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When I dive Persistence, I dive it neutral
buoyant. There is an additional safety factor if you dive slightly
positive but you can't stay in position near the bottom without
constantly fighting the lift. If I want to ascend quickly without
electrical power I just give a short burst of air into any of the ballast
tanks. Normally I descend and ascend with thrusters.
Once, I let someone dive solo in my sub. He
was familiar with the sub but I coached him through the diving sequence from
topside on the radio anyway. I had him set about five pounds
positive. It was relatively shallow and he couldn't get into much trouble
being positive buoyant as R/Jay mentioned.
If you were fifty pounds positive, I doubt you
would be able to get your sub to submerge. Fifty pounds of downward thrust
is a lot to generate! There is a lot of mass to the sub so things happen
slow but five pounds heavy or light will make the sub move up or down
nicely. Ten pounds moves you a little faster. If you were only going
thirty feet to the bottom five pounds is enough. If you were going two
hundred feet down, more weight would get you there faster.
Your ballasting system doesn't have to be able to
adjust a lot. I move lead weights in and out depending if I have a
passenger and how much they weigh. I have 20 LB, 10 LB and 5 LB
weights. A total of about 400 Lbs. of lead. My variable
ballast tank is only about 35 pounds in total.
My favorite way to dive is neutral and drifting a
few feet above the bottom. It's really quiet when the sub is drifting and
only powered once in a while.
Dan H.
----- Original Message -----
Sent: Wednesday, March 01, 2006 11:33
AM
Subject: Re: [PSUBS-MAILIST] ambient
ballast control
As for buoyancy - Does anyone have a feeling for
how much "positive" bouyancy your submersible should maintain at depth?
For example, if you are displacing 3000 pounds of water and you weigh 2800,
will you be able to power under with dive planes and motors? Do you have
to be at 2950? Are you dangerously over neutral at 3100? I
know ideally that we should be right at the numbers - but doesn't the
ballast system give us some variability? I am intending to build a
two man sub - and that means the weight will
not be the same for each dive. Are there published "rates of ascent"
tables or percentages figures we can use in our calculations?
Skeet Stribling
----- Original Message -----
Sent: Wednesday, March 01, 2006 8:25
AM
Subject: Re: [PSUBS-MAILIST] ambient
ballast control
Typically the main ballast tanks should be flooded fully and the flood
valves at the bottom of the tank left open (or in the case of a K-250, the
bottom of the tanks are open). To dive the hard (rated to max.
pressure sea and/or blow system failure) is then flooded to get either
neutral or slightly positive trim. Submergence is accomplished with
dive planes and drive down with the main motor(s) (and in the case of a
K-350, additionally using the auxilliary motors). Being slightly
positive will benefit in the case of a loss of power as the sub will slowly
return to the surface on its own.
A normal surfacing evolution is accompllished by powering to near the
surface and them blowing main and trim ballast tanks thus conserving the
limited supply of air carried aboard. Only in an emergency are main
and trim tanks blown at depth (and then dropping the emergency weight if
neccessary).
R/Jay
-----Original Message-----
From: "Dan H."
Sent: Mar 1, 2006 8:39 AM
To:
personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] ambient
ballast control
Joe,
What I was questioning is the need for a
pressure relief in a closed ballast tank where the tank is built strong
enough to withstand the system pressure. If your tank is strong enough
to withstand the system pressure, then it won't blow.
As was pointed out to me by Jay, it's still
possible to get a regulator failure, and unless the ballast tank was built
to SCUBA tank pressure, it could possibly be ruptured if that
happened. Good point. My sub is a one atmosphere
design.
That is the one scary thing about an open
ambient sub. Then deeper you go the more air it takes not to go
any deeper. A situation that can be a runaway disaster.
I don't know about an ambient sub, but
in my one atmosphere, when diving, I keep my main ballast tanks
totally full of water , vents opened, and my main ballast tank,
adjusted to neutral buoyancy of the sub and all valves shut.
Dan H.
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