Ok, makes sense. I do read it, but my memory lacks appearently. ;p Apologies for making assumptions about your peeve.

//off off-topic

while on the bow planes at 400' on a 350' diesel submarine (USS Salmon SS-573)

I will never forget the Officer of the deck and the Chief of the watch yelling blow negative at the same time.

We came up very fast :yep:

for a more qualified study of surfacing a submarine you can go here:

http://www.heiszwolf.com/subs/tech/tech01.html

Quote:

To surface the boat, the water in the MBT's is forced out by pressurized air. When the boat is deeply submerged, the water is forced out using high pressure air to overcome the water pressure. Once the boat is near the surface, the blowing of the MBT's proceeds with low pressure air. Once at the surface, the Russian boats close the Kingston valve and then opens the main vent valve briefly to equalize the air pressure in the MBT with that of the atmosphere. In the USA/UK boats, the main vent valve remains shut to keep the air in the MBT under pressure. The pressure inside the tanks remains equal to that of the low pressure air system.

Correct. As you go shallower the water pressure decreases allowing more of it to be discharged from the ballast tanks by the higher pressure air. As more water is expelled from the tanks the air pressure will drop as it has more room to expand. As far as compensation is concerned, yes they could open the MBT vents to vent some of that air. But the $100K question is WHY would they want to do that? Like I stated before if you merely wanted to change depth you use your planes to proceed to the new depth. You don't blow ballast to go to a shallower depth.

Actually it's Onkel Neal.:O:
Busted again.:D

Very true. My German isn't what it once was. In fact my German never was what it once was.

"Your" absolutely right.

I meant as a way of slowing the ascent or even to prevent it from speeding up as more and more balast is expelled by the expanding air already in the tanks.
Perhaps similar to rising and descending in a gas balloon?

On the other hand, would decreasing water density compensate anyway by reducig the boat's bouyancy as she rises?

Well in a hot air balloon lift is generated by the hot air in the balloon itself. As the air cools it descends thus making it necessary to heat the air trapped within the balloon to maintain altitude or to ascend. That's pretty well know.

A submarine operates on a completely different principle. When it submerges it takes on water in its ballast tanks to maintain a state of neutral buoyancy. Depth changes are accomplished by the use of the control planes. Once submerged the main ballast tanks aren't touched except to surface the boat. There's no need to blow water out or take water on except for when your displacement would change (consuming food stores, firing torpedoes, etc) and there is an auxiliary system for this purpose.

As for your second question about water density affecting buoyancy, I'll hold off on answering that for now.

I know about hot air balloons, I was making the comparison with gas balloons which are rather different.

Do they close the valves at the bottom of the ballast tanks once blowing has been done and the vessel is on the way to the surface?

My mistake about the balloons. In that case the comparison is even further apart.

Now about your question here, I can only speak to American submarines. The US boats have a vent valve on the top of each ballast tank and flood grates on the bottom. This maintains the water pressure in the ballast tank equal with that of sea pressure on the outside. It is my understanding that Russian boats have valves on the top and bottom of their ballast tanks. They submerge by opening both sets of valves. Once the ballast tanks are full of water both valves are shut. This maintains a constant pressure of the water in the tanks, that which would be nearer to surface water pressure. My conjecture is that you could blow HP air into the tanks and then open the bottom set of valves to force the water out quickly even at great depth. I would imagine that the valves would be closed again once the tanks were blown dry. I either used to have a diagram of this or saw one somewhere but damned if I can find it now. :hmmm: