Physics of subs

I enjoyed the “Figuring Physics” piece in the Dec. 2015 issue,¹ partly because I once served on a nuclear submarine when in the U.S. Navy. We were all very much aware of the factors that affected our buoyant force and we were always striving to keep ourselves neutrally buoyant. The main factor that affected us on a daily basis was the water temperature. As Paul Hewitt mentions in his explanation, water density does change with temperature. If our sub had a displacement of 7000 metric tons and our buoyant force was equal to the weight of the water displaced, when we submerged from near the surface to a depth of 100 meters, the water temperature could drop by 10 °C. This would cause the density to increase by about 0.2%, corresponding to a buoyant force increase of 14 metric tons. As a result the boat would tend to rise...