 valves at the same
	time to conpensate.
	
	Is that a variable ballast system? Is there a difference between those two
	systems (main and variable)?
	
	I'm a bit confused with that! I'm looking for something as simple as
	possible.
	
	I tought that some of you guys who are more experienced surely have the
	answer to my problem!
	
	Thanks for your help!
	
	Pierre Poulin
	From cold Québec, Canada.
	
	
	
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