RE: Fw: [PSUBS-MAILIST] Free Surfaces solution!

Rick,

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 -----
From: rick miller

To: personal_submersibles@psubs.org

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.

go to link to see drawing of this http://www.frappr.com/?a=myfrappr&id=386116

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