Stopping underwater. Any figure?
 

It's virtually impossible to completely balance a sub. So, once you can't use the depth planes to control your depth, you are going to either sink or float. However, this should be quite slow and as long as you don't stay for too long, it shouldn't be a problem.

Another problem would be the trim. Again, without speed to control your attitude, your U-boat is goint to pitch up or down, and this may be much more of a problem.

I would be interested to read real figures about these processes. How fast they happen. Could real U-boat stop for a few minutes? And if they could, did they do it actually? If they couldn't, what was the minimum speed needed to steer the boat?

Re: Stopping underwater. Any figure?
 

You can also control your depth by rasing one or both periscopes couse that way you increase your deplacement. But periscope motors are quite loud, so in a stealth situation that might not be a good option unless you have the possibilitie to raise the periscope manualy without or with little noises.

The trim wouldn't be sutch a problem. If the boat rock upwards than you send a men towards the bow and that should help to trim out the boat for a while.

But that's why an u-baot always dive at least one time per day and trim out the boat for the new water conditions and changed weight and weight distribution on the boat. Trimming calculations are performed regularly in order to have best possible trim when it has to crash dive.

I remember one occasion in WWI where an u-boat had to stop all engines cose there was a hunter group above it at night one boat puted it's hydrophone in to water and listened prior to that the boat stoped and went dead silent. After around 30 minutes, if i remember right the commander ordered to surface the boat very slowly and silent. After they arrived on the surface the commander went on the bridge and saw a few hundred meters away the hydrophon boat the crew manned both duck guns and gaved him a rough wake up call Muahahaha :lol:

Well that depend on how good the boat is trimmed. If it's trimmed very good than ahead slow could be already enough but when the boat dives deeper in colder water layers than it starts to sink faster and they need to press water out of the buoyancy tank quickly or increase the speed significantly. Often, and especialy when the boat dives with a rooky crew in deeper colder layers the boat is being trimmed to a positive buoyancy and than being keept on a curtain depth with the propulsion and a small negative angle. The order the commander gives for that might sound something like this:"Set new depth 30 meters, straight and level 1° downbouble for both engines ahead standart", so the chief navigator will trim the boat in a way that keep that depth with 1° downbouble and both engines ahead standart. Especialy when diving with rooky and it screw up something and the engines would fail for example than the boat would float to the surface or when go down deep in to a colder water layer won't drop immediatly.

wow, so complicated??

Is this similar to modern nuke boats or they don't have such issues as staying still, etc?

What ? About whitch similarity are you talking about ?

A sub can maintain its depth without moving due to the ballast tanks

Yes, of course, but the talk was i think to keep the depth under sealth conditions where you maybe couldn't use high pressure or pumps due to stelth restrictions.

funny I saw this thread, I had no idea that this was modelled, but apparently it is....

I was at full stop, at periscope depth ahead of this convoy, waiting for it to pass by. I got bored just waiting, so I cranked the TC a bit until the convoy got closer.

Once the Sonar man reported that they were starting to cross my bow, I popped to the periscope station, and found that I had to raise it ALL THE WAY to barely be able to see the surface (seas were reasonably calm, no more than 5mps wind speed)

I then noticed that my depth was now 15m... so now I was curious, ignored the convoy and turned up the TC again.... I started sinking (VERY VERY Slowly, mind you) but I was most certainly sinking while at full stop....

Impressed me to find that this was modeled..

Hmm, wouldn't the trim tanks be used for that ?

i mean do modern nuke subs have to play with trimming, and do they also sink if they are stationed in the water at no speed?

Yup, they have to deal with trim. They also have dedicated trim-tanks to help fine-tune their bouancy situation. But, nothing is perfect, and as mentioned they would either sink or float if left at a dead-stop while submerged for too long.