What was a trim dive?

The objective of a trim dive was to get the boat into neutral trim so that future dives could be done quickly and safely. This was particularly important for the first dive after leaving port to make sure the boat had been compensated for the recent weight changes. There was a trim dive each day while in transit to and from the patrol area. This was needed because of the increased weight of the boat as fuel was used. That was at least partially offset by the food that was consumed, any ammunition used for target practice, supplies that were used, and so on.