During a trip the "trim" of the boat alters continually. She is using fuel, ammunition, food, and water, and calculation is necessary to allow for this. Certain tanks are used for this compensating, so that on all occasions when a rapid dive is necessary, there is nothing to do but flood the big external tanks, and yet know that the boat will be in hand when under. If mistakes are made, they will show at once. If too much has been put into the "internals" to compensate, the boat will run on down to the bottom in spite of "up helm" and full speed. If too little, you have to flood internals according to an estimate of what is needed as she ploughs along half-submerged; the latter case is one to be avoided, as you may be killed by the enemy while flooding. The usual war practice is to compensate on the "heavy" side--_i.e._, let her go with a rush and blow tanks so as to catch her and hold her at sixty feet; then you can bring her up to patrol depth at your leisure.
It can be seen, then, that the description of a boat as "bouncing" is not incorrect. When going to the bottom for the night it is a common occurrence, if too rough a "landing" is made, to proceed like a tennis-ball along the sand for a couple of hundred yards. It is a curious thing that both in the Cattegat and the Sea of Marmora, boats have been able to lie for the night with motors stopped at depths of from thirty to seventy feet. In the Marmora the junction depth of the salt and fresh water is about seventy. A boat trimmed with about two hundred pounds of negative buoyancy out there will, if she stops her motors, sink slowly through the upper layer of fresh (or brackish) water, till she meets the denser salt below; on reaching this she will be in a state of "positive" buoyancy, and after a little bouncing up and down to find her "zero" stratum, she will settle at a steady depth. The same sort of thing--a blessing in the Marmora--is a nuisance in the Bight. A boat crossing the mouths of the German rivers may be at one moment diving comfortably with zero helm on the hydroplanes--the next, she meets a layer of fresh water from the Jahde ebb and is bumping on the bottom with "hard-up" helm and the pumps working on the tanks.