You are so right, the vessel stability issues -vs- aircraft, does increase
my Excedrin use on occasion! Some elements such as the datum, weights,
moments and arms are familiar territory but, the changes during submergence,
are not quite firmly grasped yet.
But you and Rick L have given me yet another epiphany! Rick L for his
mentioning the word "Isolated" and your use of a "spherical" hard tank at the
cg for use as my VBT! Up until this moment, I had not considered anything
as 1 atm aboard Octopus.
I'm a bit tired at the moment from a hard day today but, at this moment,
nothing is glaring at me as to why I shouldn't use such a system for very
small changes in my buoyancy control profile when going below snorkel depth. I
might put a bow thruster in a tube to nudge me around the vertical axis
because she is long....big maybe but, I am not inclined to start adding a
bunch of thrusters to this. I much prefer to have tight buoyancy control.
My "free surface" problem is only when she would be running surfaced with
the tanks full of air. I think I have that problem licked As
you Jay, Dan and the textbooks have pointed out, these volumes "soft
tanks", will be vented completely on every dive. Octopus would have no free
surfaces while submerged except for the little silly square ambient VBT I was
contemplating until you two spoke up.
A million thanks to both of you...this is precisely what this forum is
about!
Now I have another little buoyancy problem and that is...what to do with
the 10 gal diesel tank?The old fleet boats admitted seawater into the tanks to
replace fuel as it burned off but, I don't know how practical this would be in
a 10 gal tank. Any suggestions?? :),,,bladder perhaps??
Thanks again
Joe
From: "rick miller" <rickm@pegasuscontrols.com>
Reply-To:
personal_submersibles@psubs.org
To:
<personal_submersibles@psubs.org>
Subject: Fw:
[PSUBS-MAILIST] Free Surfaces solution!
Date: Mon, 6 Mar 2006
13:30:32 -0800
----- Original Message -----
Sent: Monday, March 06, 2006 1:27 PM
Subject: Fw: [PSUBS-MAILIST] Free Surfaces solution!
Joe
your center of gravity vrs
center of buoyancy difficulties are probably based upon your aircraft
experience. in an aircraft the center of buoyancy is the center of lift
of the wing and does not greatly change do to attitude. where as in a vessel
the cb is constantly changing due to attitude and store consumption. your
center of buoyancy is based on the displacement vrs weight. in a sub where
you have a minimum of four different configurations you must figure for all
of them i.e., surface ,negative , neutral, positive.
then you must work out the
stability of the vessel its tendency to right its self after a roll. most
commonly a problem at the surface. because unlike a aircraft where the plane
of the lifting force follows the attitude of the aircraft the force of
buoyancy is always straight up.
for smaller subs with
minimum consumables HP air the best way to deal with this is to keep
the stores as close to the center of buoyancy as possible. the biggest item
for change is in the hard ballast tanks. a spherical tank place at the
center of buoyancy on the longitudinal axis will minimize the free surface
effect . as the bow goes down. the water in the tank shifts fwd but the
tank shifts aft in respect to the center of buoyancy canceling the shift. an
18 sphere at the center will give you up to 110 lbs buoyancy without having
a free surface problem.
spherical tanks will self level. square tanks
will go 45 degress down 45 degress up or level these are the equalibrium
states. cylinder tanks will have equalibrium points farther off
as then square tanks.
fianl conclusion any tank
exposed to free surface effect should be spherical.
Rick m