What is a trim dive?

The actual weight of the submarine matters a great deal when it was submerged. If it were too light, it would tend to rise and broach the surface. If it were too heavy, it would tend to go deeper, possibly to unsafe depths. Therefore, the crew worked to keep the boat in neutral trim which was safer and makes depth control easier. We want to weigh the same as the water we displace.

The challenge was that the weight of an operating submarine was constantly changing. Consuming food and using supplies make the boat lighter. Burning fuel makes the boat heavier. (See the section on diving and surfacing the submarine.) Changes in the temperature or salinity of the water can make the boat seem heavier or lighter.

As a result, at least once each day, the submarine would have been submerged so that the diving officer can get the boat back into neutral trim. That was the trim dive. It probably takes no more than 10 or 15 minutes. Once the boat was back in trim, it would have been surfaced and previous operations resumed. Trim dives were most common during lengthy transits from one location to another.