Main ballast tanks are always completely empty or completely full. Their purpose is to provide or eliminate the overriding postive buoyancy that holds the boat on the surface. The trim tanks are used to regulate the buoyancy of the boat.
While in port between patrols, a careful accounting of how much fresh water, fuel oil, lube oil, hydraulic fluid, torpedo alcohol, stores, etc., etc. is brought on board and the weight of all this is used to figure the initial compensation. One of the first things that is done after clearing the harbor outbound is to conduct a trim dive. It is not uncommon for the boat to have trouble submerging or maintaining depth at this time. As much time as is needed it taken to properly adjust the ballast compensation by flooding water into or pumping it out of the trim tanks. The goal is to obtain perfect neutral buoyancy so that a minimal use of the planes will maintain depth. The whole time the boat is at sea, a running tab is kept on consumables useage and the compensation is periodically adjusted as needed. Trim dives usually took place once a day.
In reality, on the fleet boats, perfect neutral buoyancy is virtually impossible to achieve. The manually operated trim and drain system, while a very good design, was just not sensitive enough to get to that point.
From a submariners standpoint, if you can't get to neutral buoyancy you would default to a slight positive buoyancy. Contrary to popular belief, true crash dives did not happen all that often with our boats (radar gave you enough warning). If you had to go down quickly you could flood the negative tank and you would go down like a rock. I heartily agree with the other posts that it is far better to broach (although obviously not desireable) than to sink. You at least have some chance at survival if you broach, but if you exceed crush depth you WILL die.
Dave
www.pigboats.com