During ascent and descent of a submarine, which components are adjusted to control depth and trim?

• A. Adjusting ballast valves only.
• B. Using ballast pumps and depth sensors.
• C. Ballast tanks and navigation charts.
• D. Adjusting ballast tanks and control surfaces to control depth and trim.

Answer: (D)

When a submarine changes depth, two things work together: buoyancy and attitude. Ballast tanks control buoyancy by taking in or releasing water. Filling ballast tanks increases the submarine’s weight and reduces buoyancy, causing descent; expelling water decreases weight and increases buoyancy, causing ascent. To keep the hull at the desired angle during that vertical movement, horizontal control surfaces—the dive planes or hydroplanes—adjust the pitch, helping to trim the submarine so it remains stable or moves at the chosen angle.

Depth is mainly about changing buoyancy with ballast; trim is about attitude with the control surfaces. Ballast valves alone would change buoyancy but not fine-tune the angle; ballast pumps and sensors contribute to the system but don’t directly provide the combined actuation for depth and trim; navigation charts don’t affect depth or attitude. Therefore, the combination of ballast tanks and control surfaces is what manages depth and trim.