The torpedoes stored at each end of the submarine weigh aproximately one-ton each. The sudden loss of this much weight from the boat when a torpedo is fired would dramatically change the trim of the boat unless compensated for in some way. Because the torpedo tubes are located at the extreme ends of the boat the effect on fore and aft trim would be very severe. This problem is solved through an ingeneous set of procedures.
At either end of the boat, quite near the location of the torpedo tubes are water tanks whose level can be varied. These tanks are named WRT tanks (Water Round Torpedo tanks). When leaving port with a full load of torpedoes on board the WRT tanks contain sufficient water to fill the tubes (i.e.: the space remaining around and in front of the torpedo which is in each tube). Because the filling water is already on board making up a part of the submarine's total weight, the filling process will not result in a change in the boat's overall weight. Also, because the two WRT's are located very near the tubes there will be very little change in fore and aft trim.
When a torpedo is fired and leaves the boat the space in the tube left behind by the torpedo will be filled by seawater entering via the still open outer door. Since a torpedo weighs approximately the same as the amount of sea water it displaces there will, again, be no net change in the weight of the boat nor in the fore and aft trim.
The outer door is now closed and all of the water drained into the WRT, which now will contain the original quantity of water plus an amount approximately equal to the weight of the torpedo which was fired.
Thus, the balance and trim of the boat will not have changed significantly due to the firing of the torpedo!