With low speed it is difficult to maintain depth the deeper you go. Your boat gets compressed by the pressure, reducing its cubic volume. This makes you sink further so the only way to maintain depth is to have water flowing over your control surfaces. It is a delicate balancing act. I was once so deep that I could not maintain depth and she kept dropping lower. I was scared I would get crushed so blew some ballast to come up another 50 metres.
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