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Re: [PSUBS-MAILIST] Re: Variable Ballast Calculations, Bill



Hi Paul.
 
Answering your subjects in the order you wrote them.
 
 
Your Navy dive tables are less conservative than the P.A.D.I. dive tables are. P.A.D.I. does that to build in a WIDE safety factor, since P.A.D.I.
 
knows that many sport divers are not necessarily highly trained or diciplined and might make mistakes on depths or times they stay.
 
The Navy knows its divers are professional and highly trained and gives more liberal and longer bottom times.
 
 
I would say a hybrid would be somewhat more complicated. If you intended to only dive to a maximum of 4 atm/99ft and while doing so
 
keep your interior hull pressurized at 2 atm so you could stay 205 minutes (P.A.D.I. no deco limit) without deco as opposed to a typical ambient's 20 minute limit
 
(P.A.D.I. no deco limit) without deco, then your hull has to be strong enough to withstand 2 atm of outside water pressure with 1 atm in the hull interior. So it would be
 
more complicated than a typical ambient hull that requires no real pressure strength because it uses internal air to counteract the outside pressure
 
and therefore doesn't need a pressure hull. But it would be much simpler, cheaper and less thick than a full 1 atm sub's hull would have to be. You
 
only need to have a hull that would be like a regular 1 atm sub at 33ft. Not that much pressure, but still more than a typical ambient hull could withstand if that ambient's interior was at 1 atm.
 
So it would probably be a bit more complicated than a typical ambient sub construction, but think of it, you could go to 99 ft for 205 minutes instead of just 20 minutes,
 
that would make it worth it I believe. Most of the largely unexplored wrecks in my area are just beyond normal scuba depths. The ones AT scuba depths have all been
 
picked over mostly. So a lot of the good stuff is deeper than normal P.A.D.I. scuba depths allow you to go or else to stay very long. Even though P.A.D.I. dive tables allow
 
an absolute maximum no deco safe scuba depth of 140ft for only 8 minutes, not many divers except for the advanced techs like to go that deep because they cannot stay long enough to
 
enjoy anything with just 8 minutes without deco. So imagine yourself at only 2 atm or 33 ft of pressure on your body, but actually diving to 99 feet and staying for a maximum of 205
 
minutes without any deco! Wouldn't we scuba divers love it if we could do that! A hybrid like this would allow us to. I would say it would be worth the extra trouble. What do you think Paul?
 
Now I would like to know what hull strength I would need for a 1 atm pressure vessel to withstand 2 atm of outside water pressure. Probably have to build it with a safety factor for 3 or so atm
 
just to make sure it was safe. I wonder what steel thickness I would need or if a 1/2 inch thick kevlar wing tank would withstand 2 or 3 atm with 1 atm inside it? Any help here fellas?
 
 
 
Bill.
 
 
 
 
----- Original Message -----
Sent: Monday, November 14, 2005 1:32 PM
Subject: Re: [PSUBS-MAILIST] Re: Variable Ballast Calculations, Bill

Thanks Bill for the great explanations.  I like those ideas - both work creatively with underwater physics and physiology.  How about 'regenerative ballasting' for a term?   You'd expect a hybrid ambient sub would need some sort of regenerative system, like any other hybrid vehicle.  :-)

It sounds good to me to limit cabin pressure to two atm total.  I don't have my old PADI dive tables with me but it looks like the Navy tables give 310 minutes no deco time at 35 feet.
http://home.flash.net/~table/table/p0000149.htm
That seems like a very useful dive time, considering that you're in a relatively warm and dry compartment.  And very different from the 15 minutes of bottom time that these same tables give for 120 ft.  Of course a person would probably use a dive computer or other tables and get some different numbers.


Would you guys say that a hybrid ambient would be a little more complicated, or just different, to design and operate than either a pure 1atm or a plain ambient dry sub?  The ballast tanks are plumbed into the crew compartment and so could allow water to enter.  Dan notes that the cabin would need to resist an internal overpressure (or have a good overpressure valve?).  And I'd think that the hull would have to be strong enough to handle the max operating depth with a cabin pressure of just 1atm.



Paul