Exactly how do the ballast tanks work? | Forums

[vanjast]

The diesel tanks have no bottoms.

Interesting. this means the whole fuel system was subject to high pressure and shockwaves, unless they had a way if equalising the internal/external pressure difference (Equaliser valve or just a plain valve).

I wonder if they managed to get sea water into the fuel system when they run on empty? This would completely stuff it up.

But come to think of it we've seen many pics of UBoats with their sterns in the air, but no fuel leaking out ??

[Baldricks_Mate]

I don't know the physics but a tactic I have used is to dive before a convoy - deep & quietly rise to periscope depth in the middle and shoot torps everywhere. The escorts race off to an outside point searching for a sub that isn't there. The point is that this is damm hard to do with no forward momentum and if the seas is 10m + the crew struggle to keep the boat at the correct depth, there is a real risk of broaching or the periscope not clearing the waves long enough to fire all the tubes in quick succession. It is a juggle in succession between blowing ballast and the gentle rise/ hover that Anton talks about( there is more to it than just this as well). So the boat's behaviour seems to be well modelled on reality.

Interesting thread!

[the_rydster]

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?

No that is not right the hydrostatic pressure of 1 meter of seawater is around 0.1 bar not 1 bar.

I believe that maintaining the boats depth without moving required a certain amount of skill but was difficult near surface in bad seas.

U boats used to carry equipment for measuring the sea water density for the calculations. The whole think is quite complex.

[W.Irving]

Thank you all, and lecek especially!

Very interesting topic. Never thought the ballast tanks were actually somewhat underdimensioned in the sense that the boat cannot achieve negative buoyancy.

So if I understand this correctly, the game unrealistically allows you to dive even though you have no speed?

[Dominicrigg]

Uboats could dive and surface without forward speed, it would just be very slow without the aid of dive planes.

http://www.usna.edu/NAOE/courses/en200/ch10.pdf

For the super tech heads amongst us!

[Mik1984]

Originally posted by Dominicrigg:
Uboats could dive and surface without forward speed, it would just be very slow without the aid of dive planes.

http://www.usna.edu/NAOE/courses/en200/ch10.pdf

For the super tech heads amongst us!

Nice document. However if you want to surface the standing sub by blowing the main ballast, you would have to be extremely caucious in ofder not to gain any horizontal speed, because of the fores I described in another thread. Unfortunately those forces are not moddeled in the game.

[jeroen-79]

Originally posted by Kaleun1961:
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

As you stated, water is heavier than diesel.
So if the diesel is replaced with water then the boat gets heavier and starts to sink.

It probably is because the water presses on the outside of the tank.
If their would be air in the tanks at surface pressure then the tank would collapse.
If you leave flooding holes at the bottom then the tank will be filled only with noncompressable liquid and the internal pressure would always be equal to the external pressure.

[Anton_Reinhold]

All you would need in those diesel tanks is a hole large enough to allow seawater to come in at the same rate as the diesels draw it out, OR you would have some sort of compensation system that actually pumped water into the bottom of the tank as needed. The latter would make more sense to me, since I would not want to have an actual hole open in the bottom of the tank during a DC attack or when running near or on the bottom of the sea bed where silt would get in. I would think that the trick is to keep the tanks completely full with liquid at all times to keep the fuel and water better seperated by minimizing the "Sloshing about" that an air pocket would allow. I also think that since seawater is heavier than diesel fuel, that they would have to occasionally pump water out of the trim tanks to compensate. Perhaps someone can look this up somewhere and find out for sure exactly how they designed it and how it worked...?

[Dominicrigg]

Also as someone said the seawater could get into the engines if not watched properly. There were 2 fuel tanks and the feed would be switched over before the fuel ran out so as to stop water getting into the system.

I wonder if deisal which escaped to the surface during some attacks was as a result of the boat shaking from side to side, or if it would have to roll to an impossible angle of 90 degrees plus for this to happen. I guess the latter.

[Kaleun1961]

When I said they had no bottom, that was a poor choice of words on my part, for which I apologize. I mean that the bottom is not completely closed in. There were small holes to allow the seawater to replace the drawn fuel. As these tanks were on the outside of the pressure hull, the heavier seawater replacing the diesel would not make the boat heavier.