Senior Member
The basic principle is pretty straightforward; fill with water to decrease buoyancy - fill with air to increase buoyancy.
Surprisingly, we never dealt much with fluids in the two courses in physics I took. Could someone please confirm my findings?
I am assuming filling the ballast tanks completely with water (achieving negative buoyancy) will make the boat sink like a stone - i.e. they are vastly overdimensioned.
In the game, you can change depth while you are stationary (I assume this is realistic!). This must mean that while diving, buoyancy is negative. When target depth is reached, neutral buoyancy is achieved by blowing the tanks slightly. When ascending, tanks are blown a bit more in order to make the boat slightly positive buoyant.
This means you should in fact consume air when you are performing depth changes.
If I am not mistaken, the air tanks of a VII hold 200 bar or 200 kg/cmâ². This would mean the submarine can ascend from a maximum depth of 199 metres if the air supply is full.
If the air tanks only contain 58 bar of air pressure, diving below 57 metres would mean the submarine could never surface again, doesn't it?
Senior Member
That is how the ballast works. But it is better to think of pumping water not air.
IE you pump in water to make you heavier, and pump out water to make you lighter.
Using the trim tanks this way does not always use up air because though you may add the air from your compressed, you don't have to vent it out when done.
You can pass water forward and backward between fore, aft and center trim tanks.
So if you are trying to crash dive, you might send all water from aft tank to fore tank simultaniously makeing the back lighter and front heavier. Further you might pump water into the center tank from sea to make the boat heavier.
Because you are not using air to empty your trim tanks, it matters how strong your pumps are not how much pressure your air tank has.
You do use air to empty your dive tanks. Your main ballast. But when full of water this only = about neutral. If you boat is full of supplies and food and torps to max load, you should still need a bit extra water in your trim to dive. When your boat is fully empty you should have almost full trim tanks.
Your main dive tanks are normally either full or empty. They are full when under and empty when fully surfaced. You don't normally make any adjustment to the main tanks to control depth beyond that.
When surfacing the sub uses trim or dive planes or both to reach the surface. Then it can vent the main ballast from air, tank, or even engine exaust.
You can see this in the simulation. Surface normally in calm water and notice how your sub rides about a meter lower then normally for a few minutes. This is because you are venting you tanks from exaust.
Repeat the experiment but double blow your ballast with the 'e' key (twice). Notice how your sub rides full hight right from the start.
Some subs had tanks specifically for taking on water after a torpedo fires.
Senior Member
Very interesting, guys ..!
Question(s) *
“.. double blow your ballast with the 'e' key (twice). Notice how your sub rides full [r]ight right from the start.â€Â
If doing so, would one be able to acquire greater surface speed sooner, than if waiting for the exhaust to vent tanks?
(Also, if-so) - would this be true in both, SH III and in reality?
Is this also true of all subs of the era â€" regardless of Type?
Best,
~ C.
Senior Member
Submarines also need to keep some movement in order to maintain depth. It is the force of the water moving over the dive planes that enable it maintain control over depth. It is not realistic to turn off the engines and then just sit there indefinitely at a given depth. Eventually the boat will either rise or more likely sink with the engines off.
Sharks do the same thing for the same reason. If they sat still too long they would sink. I'm no marine biologist [George Costanza, where are you when we need you?] but I think this is true. May be wrong on this one.
K61
Senior Member
Forgot to add this to my last post.
The diesel tanks have no bottoms. This may seem strange until you know why. Diesel oil is lighter than water, thus it floats on water. They pump the diesel into the tanks and as it fills, the seawater moves out through the bottom of the tank. As the boat uses diesel fuel, seawater comes in from the bottom to replace the void. This way, the crew do not have to constantly compensate for the weight and volume of the used fuel. The boat maintains balance as fuel is used.
If seawater did not replace the missing fuel oil, air would take its place and cause buoyancy problems.
K61
Senior Member
Member
I'm no George Costanza either but I thought sharks had to keep moving in order to breath. Their gills don't work like other fish, sharks need to keep water passing over their gill to exchange oxegen and waste gases, must be a pain to have to keep moving to breath huh?Submarines also need to keep some movement in order to maintain depth. It is the force of the water moving over the dive planes that enable it maintain control over depth. It is not realistic to turn off the engines and then just sit there indefinitly at a given depth. Eventually the boat will either rise or more likely sink with the engines off.
Sharks do the same thing for the same reason. If they sat still too long they would sink. I'm no marine biologist [George Costanza, where are you when we need you?] but I think this is true. May be wrong on this one.
Also as far as I know U-boats are able to maintain a hover position underwater if the boats trimmed right
Senior Member
Divedevil, I believe you are correct in that. I think if I recall correctly that they also are missing an air bladder that true fishes have, which they inflate/deflate to help them maintain buoyancy.
For those who do not understand, George Costanza is a character on the Seinfeld show. He is a loser type of person who is always getting it wrong with women. In one episode he pretends to a former high school friend that he became a marine biologist after school. She thinks that is great and begins to date him. Then they are walking on the beach and a crowd is gathered around a beached whale. Suddenly someone cries out, "Is anybody here a marine biologist?" Hysterical! George gets this look on his face like he wants the earth to open up and swallow him. One of the Seinfeld classic moments.
As for U-boats "hovering" I wonder if they had sophisticated enough control over their trim to do that?
Member
They cannot "hover" indefinitely. The water temperature changes, currents, and changes in salinity all effect the boats bouyancy. The game seems to simulate this fairly well too as I have noticed trying to sit still waiting in a harbor for night to fall so I could attack and then escape. I tried to remain at a depth of 13 meters, but I constantly had to tell them to stay at 13 meters, otherwise the boat would start to rise or fall a few meters.
Member
U-Boats had the ability to "hover". It just took constant attention on the trim to compensate for the factors mentioned in the above posts. Perhaps not a perfect, absolutely not moving "hover", but close enough.
When bouyancy = weight = "hover".
Granted, it was much easier to maintain depth with some forward momentum so that the dive planes were effective.
I wonder what they actually did in reality. I suppose it depended a lot on sea state. I would imagine that in calm seas, an attempt would be made to hover in order to prevent a perisope wake. In rough seas, it would be more important to have more control over the depth and so probably maintained some speed.
......I guess.
