MovieChat Forums > Down Periscope (1996) Discussion > Fleet Sub Operating Procedures.

Fleet Sub Operating Procedures.

posted 14 years ago by CGSailor
5 replies | jump to latest

Got a trick question for you guys. Though announcing it as a trick question probably gave away what the correct answer is because it will be the one opposite of what one might expect. Still, in answering, explain why the answer.

There are three basic groups of Seawater Buoyancy tanks based upon comonality of purpose.

MAIN BALLAST TANKS:
Used to destroy or restore positive buoyancy for diving and surfacing.

VARIABLE BALLAST TANKS:
Used to vary the weight of the sub to maintain NEUTRAL buoyancy and thus controllability of the sub while submerged.

SPECIAL BALLAST TANK GROUP:
Three tanks each with unique functions, Bow Buoyancy, Safety, Negative.

My trick question involves the Variable tank group and its use in compensation of the subs weight.

A subs weight varies as food, fuel, supplies, weapons, etc.. are either expended and used up or resupplied when in port. The Diving officer uses the Variable tanks to add weight by pumping from sea to the tanks when the subs weight lessens. When the subs weight increases, he pumps from the tanks back to sea again to lessen the weight. The overall weight of the boat must be the same at all times in order for the MAINS to reduce the Positive buoyancy to Neutral when submerged.

Now here is the question:

You are the Diving Officer.
You just had a long run on the surface running on all four diesels (doing a high-speed end-run to get in position ahead of a convoy) A lot of diesel fuel has been used up in doing so. Now you are diving ahead of the convoy.

The change in weight due to using up fuel must be compensated for on diving.

Do you:

A) Add seawater weight to the tanks to counteract the loss of sub's weight from the used up fuel.
or...
B) Pump weight to sea to lighten the Sub's weight because the sub got heavier as fuel was used up.

Choose A or B and explain why.

I joined the Navy to see the world, only to discover the world is 2/3 water!

[–] LLLg 14 years ago

B

Because the Variable Tanks have already compensated for the loss in fuel weight.

"Care to see the ruins, my friend ?"

[–] CGSailor 14 years ago

Because the Variable Tanks have already compensated for the loss in fuel weight.

It is B true, but your reasoning is wrong.

In fact, It is the very act of compensating the sub using the Variable Ballast tanks that we are considering here.

Common sense would seem to have suggested pumping FROM sea to the variable tanks to make the boat heavier to compensate for the loss of weight from the fuel used. But we don't.

Instead, the boat is heavier after burning up tons of fuel, so we have to pump to sea to lighten the boat.
Why?

I'll post the answer Monday if no one gets it.

I joined the Navy to see the world, only to discover the world is 2/3 water!

[–] michaelnewman01 14 years ago

CG, here's what I would say, though I am unfortunately not as familiar with the process as I would like.

The fuel would need to be replaced on the surface to eliminate the free surface. Whether you're diving or not, Free Surfaces really hurt stability and would be taken care of long before diving. (When that didn't happen, you lose destroyers in Typhoons...Halsey)

To keep the tanks full and eliminate the free surface, you ballast with seawater. But seawater is denser than diesel fuel, so for an equivalent volume you have much more weight.

"You feel the way the boat moves? The sunlight on your skin? That’s real. Life is wonderful."

[–] CGSailor 14 years ago

Ding Ding Ding, we have a winner!

The NFOT tanks (Normal Fuel Oil Tanks) are kept full of liquid at all times to prevent a free surface from disrupting stability.

This means the tanks are filled with seawater as the fuel itself is used up. The fuel being lighter, is siphoned off the top and the seawater remains at the bottom where it is admitted into the tanks.

As heavier seawater replaces lighter diesel fuel, the sub grows heavier overall and on compensating, Seawater ballast must be removed from the AUX tanks (part of the Variable ballast tank group) to insure the sub remains at a constant weight and thus in trim for neutral buoyancy on diving.

I never have figured out how they prevent Free Surface in the Variable tank group as by their very nature, are always only partially filled. Maybe they are more baffled than the other tanks.

The Main Ballast Tanks (MBT's) present no trouble as they are designed to always be Empty (on the surface) or completely filled (while submerged).

Only in the actual act of diving and surfacing as the tanks are being flooded or blown can an instable condition develop. On diving this period is passed rather quickly. Too quick for any real trouble to develop. On surfacing, as the tanks are being blown, any instability can be manually corrected by MBT's 2a/b, 2c/d, 6a/b, and 6c/d.

These 8 tanks, (two tanks subdivided into four each aranged port/starboard) have a restricting valve on a rocker lever in the overhead. if a list is developing to one side or the other, moving the rocker lever restricts flow of HP air into one side or the other and increases flow to the other side, thus balancing out the list.

I joined the Navy to see the world, only to discover the world is 2/3 water!

[–] liscarkat (19447) 3 years ago

CGSailor, you posted years ago, but I hope you're still around and well. You should write a book. I'm not kidding, I would buy a copy.