Displacement once submerged: the same as when floating?
 

I have this doubt:

RE: Displacement once submerged: the same as when floating?
 

Yes

The displacement weight and the weight of the sub are separate parameters. The weight of the sub can vary. You can add a lead weight or water to the internal volume of the sub to increase its weight. You displace air which weighs considerably less than lead or water.

Displacement weight, on the other hand, depends on the shape of the sub, (volume) it is a constant and cannot be changed unless the shape of the sub is changed. It is the weight of water (salt slightly heavier) equal to the volume of the sub. i.e Fill the bath tub to the brim. Place a brick in the tub. Some water spills over. Collect that water and it will weigh less than the brick. The brick sinks. Take a piece of wood same dimensions as the brick, the displaced brick water weighs more than the wood piece, the wood floats.

If the displacement weight of a model submarine is 100 pounds (freshwater, slightly more if salt) and the weight of the sub is 90 pounds, 10% of the sub will be above the water line and 90% below. If the two weights are equal, theoretically the sub is at neutral buoyancy and is submerged.

RE: Displacement once submerged: the same as when floating?
 

So when you read the specifications of a full sized submarine, the submerged displacement value is the weight of the water when the submarine is submerged with or WITHOUT filled ballast tanks?

RE: Displacement once submerged: the same as when floating?
 

It sounds to me that the submerged displacement value is the weight of the water when the submarine is submerged WITHOUT filled ballast tanks.

Regards

RE: Displacement once submerged: the same as when floating?
 

Do a search entitled: water displacement. That will answer your question.

RE: Displacement once submerged: the same as when floating?
 

The displacement of a submarine or any other vessels is the volume of water it displaces when placed in the pond or ocean. It is independent of the mass of the submarine. A submerged submarine made of lead displaces the same amount of water as a same sized submarine made of balsa (will need bigger ballast tanks to submerege ofcourse). A submarine sitting on the bottom of the sea with a full ballast tank displaces the same amount of water as submarine with empty ballast tank (assuming it is tethered to keep it down). the differeance is that the sub with the empty ballast tank has far more buoyancy. A submerged sub displaces more water than a surfaced sub by an amount equal to the non fee flooding volume of the submarine structures above the surface. A sub displaces the same in fresh or salt water, but its bouyancy is different.

RE: Displacement once submerged: the same as when floating?
 

I don't understand how this is so. A ping pong ball glued to the bottom of a fish bowl displaces much more water than a ping pong ball glued to the bottom of a fishbowl, but has two tiny holes that allow water to enter the ping pong ball.

The second bolded statement makes sense, except what happens if you have a submarine with HUGE ballast tanks and small watertight areas versus a submarine with the smaller ballast tanks and larger watertight compartments? Both submarines have ballast tanks capable of bringing the boat to the bottom, but when both boats are sitting on the bottom, each submarine is only displacing the amount of water equal to the watertight areas, right?

RE: Displacement once submerged: the same as when floating?
 

Of course if you make holes in the ping pong ball and let water freely enter then the ping pong with the holes will displace less. Now imagine you have 3 ping pongs, 1 has no no holes and the other two have holes. You fill one with 50% water anf the other with 100% water and then seal the hole so no more water can enter or leave. Now you push each ping pong to the bottom of the fish bowl. They all displace the same amount of water which is the volume of the ping pong ball, regardless if it is filled with water or any other material.
I think youi are assuming that because the water from the sea is now inside the sub ballast tank, the water belongs to the sea, but as you can see from the ping pong experiment, the water in the ballast tank can be considered part of the sub.

Another experiment. Imagine you have a oil tanker at port, with its tanks empty. Now they start filling the tanks with water from the shore, gradually the tanker sits lower in the water, its displacement is increasing. Now imagine the same tanker instead of using water from the shore, opened the valves at the bottom of the tanks and let sea water in, then they closed the ballast tank off. The tanker is again fully loaded and sitting lower in the water and displacing more water like before. thats why I said the water in the subs ballast tank is part of the sub, not the sea.

RE: Displacement once submerged: the same as when floating?
 

That makes sense, and I see what you're getting at. I suppose by using your reasoning, a real sub's "submerged displacement" is the same value, whether the ballast tanks are filled or not.

However, using simple logic and layman's terms, a submarine is not displacing water by taking on water, but you recognized my misunderstanding be saying the water in a ballast tank does not belong to the sea.

It all makes sense now, thanks.