When blowing ALL the compressed air you have stored forcing all the water out of the ballast tanks, diving immediately without charging means you have lost all the stored pressure necessary to bring you back to the surface using ballast tanks. Since there is no compressed air, you have also lost the ability to trim and control your depth. You can take on water to sink, but you cannot push it back out again. Planes and propulsion are not enough to compensate for the water you have taken on to dive. Your boat essentially becomes a rock with an engine.
If you are surfacing to save the crew with enemy nearby, then you blow emergency in order to abandon ship at the surface increasing your crew's ability to survive.
If you are blowing to get to the surface so you can make enough repairs to sail home (in the game, get to the map edge or abort the 3D sim) then there are no attacking enemy around and you have time to recharge.
Other than these two reasons, you should not be blowing emergency.
As stated above, you should only do this to save the crew and in most cases, you have probably lost the fight.
-Pv-
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